Foresight Exploiting the Electromagnetic Spectrum

State of the Science Review

Picturing people: non-intrusive imaging

Douglas J Paul Cavendish Laboratory, University of Cambridge

This review does not represent the view of the DTI or Government policy, but is an account of the state of the art in the field by the commissioned author. This document is one of four state of the science reviews produced for the four topics selected for detailed study in the Foresight Exploiting the Electromagnetic Spectrum project. Further details are available at the Foresight web site: http://www.foresight.gov.uk/ Contact the Foresight Exploiting the Electromagnetic Spectrum team at: Foresight Directorate Office of Science and Technology 1 Victoria Street London SW1H 0ET Fax: 020 7215 0054 E-mail: EM.spectrum@dti.gsi.gov.uk

1

Summary

Non-intrusive imaging affects almost every person in the UK at some level. From

close circuit television (CCTV) cameras in cities through x-rays and MRI at

hospitals to the metal detectors at airports, few people can live without crossing a

non-intrusive imaging technology at some time in their lives. The fast pace of

technology is going to increase the number of non-intrusive imaging techniques

and systems in the future.

Historically the UK has been involved in the development of a number of imaging

modalities including x-ray, radar, sonar, MRI and x-ray computed

tomography (CT). The UK has a strong presence in new emerging imaging

modalities including microwave, terahertz and infrared. The UK also has strength

in component technologies and techniques including contrast agents, image

analysis techniques and superconducting magnets.

In medical imaging there are clear drivers towards non-ionising modalities (non-

intrusive imaging) such as ultrasound, MRI and thermography rather than PET,

x-ray CT and gamma-ray, where possible. Many of the ionising technologies still

provide functional and diagnostic information that cannot yet be obtained from

the non-invasive technologies. There is also a substantial drive towards

multimodal imaging systems where the high resolution of one technique is

combined with the functional information of another to allow diagnosis which was

previously impossible or difficult with the one technique. Genetic research on

small animals is also driving new small scale imaging tools such as PET, MRI

and x-ray CT. The visible and near infrared are presently poorly utilised by the

medical community but potentially may allow new techniques especially in biopsy

and cancer screening if technical problems can be solved.

Security imaging is presently driven by the application of detection of suicide

bombers and improved airport security in the present world climate. Microwave

2

and terahertz technologies have demonstrated potential in these areas but are

still immature technologies requiring further research.

The terahertz region of the spectra is the one area where little has been clearly

achieved due to the lack of practical sources and detectors. With recent

technological advances the first systems are becoming available and first

demonstrations of a number of different applications have been achieved. All

molecules (biological, organic, inorganic, etc) have vibrational and rotational

spectra that lie in the terahertz frequency range with signatures resulting from

intra- and inter-molecular interactions. The wavelengths are short enough to

enable sub-millimetre imaging while long enough to penetrate many materials

allowing hidden objects to be imaged. The UK has a strong presence in this field

both industrially and academically and there are numerous potential applications

ranging from explosive identification and suicide bomber detection to skin cancer

imaging.

In all areas of imaging, image analysis and display is required. The UK has

significant expertise in the development of a number of the algorithms and

techniques presently used for image analysis and the extraction of additional

information. Automated image recognition along with automated diagnosis for the

medical fields are areas with good potential for breakthroughs in image analysis

software.

Database and information management along with fast delivery of imaging

information are areas which need to be addressed as the size and number of

images increases as technology improves. There is already some funding

through EPSRC for e-GRID and information extraction from images but more

investment could put the UK into a very strong world position. While the UK has

little manufacturing of the microelectronic components in computers, hard disk

drives and storage media, it does have significant expertise in computer

architecture (e.g. ARM Holdings) and computer systems design.

3

Contents 1. Introduction and Background 6 1.1 Present Exploitation of the Electromagnetic Spectrum 6 1.2 Imaging Systems 7 1.3 Limitations: Resolution, Detectivity, Screening, Speed, Cost 7 1.4 Technology Push, Application Pull 122. Drivers for Medical and Security Imaging 13 2.1 Applications Drivers, Technology Drivers and Markets for Medical

Imaging 13

2.2 Applications Drivers, Technology Drivers and Markets for Security Imaging

14

3. Imaging Technologies 18 3.1 Electrical and Magnetic Source Imaging 18 3.1.1 Background to Electrical and Magnetic Source and Impedance

Imaging 18

3.1.2 Present Electrical and Magnetic Source Imaging Technology 19 3.1.3 Future and Emerging Electrical and Magnetic Source Imaging

Technology 19

3.2 Ultrasound and Sonar 20 3.2.1 Background to Ultrasound and Sonar 20 3.2.2 Present Ultrasound and Sonar Technology 21 3.2.3 Future and Emerging Ultrasound and Sonar Technology 21 3.3 Magnetic Resonance Imaging and Nuclear Magnetic Resonance 23 3.3.1 Background to MRI and NMR 23 3.3.2 Present MRI and NMR Technology 24 3.3.3 Future and Emerging MRI and NMR Technology 24 3.4 Microwave Imaging 27 3.4.1 Background to Microwave Imaging 27 3.4.2 Present Microwave Imaging Technology 28 3.4.3 Future and Emerging Imaging Technology 29 3.5 Terahertz Imaging 29 3.5.1 Background to Terahertz Technology 29 3.5.2 Present Terahertz Technology 30 3.5.3 Future and Emerging Terahertz Technology 30 3.6 Infrared Imaging 33 3.6.1 Background to Infrared Imaging 33 3.6.2 Present Infrared Imaging Technology 33 3.6.3 Future and Emerging Infrared Imaging Technology 35

4

3.7 Visible Imaging 36 3.7.1 Background to Visible Imaging 36 3.7.2 Present Visible Imaging Technology 37 3.7.3 Future and Emerging Visible Imaging Technology 37 3.8 X-ray Imaging 39 3.8.1 Background to x-ray Imaging 39 3.8.2 Present x-ray Imaging Technology 39 3.8.3 Future and Emerging x-ray Imaging Technology 40 3.9 Radionuclide Imaging 41 3.9.1 Background to Radionuclide Imaging 41 3.9.2 Present Radionuclide Imaging 41 3.9.3 Future and Emerging Radionuclide Imaging 42 3.10 Hybrid Technologies and Data Fusion 43 3.11 Other Emerging Non-invasive Imaging Technology or Developments 444. Developments in Component Technologies 44 4.1 Image Analysis and Display 44 4.2 Phase Contrast Techniques 46 4.3 General Components: Computer and Data Storage Technology 475. Health, Safety, Ethical and Civil Liberty Issues of Imaging Technologies and

Applications 50

5.1 Health and Safety 50 5.2 Ethics and Medical Imaging 51 5.3 Civil Liberty 516. Conclusions 52 Acknowledgments 55Appendix 56I. References 56II. UK Expertise 61III. Acronyms and Technical Terms 63

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1. Introduction and Background

1.1 Present Exploitation of the Electromagnetic Spectrum

Imaging can be described as the display of the interactions of radiation of a

particular frequency or frequencies with the body being imaged. Images of a

complex body can reveal characteristics of the body such as transmissivity,

opacity, emissivity, reflectivity, conductivity and magnetisability. Images that

reveal one or more of these properties can also be analysed to extract additional

information. As an example the properties of x-rays transmitted through some

body can be used to extract the effective atomic number, physical density and

electron density.

Historically medical and security or defence imaging have been linked mainly due

to the similar requirements of sources, detectors or underlying techniques for

very different imaging applications. As an example, sonar techniques for

detecting submarines were later transferred and developed into the medical field

of ultrasound imaging. Scintillator counters and nuclear reactor prepared

isotopes were developed during the Manhattan project before appearing much

later in medical imaging systems.

Fig. 1 is a schematic diagram of the electromagnetic spectrum giving the energy

and wavelength of different photons as a function of frequency. The major

allocation of bands are shown although the precise division between different

bands and also the actual names used vary in a number of different fields. On

the right of Fig. 1, the typical type of photonic interaction with material is listed

ranging from interactions at the nuclear level at the highest frequency to

interactions with the nuclear spin at the lowest frequency. It is these interactions

which define the type of information that can be extracted using different

frequencies of radiation for imaging purposes and determine what parts of the

spectrum can be exploited.

6

Most of the electromagnetic spectrum is now exploited for imaging purposes to

some level. The terahertz is one region which is substantially under-exploited

due to a lack of cheap and compact sources and detectors.

1.2 Imaging Systems

Imaging systems can be divided into a number of different types (Fig. 2). Active

systems are where a source of radiation is used to illuminate the body being

imaged and some form of detection is used to form the image. Passive systems

rely on the detection of the emission from the body itself or of reflection or

interactions with background sources of radiation. Each of these types of system

can be subdivided into direct-detection, time-gated or heterodyne. Heterodyne

detection is where a local source close to the frequency of the detected signal is

mixed with the signal to provide amplification through interference. Time-gated

imaging includes both interferometric and tomographic imaging as both use the

time of flight of a photon to extract additional information, either by adding a

delay or by measurement at different distances or angles. Direct-detection is a

system where the photon is directly absorbed in the detector without any

additional techniques before detection to improve image quality or information.

1.3 Limitations: Resolution, Detectivity, Screening, Speed, Cost

Resolution limit: The resolution limit for a system with a lens forming the image

is derived by considering when the centre of the Airy’s disc of one point source

falls on the first minimum of the Airy’s disc of a second point source of radiation.

This defines the Rayleigh or diffraction limit of resolution as

Dfλ22.1min =∆λ (m) (1)

where f is the focal length and D is the aperture diameter. Appropriate lenses are

not available for frequencies above the ultraviolet apart from electromagnetic

lenses for charged particles such as electrons and protons1. Equation (1) states

7

that the resolution is proportional to the wavelength of the radiation being used.

Noise-Equivalent Power (NEP): The noise equivalent power of a photodetector

is defined as the power that corresponds to the incident rms (root mean square)

optical power ( 2/optrms mPp = with m the modulation index) required such that

the signal-to-noise ratio is one in a bandwidth of 1 Hz2.

Fig. 1: The electromagnetic spectrum with designated bands and the main electromagnetic

interaction mechanisms with atoms for imaging at different frequencies, energies and

wavelengths.

8

Fig. 2: The different types of imaging systems with some examples.

Detectivity (D*): The detectivity of a photodetector is defined as

NEPAD ν∆

=* (cm√(Hz)/W) (2)

where A is the cross-sectional area of the detector in cm, ∆ν is the bandwidth of

the circuit and NEP is the noise equivalent power2.

Noise equivalent temperature difference (NE∆T): The NE∆T is used

frequently in the infrared and microwave community to compare the sensitivity of

detectors. It is defined as the minimum temperature difference that can be

measured with a signal-to-noise ratio of 1 in a bandwidth of 1 Hz. In the

Rayleigh-Jeans limit this gives

ν∆=∆Τ

BnkNEPNE (K) (3)

where n is the number of modes, kB is Boltzmann’s constant and ∆ν is the

bandwidth.

Quantum efficiency: The quantum efficiency of a photodetector is the number

of electron-hole pairs generated for each incident photon

opt

p

Ph

qI νη = (4)

where Ip is the photogenerated current from the absorption of incident optical

power, Popt at frequency ν, q is the electronic charge and h is Planck’s constant2.

Skin depth: The skin depth is the length scale of the exponential attenuation of

radiation in conducting materials. It is defined as 9

πνµσδ 1

= (m) (5)

where ν is the frequency, µ is the permittivity and σ is the conductivity of the

material. It is especially important for metals where the skin depth can be very

small leading to complete screening of the radiation. This can also be important

for clothes in security screening or medical imaging or for different parts of the

body such as skin1.

Speed: The maximum time for obtaining an image is related to the application.

For security applications where real-time video rate is required, it is normally

defined as 30 frames per second (fps). In medical imaging the maximum imaging

time can sometimes be limited by the ability of people to hold their breath or to

the time a child can controllably be kept still. Longer times can therefore reduce

resolution if the body being imaged is moving during acquisition. High speed

imaging normally suggests the need for focal-plane arrays of detectors and

sources allowing parallel rather than serial data acquisition.

Cost: Reducing system cost is always important if availability and market

exposure is to increase. While for defence applications high costs can be

tolerated in a number of applications, high costs for medical imaging results in

few hospitals with access to an imaging modality. Cheap, automated systems

allow accurate diagnosis even at a GP surgery, reducing patient waiting times

and providing fast diagnosis.

Semiconductor manufacturing techniques available for sources below 1 PHz

demonstrate the ability in many regions of the electromagnetic spectrum for

cheap, large and compact arrays of sources and detectors to be realised. The

cost of sources scales in an approximate linear fashion between 1 GHz and the

visible except for a region between approximately 100 GHz to 10 THz. In this

region present sources costs are orders of magnitude above the rest of the

spectrum (Fig. 3)3.

10

There are still many areas where reduced cost could provide significantly higher

access to a particular technology especially in a number of medical applications.

Fig. 3: Schematic diagram showing approximate source costs as a function of frequency.

Between 0.1 and 10 THz the cost of sources is substantially higher than other parts of the

electromagnetic spectrum3.

11

Fig. 4: The transparency of the atmosphere at sea level. Data is taken from the HITRAN 2000

database14 with fog and rain data from15, 16.

1.4 Technology Push, Applications Pull

Most of the early imaging technologies have originally been developed by

technology push. Technology push is the normal driving force if suitable sources

or detectors are not available for an application as it is the availability of

technology that drives the application. In a number of cases it is the application

which drives new technology development. A good example is ultrasound which

was developed by using much of the technology already available for sonar but

the application of the technology to human imaging pulled the technology into a

specific application.

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2. Drivers for Medical and Security Imaging 2.1 Applications Drivers, Technology Drivers and Markets for Medical Imaging

The current global market for medical imaging is around £8.8 billion per annum.

The main drive from applications in medical imaging is related to moving to safer

imaging techniques where especially non-ionising sources of radiation are used.

MRI, infrared or ultrasound are therefore preferred technologies compared to x-

ray, gamma or PET from a safety perspective4. There is still key information

which can only be extracted by radiology or nuclear medicine modalities and

hence multimodality image fusion is a major application driving research and new

systems. There are also other ethical drivers in determining which imaging

modalities will be favoured.

There is a drive to less invasive surgical techniques which can also be helped

with new imaging modalities4. Higher resolution is also being used for

identification of better imaging targets by genome-based approaches. Such

techniques are aimed at diagnosis of disease at the molecular level5. Imaging for

medical robotic applications is also a driver where guidance through imaging

modalities can be used to guide robotics used for surgery6,7.

Technology drivers in most areas of medical imaging are cost reduction, faster

image acquisition times and higher resolution and/or sensitivity. This is especially

true in techniques such as PET and MRI which are used much more frequently in

hospitals in the US than in the UK due to cost restraints.

The increased rise in resolution in many techniques such as computed

tomography x-ray scanning and MRI has resulted in many hospitals experiencing

dramatic and often exponential increases in the amount of data generated4.

Computer speed and power along with increased memory storage, therefore, are

key technology components for image analysis and storage. The ability to

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search, process and analyse data is limited by the available computer technology

- both hardware and software. The data explosion is predicted to get worse as

multimodality imaging increases. Technology is required to allow fast automated

extraction of relevant information from large databases to allow clinical

diagnosis4.

The visible and infrared regions of the spectra have to date been poorly utilised

for medical imaging. Problems of absorption, low resolution and non-specificity

have resulted in the techniques being unpopular with clinicians. New technology

in these fields could potentially open up new imaging techniques and

methodologies.

The terahertz or far-infrared part of the spectrum has potential for medical

imaging as all molecules have parts of their vibrational and rotational spectra in

this region8. There have been a number of first demonstrations of medical

applications but this region is limited by the available technology9,10,11.

Technology is therefore the driver for the appearance of practical imaging

systems. Some oncology applications are now in pre-clinical trials but very little

detailed work has been completed. There are also no studies of the interaction of

the radiation with biological tissue and numerous ethical issues have yet to be

considered.

Table 1 gives a summary and comparison of the main medical imaging

modalities including emerging technologies.

2.2 Applications Drivers, Technology Drivers and Markets for Security Imaging

Security imaging at airports is potentially the largest market for security imaging

systems outside military markets. The US has 400 airports with a market of £3.2

billion for security imaging systems for people and luggage. The UK has over 45

airports with commercial flights and worldwide there are over 1520

14

Plan

ar X

-ray

X-ra

y C

T

Gam

ma

cam

era

imag

e

Gam

ma

cam

era

SPEC

T

PET

Ultr

asou

nd B

-sca

n +

Dop

pler

imag

e

MR

I im

age

MR

I spe

ctru

m

Hyp

erpo

laris

ed M

RI

Ther

mog

ram

OC

T

Tera

hert

z Pu

lsed

Im

agin

g

Tera

hert

z Sp

ectr

osco

py

Ionising radiation? Yes Yes Yes Yes Yes No No No No No No No No Anatomy or functional A A F F F A+F A+F F F F A(+F) A+F FData acquisition time <1 s few s 5 min 20min 20min 1 s 20min 20min 1 s <<1 s 10 s 1 min <1s Data reconstruction + process time

2 min 2 min <1 s 10min 10min < 1 s 2 min < 1 s < 1 s << 1 s <1 s < 1 s < 10 s

Patient handling time 5 min 30min 10min 30min 30min 10min 1 hr >1hr 10min 10min 10min 10min 10minSpatial resolution <1mm >1mm >5mm >5mm >5mm >30µm >1mm >1mm >1mm >1mm >15µm >20µm >20µm Depth >body >body >body>body >body >body>body >body >body 3mm 3mm3mm 3mmSensitivity (1 poor to 5 great)

2 4 2 3 4 3 4 2 4 4 3 4 5

Availability of equipment in NHS hospitals (1 poor to 5 good)

5

2

3

2

1

5

1

1

1

2

2

1

1

Capital cost of equipment

£70k £500k £300k £200k £0.5-2M £10k £0.8k £1.5M £0.9M £50k <£50k £250k £250k

Table 1: Comparison of commercial and emerging research medical imaging modalities (modified from17). Sensitivity of a diagnostic technique can be defined as the number of correct positive assessments divided by the number of truly positive cases.

15

airports registered for commercial flights suggesting a total market of around £10

billion for airport security scanning.

One of the main applications drivers at present from anti-terrorist applications is

detection of suicide bombers12. At present only hand searching can be used on

people due to safety concerns with x-ray techniques. The US military estimate

that the death of a service personnel costs around $2M including funeral, legal

and pension costs for the deceased’s family. The only present solution is a low-

dose x-ray technology which cannot be used in any western countries and the

use in a number of other countries poses serious ethical issues. Technology for

detection of human suicide bombers in airports or at an entrance to a compound

or government building can certainly be envisaged as a walk-through system with

operators remote from potential suspects.

The detection threshold distance for suicide bombers can be reduced if

appropriate protection screens can be used for equipment and operators12. For

airports, scanning rates for people require a maximum imaging time of around 20

seconds although ideally this should be well below 10 seconds. For military

personnel in hostile countries video rate (30 frames per second) is required for

soldiers walking through crowded city areas such as streets or markets. The lack

of transparency in certain parts of the atmosphere limits the technologies which

might be able to be used over large distances for suicide bomber detection

(Fig. 4).

Large vehicle bomb detection is much more difficult to envisage especially in

populated areas due to the screening properties of metals used for the vehicle

bodies. Only high energy radiation such as x-rays can penetrate such metal but

such radiation has health restrictions12.

For the UK police, firearms detection on suspects is now a major applications

driver especially with the present increase in firearms offences. Imaging time for

such an application needs to be of the order of a second and preferably real-time

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or video rate. The technology needs to be portable, battery powered and

compact. As there is no present remote solution to this problem, any new

technology could have significant market penetration especially in foreign

countries such as the US with its large number of legally carried guns.

Automotive and aircraft radar to allow navigation and target acquisition in fog is

extremely difficult in the infrared due to strong absorption (Fig. 4) but in the W-

band between 90 to 100 GHz there is a good window of operation. These

applications are major drivers to develop W-band (Fig. 5) imaging components

and systems. Most of the development work is military-based at present but there

is a substantial commercial aviation market which can be targeted with

appropriate technology.

The terahertz is a technologically developing area with many first demonstrations

of applications but little detailed research. The combination of transparency to

clothing combined with spectroscopy of illicit materials such as narcotics,

bioweapons or explosives could allow detection and potentially identification of

many different types of materials9. Imaging and spectroscopic identification of

explosives through a number of layers of clothes has been demonstrated at 1

metre distance but requires development to allow imaging at safe distances13.

This field is still very young and is presently driven by the availability of

technology.

Infrared technology has been driven by night-vision applications for the military.

Such technology is now extremely advanced with many different products

available for passive imaging particularly in the 8 to 12 µm window of the

spectrum (Fig. 4) and at shorter wavelengths. The field is still technology driven

better performance (and/or cheaper) detector technology becoming available will

open new applications.

The technology for the visible part of the spectrum is now being driven by

entertainment and consumer products such as digital cameras for imaging

17

applications. The charged coupled displays (CCDs) now available in cameras

cost around £400 for 5 million pixels and £1300 for 12 million pixels. Such CCDs

are also available in security cameras both for operation in city streets and in

cheaper systems for small businesses and homes. Such systems also use

computer technology rather than video to record and store information,

significantly reducing the cost of systems and their operation.

X-ray systems are still prevalent for airport baggage and cargo imaging. The

main driver is increased sensitivity for functional identification. There are also

low-dose x-ray systems available for weapons detection on humans but there are

significant ethical implications to the use of such ionising radiation techniques.

3. Imaging Technologies 3.1 Electrical and Magnetic Source Imaging

3.1.1 Background to Electrical and Magnetic Source and Impedance Imaging

Electrical or magnetic imaging in this section relates to techniques from ~10 MHz

in frequency down to 1 Hz. At such frequencies the wavelength is greater than

10 m and resolution is a significant issue. All the techniques described below are

both non-ionising and non-invasive. The techniques also provide additional

information to other more standard techniques.

Electrical source imaging (ESI) uses electrical field measurements to construct

maps of the underlying electrical activity. Nerve impulses are electrical currents

that propagate in neural tissue and ESI measures the electrical fields that are

produced by the current flow. ESI is an extension of electroencephalography

(EEG) and electrocardiography (ECG) that permit identification of sources of

intense electrical activity in the brain and heart18,19.

18

Magnetic source imaging (MSI) or magnetic induction tomography constructs

images from either the magnetic fields emerging from electrical current flow from

neural and other tissue18 or from measuring eddy currents induced by applied

magnetic fields20. Superconducting quantum interference devices (SQUIDs) are

used to map out the magnetic activity of numerous parts of the body but

especially the brain (magnetoencelphalography)21.

Electrical impedance imaging or applied potential tomography is based on

measuring the difference in electrical conduction and / or permittivity (potential

difference) that occurs among tissues when a voltage is applied18,20-23. Since

certain tissues exhibit large differences in electrical resistivity, high contrast

resolution should be achievable if the problems of spatial resolution can be

solved22.

For security imaging metal detection uses techniques in this part of the frequency

spectrum.

3.1.2 Present Electrical and Magnetic Source Imaging Technology

At present there are no commercial ESI, MSI or electrical impedance imaging

systems on the market. All present systems are being used as research tools.

Metal detectors either as portal walk-through systems or hand-held detectors are

a mature technology with many companies selling such systems.

3.1.3 Future and Emerging Electrical and Magnetic Source Imaging Technology

All three medical techniques described in 3.1.1 are emerging techniques that

have particular advantages for extracting specific information but have only been

used as research tools in combination with other techniques such as PET or

MRI.

19

ESI has the potential to improve the diagnosis of certain abnormalities such as

epilepsy and disorders in impulse conduction in the heart18. It could also be

useful in guiding surgical procedures and in monitoring the effectiveness of

certain drug treatments.

There are severe limitations to ESI which include:

1. Poor spatial resolution (e.g. 30 mm in detection of the thorax22).

2. Significant absorption of the radiation in bone.

3. Difficulties in identifying and localising multiple sources of electrical

activity from the remote measurement of electric fields.

MSI is a far newer technique with the first reports of the technique dated around

1992. The technique has been researched as a potential methodology for clinical

evaluation of epilepsy, migraine headaches and diabetic comas18. It may also

have potential applications in studying brain responses to a variety of conditions

including auditory, olfactory and visual stimuli. MSI is also showing promise for

studying the physiology of neural tissue and the cognitive properties of the

brain18-20.

The main limitation with electrical impedance imaging is that there is seldom a

single resistance path between two electrodes placed on the human body. The

major applications which are under research include cardiac output

measurements, pulmonary oedema monitoring and tissue characterisation in

breast cancer18,22,23.

3.2 Ultrasound and Sonar

3.2.1 Background to Ultrasound and Sonar

Ultrasound and sonar are techniques which rely on the reflection and refraction

at interfaces between two media with different acoustic refractive indices of

20

longitudinal waves. Ultrasound also extracts additional information from the

absorption of the signal as it is transmitted though different materials along with

using interference effects from the coherence of the radiation. As sound waves

propagate at significantly slower velocities than electromagnetic waves (e.g.

1540 m/s in the soft tissue of a human17) the wavelength associated with a

1 MHz sound wave is 1.5 mm and 0.1 mm at 15 MHz. For sonar systems the

velocity of sound in water is around 440 m/s corresponding to 0.5 mm at 1 MHz

and 5 cm at 10 kHz. For these wavelengths wave optics is appropriate rather

than geometrical optics and both diffraction and interference limit the resolution

of the techniques. That is, the resolution is already at the limit set by the optics

and so other techniques must be used for improvement.

3.2.2 Present Ultrasound and Sonar Technology

Ultrasound systems are presently sold by a large number of different companies

including GE Medical Systems, Siemens, Toshiba and Hitachi along with a few

UK companies including Dynamic Imaging. Ultrasound systems are cheaper to

purchase, operate and maintain than x-ray computed tomography and MRI

systems.

Sonar systems are sold by many UK and foreign companies with numerous

applications from naval, mapping of the ocean floor24, navigation in shallow

waters to detection of cables and oil rig inspection.

3.2.3 Future and Emerging Ultrasound and Sonar Technology

Developments are still being pursued for new high-density transducer arrays, one

and one-half dimensional transducers, broad band transducers, increased

scanner bandwidth and more sophisticated image formation and analysis

routines25. Functional ultrasound and quantitative measurements are also now

possible where previously it was believed that ultrasound could not obtain such

information25.

21

Ultrasound is getting much better for vascular applications particularly. Sound

waves are used to break up blockages in arteries so that the remaining pieces

are small enough to be transported away in the bloodstream from the blockage26.

Longitudinal sound waves require rigid equipment for the production of

ultrasound which is not easy to deploy to many parts of the body especially

during surgery. Transverse sound waves are being developed which can be

waveguided down flexible tubes allowing much more compatibility with present

techniques4.

Ultrasound research is also aimed at trying to produce functional information for

clinical applications. The Doppler effect can be used to monitor any changes in

blood flow and could potentially be used to monitor anti-angiogenesis cancer

therapies27 which attack the growth of new blood vessels in tumours4. At present

ultrasound is only a qualitative modality for such applications and quantitative

data is required.

There is potential for interventional ultrasound with, e.g., sound activated

liposomes. Different contrast agents are also being researched to enhance

images. Suspensions of small bubbles which are only a few microns in diameter

are used as ultrasound contrast agents and exposure to the ultrasound

frequencies resonates the bubbles increasing the reflected signal28,29. High

intensity pulses can burst the bubbles resulting in even higher contrast. Such

techniques are also being investigated as possible methods for targeted drug

delivery4.

Research in sonar systems is predominantly aimed at image analysis techniques

from efforts to obtain additional information for instance in tomographic mapping

of the ocean floor24 through wavelet analysis to study sediments on the ocean

floor30 to techniques for reducing errors from towed arrays31.

22

3.3 Magnetic Resonance Imaging and Nuclear Magnetic Resonance

3.3.1 Background to MRI and NMR

Magnetic Resonance Imaging (MRI) through nuclear magnetic resonance (NMR)

can now be used to image almost any organ in the human body4,17. The MRI

technique measures the distribution of water molecules when placed in a

magnetic field using radio waves and hence is non-intrusive and involves non-

ionising radiation. The UK has been one of the pioneering countries of MRI and

in 2003 the Nobel Prize for Medicine was awarded to Sir Peter Mansfield of

Nottingham University and Paul Lauterbur of the University of Illinois in

recognition of their work in developing the technique. MRI is derived from the

original development of nuclear magnetic resonance by Felix Bloch and Edward

Purcell in 1946.

MRI and NMR use the fact that the hydrogen nuclei within water molecules have

an inherent magnetic moment due to the nuclear spin17. If the spins are placed in

a magnetic field, quantum mechanics dictates that the nuclear spin must be

parallel (lower energy state) or anti-parallel (higher energy state) to the magnetic

field pulse of electromagnetic radiation with frequency

Bmeg

pπν

4= (5)

can excite the nuclear spins to the higher energy state (e is the electron charge,

g is the g-factor for the nucleus, mp is the proton mass and B is the applied

magnetic field). For a 1 T magnetic field, the transition frequency is 42.6 MHz.

When the nuclear spin realign with the applied magnetic field, a radio signal of

the same frequency is emitted.

MRI uses a second, weaker magnetic field with a gradient that decreases linearly

in strength across the sample. This field changes the frequency at which

hydrogen nuclei absorb and emit radiation across the sample and hence spatial

23

information can also be extracted from the sample. NMR is typically used in

chemistry and biochemistry laboratories to identify a large number of different

nuclei in a sample and obtain the relative abundances of the different species.

3.3.2 Present MRI and NMR Technology

There are numerous companies selling MRI machines at present including GE

Medical, Siemens and the UK company Resonance Instruments. The costs are

still relatively high and the availability in NHS UK hospitals is still not complete.

NMR systems are sold by the UK companies Oxford Instruments and Resonance

Instruments although the main companies for chemical NMR spectrometers are

Bruker, Jeol and Varian. UK companies such as Oxford Instruments and Magnex

Scientific also sell superconducting magnets for NMR and MRI systems.

Nuclear quadrupole systems are predominantly in development and testing

although a US company Quantum Magnetics sells explosive detection imagers

for airports.

3.3.3 Future and Emerging MRI and NMR Technology

The single greatest problem of MRI is the sensitivity of the technique. One

approach to dealing with this is to increase magnetic field strength. This has

been a widely favoured approach though it is also by far the most expensive.

Alternative strategies include improvement in radio frequency coil design and

increased polarisation particularly for the lower sensitivity nuclei such as 13C.

Work, however, has proceeded somewhat slowly, is limited in scope and - for

polarisation- still is focused in basic physics/chemistry laboratories. High

temperature superconducting (HTS) magnets are presently being researched

both for increasing magnetic fields but also to increase the operating temperature

for better signal to noise ratios32,33.

MRI scanners used during surgery must have open magnet structures to allow

24

the surgeon to have full access to the patient. Such magnet systems have limited

magnetic fields of up to 0.6 T while closed bore systems can have 3 T or more.

These lower fields produce lower signal to noise ratio and poorer resolution.

Potential solutions are unlikely to result in cheaper MRI tools. MRI guided

procedures are being used for numerous applications including neurosurgery and

cardiac operations where accuracy and risk from x-ray exposure are a prime

importance4.

Functional MRI is one of the areas with significant research interest. Techniques

to investigate the human brain, such as blood oxygenation level dependent

functional MRI34,35 along with structural, biochemical and functional pathological

information from multiple sclerosis36 demonstrate the direction in which the

research is being pushed. The functional MRI techniques are attempting to

obtain information which previously was only available by radiological techniques

such as PET, but using a non-invasive and non-ionising technology. For the brain

imaging, neurons are about 5 to 10 µm in spacing and resistance arterioles are

about 0.5 to 1.0 mm in spacing while present MRI resolution is of order of 1 to a

few mm35. A number of techniques are being researched to improve the

resolution. This is covered in another EEMS project state of the science review

‘Inside the wavelength: electromagnetics in the near field’.

Micro-MRI in conjunction with new image processing techniques and feature

extraction techniques is being used to obtain detailed structural information of

vertebrae bones37. While resolution is limited by the signal-to-noise ratio, the

voxel size achievable in vivo is of the same order of thickness as the trabeculae

(100 to 150 µm). Such techniques are being used in the study of

postmenopausal osteoporosis, male hypogonadism and secondary

hyperparathyroidism.

Genetics and MRI imaging is of real interest. A number of proofs of principle

have been achieved38,39. Basically, imaging might be difficult as a quantitative

trait marker simply because of the numbers of studies that would be needed.

25

However, it may be extremely good for:

1. Exploring heterogeneity in populations otherwise phenotypically

indistinguishable, or, if a trait is clear by imaging, refining phenotypic definition in

otherwise indistinguishable groups (e.g. pre-symptomatic Alzheimers)

2. Defining specific functional and structural features associated with the tails

of behavioural curves.

3. Helping to understand on a systems level the expression of genetically

transmitted traits (e.g., understanding the evolution of pathology in an inherited

disease as a clue to mechanisms of progression).

Hyperpolarisation is the enhancement of nuclear spin by irradiating atoms with

polarised light40,41. Typical atoms include 3He, 129Xe, 13C, which can have

increases of nuclear spin polarisation of over 104 when optically pumped40-43. The

technique therefore opens up MRI imaging of the lungs and pulmonary airspaces

including morphologic imaging of airways and alveolar spaces and analysis of

the intrapulmonary distribution of inhaled aliquots of the tracer gases40,41. While

the natural abundance of 13C in biological molecules is too low to be used for

MRI, hyperpolarisation enhancement of 104 of the nuclear spins have allowed

subsecond angiography in small animals to be obtained42. Hence the

hyperpolarisation technique allows 13C to become relevant for practical medical

and diagnostic work. Injected molecules in rabbits have been tracked by MRI of

hyperpolarised 13C in the injected molecules41.

Nuclear quadrupole resonance techniques are an emerging technology being

investigated for security imaging. SQUIDs are used for the remote measurement

of magnetic fields especially for the detection of explosives through NMR

detection of 14N. Research is presently aimed at detection of landmines44-46. One

US company sells portal detectors for people and imagers for baggage and

postal imaging.

26

3.4 Microwave Imaging

3.4.1 Background to Microwave Imaging

In this section microwaves will be defined as the region from 1 GHz to W-band

(110 GHz) (Fig. 5) where there are plenty of sources and detectors on the

market. The microwave part of the spectrum has good atmospheric transparency

to radiation over most of this range (Fig. 4) although there is a strong absorption

from an oxygen line at 80 GHz. Millimetre wave systems can penetrate poor

weather and battlefield obscurants far better than infrared or visible systems.

There are a significant number of applications using this part of the spectrum

(Fig. 6) and for non-intrusive imaging many of the main communications regions

need to be excluded to avoid interference effects.

Fig. 5: The band designations for the microwave part of the electromagnetic spectrum above 8

GHz

The region has to date predominantly been used for security imaging47 especially

to see through fog and rain (Fig. 4) but there is research on microwave

tomography for medical applications48 and also automotive imaging for

navigation in fog (Fig. 6). Systems have also been used to demonstrate

concealed weapons detection on humans as clothes are transparent in this

frequency range (approximately constant transparency between 60 and 150

GHz)49-52. The technology has also been used for the detection of illegal

passengers on lorries as 90 % of lorries in Europe have non-metallic sides53.

From Fig. 1 and Fig. 4 it should be clear that there is a drive towards lower

frequencies in the microwave region since the atmosphere and also clothes are 27

more transparent at the lower frequencies but then the wavelength is increasing

and so resolution does become an issue.

3.4.2 Present Microwave Imaging Technology

The UK is well placed in this field for passive millimetre wave imaging systems

with the work of QinetiQ49-58. Systems operating at 35 GHz54 and 94 GHz55 have

been used as demonstrators for numerous applications. The QinetiQ 94 GHz

system uses mechanical scanning along with compact folded optics54. These

systems operate at video rate with a field of view of 60 degrees by 30 degrees

with diffraction limited performance (that is, at the limit of the optical system) over

two thirds of the field of view. They have the potential to be used not just for

military imaging applications but also for airport security and the illegal smuggling

of people at ports.

Fig.6: The designated frequencies and bands for a number of different applications in the

microwave part of the electromagnetic spectrum

Compared to many imaging technologies the systems are not expensive, the

major costs are probably the Indium Phosphide (InP) MMIC used as detectors.

There is therefore the potential to reduce the cost of systems as new technology

becomes available.

There is plenty of research in microwave tomography for medical applications48,49

but few systems are being marketed commercially.

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3.4.3 Future and Emerging Imaging Technology

The component technologies required for microwave imaging systems continue

to improve due to the relentless improvement in semiconductor technology. InP

MMICs are presently the main semiconductor technology for W-band power

amplifiers and low noise amplifiers (LNAs) while at the lower frequencies GaAs

technology dominates. At the lowest frequencies (1 GHz) Si is now a competing

technology60 and SiGe HBTs now provide many power amplifiers and LNAs in

mobile phones and in consumer products below 10 GHz60. The cost of the

detectors will therefore continue to fall as semiconductor technology progresses.

Microwave tomography in medical imaging is being aimed at a number of

applications including cardiovascular applications48.

3.5 Terahertz Imaging

3.5.1 Background to Terahertz Technology

For this review it is practical to define the terahertz region of the electromagnetic

spectrum as that from above W-band (110 GHz) to 10 THz. This then defines the

terahertz region of the spectrum as that which has not been fully utilised due to

the lack of practical, cheap and coherent sources. It is only in the last 6 years

that suitable sources and detectors have become available in the research

laboratories (Fig. 7) and the first commercial imaging system (TeraView Ltd.)

became available in 20029. All molecules (biological, organic, inorganic, etc)

have vibrational and rotational spectra that lie in the terahertz frequency range

with signatures resulting from intra- and inter-molecular interactions (Fig. 1)8,10,11.

The wavelengths are short enough to enable sub-millimetre imaging while long

enough to penetrate many materials allowing hidden objects to be imaged.

Techniques, therefore, allow 3D imaging of many objects8,9.

While there have been many first demonstrations of terahertz imaging in different

29

applications, there are very few, if any detailed studies as yet.

3.5.2 Present Terahertz Technology

At present there is a single terahertz imaging system commercially available

which is sold by TeraView Ltd. in Cambridge7. The UK is therefore extremely

strong in this emerging field both from an industrial but also academic basis. The

imager uses photomixer technology with a femtosecond pulsed laser illuminating

low temperature GaAs as the broadband terahertz source (300 GHz to 3 THz).

The system uses rastering of a single spot to build up an image and the detected

signal is mixed optically with the original laser source to provide coherent

detection. Imaging time is between 10s of seconds to minutes depending on the

resolution used. The femtosecond laser pushes the cost of such systems to over

£250k.

3.5.3 Future and Emerging Terahertz Technology

There are very few terahertz components such as power amplifiers, LNAs or

mixer technologies commercially available and this is where most development is

still required before cheap, efficient and compact terahertz systems can be

produced (see Fig. 7 for present sources). The terahertz is predominantly being

driven by technological development as better sources and detectors are

required before applications can be pursued.

30

Fig.7: The present sources of radiation between 100 GHz and 10 THz10, 11, 61-64. QCL=quantum

cascade laser, RTD=resonant tunnelling diode, MMIC=monolithic microwave integrated circuit.

There are many first demonstrations of applications in the terahertz with regard

to non-intrusive imaging and these will be reviewed below.

Medical imaging and diagnostics: Work at Cambridge has demonstrated the

ability to detect basal skin carcinomas8,65,66 (the most prevalent form of skin

cancer). The ability of terahertz pulsed imaging to recognize high contrast

between small changes in the density of soft tissues suggest great promise in

cancer detection. Clinical trials are now being started on this technique. The

current global market for medical imaging is US $14.1 billion per annum and

business analysis has suggested that terahertz might be able to capture 10 % of

this within 10 years.

31

Dental imaging: Terahertz has been used to image dentine, enamel and pulp

cavity in teeth67. It has also demonstrated imaging of early caries (tooth erosion -

Fig. 2) therefore allowing dentists to prevent cavity formation and may aid the

identification of periodontal disease68. Terahertz imaging has also been used to

measure the thickness of enamel (Fig. 2) and the amount of demineralisation in

teeth, a growing problem with the large consumption of highly acidic, fizzy

drinks9,67. Terahertz produces higher contrast images than x-rays and is non-

ionising.

Security scanning: While x-rays can be used to detect metals and numerous

sharp objects, health concerns limit their use in scanning humans. Terahertz,

however, has been used to detect metal, plastic, GRP and ceramic knives

underneath a number of layers of different clothing materials69,70. Transmission

through cotton teeshirts is almost 90 % while leather transmits 8 % and a

computer laptop bag 3 %70. One major problem is the complete absorption by

metals which limits the applicability without other techniques such as x-ray for

baggage inspection. Initial research has also demonstrated the potential for

identifying unknown substances such as drugs or explosives69. Explosives

detection on people through a number of layers of clothes has demonstrated

explosives detection on people at a distance of 1 m in the research laboratory

with spectroscopic identification of the explosive13.

Postal scanning: Initial results on letters have shown that THz can detect the

number of pages in a letter and also detect materials between pages including

ink69. There is therefore the possibility of scanning mail to identify dangerous

materials71.

Biological weapons detection: The US Army Research Office has been

funding research on detection of a number of pathogens including anthrax.

Similar DNA species of fish have been differentiated which is useful in the first

stages of determining whether biological material is dangerous but unique

determination of pathogens has yet to be demonstrated by any technique 72.

32

Landmine detection: Modern plastic landmines are significantly more difficult to

detect than older mines with metallic parts. THz detection potentially provides a

more robust and accurate detection technique than the present schemes73.

3.6 Infrared Imaging

3.6.1 Background to Infrared Imaging

Infrared technology has been driven by predominantly military applications for

night-vision imaging72. The technology has also been applied for thermal security

cameras and for the fire service to image through smoke. As almost everything

emits a blackbody spectrum in the infrared most imaging systems are passive.

On the medical side there is work on thermography but the non-specificity of the

technique has limited the potential applications and use73. With new technology

now offering better detectivity and NE∆T of 0.04 K or better at video rates, there

may be a new opportunity for thermography as a non-ionising and non-intrusive

technique.

Predictive maintenance by using thermal imaging of electrical and production

machinery is also a growing market for infrared non-invasive passive imaging

systems.

3.6.2 Present Infrared Imaging Technology

Available infrared detector technology includes:

1. Bolometers and Microbolometers: operate by measuring the resistance of a

material with a temperature dependent resistance. Microbolometers can be

fabricated using microelectronics fabrication techniques allowing large arrays to

be fabricated at low cost.

33

Fig. 8: The present detectors in the infrared and visible. Thermal detectors are in green2, 72, 74,

photoconductors are in red2,quantum well infrared photo detectors are cyan2, 75 and photovoltaic

detectors are in blue2.

2. Ferroelectrics: the dielectric constant of the material varies with temperature.

3. Quantum well infrared photodetectors (QWIPs): the current is proportional

to the number of absorbed photons. These are not broadband sources.

4. Photovoltaic: HgCdTe, InSb and Schottky barrier PtSi detectors whose

voltage varies with the number of absorbed photons. InSb provides the best

performance between 0.2 to 5 µm or so at 77 K while HgCdTe provides the best

performance between about 2 and 15 µm at 77 K. PtSi devices have poor

quantum efficiency.

34

5. Thermal detectors: Golay cells operate by detecting the change in pressure

of a gas as a function of temperature. These detectors are slow and cannot

easily be made into arrays.

The best detector is a function of the precise frequency required to be detected.

CMOS fabricated microbolometers are just emerging and allow large focal plane

arrays of detectors to be fabricated cheaply and offer excellent detectivity

at 300 K below about 5 µm (Fig. 8) with NE∆T of about 0.2 K76. For higher

detectivity, detectors require cooling. QWIPs at 77 K have demonstrated the

highest detectivity below about 9 µm and photoconductors at 4 K are required for

high detectivity in the far infrared. At 77 K QWIP focal plane arrays can now

achieve NE∆T of 20 mK with a detectivity of 1011 cm√Hz/W 77.

Complete thermal imaging systems exist for many different applications and are

manufactured and sold by a large number of UK companies. The UK has

significant expertise in this field.

3.6.3 Future and Emerging Infrared Imaging Technology

Research is still aimed at improving passive detector technology especially

techniques which can produce large focal-plane arrays of detectors which have

better detectivity, higher operating temperatures and better NE∆Ts.

Skin and subcutaneous tissue are partially transparent in the visible and near-

infrared78. Breast cancer detection is being researched in this part of the

spectrum combined with the visible as a non-invasive and non-ionising

alternative to x-ray techniques78.

A developing field is that of chemical imaging cameras where infrared technology

using focal-plane detector arrays from military technology are used to produce

chemically specific composition and distribution in samples in seconds80,81.

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3.7 Visible Imaging

3.7.1 Background to Visible Imaging

Visible imaging is probably the oldest field in this review but to some extent it is

limited to imaging surfaces or near subsurfaces by the poor penetration depth of

most materials in the visible. There are now numerous security cameras in most

UK cities, all linked to the police to provide security imaging for crime prevention

applications.

Medical imaging also uses visible techniques, predominantly for skin problems or

near surface imaging but also for many surgical procedures such as with

endoscopes. While such techniques are invasive, they have significantly

improved patient recovery time for certain surgeries where key hole techniques

can be used. The ability to guide visible radiation down flexible fibre optic cables

can provide information which is not easy to obtain using other imaging

modalities. Such systems are also extremely useful for many other applications

such as pipe inspection in hazardous environments, etc.

Optical coherence tomography (OCT) is an emerging biomedical imaging

technique that now performs high resolution, cross-sectional tomographic

imaging of microstructure in biological systems82,83. The commercial systems are

based on a fibre optic Michelson interferometer with a low-coherence length light

source. OCT has achieved image resolutions of 1 to 15 µm84 with the penetration

depth in tissue limited by optical scattering to 2 to 3 mm85. Although the depth is

shallow compared to many other modalities, the resolution is 2 to 3 orders of

magnitude finer than conventional ultrasound, MRI or x-ray CT. The resolution is

approaching that of conventional histopathology.

OCT technology provides in-situ imaging in real time allowing surgical guidance,

guidance of biopsy or investigation of dynamic response to therapeutic agents.

OCT imaging may be carried out using non-invasive or minimally invasive

36

delivery systems such as microscopes, handheld probes, endoscopes, catheters,

laparoscopes and needles. OCT can also perform functional imaging such as

spectroscopic imaging of tissue properties, Doppler blood flow measurement and

quantification of blood oxygenation or tissue birefringence83.

3.7.2 Present Visible Imaging Technology

The original drive for large focal plane arrays of detectors in the visible and the

corresponding optics was driven by spy planes and satellites for military

reconnaissance. Charge couple devices (CCDs) in commercially available digital

cameras now offer 5 million pixels for under £400 and 12 million pixels for £1300.

These are fabricated using silicon microelectronics and as such the UK does not

have any native manufacturing companies. The major profit, however, is related

to the system, architecture and operation of such systems. The UK has many

companies which design and sell complete visible imaging systems for many

different applications. Complete security systems for homes or small businesses

are now available which use computer technology rather than video to record

images from digital cameras thus substantially reducing costs.

There are many visible and laser-based imaging systems for medical

applications. Most are aimed at simple inspection while research is aimed at

obtaining more information from biological specimens86,87. OCT systems are

dominated by the US company Carl Zeiss Meditec with over 1500 tools in

hospitals worldwide.

3.7.3 Future and Emerging Visible Imaging Technology

Medical optical imaging remains one of the great unrealised areas of potential. If

problems of detection sensitivity, tissue absorption and scattering could be

satisfactorily solved, it offers a new approach to highly specific “tag imaging” (like

PET) without radiation. Applications include breast cancer detection78, optical

37

biopsy87, and molecular imaging87. Research is also aimed at finding contrast

agents to improve present technology88.

OCT is heavily dependent on the technology base of photonics and lasers. The

recent short pulse laser sources have enabled the image resolution to be

reduced to the order of 1 µm. The fibre and diode femtosecond lasers being

developed may be able to reduce this further. The high resolution allows cellular-

level image resolution producing visualisation of mitotic cycles and a track for cell

migration84,89. Single photon detection will also improve the OCT technique.

Superconducting / insulating / superconducting (SIS) detectors were the first to

achieve single photon detection sensitivity but are limited to liquid He

temperatures90. HTS may improve this to 77 K but there are technological

problems with the ceramic HTS materials. Semiconductor quantum dots have

already demonstrated single photon detection at 4 K integrated in a conventional

field effect transistor (FET) to provide an amplified electrical output91. Quantum

dot technology is now close to 77 K and has the potential with development to

reach room temperature operation. Using semiconductor materials it also has the

potential to be fabricated into large arrays at cheap cost.

Broadband light sources are also attractive for spectroscopic OCT imaging

providing functional information83. The different optical properties of tissues allow

enhanced tissue contrast through spectroscopic imaging. In addition, it is

possible to perform quantitative imaging of tissue chromophores or functional

state. Metabolic indicators such as tissue hydration or oxygenation of

haemoglobin in vascular tissue can be measured92. Other tissue properties such

as birefringence can be imaged and quantitatively measured93.

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3.8 X-ray Imaging

3.8.1 Background to x-ray Imaging

X-ray imaging is the original medical imaging technology with the first medical x-

ray image being taken in Birmingham in January 1896 after the discovery of x-

rays by Roentgen at the University of Wurzburg in November 1895. The

transmission and detection of x-rays is still the basis of numerous medical

imaging techniques including radiography, angiography, fluoroscopy and

conventional mammography examinations. Since x-rays are ionising radiation,

such techniques are not completely non-invasive although in applications such

as imaging the lungs, other cheap and practical techniques are far more invasive.

For security monitoring a large number of items can be scanned without the

requirement of opening every item, reducing time and cost of weapons or

narcotics detection.

3.8.2 Present x-ray Imaging Technology

There are many different companies selling x-ray tools for medical imaging

including GE Medical, Philips, Siemens, Toshiba and Hitachi. These tools are

used for many different applications or modalities in medical imaging including

radiography, angiography, fluoroscopy and conventional mammography

examinations.

Medical systems range from planar x-ray systems to modern computed

tomography (CT) systems designed to produce a series of 2D slices through the

body. The planar x-ray systems can be divided into cassette-based computed

radiography systems94 and instant-readout digital radiography systems95. Digital

radiography systems can be further divided into direct or indirect conversion

systems where the x-ray is either directly converted into an electrical signal or for

the indirect process first converted into a photon which is then converted into an

electrical signal. Modern equipment has moved from film-based systems to

39

digital systems which use caesium-iodide scintillators coupled to flat-panel

detectors. Charged coupled device arrays (CCD) are also used for detection in

some systems.

CT systems are also available in either sequential scanning of consecutive slices

or multi-slice spiral systems which now allow resolution down to ~1 mm96. The

latter systems are substantially faster and can have reduced radiation doses (up

to 50 %) compared to the former with appropriate techniques96.

For x-ray security scanning for baggage and cargo there are a large number of

commercially available systems.

3.8.3 Future and Emerging x-ray Imaging Technology

A significant part of research in x-ray technology is the development of arrays of

detectors with higher sensitivity particularly for medical imaging where such

detectors allow the reduction of radiation dose to the patient94-98.

Fast CT has a real role, particularly because of clinical practicalities (e.g.

perfusion imaging, angiography), but may have something of a limited range of

entirely novel applications, just because x-ray computed tomography contrast

mechanisms are limited. As scanning speeds improve Dynamic CT imaging is

also developing as scanning speeds improve where reconstructing image

sequences to see the dynamic nature of the living human body is of primary

interest99.

CT is presently being fused with other techniques such as PET and MRI (since it

provides higher resolution) to provide anatomical information, but, for example,

PET is especially good at mapping areas of metabolic hyperactivity.

40

3.9 Radionuclide Imaging

3.9.1 Background to Radionuclide Imaging

Unsealed radiopharmaceutical imaging in the form of gamma camera, single-

photon emission computerised tomography (SPECT) and positron emission

tomography (PET) cannot strictly be considered non-intrusive forms of imaging

as ionising sources of radiation are used but the associated low morbidity of

patients does allow these functional techniques to be classed as such. These

techniques all rely on the detection of gamma-ray, x-ray or positron emission

from radionuclides in the human body which are used to label substrates,

ligands, drugs, antibodies, neurotransmitters and other compounds which are

tracers of specific biochemical or physiological processes. As such these are

functional imaging modalities that provide clinical information regarding

biochemical and physiological processes in patients. Since changes in the

biochemical or physiological processes are frequently the first manifestations of

disease before structural changes can be imaged, the techniques provide an

important role in evaluating and managing patients with disease100.

3.9.2 Present Radionuclide Imaging

Anger designed the original so-called Anger gamma camera in the 1960s and the

gamma cameras101 found in most routine nuclear medicine departments

(manufactured by GE Medical, Philips Medical and Siemens) and very little has

changed from the original design. The basis of the camera design for all three

techniques relies on scintillations (NaI(Tl) for gamma-ray and SPECT with

bismuth germinate (BGO) for PET). Gamma-ray and SPECT require collimation

of the beam while PET uses detection coincidence of the two photons created by

annihilation and travelling in the opposite directions. Photomultiplier tube arrays

are then used to collect the visible photons and convert the light into electrical

signals.

41

There are about 400 PET scanners in hospitals worldwide with 12 in UK research

institutes or hospitals and 3 commercial organisations offering PET scans in the

UK. There are more gamma camera systems but again numbers are limited in

UK hospitals. As cyclotrons are required to generate the short half-life

radionuclides (e.g. 15O) for PET systems, the costs of complete systems

including the preparation of the radionuclides are relatively high (~£2M). A

gamma camera typically costs £300k to £400k.

3.9.3 Future and Emerging Radionuclide Imaging

The major disadvantage of gamma camera, PET and SPECT systems is the

relatively poor resolution. Typical resolution is limited to around 5 mm or greater.

The clear benefits of these functional imaging techniques to the UK have been

stated102,103 but research is required on improved resolution.

One of the promising approaches to gamma ray detection is the use of

semiconductor detectors such as CdTe104 and CdZnTe105. Both can be used at

room temperature and convert single photons of tens to hundreds keV energy

directly into electrical signals. The increase in spatial and energy resolution of

these techniques is translated into higher contrast resolution and improved

detection of abnormalities. The second technique being pursued is the use of

silicon CCD detectors in place of the photomultiplier tubes105. Both technologies

reduce the weight of the camera to a level which allow more portable applications

rather than a fixed portal system.

Attenuation correction systems are also being developed to circumvent the

physical phenomena that degrade the degree to which the detected counts

represent tracer activity and thus degrade diagnostic accuracy106.

There have been a number of attempts at reducing the cost of systems, as this is

the major factor in the low number of systems in hospitals worldwide. The cost

42

and imaging performance are directly related to the total number and type of

detector surrounding the patient. The standard BGO geometry detectors are

normally arranged in ring formation and new geometries have been attempted to

reduce the number of detector elements and hence cost whilst maintaining the

imaging performance107,108.

3.10 Hybrid Technologies and Data Fusion

There is substantial research at present to develop the potential for integrating

different imaging modalities in a simple way. For example, it is now possible to

obtain electroencephographic and functional imaging and structural imaging data

all in one examination. The techniques, however, remain cumbersome.

Similarly, demonstrations in animals have shown how PET and MRI data can be

obtained together. Finding these approaches to joint data acquisition and mutual

information promises to enhance the potential of the techniques4,5.

Hybrid imaging systems are also a major emerging area with PET/SPECT,

PET/MRI and PET/CT scanners being developed especially for cancer imaging.

PET is especially good at mapping areas of metabolic hyperactivity (a high

indication of cancer being present) while multi-slice CT has sub-millimetre

resolution providing a clear picture of anatomical detail. By combining the two

techniques, tumours can be accurately located and the size and shape

assessed. This is potentially valuable when characterising or staging tumours

when they are first diagnosed and then monitoring their response to treatment.

PET has a role both in metabolic imaging (with resolution enhancement in part by

data fusion with structural MRI) and in rather cutting-edge techniques such as

receptor mapping, benefiting from the high sensitivity and potential specificity.

There are also major developments in high resolution, low-cost micro-PET

imagers for small laboratory animals to allow research in genetics5,100. There are

three options for generating combined systems such as the PET / CT combined

system: a) collect the PET and CT images separately and then use software to

combine the data, b) fix a patient to the bed and then quickly obtain both images

43

in separate machines or c) develop hybrid machines with both imaging

technologies. The first two techniques have significant disadvantages but can be

used with minimum new capital costs. The third is that preferred by the

equipment manufacturers mainly because of the ability to sell new machines.

3.11 Other Emerging Non-invasive Imaging Technology or Developments

One of the emerging or longer term areas of imaging for the medical field is out-

of-hospital or health monitoring in the home109-111. With advancements in sensor

technology along with communications and IT, there are an increasing number of

papers in the literature discussing options for wearable sensors and imaging.

While much of the literature is aimed at non-imaging applications, similar local

area networking (LAN) reading of sensors and transmission of data to a local

doctor or hospital can be envisaged for imaging applications. It is more likely that

local doctors will use such technology first to send data and gain diagnosis from

a specialist in a non-local hospital rather than true health monitoring in the home.

Cost is likely to be the major limitation to such technology being adopted

although the communications infrastructure is becoming more widespread.

4. Developments in Component Technologies 4.1 Image Analysis and Display

Powerful new approaches to analysis of outcome data based on collection of

information from multiple sources including clinical, testing, imaging are being

incorporated into novel clinical trials. These approaches include adaptive

designs, in which prospectively acquired information defining, for example,

subgroups can be used to provide the most sensitive outcomes measures. This

should be particularly useful in finding efficient strategies for early Phase II trials.

Radiology is in general still dominated by practitioners who read simply by visual

44

inspection. This approach suffers from considerable observer bias along with

variable sensitivity and specificity. Automating the process of extracting or

recognising objects out of an image is useful in many imaging applications as

potentially allowing automated diagnosis or recognition of dangerous events or

objects in military or security applications112. There are a variety of modern

techniques for comparison between normative data basis with specific image

data in order to identify pathology. Demonstrations of the potential of this have

been made in research laboratories, but refinement and wide practical

implementation of this technology has yet to be performed. There is a need for

research concerning problems of effectively matching tissues between images

(problems such as non-linear warping), developing tissue classification

approaches based on multimodal datasets and incorporation of prior information

in effective ways.

Image segmentation, that is, the process of defining objects in images, remains

the most challenging problem in image processing. Object regions manifest

themselves in images with a heterogeneity of image intensities owing to the

inherent object material heterogeneity, and to artifacts such as blurring, noise

and background variations introduced by the imaging system. Knowledgable

observers can perceive object regions as a gestalt despite this graduation of

intensities. The ‘fuzzy correctedness’ framework is one image analysis technique

which defines how the image elements hang together spatially in spite of their

graduation of intensities113. In defining objects in a given image, the strength of

correctedness between every pair of image elements is considered, which in turn

is determined by considering all possible connecting paths between the pair. This

research technique is being used in the fields of multiple sclerosis of the brain,

magnetic resonance and computer tomographic angiography, brain tumour,

mammography, upper airway disorder in children and colonography112,113.

CT is a computationally intensive process and one attempt to produce lower cost

systems is the use of fast Fourier transforms (FFTs) for Fourier-based

reconstruction114,115. These techniques can be implemented cost effectively using

off-the-shelf components such as digital signal processors (DSPs) which are

45

used in all forms of imaging applications and already have a strong market in

graphics cards for personal computers.

The Fourier reconstruction requires images to be acquired using a fan beam data

scanner to obtain a radial set of projections. Once the data is obtained it goes

through preprocessing (filtering), weighting and a rebinning before the Fourier

reconstruction is implemented14.

The hybrid imaging discussed in section 3.10 has produced a number of software

systems aimed at registration of parts in different images116 along with automated

procedures for aligning images from different modalities or detecting large

misregistration117.

4.2 Phase Contrast Techniques

Contrast / Imaging agents are an increasingly exciting field. Medical imaging is

now moving from simpler techniques which highlight structural ‘contrast’ between

different tissue types, to sophisticated ‘molecular imaging’ agents, capable of pin-

pointing very early stage pre-symptomatic functional changes in tissues.

Examples under development include hypoxia for cancer (PET), cell tracking

(PET, MRI) in cell therapy, microglia activation (PET), gene expression tracking

(optical, MRI). Ultrasound cavitation may also be an appropriate contrast

mechanism in certain situations, though it lacks the molecular ‘precision’ of

nuclear imaging29. Imaging agents have the transformational potential to drive

earlier diagnosis and as a result better and more cost effective health outcomes.

Regulation and poor uptake of new agents inhibit research in this area in the UK.

Despite this the UK-based company Amersham is the world leader in this field.

Over the last 5 years, our knowledge of true molecular biology has expanded

exponentially through the decoding of the human genome. It is the belief of

some that the major scientific challenge of the early 21st century is to translate

46

this ‘raw’ molecular knowledge into useful biomedical products. There is

consensus that advances will come quicker in molecular diagnostics than in

genomically-derived therapeutics, which will take longer to develop. The major

medical challenge is therefore to transfer this genomic and proteomic knowledge

into practical imaging products. Early diagnosis has the potential to transform the

cost and quality of health services along with creating a significant export base

for UK industry. Early diagnosis also has the power to enhance the effect of

prevention campaigns. These often founder due to the individual’s ignorance of

early pre-disease changes taking place in their bodies, which, if unmanaged, will

lead to debilitating and expensive clinical conditions in later years. The

development of such products will demand a multidisciplinary approach involving

experts from the fields of biological sciences, chemistry, computer science,

engineering, medicine and physics coupled with industrialists to fully address the

requirements of these molecular imaging techniques.

Phase maps in an image may contain additional information about the body

being imaged. For example the velocity of blood flow can be encoded in the

phase of MRI signals118. Phase unwrapping is the technique used to attempt to

extract such information with a number of different analysis techniques119. The

UK has a number of strengths in this area. Phase-based frequency estimation is

a related field which uses analysis to remove unwanted noise which may distort

the phase information120.

4.3 General Components: Computer and Data Storage Technology

Many of the techniques described above rely on data processing and analysis to

enable the imaging technology, to enhance the images obtained or to extract

additional information from an image. Key to all these imaging modalities is the

available computing power and also the ability to store such data. The

semiconductor industry publishes a roadmap60 every 2 years to analyse what is

required for the industry to maintain the exponential improvement in technology

which has been achieved over the last 40 years (and arguably centuries). Recent

47

analysis suggests that this exponential increase in performance is unlikely to be

halted before 2015 at least from technological issues121,122.

The basis of the improvement in performance has been termed Moore’s law123

after Gordon Moore who first analysed the technology scaling in 1965. He

demonstrated that transistor feature size decreases by a factor of 2 roughly every

18 months. This is predicted to continue until around 2015 where the feature size

becomes less than 10 nm and quantum mechanics produces a number of

problems preventing further transistor miniaturisation. The decrease in feature

size produces an increase in the number of transistors and this accounts for the

improvement in computer processing unit (CPU) performance (Fig. 9) and the

increase in dynamic random access memory (DRAM) density60 (Fig. 10).

Fig. 9 plots data from the literature and data sheets from companies of different

processors and supercomputers124. While the actual performance of a CPU is

dependent on the application, software and efficiency, the graph plots the

number of floating point operations that have been achieved by different

processors per second as this gives a good software- and applications-

independent performance estimate. Fig. 10 plots the major semiconductor

memory along with magnetic hard disk data from IBM125. While DRAM is

predicted to scale until at least 2015, present magnetic disk technology will hit

problems within 5 years when the superparamagnetic limit of the present

technology is reached. A number of new technologies are being researched to try

to circumvent this problem. For imaging processing, latency is the biggest

problem which is related to the bandwidth of the buses in the different parts of

the computer. The off-microprocessor address bus is typically running at about

one tenth the microprocessor clock speed and therefore is also increasing

exponentially with time. This accounts for the semiconductor memory to

microprocessor data transfer (and vice versa). The access time to hard disks has

been reducing much more slowly and this is likely to be the limiting factor. As

DRAM and flash become cheaper and magnetic RAM becomes more prevalent,

48

direct writing into the semiconductor memory will also reduce latency. Recalling

stored images from storage databases will also be significantly slower.

Computer grid and database technologies are also important areas especially in

hospitals where multiple image modalities may be used for diagnosis127.

Communications and networking technologies are key components of such

systems. At present the bandwidth of data transfer is still increasing at significant

rates but limits are being imposed by junk such as spam on internet systems.

Fig. 9: A plot of the floating point operations per second for central processing units (CPUs) and

supercomputers commercially available versus year. The ITRS roadmap60 suggests the

performance increase should continue to at least 2015 and present supercomputer

performance124 is already above the predicted 2015 CPU performance.

49

Fig. 10: A plot of magnetic hard disc drive and dynamic random access memory (DRAM) areal

memory density as a function of year. For present magnetic drives, the super-paramagnetic limit

to the technology is also plotted60, 125.

5. Health, Safety, Ethical and Civil Liberty Issues of Imaging Technologies and Applications

5.1 Health and Safety

Health and safety considerations are always required when developing new

techniques. Imaging modalities with ionising radiation have always had to be

designed with the health and safety of both operator and the patient in mind18.

For new technologies where non-ionising radiation is being produced,

consideration must be given to interactions which may be harmful to humans.

Examples of emerging areas include microwave ovens and mobile phones which

operate either at or close to the vibrational frequency of water molecules.

Terahertz is a good example where the radiation corresponds to the vibrational

and rotational frequencies of most biological molecules.

50

5.2 Ethics and Medical Imaging

To a large extent the ethics of medical imaging, and in particular the relationship

of the companies which manufacture and research medical imaging modalities,

has a large number of common features with the pharmaceutical industry108.

There are three main ethical concerns with medical imaging:

1. The possibility that associations between doctors and medical imaging

companies may serve commercial objectives of industry and acquisitive interests

of clinicians rather than legitimate care, educational or research goals, thereby

compromising the primary ethical obligation of doctors to patients, dividing the

loyalties of doctors and undermining the basic trust on which clinical relationships

depend.

2. The risk that product promotions from companies will inappropriately influence

doctor’s decisions.

3. The danger that industrial involvement in research will lead to distortions in

scientific evidence and prevent independent assessment of data.

5.3 Civil Liberty

There is a growing debate about the use of the numerous images which are now

readily available to the police, law courts, security services and government. With

CCD cameras in every major UK city and medical imaging data of patients stored

on computer networks in hospitals, the issue is one of confidentiality and of when

data can be used ethically or for legal purposes without the individual’s consent.

As computer databases of images increase, this problem is going to become

more significant in the future. To a large extent civil liberties will be controlled

through the legal process such as the Data Protection Act 1998, but as science

and technology is progressing at such a fast pace, the law does not always keep

up. This issue is far more complex when security imaging is discussed as many

lives may depend on the information which has been obtained from an imaging

51

modality. With a number of security scanning systems able to image through

people’s clothing, there are also issues over the right to capture such images,

confidentiality and discreet use of the images produced.

6. Conclusions

Non-intrusive imaging is a field which is presently growing and has substantial

world-wide markets. The medical imaging market is approximately £8.8 billion

per annum, airport security is around £10 billion and anti-terrorism and defence

sectors are certainly >£10 billion per annum. The UK has had a strong position

historically of being involved in many of the developments of modern imaging

technologies for numerous applications. These include the first use of x-rays, the

development of MRI and x-ray computed tomography. The UK also has a strong

position in world terms with a number of imaging modalities which are presently

emerging technologies including microwave, W-band, terahertz and infrared

technologies from both industrial and academic perspectives. There are a

number of areas of imaging research and development where government help

and steer would significantly help UK industry.

In medical imaging there are substantial benefits to UK society but the research

cannot always be commercially driven. One of the problems in addressing MRI

research issues has been that while industry can see the possible benefits, any

individual company also appreciates that the results of investment in developing

the technology would be shared and thus not clearly provide a “competitive

edge”. This is exactly the sort of situation in which Government needs to step in,

develop and to some extent nurture technologies where appropriate and make

them available for industrial exploitation in a way that maximises competition.

Table 1 shows a comparison of some of the important parameters in medical

imaging. The clear drivers in medical imaging are aimed at moving to

technologies which are less invasive and non-ionising such as ultrasound, MRI

and thermography rather than PET, x-ray CT and gamma-ray. The real need is

52

development of cheaper technologies which provide functional information. While

ultrasound and thermography can be qualitative and in some applications non-

specific, there is a drive for attempting to develop these technologies into full

diagnostic tools in the hope of replacing the present expensive and/or ionising

modalities. There is also a substantial drive for multimodal imaging systems

where for instance the high resolution of one technique may be combined with

the functional information of another.

Imaging research is also being driven by genetics applications where miniature

PET, MRI and CT scanners are being developed to image mice which are being

studied in genetics research. The visible and near infrared part of the spectrum is

presently poorly utilised by the medical imaging community due to the poor

detection sensitivity, tissue absorption and scattering. If these could be solved

there is great potential for a number of applications including breast cancer

screening.

The clear applications drivers in security imaging at present are detection of

suicide bombers along with explosives identification for airports. A number of

microwave and terahertz technologies have shown through first demonstrations

that they have potential in these areas but significantly more research is required

before products will be available.

The terahertz region of the spectra is the one area where little has been clearly

achieved due to the lack of practical sources and detectors. As technology

progresses, systems are now available and first demonstrations of a number of

different applications have been achieved. All molecules (biological, organic,

inorganic, etc) have vibrational and rotational spectra that lie in the terahertz

frequency range with signatures resulting from intra- and inter-molecular

interactions. The wavelengths are short enough to enable sub-millimetre imaging

while long enough to penetrate many materials allowing hidden objects to be

imaged. The UK has a strong presence in this field both industrially and

53

academically and there are numerous potential applications ranging from

explosive identification and suicide bomber detection to skin cancer imaging.

In all areas of imaging, image analysis and display is required. The UK has

significant expertise in the development of a number of the algorithms and

techniques presently used for image analysis and the extraction of additional

information. Automated image recognition along with automated diagnosis for

the medical fields are areas with good potential for breakthroughs in image

analysis software.

Database and information management along with fast delivery of imaging

information are areas which need to be addressed as the size and number of

images increases as technology improves. There is already some funding

through EPSRC for e-GRID and information extraction from images but more

investment could put the UK into a very strong world position. While the UK has

little manufacturing of the microelectronic components in computers, hard disk

drives and storage media, it does have significant expertise in computer

architecture (e.g. ARM Holdings) and computer systems design.

54

Acknowledgements This review is funded by the UK Department of Trade and Industry in the

framework of the Foresight ‘Exploiting the Electromagnetic Spectrum’ (EEMS)

project.

The author acknowledges the following for contributions during the

preparation of this report:

• Foresight EEMS – ‘Picturing people: non-intrusive imaging’ action group:

o Brian Allen, e2v Technologies o David Burrows, DSTL o Prof. David Delpy, University College London, London o Prof Paul Matthews, Professor of Neurology; Director, Centre for

Functional Magnetic Resonance Imaging of the Brain, University of Oxford

o Prof Mike Pepper, Cavendish Laboratory, University of Cambridge o Dr Clifford Smith, Amersham Health Research o Mike Brady, University of Oxford o Allyson P C Reed, Qinetiq o Brian Maddison, RAL o Andrew Mackintosh, Oxford Instruments o Mel King, Department of Health, Medicines and Healthcare product

Regulatory Agency o Lizzy Hylton, EPSRC o Peter Dukes, MRC o Delyth Morgan, MRC o James Barlow, Imperial College

• Dr Alison Bolster, Consulting Clinical Physicist, Royal Hospital, Glasgow • Dr Mike Kemp, TeraView Ltd., Cambridge • Dr Edmund Linfield, Cavendish Laboratory, University of Cambridge • Dr Alistair Rose, Crime Prevention Programme Manager, EPSRC • Foresight: A. Jackson, F. Harrison, S. Bass, C. Hepworth, N. Pitter

55

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Trans. Med. Imaging 22, 1407 (2003). 117C.E. Rodriguez-Carranza and M.H. Loew “Design and evaluaion of an automatic procedure for detection of large misregistration of medical images” IEEE Trans. Med. Imaging 22, 1445 (2003). 118I.R. Yound and G.M. Bydder “Phase Imaging” in Magnetic Resonance Imaging 2nd Ed, D.D. Stark and W.G. Bradley, Eds., Mosby Year Book (1992). 119S. Chavez et al “Understanding phase maps in MRI: A new cutline phase unwrapping method” IEEE Trans. Med. Imaging 21, 966 (2002). 120M.L. Fowler “Phase based frequency estimation: a review” Digital Sig. Processing 12, 590 (2002). 121House of Lords Select Commitee on Science and Technology “Chips for Everything: Britain’s opportunities in a key global market” The Stationery Office (2002). 122R. Compano “Technology Roadmap for Nanoelectronics” EC (2000) (http://www.cordis/lu/ist/fetnid.htm). 123G.E. Moore “Cramming more components onto integrated circuits” Electronics 38(8), 19th April (1965). 124http://www.top500.org/ 125http://www.almaden.ibm.com/st/magnetism/ms/index.shtml 126M.W. Vannier et al., “Matching clinical and biological needs with emerging imaging technologies” Proc. IEEE 91, 1562 (2003). 127P.A. Komesaroff and I.H. Kerridge “Ethical issues concerning the relationship between medical practitioners and the pharmaceutical industry” Med. J. Aus. 176, 118 (2002). II. UK Expertise

a) Industrial

Security Imaging: QinetiQ, e2V, BAE Systems, Forensic Alliance Ltd., Forensic Science Service Ltd., Spatial Imaging Ltd. Sonar: Ultra Electronics, Chelsea Technology Group, BAE Systems, Thales (Thomson Marconi), GeoAcoustics Ltd., Son-ix Technologies Ltd. Terahertz: TeraView Ltd., QMC Instruments, Thomas Keating Ltd. Medical Imaging: Amersham (now part of GE), Xograph, Oxford Instruments (components), Marconi (components), e2V (components), Dynamic Imaging Ltd. Image Analysis: Perceptive Instruments, Phase Vision, Image Associates, Dynamic Data Links, Image Scan Misc. Imaging Components: Leica Microsystems UK Ltd., Alrad Instruments Ltd.

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b) Academic

Medical Impedance Imaging: Brian Brown or David Barber, University of Sheffield MEG: Krishna Singh, University of Aston Ultrasound: RW Prager, University of Cambridge, Peter Wells, United Bristol Healthcare Trust: Jeff Bamber, Institute of Cancer Research and Royal Marsden Hospital NHS Trust. MRI: Paul Matthews, University of Oxford; Ian Young, Imperial College; D. Rueckert, Imperial College; S. Webb, Institute of Cancer Research; Y Zheng, UCL; A. Connelly, UCL; N.G. Papadakis, University of Sheffield; D.L.G. Hill, King’s Colege London; D. Atkinson, King’s College London; D.C. Alexander, UCL; C.A. Clark, St. Georges Hospital Medical School Terahertz: Mike Pepper, Ed Linfield, Richard Pye, University of Cambridge; Bob, Miles, Giles Davies, E. Berry and M.A. Smith, University of Leeds; D.A. Berk, University of Manchester; Martin Chamberlain, University of Durham Visible / NIR: David Delpy, UCL; Jeremy Hebden, UCL; A.P. Gibson, UCL X-ray / CT: K.D. Rogers, Cranfield University PET: A.J. Reader, UMIST Optical tweezers / near field: Kishan Dholakia or Wilson Sibbett (St. Andrews University) Component Technology: R.D. Speller, UCL Image Analysis Technology: MC Fairhurst, Leigh House Hospital; M. Niranjan, University of Sheffield; S.M. Smith, University of Oxford; D. Atkinson, King’s College London; J.P. Siebert, University of Glasgow; Y. Demiris, Imperial College; R.J. Lapeer, University of East Anglia; P.R. Hoskins, University of Edinburgh; F. Bello, Imperial College; A. Hoppe, Kingston University; J.M. Brady, University of Oxford; D. Rueckert, Imperial College; A. Bhalerao, King’s College London; G.Z. Yang, Imperial College, A. Todd-Pokropek, UCL Computing and Database: J.M. Brady, University of Oxford; N.R. Shadbolt, University of Southampton; T.A. Rodden, University of Nottingham; P.N. Angel, University of Glamorgan; C.J. Taylor, University of Manchester, P.C.W. Beatty, University of Manchester; J.V. Hajnal, Imperial College; D.L.G. Hill, King’s College London; A. Todd-Pokropek, UCL; S.R. Arridge, UCL; D.T Delpy, UCL; D.J. Hawkes, King’s College London; L. Tarassenko, University of Oxford; J. Graham, University of Manchester; A.D. Linney, UCL; S.M. Smith, University of

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Oxford; S. Astley, University of Manchester; C.J. Taylor, University of Manchester; T.F. Cootes, University of Manchester; A. Jackson, University of Manchester; P.K. Marsden, King’s College London Security Microwave / mm-wave: Y. Petillot, Heriot-Watt University Sonar: P.J. Probert Smith, University of Oxford Terahertz: Mike Pepper, Douglas Paul, University of Cambridge; Giles Davies, Bob Miles, University of Leeds Infrared: G. Bullard, Heriot-Watt University Imaging Technology: M. Petrou, University of Surrey; J.M. Girkin, University of Stathclyde; A.J. Harris, University of Glamorgan; M.P. Evison, University of Sheffield; H.N. McMurray, University of Wales Swansea Image analysis: G.A. Jones, Kingston University; G.E. Pike, Open University; C.J. Solomon, University of Kent; F. Deravi, University of Kent; S. Gong, Queen Mary London; P. Hancock, University of Stirling; J. Ming, Queens University Belfast Computing and Database: A. Hirschfield, University of Liverpool; Q. Shen, University of Edinburgh; D.R. Bull; University of Bristol; D.R. Parish, Loughborough University III. Acronyms and Technical Terms CCD - charged coupled device CCTV - close circuit television CPU - central processing unit CT - computed tomography DRAM - dynamic random access memory DSP - digital signal processor ECG - electrocardiography EEG - electroencephalography ESI - electrical source imaging FFT - fast Fourier transform GRP - glass reinforced plastic HBT - heterobipolar transistor HTS - high temperature superconductor hyperpolarisation - the optical pumping of an atom to enhance the nuclear spin polarisation LAN - local area network LNA - low noise amplifiers MMIC - monolithic microwave integrated circuit MRI - magnetic resonance imaging

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MSI - magnetic source imaging NE∆T - noise equivalent differential temperature NEP - noise equivalent power NMR - nuclear magnetic resonance OCT - optical coherence tomography PET - positron emission tomography QCL - quantum cascade laser quantum efficiency - the number of electron-hole pairs created in a photodetector per incident photon QWIP - quantum well infrared photodetector RAM - random access memory RTD - resonant tunnelling diode SIS - superconductor-insulator-superconductor (Josephson junction) skin depth - the length scale of the exponential attenuation of radiation in a material SPECT - single photon emission computerised tomography SQUID - superconducting quantum interference device