Advances in Computed Tomography and Digital Mammography · 1 Advances in Computed Tomography and...
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Advances in Computed Tomography and
Digital MammographyRuvin Deych and Sorin Marcovici
Analogic Corporation
Peabody, MA 01960, USA
ISMART, November 18, 2008, Kharkov
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• Advances in Medical X-Ray CT
– World market of X-Ray CT
– Principles of third generation CT
– Main performance parameters of modern CT
– Data Measurement System in modern X-Ray CT
– Future trends in X-Ray CT
• Advances in Digital mammography
– Selenium/TFT technology
– Performance of Selenium based mammography detectors
– Manufacturing
– Tomosynthesis
Outline
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0
500
1000
1500
2000
2500
3000
3500
4000
4500
CT
Mark
et
($U
S M
il)
2004 2005 2006 2007 2008 2009
US Market ($US Mil) Rest of World
Source: Philips Medical, Fuji-Kezai, Analogic
28% U.S. Decline
Note: U.S. data in orders
Global CT Market
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Source: 2006 Frost & Sullivan
Market Share by Product Tier (2006) Based on unit volume
20%
29%
10%
41%
Up to 10 Slice
16 Slice
32 Slice
64 Slice
US CT Market Distribution
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CT Market Pricing Trends
Source: 2008 Frost & Sullivan
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Evolution and Present Status of Medical
X-Ray CT Imaging
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Medical CT
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CT Systems Medical
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Analogic in CT
• CT Subsystems– Data Management
Systems (DMS)
– Data Acquisition Systems(DAS)
– Detector Arrays
– Gantries
– PowerLink – Non-ContactPower Transfer
– Collimators
– Reconstruction Software
– Motion Control
– Patient Table
– Operator Contol Station
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CT Systems Security
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Milestones in X-ray CT
• Sequential scanning of consecutive slices: 1970s
• Spiral scanning-CT acquisition with continuous
translation of patient: beginning of 1990s
• Multislice spiral CT: end of 1990s
• Dual source CT: 2000s
• Main trend: increasing speed of acquisition and axial
coverage (slices)
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Multislice Spiral CT Scanning
• Mid 1990s: 300 mm lung or
abdominal examination
with narrow slice requires
200 s long scan
• Mid 2000s: same
examination takes 1-3 sec
• High speed of CT allows
large scan within one
breath hold, and to acquire
images of moving organs,
such as heart.
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CT applications
• Coronary Angiography
–Rest phase of coronary
arteries is 60 msec!
–Non-invasive emergency
diagnosis for cause of chest
pain: coronary blockage,
pulmonary embolism, aortic
aneurism.
–Alternative: 6 hour long
invasive catheterization
procedure with 1% risk of
serious complications,
including death.
Courtesy of Toshiba Medical Corporation
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Main drivers in X-ray CT
• Short scan time and large axial coverage for reliable
anatomical and functional measurements of whole
organs (perfusion of heart, brain, lung)
• Ultra-High spatial resolution
• Anatomical/Functional multi-modality imaging in
SPECT-CT, PET-CT
• Patient dose reduction
• Is slice war over?
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DMS for X-Ray CT
• Scan time: 0.3 sec
• XRT Power: 100 kW
• Axial coverage: 40-160 mm
at isocenter
• Spatial resolution: ~0.5 mm
• Remarkable progress in X-
Ray CT in the past decade is
largely explained by fast
development of the Data
Measurement System (DMS)
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Main DMS Parameters
Parameter Typical value Parameter Typical valueX-ray energy, kVp 80-140 Rotation time, sec 0.3
XRT power, kW 100 Data transfer rate,
GBit/sec
10
FOV, cm 50 Operational
temperature range
15-45°C
Isotropic resolution at
isocenter, mm
0.5-0.6 Number of x-ray
photons per sample at
peak power
300,000
Number of channels per
row
700-1000 Conversion efficiency,
el/eV
0.01-0.02
Number of rows 64, 128, 320 Sampling rate, Hz 3000
Typical element size,
mm
1x1 Data resolution, bit 18-20
Typical detector
distance from XRT focal
spot, mm
1000 Dynamic range, bit 16-18
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Detector Channels per CT - 2008
1000Low End (single slice)
57,00Mid Range (16 slices)
up to 300,000High End (128-320 slices)
Analogic DAS/DMS•From 400-1000 channels in 1993 to 300,000 channels in 2008
•DAS/DMS complexity increases at constant:
•Cost
•Power consumption
•and almost same Mechanical Envelope
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Charge Integrating CT Detector
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Uniform and Adaptive
Detector Configurations
Adaptive arrays have fewer septa, and DAS channels. Used primarily in CTswith 16 slices.
Uniform coverage in axial direction.
Used in most 64, 128, 256 slice
Scanners.
Adaptive arrays have fewer septa and
DAS channels. Used in CT with 16 or
fewer slices.
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CT Subsystems
• DMS
– 1 – 64 Slice+
– Integrated DAS and
Detector Assemblies
– X-ray Beamline Design
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16x64 CT Detector
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Requirements for Scintillators
in X-Ray CT
Used with Si photodiodes400-1000Emission spectrum, nm
DMS cost, cost of removalLow cost, non-toxic
100 kGyLifetime dose
~0.1 Gy/sDose rates at detector
Image artifacts if channels
non-uniform
<0.3 %/°CLO temperature coefficient
Dynamic range reduction<20 %Lifetime degradation
Image artifacts if channels
non-uniform
<1 %/GySusceptibility to radiation
damage
Image artifacts<10 ppm at 3 msAfterglow
To support >10 kHz DAS
rates
<10 sDecay time
Image noise at high
attenuation
>40,000 ph/MeVLight output (LO)
Image noise, dose
reduction
>95 %DQE (0)
20-140 keVEnergy range
ImportanceRequirementParameter
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Main Scintillators in X-Ray CT
CdWO4 7.9 2.6 1 495 2, 15 <0.1
Gd2O3:Eu+3 7.55 2.6 - 610 - -
(Y,Gd)2O3:Eu 5.9 6.1 1.52 610 1000 5
Gd2O2S:Pr,Ce,
F
7.34 2.9 1.8 520 2.4 <0.1
Gd2O2S:Tb(Ce) 7.34 2.9 1.8 550 600 0.7
La2HfO7:Ti 7.9 2.8 0.45 475 10 -
Gd3Ga5O12:Cr, 7.09 4.5 1.38 730 150 <0.1
aThickness to absorb 99 % of x-ray photons generated by tungsten anode x-ray tube at 140 kVp.
bRelative light output measured using silicon photodiode, under 140 kVp tungsten anode XRT excitation.
Primary
decay
( s)
Afterglow
(% at 3
ms)
Scintillator Relative
Light
outputb
Emission
band
maximum
( )
Density
(g/cm3)
Thicknessa
to
stop 99 %
(mm)
General Electric introduced fast “Gemstone” garnet based ceramic scintillator in
2007.
Limited data in public domain.
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Silicon Photodiodes for X-Ray CT:
Main Requirements
P-i-n structure, Photovoltaic, 0V biasMode of operation
20 pF, max, @0V, 10 kHzTerminal capacitance for 1x1 mm2
0.1 %, maxCross-talk
5 pA, max,@10 mV bias, 25 C,Leakage current for 1x1 mm2
+/-2% ch-to-chUniformity of photosensitivity
0.3 A/W at 500 nmPhotosensitivity, typ
320-1060 nmSpectral response range
<1 sRise, fall time
<3·10-15 W/ HzNEP
1.0 mmPitch in z-axis, typ
1.0 mmPitch in x-axis, typ
>64 per pitchInterconnect density
512Elements number per chip, typ
ValueParameter
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Data Acquisition Electronics:
Main Requirements
ASIC64-128 Channels
AMP/ADC
Packaging
3 mW / ChannelPower Consumption
30 ppm R /°C
± 1 ppm FSR
Differential Non
Linearity
R = Reading 200 ppm R /°C
± 2 ppm FSR
Integral Non Linearity
± 50 ppm FSR /°CGain Stability
FSR = Full Scale
Reading
1.5 ppm FSR /°COffset Stability
10 kHzSampling Rate
= Photon Noise (1/6) ( 2 + e2)1/2Digitization Interval
,m = Minimum Photon
Noise
(1/2) ,mElectronic Noise
e
NotePerformanceParameter
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Trends in Medical CT
1. New CT scanning geometry: Dual-Source, Multi-Source,Inverse-geometry
– Advantages: Faster acquisition, Cone-beam artifact reduction
– Requires multiple DMS, or area detector, expensive
2. Energy-sensitive CT
– Advantages: elimination of beam hardening artifacts, materialdiscrimination, better contrast at lower dose
– Solutions
• Dual-layered detectors
• Dual-source CT, kVp switching
• Photon Counting Detectors with multiple energy bins
3. Multimodality CT: SPECT/CT, PET/CT, Preclinical systems
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Trends in Medical CT
4. Phase-contrast imaging (more distant future)
– Phase-contrast imaging, based on difraction is more
sensitive in 10-150 keV range then attenuation based
imaging.
– Requires interferometry and difractometry detection
technique.
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Multiple Source CT
• Multiple source/detector systems-old idea becomes
a desirable development
• Higher rotation rates require increase in X-Ray
power, not achievable with present X-Ray
technology
• Fraction of rotation is required for a full scan
• Issues: scatter reduction, high cost
• Siemens introduced commercial Dual Source CT in
2004
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Energy sensitive CT:
Dual Energy Detector
• Two crystals with different
emission bands are used.
• Radiation is hardened by the
first crystal.
• Optical band pass filters limit
diodes to see signal from only
one crystal.
• Advantages:
– Simultaneous acquisition of Low
and High Energy samples.
– High Quantum Efficiency
– Planar silicon PDA technologyIncident X-ray Photons
Light Photons from High Energy Scintillator
Light Photons from Low Energy ScintillatorR. Deych, US Patent 7,388,208 B2 2008
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Contrast-to-Noise
Model Results
(Teflon detail in water background)
0
1
2
3
4
5
6
0 0.2 0.4 0.6 0.8 1
LE thickness (g/cm2)
CN
R
CsI:Tl(LE)/CdWO4(HE)
ZnSe/CdWO4
GOS/CdWO4
GGG/CdWO4
CsI:Na/CsI:Tl
ZnSe/LSO
GGG/LSO
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TimeTime
VV
Single Pixel Analog LineSingle Pixel Analog Line
SPC ThresholdSPC Threshold
NoiseNoise
X-PhotonsX-Photons
[ ]dt S =S =CICI
S = NS = NSPCSPC
Energy sensitive CT,
Single Photon Counting
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SPC in Computed Tomography
• New medical applications and capabilities
– Contrast media removal in images
– Multiple contrast agents
– Reduction of beam hardening artifacts
– Patient dose reduction
• Requires high counting rates up to 109 (!)
photons/sec/mm2
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Direct Conversion Detectors
• Dual energy CZT based detector tested in
LightSpeed GE CT scanner, IEEE 2007
• Pre-clinical CT scanner with 6 energy bands based
on CZT technology tested by Philips, IEEE 2008
• Main drawbacks:
– Long carrier transit time, insufficient speed
– Material polarization at high exposure rates
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Scintillator Based SPC
• X-ray CT will require fast scintillators and internal
gain in photodetectors
• Fast scintillators with Solid State PM are being
proposed for CT
• Potential Available Fast Scintillators: LSO, LYSO,
LaBr3
• New faster scintillators with 1-10 nsec decay time
are required
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AC
ECT With AC
Image Fusion
ECT NC
Attenuation MapCourtesy of General Electric, Functional Imaging
Multimodality CT:
SPECT/CT, PET/CT
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Advances in X-ray CT:
Conclusions and Predictions
• CT scanners with 320 slice acquisition in 0.3 sec areavailable
• The “slice war” between major medical imagingcompanies is over!
• New CT systems will include novel scanningtechniques: multiple sources, inverse geometries
• Multi-energy CT will be needed to obtain bettertissue discrimination at lower patient dose.
• Photon counting detection may replace chargeintegration
• X-ray CT will become new market for ultrafastnanosecond scintillators.
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Advances in Digital
Mammography
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Increasing mammography clinical diagnostic’s
sensitivity and specificity while optimizing
patients’ flow and reducing operational costs.
Stating the Problem
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2005 450
2006 900
2007 1650
2008 2000 est.
2009 2100 est.
Digital Mammography Installed Units
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Film-based analog mammography:
12 – 15 minutes
Se-based digital mammography:
5 – 6 minutes
Average Time/Patient
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• Two Step Conversion - INDIRECT
X-Ray to light to electrical charge
• One Step Conversion - DIRECT
X-Ray to electrical charge
Digital Radiography
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• Generation I:
dielectric isolation layer deposited on top of two layer “p” Se
structure
• Generation II:
single Se deposition process with real time doping to create
three layer pin or nip structures
a: Se Technologies
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General Characteristics
Technology: amorphous Selenium
Active area: 24 cm x 30 cm
Resolution: 2816 x 3584 pixels
Pixel pitch: 85 μm
Acquisition speed: 2 frames/second
Digitization: 14 bits
Mammography Detector
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• Atomic Number: 34
• Conversion Efficiency: 50 eV / e-h
• Evaporation Temperature: 217 deg. C
• Crystallization Temperature: 60 deg. C
• Expansion Coefficient: 40 ppm/deg. C
Se Characteristics
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X-Rays
Amorphous
Selenium
Layer
TFT Array
Charge amplifier
a: Selenium Detector Structure
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RT, Radiography
40 keV < Ex < 120 keV
Mammography
25 keV < Ex < 40 keV
1 – exp[-aL]
= a (E,Z,r)
mostly photoelectric
Attenuated X-Ray fraction
or Quantum Efficiency:selenium
layer
pixel
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.2 0.4 0.6 0.8 1
Tickness (mm)
Att
en
ua
ted
F
ra
cti
on 53 keV
25 keV
X-Ray Absorption
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Number of electron-hole pair created:
Ex / W+/-
W+/- = W+/- (E,F)
selenium
layerF
-10kV
for F = 10 V/mm, W +/- ~ 50 eV
Charge Generation
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μ: mobility : lifetime
μ E: mean free path
Charge Collection (induction) efficiency:
= μ E/L { 1-exp[-L/μ E] }Typical values for a-Se:
μ E(e) = 3-4 mm μ E(h) = 3-20 mmFor a good detector μ E >> L
Charge Drift
selenium
layer
TFT glass
Gate
Linedata line
E
data line
L
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150 um: Real-Time, GR
85 um: Mammography
scan line
data
lin
e
to charge amp
pixel
electrodestorage
capacitor
TFT
switch
• Pixel pitch is larger than the pixel electrode (geometrical fill factor)
TFT Pixel Architecture
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TFT Array Sequential Readout
data
lin
e
scan line-10V
-10V
-10V
-10V
-10V
+20V switch line#1data
lin
e
scan line-10V
-10V
+20V switch line#2
data
lin
e
scan line
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FFT
selenium
layer
10 μm wide
Tungsten slit
Line Spread Function (LSF)
a
Sinc(a,f) =Sin(a f)
(a f)
Detector Spatial Resolution
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• The first zero-crossing of each sinc function corresponds to physical pixel pitch: 6.6 lp/mm for 150 μm pixel
11.7 lp/mm for 85 μm pixel
• Experimental MTF’s are close to their corresponding sinc functions
Detector Modulation
Transfer Function (MTF)
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5 6 7 8 9 10 11 12
frequency (lp/mm)
MTF
FPD14 data
sinc 150um
LMAM data
sinc 85 um
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If is the number of X-Ray incident on the detector then
where Ne is the electronic noiseSNR =
N
Detector Performance:
Signal to Noise Ratio (SNR)
2
e +
The SNR curve follow
a sqrt behavior above2μR
1
10
0 1 2 3 4 5 6 7 8 9 10
dose (uR per frame)
SN
R
FPD9 data
C*sqrt
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frequency
domain
Detector Detective
Quantum Efficiency (DQE)
DQE =
(SNR) 2
det
(SNR) 2in
: X-Ray fluence
NPS: noise power spectrum
G: conversion gainDQE(f) =
G 2
x NPS(f)
MTF (f)2
Ö
DQE remains high for
high frequency values
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 1 2 3
Spatial frequency (lp/mm)
DQ
E
12.1 uR
8.6 uR
6.8 uR
5.5 uR
3.9 uR
2.4 uR
1.3 uR
0.6 uR
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Manufacturing Steps
• Deposit in vacuum amorphous Se on TFT
• Deposit top metal electrode on a-Se the
• Attach high voltage contact to electrode
• Deposit isolation on multi-layer structure
• Attach peripheral ASIC electronics to TFT
• Assembly the detector in final enclosure
• Perform parametric and imaging tests
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Selenium Coater
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Selenium Coater
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Mammography Detector
Packaged detector
Electronic sub-assembly
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Tomosynthesis
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Acknowledgements
The author acknowledges the contribution of Dr. Olivier Tousignant,
Anrad Corporation, Saint-Laurent, QC, Canada who made the
characteristic parameters measurements of the LMAM detectors.