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Science Discussion

Medical Scanners

Marge Rose

16th November 2012

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Science Discussion

• Confusion – they all look the same

• CT, MR, SPECT, PET, Ultrasound

• A plethora of names

• Why a scan?

Introduction

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Science Discussion

• CT – computed tomography (was CAT)• SPECT – nuclear medicine (was radioisotopes) • MR(I) is based on NMR• PET stands for positron emission tomography

Names

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Science Discussion

• An aid to diagnosis• Localisation• Screening• Assessment of function• Treatment planning and monitoring• Research• Reassurance

Why a scan?

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Science Discussion

We’ll look at• History• Importance• Probe• Signal – few natural ones• Detector – match to signal• What is it detecting?

For each modality

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Science DiscussionEM spectrum

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Science DiscussionIonisation

Certain types of radiation can ionise atoms

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Science DiscussionUltrasound• Sound is experienced by our ears• Caused by longitudinal pressure waves• We can hear from 20 Hz to 20 kHz • Above 20 kHz - ultrasound

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Science DiscussionUltrasound scan

1980

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Science Discussion

CT scan – uses x-rays

Here is the very first x-ray – it shows his wife’s hand and was taken in 1895. The first medical use was just a few months later in 1896.

X-rays are the most important and widespread of the modalities we will look at in this talk.

The method of production is essentially unchanged.

They were discovered in 1895 by Röntgen.

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Science DiscussionX-ray tube and image

But x-ray tubes and images have improved a great deal in over 100 years

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Science DiscussionThe naked CT

X-Ray tube120-140kVDetector Array

Detector Amplifiers &A/D Converters

Cooling oil pump

Cooling heatexchanger

High Voltage Generator

Collimator

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Science DiscussionTomography

“Atom” derived from Greek atomos meaning “uncut, indivisible”“Tomography” is from the Greek tomē meaning “cut” or tomos meaning

“section” and graphein meaning “to write”

Reconstruction of the data by Back projectionX-ray tube

Patient Grid

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Science DiscussionCT slice through abdomen

Probe120kV X-ray

InteractionPhotoelectric, Compton

PropertyX-ray attenuation

Image3D reconstruction from multiple projections

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Science DiscussionSPECT – uses γ rays

Becquerel discovered radioactivity in 1896

The Curies researched into it and Marie opened the first Radium Institute in 1914

Radioisotopes were first used in diagnosis after World War II when radioiodine became readily available

Rectilinear scanner appeared in 1951

Anger camera was invented in 1957

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Science DiscussionGamma camera detector

First Anger camera I ever saw in use was in 1975

The most common radionuclide used is still Technetium 99m despite supply difficulties

Gamma ray energy 140keV

Half life of 6 hours

‘No’ beta emission

Flexible chemistry

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Science DiscussionNuclear Medicine

X-ray images show anatomy whereas Nuclear Medicine images show function

Uses unsealed radioactive sources introduced into the patient. Patients can still be radioactive when they leave the hospital

Gamma cameras are much less common than x-ray machines

Very few Nuclear Medicine tests are diagnostic – generally they are highly sensitive but are of low specificity

‘Scans’ can comprise of static or dynamic images, whole body, gated images or SPECT (single photon emission computed tomography)

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Science DiscussionWhole body imaging

A type of static imaging –

A whole body bone scan is a very common example

R-R interval

24

1 2 3 4Frame or bin

Gated images – the MUGA

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Science DiscussionSPECT studies – Myocardial perfusion scan

ProbeGamma emitting isotope

InteractionUptake of radiopharmaceutical

Property Concentration of pharmaceutical in organ

ImageSpatial distribution of countsSPECT – 3D

PET – Positron Emission Tomography

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Science DiscussionAntimatter

• Each fundamental particle has an antimatter equivalent• Same mass but opposite charge• Positrons are positive electrons• Collide with the first electron they come across to produce annihilation radiation

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Science DiscussionPositron annihilation

e.g.18F

511 keV

511 keV

+

e- Coincidence Unit

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Science DiscussionPET images

Normal Pre-therapy Post-therapy

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Science DiscussionMR scanner

An MR(I) Scanner

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Science DiscussionMR – souped up NMR

• If placed in a magnetic field, the nucleus precesses around in the direction of that field

• Direct in an RF (radiofrequency) pulse and the nucleus can flip to the higher energy state, opposing the field

• When it relaxes back, it gives off an RF signal which is dependent on the chemical environment

• A hydrogen nucleus has spin

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Science DiscussionGradient coils

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Science DiscussionMR

• Probe– EM pulses

• Interaction– Resonant energy exchange

changes nucleus spin state• Property

– proton density, proton microenvironment

• Image– Map EM signal– 3D reconstruction

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Science DiscussionArtifacts, Hybrid scans

The End