6.5.2Particles and Medical PhysicsCore

Medical Tracers, Gamma Cameras and PET

Nuclear imaging follows the distribution of a labelled substance. A gamma camera detects emitted gamma photons directly; PET detects the paired photons produced after positron annihilation.

40 min 6.5 Medical Imaging

What you'll be able to do

Choose between technetium-99m and fluorine-18 applications
Explain every gamma-camera conversion stage
Explain PET coincidence imaging and evaluate equipment choices

A tracer needs useful chemistry and decay properties

A labelled compound should reach the tissue or process being investigated and remain detectable during preparation and scanning. A short physical half-life limits prolonged activity but must still allow time for the procedure. Technetium-99m is a gamma-emitting tracer with a half-life of about 6 h; fluorine-18 is a positron emitter with a half-life of about 110 min.

The radiation and chemical carrier determine the application. A labelled fluorine-18 glucose analogue can reveal glucose uptake, while technetium compounds can target different organs depending on the carrier. A scan maps tracer distribution rather than automatically showing an anatomical boundary.

The gamma-camera chain

A lead collimator absorbs photons travelling in unwanted directions so the detected position relates to source position. A scintillator converts absorbed gamma energy into a flash of visible light. Light reaching a photomultiplier releases photoelectrons at a photocathode; dynodes multiply their number to give a measurable pulse.

Comparing signals from several photomultipliers locates the scintillation, and a computer builds an image from many events. Collimation improves directional information but discards many photons, reducing sensitivity. Distinguish gamma photons entering the crystal from the visible photons and electrons generated later.

PET and the choice of scanner

A fluorine-18 nucleus can emit a positron, which slows and annihilates with an electron. Two photons of about 511 keV usually leave in nearly opposite directions. Detectors registering a coincidence define a line on which the annihilation occurred; many such lines allow reconstruction of tracer uptake.

A single coincidence line does not identify an exact point by itself in a simple PET model. PET can provide functional information but involves tracer radiation exposure, specialist facilities and substantial cost. Compare diagnostic benefit, resolution, access and resources rather than assuming every hospital can or should use the most expensive scanner.

Work through the reasoning

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.

Example 1A fluorine-18 sample has activity 80 MBq initially. Use half-life 110 min to find activity after 220 min.

1220 min is two half-lives.

A=80(1/2)^2=20

MBq.

Answer20 MBq, ignoring biological removal.

Example 2Show why annihilation at rest produces about 511 keV per photon. Use electron mass

9.11\times10^{-31}

kg.

1Total rest energy of the pair is

2m_ec^2

2For two equal opposed photons, each has

E=m_ec^2=(9.11\times10^{-31})(3.00\times10^8)^2=8.20\times10^{-14}

3Divide by

1.60\times10^{-16}

J per keV.

AnswerAbout 512 keV with rounded constants, conventionally quoted as 511 keV.

Common mistakes to avoid

Calling PET photons emitted positrons.

Positrons are particles that subsequently annihilate; detectors register gamma photons.

Choosing a tracer by half-life alone.

Also consider emission, chemistry, uptake and the procedure time.

Key takeaways

Tracer uptake gives functional information.
Gamma cameras combine collimation, scintillation and photomultiplication.
PET uses coincident annihilation photons.

Test yourself

Answer a few questions, then review anything you missed.

Next step

Keep practising

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