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Radionuclide imaging and PET
Every other scan shines something through the body. This one turns the patient into the source: a chosen radioactive tracer goes where the biology takes it, and the radiation it sends out reports back on function as well as shape.
Builds on Antimatter and photons and Radioactive decay and half-life.
IN THIS TOPIC
- Explain what a medical tracer is, and why its half-life and emission type must be chosen carefully.
- Describe how emitted gamma rays are used to build an image of where the tracer went.
- Explain PET: annihilation into two 511 keV photons, and coincidence detection locating the source.
WHAT YOU PROBABLY THINK
A PET scanner works by sending radiation into the body.
The patient as the source
A tracer is a radioactive nuclide chemically attached to a molecule the body uses: a glucose analogue, a bone-seeking compound, whatever the question is. Injected or swallowed, the molecule goes where the biology sends it, and the nuclide radiates from wherever it accumulates. Detecting that radiation maps function: which tissue is consuming, which is inflamed, which is not working at all. An X-ray shows what the body looks like; a tracer shows what it is doing.
The nuclide must be chosen with care. It should emit gamma, which escapes the body to be detected, rather than alpha or beta, which would deposit their energy in the patient for no image at all. And its half-life must suit the job: long enough to prepare and image, short enough not to irradiate the patient for weeks. The workhorse is technetium-99m, half-life six hours, a near-pure gamma emitter made fresh each morning for exactly these reasons: hospitals keep a generator of its parent, molybdenum-99, and draw off the technetium daily as it forms.
Two refinements complete the picture. A tracer leaves the body biologically as well as decaying physically, so the dose falls with an effective half-life given by 1/TE = 1/TB + 1/TP, always shorter than either. And the same radiation that images can treat: radiotherapy turns the dose into the point, from iodine-131 taken up by the thyroid it is meant to ablate, to beams of high-energy X-rays crossed on a tumour from many directions so the target takes far more dose than any one path through healthy tissue.
Catching the gammas
The emitted gammas are collected by a gamma camera: a lead collimator that admits only photons travelling nearly parallel to its channels, a scintillator crystal that converts each gamma to a flash of light, and detectors that locate each flash. The result is a map of tracer concentration, of activity, built one photon at a time.
The picture is blunter than a CT slice, and that is acceptable: the question a tracer answers is where the activity is, not what the anatomy looks like. The two kinds of scan are routinely overlaid, structure from one, function from the other.
PET: annihilation as a beacon
Positron emission tomography sharpens the idea with antimatter. The tracer, commonly fluorine-18 in a glucose analogue, emits positrons. Each positron travels a millimetre or so, meets an electron, and the pair annihilates: both particles vanish and their rest energy becomes two gamma photons of 511 keV each, flying in opposite directions to conserve momentum.
The scanner is a ring of detectors watching for coincidences: two 511 keV photons arriving at opposite sides at effectively the same instant. Each coincidence pins the annihilation to the straight line between the two detectors, and hundreds of thousands of lines intersect at the places the tracer gathered. The 511 keV is no accident: it is the electron's rest energy through E = mc2, the same physics as the antimatter lesson, working a hospital shift.
THE EXAM BIT
- Tracer questions want both halves of the choice: gamma emission so the radiation escapes to be detected, and a half-life matched to the procedure. Name technetium-99m and its six hours where an example is asked for.
- Function against structure is the comparison mark: tracers image what tissue is doing, X-ray and CT image what it looks like.
- The PET chain in order: positron emitted, annihilates with an electron, two 511 keV photons in opposite directions, detected in coincidence, source on the line between the detectors. Five steps, five marks.
- The 511 keV comes from the electron rest mass via E = mc2. Quote it rather than deriving it unless asked.
- Opposite directions is a momentum statement: the pair is nearly at rest when it annihilates, so the two photons must carry equal and opposite momentum.
CHECK YOURSELF
Explain why the two photons produced in PET annihilation travel in opposite directions, and why both must be detected for the event to be useful.
Show a hint
Start from the momentum of the electron-positron pair just before annihilation.
Show the answer
The pair is very nearly at rest, with total momentum close to zero. Momentum is conserved, so the two photons must leave with equal and opposite momenta: back to back.
One photon alone gives only a direction from one detector. Two in coincidence define the whole line through the annihilation point, and it is the intersection of many such lines that locates the tracer.
A tracer maps function: gamma out, half-life to fit the job.
PET's beacon is annihilation: two 511 keV photons, back to back.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
CHECK YOUR PROGRESS
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- Explain what a medical tracer is, and why its half-life and emission type must be chosen carefully.
- Describe how emitted gamma rays are used to build an image of where the tracer went.
- Explain PET: annihilation into two 511 keV photons, and coincidence detection locating the source.
Open the full revision checklist to track your progress across the whole unit.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.