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Radionuclide imaging and PET questions
Most scans send radiation through the body from outside. In radionuclide imaging the source is inside: a tracer chosen so the biology carries it to the organ of interest, emitting gamma rays that a gamma camera detects. That makes it a picture of function rather than only of shape, and PET is the same idea with positron emitters.
18 original questions · 49 marks · the radionuclide imaging and pet notes · Medical physics
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State what is meant by a medical tracer.
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A radioactive nuclide chemically attached to a biologically active molecule, such as a glucose analogue, which the body takes to wherever that molecule is used (1). The radiation it emits from there is detected to map the tracer's location (1).Explain why a tracer used with a gamma camera should be a gamma emitter, and why its half-life must be chosen with care.
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Gamma photons are penetrating enough to escape the body and be detected outside (1); alpha particles are absorbed within a fraction of a millimetre of tissue and beta minus particles within a few millimetres, so both deliver dose but no image (1). (A PET tracer is a positron emitter, but there too it is photons that leave the patient: the pair of 511 keV gammas made when the positron annihilates.) The half-life must be long enough for the tracer to be prepared, administered and imaged, but short enough that the patient is not left radioactive for weeks (1).An X-ray image and a tracer image answer different questions about a patient. State the difference.
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X-ray and CT images show structure: what the anatomy looks like (1). A tracer image shows function: where the body is actually using the labelled molecule, so which tissue is active, inflamed or failing (1).Technetium-99m has a half-life of 6.0 hours, so a dose made at a distant factory would decay before reaching the hospital. Explain how hospitals nevertheless have technetium-99m available every morning.
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The hospital keeps a generator containing the longer-lived parent nuclide, molybdenum-99 (1). Technetium-99m forms continuously as the molybdenum decays, and is drawn off each day as it is needed (1).In PET scanning for tumours, the positron emitter fluorine-18 is attached to a glucose analogue. Suggest why a glucose analogue is a good choice of carrier molecule.
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Many tumours are metabolically very active and consume glucose faster than the tissue around them (1), so the tracer accumulates in the tumour and the annihilation photons come mostly from where the disease is (1).Describe the roles of the collimator and the scintillator crystal in a gamma camera.
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The lead collimator admits only gamma photons travelling nearly parallel to its channels, so each detected photon's origin lies along a known line (1); oblique photons are absorbed by the slats (1). The scintillator converts each admitted gamma into a flash of light whose position is recorded, building a map of the tracer's location one photon at a time (1).Technetium-99m has a half-life of 6.0 hours. Calculate the fraction of a dose's activity remaining 24 hours after preparation.
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24 hours is four half-lives (1)
A/A₀ = (1/2)⁴ = 0.0625 (1)
≈ 1/16, about 6% (1)Describe what happens when a positron emitted by a PET tracer meets an electron, and state the energy of each photon produced.
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The positron and electron annihilate, so both particles vanish (1) and their rest energy becomes two gamma photons travelling in opposite directions (1). Each photon carries 511 keV, the rest energy of one electron through E = mc² (1).Show that the rest energy of an electron is about 8 × 10⁻¹⁴ J, and convert it to keV.
Explain why the two annihilation photons travel in opposite directions.
A tracer has a physical half-life of 6.0 hours and is also cleared from the body biologically with a half-life of 12 hours. Calculate the effective half-life of the dose, and explain why it is shorter than either value alone.
Iodine-131 is given to a patient to destroy overactive thyroid tissue. Explain how this treatment uses the same principle as a medical tracer, and how its aim differs from imaging.
Explain how a PET scanner uses coincidence detection to locate the tracer.
Calculate the frequency of a 511 keV annihilation photon.
Fluorine-18, used in PET, has a half-life of 110 minutes. A hospital needs at least one eighth of the delivered activity remaining at scan time. Calculate the maximum delay between delivery and scanning.
A patient asks whether a PET scan means being irradiated by the machine. Explain what the scanner actually does, and where the dose really comes from.
A PET detector ring has a radius of 40 cm. An annihilation occurs 4.5 cm from the centre of the ring, on the line joining two opposite detectors. Calculate the difference in the arrival times of the two photons at the detectors (c = 3.00 × 10⁸ m s⁻¹), and comment on what this means for the scanner's timing electronics.
Three radionuclides are available for a kidney investigation in which imaging ends 30 minutes after the tracer is administered. Nuclide P emits beta minus particles and has a half-life of 8.0 days. Nuclide Q emits gamma photons and has a half-life of 6.0 hours. Nuclide R emits gamma photons and has a half-life of 90 s. Deduce which nuclide should be chosen, giving a reason for rejecting each of the other two.
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