(d) positron emission tomography (PET) scanner; annihilation of…

The idea

A PET scanner images the distribution of a positron-emitting radionuclide, often fluorine-18 in fluorodeoxyglucose, which is taken up by metabolically active cells. When a positron is emitted, it travels a short distance and annihilates with an electron. The annihilation converts the total rest mass into two gamma photons of equal energy, each about 511 keV, emitted in almost opposite directions. A ring of detectors surrounds the patient. When two detectors on opposite sides register photons within a very short coincidence time window, the annihilation event is assumed to lie on the line joining them. The computer collects many such coincidence events and reconstructs a three-dimensional image of tracer concentration. The technique gives functional information about metabolism and is often combined with CT for anatomical reference.

What earns the marks

• A positron-emitting radionuclide is administered and accumulates in metabolically active tissue.
• A positron emitted from the nucleus travels a short distance before annihilating with an electron.
• Annihilation converts mass into two gamma photons of equal energy, each about 511 keV, emitted in nearly opposite directions.
• A ring of detectors records coincident gamma photons within a short time window, defining a line of response through the annihilation point.
• The computer reconstructs a three-dimensional image of tracer distribution from many coincidence events.

Common mistakes and how to put them right

• Thinking the positron itself is detected.

  Put it right: The positron annihilates and the two gamma photons are detected.

• Believing the two gamma photons have different energies.

  Put it right: They each have about 511 keV, equal to the electron rest energy.

• Assuming the photons are emitted exactly 180° apart in every case.

  Put it right: They are emitted in nearly opposite directions, and small deviations affect resolution.

• Confusing PET with gamma camera imaging.

  Put it right: PET detects coincidence gamma pairs from annihilation, while a gamma camera detects single gamma photons with a collimator.

Exam tips

• State the annihilation process clearly: positron plus electron gives two gamma photons of about 511 keV each.
• Explain coincidence detection: two detectors on opposite sides must register photons within a very short time window.
• Link the line of response to the reconstruction of a three-dimensional image.