A ring of detectors surrounds a patient's torso for imaging a fluorine-18 tracer by…
PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Nuclear medicine, dosimetry and radiotherapy · Nuclear medicine, dosimetry and radiotherapy
Life at Thermal Energy, Ch Images from attenuation and from decay
Problem
A ring of detectors surrounds a patient's torso for imaging a fluorine-18 tracer by positron emission tomography (PET). A colleague suggests fitting the same ring with a lead parallel-hole collimator, as used in a gamma camera, in the hope of sharpening the image further.
((a)) Explain, in terms of what physically fixes the line along which each detected event occurred, why a collimator is unnecessary for PET but essential for a gamma camera imaging technetium-99m.
((b)) A parallel-hole collimator of the kind used in a gamma camera passes about 10−4 of the photons emitted isotropically by a point source. If such a collimator were fitted in front of every detector on the PET ring, so that each of the two back-to-back 511keV photons of an annihilation had to pass through a hole to reach its detector, estimate the factor by which the coincidence count rate would fall relative to having no collimator at all, and say whether fitting one is a sensible design choice.
((c)) Name one physical factor, other than the timing resolution of the detectors, that limits how precisely the coincidence line locates the point where the positron was emitted.