Positron Emission Tomography

12.6.1 The PET Camera

In a PET camera, there are rings of detectors around the patient. The two photons resulting from annihilation fly in opposite directions and will be detected by two detectors at almost the same time. The line of event can be determined by con­necting the two detectors (Figure 12.12). So, in contrast to gamma cameras, we do not need a collimator for PET imaging, and consequently both the sensitivity and spatial resolution of PET are better than those of a regular gamma camera for human imaging. Moreover, since the pairs of photons hitting a particular pair of detectors always travel the same path length inside the patient’s body (independent of the position where the annihilation occurred along the line), the correction for attenuation is more straightforward.

The rings can simultaneously detect radiation emitted in all directions, so PET is capable of acquiring dynamic tomographic studies as well. Utilizing this, in the beginning, PET was mostly used for research purposes, primarily for pharmaco — and receptor kinetic brain studies. Today, most of the PET studies are clinical: they are used to search for tumors and metastases.

PET scanners are rather expensive, and we need positron-emitting radionuclides produced in cyclotrons, both limiting the number of studies. The price of a PET study is typically higher by an order of magnitude than that of a gamma camera procedure.

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