Positron Emission Tomography (PET)

Positron Emission Tomography (PET) is an exciting modality, still in the early stages of general clinical acceptance [1]. The procedure has the potential to reduce healthcare expenditures for patients who have been diagnosed as having cancer. This is because PET imaging can provide comprehensive and accurate staging information which is not available from CT or MRI. Appropriate use of PET studies can lead to modification of treatment, for example from an aggressive approach to one of palliation when the staging information suggests a poor prognosis. PET images the metabolic activity within various types of cancer by detecting the uptake of a radioisotope. This is a reflection of the characteristics of the cancer, indicating the maturation rate as well as the vascularization. The information obtained is complementary to the anatomical information provided by CT, MRI or ultrasound. Since increased metabolic activity occurs before anatomical changes, and can be detected many months earlier, PET imaging can provide an earlier diagnosis of cancer and accurate staging. In addition, the PET studies can provide data for improving the accuracy of radiation therapy treatment planning, as well assessing the effects of various forms of therapy. PET imaging systems There are two types of PET imaging systems: dedi-cated PET, and single-photon emission computed tomography (SPECT) systems with coincidence imaging capabilities (coincidence systems). Both detect the presence of a radioisotope introduced into the body, and the principle of detection is the same. When a radioisotope decays it emits a positron- a positively charged electron- that immediately interacts with an electron. The combined particles undergo an annihilation reaction, which results in the release of two 511-keV photons in opposite directions. The imaging system is equipped with two detectors directly opposed to each other, i.e. in coincidence. Further, the annihilation event is known to occur along a line connecting the points of interaction in the two detectors (Figure 1). In the dedicated PET system, banks of detectors are located at fixed positions around the body, resulting in paired sets of opposed detectors. The Philips ADAC-CPET system uses six detectors located in a hexagonal arrangement that encircles the patient (Figure 2). For SPECT systems lik