Positron Emission Tomography Applications of EGS ’

The use of positron emitters in medical imaging was first suggested by Wrenn and coworkersl, and Sweet’, and the first prototype Positron Emission Tomography (PET) scanner WM built by Brownell and Sweets as shown in Fig. 22.1. A radiopharmaceutical labeled with a positron emitter is distributed within a biological target. The emitted /3+ annihilates with an electron of the surrounding tissue to produce two photons of 511 keV back-to-back (in the CM system). By detecting these two gammas in coincidence, one constrains the annihilation event to the volume spanned by the two detectors. The original distribution of radioisotope is reconstructed from a series of “projections”, obtained by moving and rotating the scanner around the patient. The “electronic collimation” avoids the use of passive collimators (typical transmission efficiency lo-’ lo-‘) which are riecessary in Single Photon Emission Computed Tomography (SPECT) with r-emitting radionuclides; this implies not only a reduction of the radiation dose delivered to the’patient, but also a higher spatial resolution and a better quantization of the imaging results.