P.E.T. Image Acquisition     Positrons are emitted from the radioactive nuclide bound to the radiotracer. Positrons are identical in mass to electrons, but they possess positive instead of negative charge. After a positron is emitted, it is rapidly slowed by interactions within the surrounding tissue until all its kinetic energy (velocity) is lost. At this point, the positron combines momentarily with an electron. The combination of particles (positron and electron) then totally annihilates or disintegrates and results in two diametrically (1800 apart) photons of exactly 511 keV energy. This occurs because the energy of one photon equals rest mass of the electron and the energy of the other photon equals the rest mass of the positron. Since the two particles are identical in mass, the photon energies are also identical (E=m0c2). The pairs of photons are emitted in all directions equally from the body, however, only those pairs of photons that impinge upon two detectors in the P.E.T. camera as shown in the figure below are recorded. In general, several million events (photon pairs) are accumulated for each P.E.T. image. Currently three Siemens scanners are being used: A Biograph TruePoint PET/CT which is comprised of a 40 slice CT scanner and a 3 ring PET scanner with LSO crystals and pico electronics. A Biograph which is comprised of a 2 slice CT and a 3 ring PET scanner with LSO crystals. And the third PET camera which is an HR+ ECAT EXACT with BGO crystals. For animal research, a Phillips Mosaic is being used.     Patients are positioned comfortably on a table which moves through the opening of the scanner. Some patients require only one field of view (10-15 cm) to visulize a particular area of the brain or body while others are moved through the scanner using 7-8 bed positions to complete whole body imaging. The patient is prepared for imaging in a patient prep area and does not see the cyclotron or chemistry areas of the P.E.T. Center. A research study may require the patient or normal subject to be in the P.E.T. Imaging Center for up to 4 hours. Clinical studies may be done in as little as 2 hours. There are no known side effects and sedation is rarely used so patients are discharged upon completion of the imaging procedure. Many patients under going P.E.T. imaging are ambulatory and often drive themselves home.     Quantitative P.E.T. imaging normally requires additional information about the arterial concentration of the radiotracer during the scanning procedure. This is obtained by measuring in real-time the positron radioactivity concentration. A small catheter is placed in the radial artery by the P.E.T. Center nurse. During the imaging sequence for O15, arterial blood flows (3-5 ml/min) through the positron detector shown in the figure above. When imaging a non O15 radiopharmaceutical, manual, discreet sampling is performed. The arterial blood curve, P.E.T. image data and a physiological model representing the uptake and distribution of the radiotracer as a function of time are all combined to create functional images of flow, metabolism or receptor density. Related Pages Pet Imaging Radio Chemistry Radionuclide Production

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