P&S Journal: Fall 1994, Vol.14, No.3
By Judith Sulzberger
An advanced research and diagnostic facility containing a PET scanner and a cyclotron officially opened in May at CPMC. The project is a collaborative venture of the departments of radiology, medicine, and psychiatry and has received strong support from both P&S and Presbyterian Hospital.
PET, or positron emission tomography, differs from other tomographical cross-sectional scanners, such as CAT, because it alone allows the use of radioactive carbon, oxygen, and nitrogen, which, when radioactive, emit positrons. These decay by the simultaneous loss of two photons exactly 180 degrees apart, which are detected by the scanner to make a computerized image.
Radioactive carbon, oxygen, and nitrogen have extremely short lives (one to two minutes), so it is necessary to have a cyclotron (located in the basement below the PET scanner) to manufacture these isotopes for immediate delivery and administration. The patient ingests, inhales, or is injected with one of these substances and is placed on a pallet that moves automatically into the scanner. The pallet is surrounded with a ring of multifaceted crystals, and the patient can be moved under the ring for a total body scan or left in one spot for a local picture. The crystals record only the photons hitting on diametrically opposite sides of the ring at precisely the same time to ensure that they are the positron decay products of the given radioactive element. The scanner provides a high-resolution computer image of the areas of uptake of whichever element is being used and reveals how the body utilizes that substance. As a scanner, PET is unique, because it examines body functions that no other imaging instrument can evaluate.
One of the studies to be undertaken with the PET scanner determines how the brain responds to the use of various psychoactive drugs. This may be demonstrated by the pattern of oxygen uptake in the brain or by the uptake patterns of radiolabeled neuroreceptor compounds. Virtually any chemical compound found in nature ultimately can be tracked with PET because so many contain carbon, nitrogen, or oxygen.
Another positron-emitting radioactive element is fluorine. This binds easily to glucose, which is utilized in the body as a metabolite and is abundantly accumulated in the brain and cancerous tumors: When radioactive fluorinated glucose is given to patients with Alzheimer's disease, glucose uptake in the parietal, temporal, and, sometimes, the frontal lobes of the brain decreases, which is of diagnostic value. In malignant tumors, which also utilize large amounts of glucose, the PET scanner is particularly useful for determining if there has been metastatic spread, thereby avoiding unnecessary surgery and allowing for early alternative treatment.
PET scanning using fluorinated glucose is also useful in heart disease. Heart muscle, when healthy, utilizes various fatty acids as metabolites; however, the heart muscle uses glucose when the oxygen supply to the muscle is compromised. If the heart muscle is totally destroyed it metabolizes nothing, and a scan of the injured area shows no glucose accumulation. Since it is useless to put a bypass graft in an area that has no viable muscle, the scan can screen out patients who will not benefit from surgery. This procedure successfully used at UCLA for several years will now be available at CPMC.
Approximately 50 PET-cyclotron centers are located around the country, three in the New York area (Sloan-Kettering, North Shore Hospital, and CPMC). The high costs of radiotracer production and the scanner itself make PET an expensive procedure. PET scanning is not a new procedure, but the scanner at CPMC is the newest state-of-the-art model.