Abstract
Compared with single photon emission CT, positron emission CT posesses several dominant features which are dependent on the properties of positron emitters. The superiority of positron emission CT from a practical point of view is consisted as follows: (1) improvement of the spatial resolution, (2) potential ability of the quantification, and (3) availability of the isotopes of the life-constituent elements such as carbon-11, nitrogen-13, oxygen-15, and fluorine-18; in other words, availability of in vivo autoradiography using these tracers.
The National Institute of Radiological Science developed a newly designed positron CT, named POSITOLOGICA. This device has unique and outstanding features in systems with unequally spaced 64 BGO detectors arranged in a circle and a continuously rotating detector ring. PUSITOLOGICA has been applied for neurosurgical patients. The radioparmaceuticals administered are N-13-ammonia, C-11-CO, and F-18-fluorodeoxyglucose, which are produced and synthesized in the Institute using the cyclotron. N-13-ammonia and F-18fluorodeoxyglucose are administered by intravenous injection but C-11-CO is administered by inhaling. N-13ammonia acts as a diffusible tracer and is readily metabolized to glutamine in the brain tissue, but the extraction fraction of the glutamine is so slow that the N-13-ammonia imaging reflects the distribution of the cerebral perfusion.C-11-CO is combined with the hemoglobin and undiffusible in behavior. This makes the C-11CO images conduct the cerebral blood pooling distribution. F-18-fluorodeoxyglucose is an analogous substance of glucose and is transported within the brain tissue competitively. Then, fluorodeoxyglucose is metabolized to fluorodeoxyglucose-6-phosphate but no further. This characteristic property of fluorodeoxyglucose let F-18fluorodeoxyglucose images convey the local cerebral metabolic rate of glucose.
In normal volunteer subjects, the distribution of N-13-ammonia and F-18-fluorodeoxyglucose appeared in a similar fashion, and was in accordance with the brain tissue. Both tracers were accumulated in particularly high concentrations in the gray matter and the basal ganglia. C-11-CO activity was prominently accumulated in various dural sinuses and the vascular areas. In stroke patients, old lesions showed a lack of perfusion but fresh lesions showed hyperperfusion surrounded by a decreased perfused area. F-18-fluorodeoxyglucose activity was decreased in the lesion in spite of the hyperperfusion. Even in a TIA case in which X-ray CT images appeared normal, decreased perfusion and the lowered glucose metabolism were recognized in the area matching the neurological signs.
Positron CT's will flourish in the near future because of their ability to obtain the physiological and biochemical information of the brain.
