Introduction
SPECT(single photon emission computed tomography) and PET(positron emission tomography) are recently being used as diagnostic imaging techniques for noninvasive evaluation of central nervous system (CNS) function. These in vivo imaging techniques provide a powerful tool for the evaluation of CNS function in health or disease states.´-" Emission computed tomographic instrumentation has developed in two directions according to the emission characteristics of the two types of radionuclides that are used: (1) detection of the two coincident 511 kev photons resulting from the annihilation of a positron in matter as a result of an interaction with a negatively charged electron and (2) detection of gamma radiation from nonpositron photon emittors. The most influential factor in the development of PET has been the recognition that a number of PET radionuclides
kretain chemical characteristics which render them particularly useful in the study of biochemical processes of fundamental importance in biology and medicine. In spite of the short half-lives of PET radionuclides, they have been extensively used in the study of chemical processes of living organisms. However, the short half-lives of most PET isotopes require an expensive on-site accelerator for production and also increase the complexity in the synthesis of certain compounds. SPECT has the practical advantage of using readily gamma-emitting radionuclides which have relatively long half-lives.
The utilization of SPECT has significantly increased with the development of appropriate detectors, radiopharmaceuticals, and reconstruction algorithms, coupled with rapid advances in the areas of computers and electronics. Generally, it is well recognized that PET has higher resolution, higher sensitivity and better quantitation capability then SPECT. Researchers in nuclear medicine have attempted to interpret apparent regional variations in radioactivity localization as representing the blood flow or receptor distribution. An observer interpreting these images applies a model incorporating his or her knowl-
plemented on a Macintosh computer (developed by Dr. Michael Graham at University of Wahington) or by nonlinear regression using a LevenbergMarquardt least squares minimization procedure implemented in MATLAB on a Macintosh computer (developed by Dr. Mark Laruelle at Yale University ).31
Conclusion
Dramatic progress has been made over the past few years in in vivo SPECT and PET neuroreceptor imaging in human brain. The most powerful aspect of SPECT and PET imaging is using the tracer technique to measure physiological and pharmacological processes in humans. With advancements of both SPECT and PET instrumentation and the selective ligands, the potential of SPECT and PET in research and clinical diagnosis in CNS disorders will greatly be recognized.
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