Functional neuroimaging with single photon emission computed tomography (SPECT).

Single photon emission computed tomography (SPECT) is a technique for producing regional maps of the in vivo distribution of radioactively labelled tracers without either the complexity or the cost of positron emission tomography (PET). Use of commercially available single photon emitting tracers such as 99mTc, 123I, or 201Th with longer half-lives than positron emitters eliminates the need for an on-site cyclotron and greatly simplifies the radiopharmacy requirements. In addition, the ability to produce images using gamma cameras which are routinely available in most nuclear medicine departments has considerably reduced the capital asset cost of imaging. SPECT is not an inexpensive procedure but it is much cheaper than PET. It is not possible to use the ideal biological labels of carbon, nitrogen, or oxygen with SPECT or to measure metabolic rates for oxygen or glucose. It is, however, now possible to image the distribution of cerebral blood flow with a reasonably well-validated technique, to investigate tumour viability, and to study an ever-increasing range of neurotransmitter receptor systems using SPECT. SPECT may have its technical limitations but it is the functional imaging technique which is likely to be available to most clinicians and, as experience with its application to a variety of pathological conditions grows, a much broader benefit from functional neuroimaging than could be produced by PET alone will result. The purpose of this review is not to compare SPECT with PET, but to give an overview of how SPECT works and what has been established in studies of various pathologies. In some cases, the clinical role of SPECT has already been established and in some it is emerging, but in other cases SPECT is a measurement tool for research purposes which is unlikely ever to be used routinely.