Single-photon emission computed tomography (SPECT) is an instrument for the evaluation of regional cerebral perfusion and receptor density studies. For the performance of SPECT, a flow tracer or a receptor-binding substance is tagged with a radionuclide and injected intravenously. The flow tracer is assumed to accumulate in different areas of the brain proportionally to the rate of delivery of nutrients to that volume of brain tissue. SPECT images are generated using gamma cameras or ring type imaging systems that record photons emitted by the tracer trapped in the brain. SPECT results in better image quality than two-dimensional or planar imaging because focal sources of activity are not superimposed on one another. Thus, SPECT provides functional information not available by conventional CT or MRI at a cost similar to that of CT. This technique has not been compared with conventional angiography, but it is more convenient than angiography when the diagnosis of death based on neurological criteria requires an arterial perfusion study. Combined with genetic information, imaging studies of patients at risk may be helpful in monitoring disease progression in early symptomatic or presymptomatic stages. Disease progression also should be monitored to test the effectiveness of new therapies aimed at halting neurodegeneration.
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