SPECT provides images of cerebral blood flow and brain activity. For this particular scan, a radioactive tracer attached to a drug, generally Technetium-99m-HMPAO (hexamethylpropylene amine oxime) or technetium-99m-ECD (ethyl cysteinate dimer), is administered intravenously. HMPAO and ECD are lipophilic drugs and are able to diffuse across the blood-brain barrier and into neurons. Once inside the cell, they are converted into hydrophilic compounds and are unable to diffuse out of the cell. Physical decay of the radionuclide attached to HMPAO or ECD leads to high-energy photon emissions that are measured by a SPECT detector. A computer creates visual images from this information, using various algorithms and filtering techniques to correct background noise and motion. Tracer uptake and cerebral blood flow are high in the gray matter, where neuronal bodies and synapses reside, and are low in the white matter, which is composed of metabolically less active axons. Thus, the cortex and subcortical structures will appear bright, or "hot," on SPECT, whereas white matter will appear "cold" and dark. The ventricles will appear even darker.

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