Single photon emission computed tomography (SPECT) is the most commonly employed nuclear medicine technique in use for imaging the brain. The SPECT assessment of CBF is based on the rapid diffusion of 99Tc-ECD across the blood-brain barrier, where it is rapidly hydrolyzed from a highly lipophilic form to a water-soluble acid. The water-soluble form is too polar to diffuse rapidly out of the intracellu-lar compartment of the brain. Imaging is then performed with a conventional planar gamma camera, which is rotated at various angles about the head. A typical image might involve acquisition of 60-120 planar images over a 360° rotation of the gamma camera. Frequently, the acquired SPECT scan is then co-registered with MRI of the same patient to provide anatomic correlation to the functional image provided by the SPECT. In two large prospective studies of ischemic stroke, SPECT had a sensitivity of 61-74% and a specificity of 88-98% . The sensitivity is somewhat higher for lacunar strokes.
The central clinical role of SPECT examination is to assess cerebrovascular reserve, both in the setting of cerebro-occlusive disease and prior to planned sacrifice of a major cerebral artery (Fig. 17.4). In some institutions, SPECT is used to diagnose and/or confirm brain death in patients in whom the neurologic examination is unreliable, such as the setting of barbiturate coma. SPECT is also used at many institutions in the evaluation of epilepsy: in the epilepsy-monitoring unit, 99Tc-ECD is injected during the ictal phase, within 40 s of onset of seizure activity. Subsequent SPECT examination demonstrates a pattern of markedly increased cerebral blood flow in the epileptogenic region. In temporal lobe epilepsy, the increased blood flow is seen within the mesial structures of the temporal lobe and often within the antero-lateral neocortex. SPECT has been of substantial experimental use for the study of stroke, subarachnoid hemorrhage, arteriovenous malformations, head injury and vascular dementias. However, the clinical utility of the study for these disorders has thus far been limited.
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