Blood flow to the brain is represented on a color scale, where dark areas have no flow and bright yellow areas have good blood flow. The dark “butterfly-shaped” area in the center of the brain is normal, but the dark area on the right of each picture is the region of impaired blood supply (ischemia) that corresponds to the patient’s acute stroke.
Prior to t-PA, the SPECT scan showed a large area of the brain that has lost its blood supply (arrow).
Several hours after treatment with t-PA, the blood flow has dramatically improved, although there is still a small area ischemia (arrow).
SPECT studies combine nuclear medicine (the use of radioisotopes in the diagnosis of disease) with computed tomography. In this technique, the patient either swallows or is injected with a radioisotope, which travels to a target organ. Concentrating in the target organ, the radioisotope emits radiation, which is detected by a gamma camera that rotates around the patient. The information obtained via the gamma camera is analyzed by a computer, which creates a cross-sectional image of the target organ. SPECT scans are frequently used to determine if a specific area of the body is receiving adequate blood flow.
Used in early hours after infarction, cerebral SPECT is able to reveal a deficit in local blood flow before changes appear on CT or MRI [Mohr JP, 1992. Dunbabin DW, Sandercock PAG, 1991.]. However, SPECT does not reliably distinguish between hemorrhage and infarction, and it is unclear whether the method will predict the potential for clinical recovery. Because it can provide information regarding cerebral perfusion, SPECT (usually in combination with transcranial Doppler) has been helpful in following the course of vasospasm in patients with subarachnoid hemorrhage.
Stable xenon-enhanced contrast CT, which uses the inert gas xenon to measure cerebral blood flow (CBF) in various brain regions, is an alternative to SPECT. [Dunbabin DW, Sandercock PAG, 1991]. Stable xenon CT, whereby a patient inhales a mixture of xenon and oxygen over the period of a few minutes, allows measurement of their increased in density caused by the gas in brain tissue [Haubitz B, et al, 1993] and can be incorporated into all existing CT technologies [Yonas H, et al, 1996]. This method can be used to determine local cerebral blood flow in an area as small as 1 x 1 x 5 mm3 area, and can be repeated within 20 minutes, allowing the assessment of hemodynamic states [Yonas H, et al, 1996], including (in certain well-defined settings) the evaluation of acute stroke, occlusive vascular disease, carotid occlusion testing, vasospasm, arteriovenous malformations, and head trauma management [Johnson DW, et al 1991].