SPECT SCAN  [ back to Neuroimaging ]
SPECT Scan or Single Photon Emission Computed Tomography uses a rotating gamma camera to image internal organs of the body in slices, which can be viewed individually or stacked to replicate the 3 dimensions of the organ. The patient is administered an intravenous dose of radioactive technetium, is challenged with mental tasks and then placed under the camera.

Unlike PET there are no collisions between positrons and electrons. Rather the technitium compound spontaneously releases gamma rays as it courses through the blood vessels in the brain. The camera records the location of the collisions, and after digitization, the data is processed by a computer which generates a map of regional cerebral blood flow.

The rate of blood flow in the brain over time is called cerebral perfusion. Traumatic brain injury tends to create a picture of markedly unequal cerebral perfusion, high in the healthy parts of the brain and low in the damaged ones. SPECT scans are much cheaper than PET scans, because the camera is much cheaper to buy. However, a real drawback is that the clarity of definition in SPECT is significantly less than PET. To the untrained, a SPECT scan looks like a smudged fingerprint or a Rorschach inkblot. Its hard to see the basic structural dividing lines of brain anatomy. It is this imprecision which makes SPECT less desirable than PET, if PET can be afforded.

This may change. In the late 1990s some manufacturers of gamma cameras designed solely for SPECT have converted them to a combination SPECT/PET scanner, which makes economic sense since PET scanners are very expensive and used far less often than SPECT scanners. The trick behind the conversion is to use a thicker sodium iodide crystal stip in the gamma camera which works well for both techniques.