PET and SPECT Scans of Brain Function

by Steve Holder on June 27, 2008

PET and SPECT images can identify brain injuries undetectable in CT scans or MRIs. CT and MRI are both structural imaging methods. They tell us what the tissues look like inside the body. PET and SPECT scans, on the other hand, provide functional imaging. They tell us how well the tissues are working.

PET Scans

CT/PET ScannerPositron Emission Tomography (”PET”) uses radioactive tracers to obtain images of organs and internal body structures. The PET scanner resembles a CT scanner but instead of measuring X-rays passing through the body, it measures gamma rays emanating from tissues where tracer elements have accumulated.

For brain imaging, different types of tracers are ingested or injected in the bloodstream depending on the type of information the doctor is looking for. Based on the tracer selected, images can focus on showing microscopic blood flow, oxygen or glucose consumption associated with brain activity, or variations in dopamine concentrations that can signal damaged areas.

Because the resulting brightly colored images show where biochemical processes are abnormal, the physician can more clearly see the degree and extent of an injury.

Though the images from PET scans are not as detailed as CT or MRI, new techniques are allowing doctors to fuse PET and CT images together. Newer machines perform a CT scan immediately after the PET scan while the patient is still on the table and in the same position. When the PET and CT images are combined, the anatomical and biochemical data are shown together so the doctor can see both what is wrong and precisely where.

Note that some insurance companies consider PET tests “experimental,” and will not cover their cost.


Single Photon Emission Computed Tomography (”SPECT”) is very similar to PET. It is a simpler technology, though, and is limited to showing blood flow. Still, it can be very effective in identifying damaged areas of the brain, and is less expensive than PET.

SPECT also requires a radioactive tracer based on the specific type of imaging desired, but the machinery is different from PET and CT scanners. Instead of the patent’s head being inside a donut-shaped ring, the SPECT camera rotates around the patient.


The radioactive tracer is typically injected about one hour before the scan so that the targeted tissues can absorb it. The amount of radioactivity involved is so low that there is no risk to the patient even from having several scans done in a short period. On very rare occasions, someone may have an allergic reaction to the tracer compound.

Both PET and SPECT scans require the patient remain very still throughout the 30 to 90 minutes of the procedure. This can be uncomfortable and some patients may receive a sedative to help them relax.

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