What is single photon emission computed tomography (SPECT)?

Indications and Procedures

Single photon emission computed tomography (SPECT) uses the radioisotopes xenon 133, technetium 99, and iodine 123 to acquire information about blood flow. A small amount of radioisotope is injected into a patient’s vein to observe the flow of blood and metabolic pathways during the digestion of food. These radioisotopes are the radioactive forms of the naturally occurring elements of xenon, technetium, and iodine. These forms are referred to as radioactive because they emit gamma rays. These gamma rays can be measured directly by using a gamma ray detector containing a series of crystals that convert the gamma rays to photons of light. Photomultiplier tubes amplify the photons into electrical signals, which are then converted by a computer into detailed three-dimensional visual images on a screen.

SPECT is one of several nuclear imaging techniques used in medicine for diagnosis. Imaging is important as a noninvasive method of seeing inside the body, without requiring surgery. Other common techniques include x-rays, magnetic resonance imaging (MRI) scans, computed tomography (CT) scans, and ultrasound. The other nuclear imaging techniques include cardiovascular imaging, bone scanning, and positron emission tomography (PET). All these techniques assist in the detection of inadequate blood flow to tissues, aneurysms (weak locations in the walls of blood vessels), various blood cell disorders, and tumors.

Of these techniques, SPECT is the most similar to PET, but SPECT is less expensive and more readily available. SPECT radioisotopes emit single gamma rays with longer decay times than in PET, and thus have the disadvantage of producing less detailed images than PET.

Tomography refers to the technique of using rotating x-rays to record an image within the body. With today’s computers, the terminology of computed tomography (CT) is used. The imaging process of SPECT combines CT with the use of radioisotopes. These radioisotopes are often referred to as tracers because they allow physicians to follow the pathway traveled by the blood through the body. Tracers emit gamma rays that are collected by a computer, which then translates the data into two-dimensional cross sections that are added together to form a three-dimensional image. These radioactive tracers decay within minutes to hours and are eliminated in the urine, thus posing negligible harm to the body.

Uses and Complications

The sharp images that can be obtained using SPECT enable it to be a useful diagnostic tool for a variety of cardiovascular, cerebrovascular, and neurological disorders. SPECT is more sensitive than an electrocardiogram (ECG) for detecting ischemia. In order to diagnose ischemic heart
disease, SPECT scanning enhances myocardial perfusion imaging (MPI) after a patient exerts stress in order to compare images from before and after stress to assess blood flow. SPECT has become an extensively used tool to diagnose coronary artery disease (CAD). Because it is such a useful tool for detecting reduced blood flow, SPECT has also been widely used to detect tumors. For example, as part of the diagnosis of patients suspected of having aneurysms or tumors at the base of the skull, the internal carotid artery temporary balloon occlusion (TBO) test is enhanced by the use of SPECT to evaluate the cerebral blood flow. SPECT is also used to detect lymphoma tumors in the chest and abdomen, neuroendocrine tumors, stress fractures and stress reactions in the spine (known as spondylolysis), and liver
lesions.

The high resolution of SPECT allows it to be a very useful tool for obtaining images of the striatum, a specific area of the brain containing the neurotransmitter dopamine. This dopamine activity can be monitored to help diagnose schizophrenia and various mood and movement disorders, including epilepsy, Alzheimer disease, dementia, and obsessive-compulsive disorder.

Perspective and Prospects

Although a SPECT scan exposes the body to less radiation than does a CT scan or a chest x-ray, pregnant or nursing women should not receive a SPECT scan. A nuclear medicine technologist will inject a patient with a small amount of radioactive tracer. After enough time is allowed for the tracer to travel to the brain (usually ten to twenty minutes), a special camera called a gamma camera is used to acquire multiple images from multiple angles by rotating around the head. This gamma camera detects the gamma radiation emitted by the radioactive tracers. Thus, the patient needs to remain motionless during the scanning process so that clear images can be obtained. After the scanning process is finished, it is important for the patient to drink fluids to remove the radioactive tracers from the body.

Bibliography

American Academy of Neurology, Therapeutics, and Technology Assessment Subcommittee. Assessment: Brain SPECT. Minneapolis: American Academy of Neurology, 1995.

American Heart Association. "Single Photon Emission Computed Tomography (SPECT)." American Heart Association, November 6, 2012.

Frankle, W. G., et al. “Neuroreceptor Imaging in Psychiatry: Theory and Applications.” International Review of Neurobiology 67 (2005): 385–440.

Masdeu, J. C., et al. “Special Review: Brain Single Photon Emission Tomography.” Neurology 44 (October, 1994): 1970–1977.

Mayo Clinic. "SPECT Scan." Mayo Clinic, March 4, 2011.

Van Heertum, R. “Single Photon Emission, CT, and Positron Emission Tomography in the Evaluation of Neurologic Disease.” Radiologic Clinics of North America 39 (May, 2001).