Single photon emission computed tomography (SPECT)
Single photon emission computed tomography (SPECT) is a nuclear imaging technique used in medicine to visualize blood flow and metabolic processes within the body. By injecting a small amount of radioactive isotopes, such as xenon-133, technetium-99, or iodine-123, into a patient's vein, SPECT captures detailed images of how blood circulates and how tissues function during various physiological states, such as digestion. This imaging technique is non-invasive and provides critical diagnostic information about a range of conditions, including cardiovascular diseases, tumors, and neurological disorders.
SPECT operates by using gamma rays emitted from the injected isotopes, which are detected by specialized cameras that convert these signals into three-dimensional images. Although it is less costly and more accessible than similar technologies like positron emission tomography (PET), SPECT typically produces images with less detail. The procedure is particularly valuable for assessing ischemic heart disease and monitoring conditions affecting brain health, such as Alzheimer's disease and schizophrenia. While SPECT is generally safe, precautions are taken for pregnant or nursing women, and patients are advised to hydrate post-procedure to assist in eliminating the tracers from their bodies.
Single photon emission computed tomography (SPECT)
Also known as: Single photon emission tomography (SPET)
Anatomy or system affected: Blood, blood vessels, brain, circulatory system, head, heart
Definition: A nuclear imaging test used to provide three-dimensional information about the flow of blood through arteries and veins to diagnose a wide range of health conditions, including strokes, epilepsy, dementia, and tumors
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. They are referred to as radioactive because they emit gamma rays, which can be measured directly 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. 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, the one most similar to SPECT is PET, but SPECT is less expensive and more readily available. SPECT radioisotopes emit single gamma rays with longer decay times than those used in PET and 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 modern 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 combined to form a three-dimensional image. These radioactive tracers decay within minutes to hours and are eliminated in the urine, posing negligible harm to the body.
Uses and Complications
The sharp images that can be obtained using SPECT make it a useful diagnostic tool for a variety of cardiovascular, cerebrovascular, and neurological disorders. SPECT is more sensitive than an electrocardiogram (ECG or EKG) for detecting ischemia. To diagnose ischemic heart disease, SPECT scanning enhances myocardial perfusion imaging (MPI) after a patient exerts stress 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 cerebral blood flow. SPECT is also used to detect lymphomas 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 makes it 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's disease, dementia, and obsessive-compulsive disorder.
Perspective and Prospects
Although a SPECT scan exposes the body to less radiation than a CT scan or a chest X-ray, pregnant or nursing women should not undergo the procedure. When the procedure is performed, a nuclear medicine technologist will inject the patient with a small amount of radioactive tracer. After enough time has passed 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. The patient must 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
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Van Heertum, Ronald L., et al., editors. Functional Cerebral SPECT and PET Imaging. 4th ed., Lippincott Williams & Wilkins, 2010.