Single Proton Emission Computed Tomography

Definition

Single proton (or photon) emission computed tomography (SPECT) allows a physician to see three-dimensional images of a person's particular organ or body system. SPECT detects the course of a radioactive substance that is injected, ingested, or inhaled. In neurology, a SPECT scan is often used to visualize the brain's cerebral blood flow and thereby, indicate metabolic activity patterns in the brain.

Purpose

SPECT can locate the site of origin of a seizure, can confirm the type of seizure that has occurred, and can provide information that is useful in the determination of therapy. Other uses for SPECT include locating tumors, monitoring the metabolism of oxygen and glucose, and determining the concentration of neurologically relevant compounds such as dopamine.

Currently, a clinical trial is underway in the United States to evaluate the potential of SPECT to study brain receptors for the neurotransmitter acetylcholine. The study will help to determine the usefulness of the technique in charting the progress of the brain deterioration associated with Parkinson's disease.

Precautions

The exposure to radiation, particularly to the thyroid gland, is minimized as described below in the sections on preparation and aftercare.

Description

Since its development in the 1970s, single proton emission computed tomography has become a critical and routine facet of a clinician's diagnostic routine. A SPECT scan is now a typical part of the diagnosis of coronary artery disease, cancer, stroke, liver disease, bone and spinal abnormalities, and lung maladies.

SPECT produces two-dimensional and three-dimensional images of a target region in the body by detecting the presence and location of a radioactive compound given prior to the test. The photon emissions of the radioactive compound can be detected in a manner that is similar to the detection of x rays in computed tomography (CT). The image produced is a compilation of data collected over time following introduction of the tracer.

The radioactive compound that is introduced typically loses its radioactive potency rapidly (this is expressed as the half-life of a compound). For example, gamma-emitting compounds can have a half-life of just a few hours. This is beneficial for the patients, as it limits the contact time with the potentially damaging radioisotope.

The emitted radiation is collected by a gamma-camera through thousands of round or hexagonal channels that are arranged in parallel in a part of the machine called the collimator. Only gamma rays can pass through the channels. At the other end of the channel, the radiation contacts a crystal of sodium iodide. The interaction produces a photon of light (hence, the name of the technique). The light is subsequently detected and the time and body location of the light-producing radiation is stored computationally. At the end of the SPECT scan, the stored information can be integrated to produce a composite image.

Typically, a patient is stationary. The SPECT scanner can move completely around the patient. Usually the patient will lie on a bed with their head restrained in a holder. Scans are taken for periods up to six hours following the injection of the tracer.

Monitoring of the heartbeat (electrocardiogram), respiration, and blood pressure are accomplished just prior to the start of the scan, five minutes after the introduction of the tracer, and 30–60 minutes after injection. Blood and urine samples are often collected towards the end of the scan.

Preparation

On the night before a scan, the patient takes an oral dose of potassium iodide. This protects the thyroid gland from the radioactive tracer. If a patient is allergic to potassium iodide, potassium perchlorate can be taken instead. Just prior to a scan, small radioisotope markers that contain the element 99Tc are attached with adhesive to the patient's head. Two intravenous catheters are usually placed in veins, through which the radioactive tracer is injected, and so that blood samples can be withdrawn during the scan.

Aftercare

Oral doses of potassium iodide or potassium perchlorate are taken daily for four days following a scan. Patients are asked to urinate every two hours for the first 12 hours following the scan to eliminate the tracer from their body as quickly as possible.

Risks

The use of radiation poses a risk of cellular or tissue damage. However, the injection of the radioactive tracer results in the swift movement of the tracer through the body, and its rapid elimination.

Normal results

The image of the target region of the body is compared to an image of the healthy target region. Analysis of the images by a qualified physician determines the result.

BOOKS

Brant, Thomas. Neurological Disorders: Course and Treatment, 2nd. ed. Philadelphia: Academic Press, 2002.

OTHER

"Psychopharmacology—The Fourth Generation of Progress." Positron and Single Photon Emission Tomography. Principles and Applications in Psychopharmacology. American College of Neuropsychopharmacology. (January 27 2004). <http://www.acnp.org/g4/GN401000088/CH087.html>

Brian Douglas Hoyle, PhD