Single-photon emission computed tomography (SPECT) is part of the examination spectrum of nuclear medicine. Its purpose is to assess metabolism and thus function in various organ systems. This is made possible by means of a radiopharmaceutical administered to the patient, the distribution of which in the body is made visible in the form of cross-sectional images with the aid of gamma cameras.
What is single photon emission computed tomography?
Single photon emission computed tomography (SPECT) is part of the examination spectrum of nuclear medicine. Its purpose is to assess metabolism and thus function in various organ systems. Single photon emission computed tomography is often known by the abbreviation SPECT, which is the abbreviation for the English name of this examination (Single Photon Emission Computed Tomography). It should not be confused with CT (computed tomography) because of the similar name: While CT uses X-rays and a classic contrast agent, single photon emission computed tomography is based on the administration of a tracer (technetium-99m is most commonly used here), whose path through the body is followed by gamma cameras due to its radiation – harmless in the dose used – i.e. without any exposure to X-rays. In principle, the examination is similar to the frequently performed scintigraphies, for example of the thyroid gland or the lungs. A distinction is made between two variants of this nuclear medical diagnostic method: in the static method, the location of the radionuclide in the body at the time of examination is measured only once; in the dynamic examination, repeated images also reveal changes over time.
Function, effect, and goals
The task of single-photon emission computed tomography is to check the functioning of certain organ systems and to detect any disorders. This is made possible with the so-called tracer, a weakly radioactive substance. The substance is usually injected into the patient’s arm vein at the start of the examination, but for special organ examinations it can also be swallowed or inhaled. The slightly radiating radionuclide spreads in the organ to be examined and emits the weak gamma radiation there for a certain period of time. This is detected by special cameras, the so-called gamma cameras. The measuring heads of the cameras rotate around the patient’s body during the examination and record the radiation from different directions. Beforehand, a waiting time that varies depending on the reason for the examination is observed so that the tracer can accumulate optimally in the body. This accumulation is recorded by the gamma cameras and reproduced in cross-sectional images using computer-aided calculations. These are at least two-dimensional, sometimes even three-dimensional, and for this reason form a meaningful basis for the nuclear medicine physician’s diagnostics. In the case of questions in which the metabolism in the organ, i.e. the course of the distribution of the radiopharmaceutical, is of primary importance, the image is repeated after a certain period of time, which can be minutes or even hours. A common type of examination in the field of single-photon emission computed tomography is SPECT of the heart: it gives the cardiologist important information about the blood supply to the heart muscle tissue and can also be used in conjunction with the ECG (gated SPECT). Indications of constricted coronary vessels or heart failure are thus often detected at an early stage, so that appropriate prophylaxis can be initiated, for example to prevent a heart attack. Single-photon emission computed tomography also plays an important role in checking brain function: From circulatory disorders that can trigger a stroke to degenerative processes such as Parkinson’s disease, the diagnostic range is extensive. Nuclear medical examinations are also used in the examination of epilepsy patients or for certain tumor diseases. The cross-sectional images also provide meaningful information about the metabolism in the bones, so that diagnostics is also used in this area and, for example, provides the image basis for adequate therapy in the case of inflammations or loose prostheses.SPECT is also used to detect NET, the neuroendocrine tumor that is usually found in the digestive system. A special combination is the so-called SPECT/CT, which is realized with the help of special equipment. It combines the ability of single-photon emission computed tomography to show functional processes in the body with the advantage of CT in showing morphological structures. Single photon emission computed tomography is usually performed in the supine position. In most cases, it does not require any special preparation. Only when examining certain organ systems may it be necessary to perform this examination when the patient is fasting.
Risks, side effects, and hazards
Single-photon emission computed tomography – like conventional scintigraphy – is a very low-risk examination method. This is due, on the one hand, to the fact that the patient is not exposed to X-rays during this examination (with the exception of the special examination SPECT/CT). In addition, instead of a classic contrast agent, which – especially in the case of iodine-containing substances – could lead to an allergic reaction in some patients, the radioactive tracer (in many cases technetium) is used, which does not usually lead to side effects. The half-life of the radiopharmaceuticals used is very short, so that there is no danger to persons who come into contact with the patient after the examination. Only close physical contact with pregnant women or small children is not recommended for the day of the examination – similar to the frequently performed thyroid scintigraphy, for example. The nuclear medicine physician also recommends certain precautions for nursing mothers, which are only relevant for a short time. Patients who drink a lot after the examination can further accelerate the elimination of the already low radioactivity from their bodies. The SPECT device cannot be compared to the narrow tube of the MRI, which many patients find stressful. The open portions on the unit make single-photon emission computed tomography easy to perform even for patients with claustrophobia.
