Single photon emission computed tomography (SPECT; German: Einzelphotonen-Emissionscomputertomographie – from the ancient Greek: tome: the cut; graphein: to write) is a functional imaging technique used in nuclear medicine to produce cross-sectional images of living organisms based on the principle of scintigraphy. Similar to positron emission tomography, the production of radiological images is made possible by the distribution of weak radioactive substances. The SPECT method, which is the basis of today’s PET scan, requires the application of a so-called radiopharmaceutical (tracer; tracer substance: chemical substance that has been labeled with a radiologically active substance), through which the biochemical activity of certain areas of the body under examination can be displayed. The metabolic activity of these areas under investigation is almost linear to the accumulation of the administered radioactive substances.
Indications (areas of application)
- Huntington’s disease (St. Vitus’ disease)
- Representation of the blood flow to the heart
- Eating disorders (to detect dysregulation of neural circuits in the limbic system, indicating altered eating behavior).
- Focal epilepsy
- Brain tumors (gliomas: e.g., glioblastoma)
- Pituitary adenoma (benign tumor of the pituitary gland).
- Bone lesions
- Parkinson’s disease
- Differentiation of neuro-degenerative and non-neurodegenerative Parkinson’s disease or tremor syndromes by SPECT studies using the 123I-labeled dopamine transporter ligand FP-CIT (DaTSCANTM).
- Wilson’s disease (copper storage disease).
- Vertigo (dizziness)
- Cerebral circulatory disorders – to size the penumbra (penumbra (Latin : penumbra) is the term used in cerebral infarction to describe the area immediately adjacent to the central necrosis zone and still contains viable cells) and to determine myocardial viability, for example, after myocardial infarction (heart attack).
The procedure
The principle of single photon emission computed tomography is based on the technique of scintigraphy. As described earlier, the use of tracers is indispensable for SPECT because the radioactive radiation emitted by them can be measured by so-called gamma cameras. The function of the cameras is based on the use of collimators, through which the bundling of radioactive radiation can be achieved. Moreover, the detection of radiation is limited to certain spatial directions; obliquely incident photons are simply absorbed as a result. The apertures of the collimators are mainly made of lead. On the SPECT procedure:
- After intravenous application of a radiopharmaceutical, there is a pause of several hours during which the tracer is to be distributed in the area to be examined. During this time, the patient is encouraged to physically rest to ensure patient performance-related values. The injection is generally administered into the superficial veins of the arm. Technetium is used in the majority of SPECT scans today.
- Following this, detection of the gamma radiation emitted by the radionuclides is performed using the gamma camera. The patient remains on a couch while the cameras rotate around the body and create a three-dimensional image of the distribution of the radiopharmaceuticals. Depending on the type of device, the detection of the radiation is done with several cameras simultaneously. The large number of cameras ensures a location-dependent registration of the radionuclide radiation, which can additionally be supplemented by a dynamic examination.
- The dynamic scan is based on the principle of multiple measurements, so that the distribution of radioactive substances can be observed in a time-dependent manner.
However, the use of a SPECT scanner today is largely limited to imaging cerebral blood flow using the lipophilic radiopharmaceutical technetium 99m. By means of SPECT examination, there is the possibility to assess the blood flow disturbances causing the insult (synonyms: cerebral insult; apoplectic insult; apoplexia cerebri; stroke; medical colloquialism often also: apoplex or insult) and to visualize the penumbra (lat.In the case of a cerebral infarction, the area immediately adjacent to the central necrosis zone and still containing viable cells) can be evaluated both in terms of the extent of the damage and its expansion. As with PET, combination of the procedure with conventional computed tomography is possible, allowing more precise localization of metabolically active areas. Furthermore, when performed simultaneously, the resolution of SPECT increases, resulting in significantly improved precision. Although research in this area is still in its infancy, the combination of the two techniques is already leading to improved functional and morphological observation, as the computer system can use both methods for image analysis. SPECT/CT has made it possible to better assign altered metabolic properties to anatomic structures.
