SPECT Scan

SPECT (Single-photon emission computed tomography) is a technique for obtaining tomographic images of the studied area after the introduction of radiopharmaceuticals that emit photons during decay. The SPECT is performed on tomographs equipped with special gamma cameras. The resulting scintigrams are subjected to computer processing and image reconstruction using a special algorithm. Single-photon emission computed tomography is used to study internal organs (brain, heart, kidneys, liver, thyroid gland, etc.), skeletal bones, as well as the whole body (oncopoisk). In recent years, a hybrid method has been introduced into clinical practice that allows combining SPECT with computed tomography, i.e. simultaneously assessing the functional and morphological state of the organs under study.

Single-photon emission computed tomography is the result of the development of such a diagnostic technique as planar scintigraphy – as is known, this technique is based on the introduction of radioactive drugs into the body with subsequent registration of their decay in the studied organs. The disadvantages of this method are the two-dimensionality of the resulting image (which is why pathological “hot” foci can overlap and not be registered) and a significant number of other image artifacts. The SPECT is largely devoid of these disadvantages due to a different approach to recording the emitted RF radiation and subsequent processing of the data obtained. The main prerequisites for the creation of the SPECT were the spread of RFP with greater activity (containing radioactive technetium, talium, xenon) and the appearance of more sensitive gamma radiation detectors along with an increase in the power of computer equipment for processing the results.

SPECT is generally based on the same principles as conventional scintigraphy – RFP is injected into the body of the subject, which has an affinity for certain tissues or organs that need to be studied. After that, the radiopharmaceutical gradually accumulates in the organ under study, and the radioactive isotopes that make up it decay with the release of photons or gamma quanta. It is on the registration of these photons and the construction of images on this basis that both scintigraphy and SPECT are based – the difference between these methods lies only in the method of registration. In single-photon emission computed tomography, several detector blocks move around the study area along a circular, elliptical or complex trajectory. Due to this, the activity of gamma radiation is recorded from different sides, which, with appropriate computer processing, allows you to build both slice images of the area under study and a three-dimensional model of the RF distribution.

SPECT is a relatively new technique, but it confidently replaces traditional scintigraphy and in some cases can serve as a worthy replacement for such methods as X-ray computed tomography or magnetic resonance imaging. The advantages of this technique can be revealed especially clearly when it is necessary to diagnose initial pathological changes or when determining the smallest tumor foci. Due to the selectivity of RFP and the accuracy of gamma radiation registration, SPECT is much superior to X-ray CT and MRI in the detection of pathological foci. The disadvantages of single-photon emission computed tomography are the increased radiation load due to the introduction of radioactive isotopes into the body and the relatively long duration of the procedure, which can reach 1-1.5 hours when examining some organs.

Indications

The main field of application of SPECT is oncology, since modern radiopharmaceuticals have the ability to accumulate in tumor tissues of various localization (bones, brain and spinal cord, thyroid, parathyroid, mammary and other glands). As mentioned above, single-photon emission computed tomography can detect even the smallest foci of the neoplastic process, which are not recorded by any other methods. In oncology, the direct indications for SPECT are ambiguous results of other diagnostic studies (CT, MRI, ultrasound, planar scintigraphy). Also, this technique can be prescribed in the presence of other signs of malignant neoplasm, for example, various biochemical markers of tumors. The combination of SPECT and X-ray CT (the so-called SPECT / CT) is becoming increasingly popular – this method allows you to determine the localization of pathological foci with high accuracy.

Often, SPECT is also used outside of oncology – for example, in the diagnosis of diseases of the cardiovascular system. Single-photon emission computed tomography makes it possible to accurately assess the degree of myocardial perfusion, based on these data, it is possible to diagnose coronary heart disease, cardiomyopathy, pre- and post-infarction conditions. SPECT is used in urology to measure the functional activity of the kidneys, in neurology and other fields of medicine. In recent years, single-photon emission computed tomography has been performed in arthrology in order to detect pathological changes in some joints. To determine the embolism of the branches of the pulmonary artery, SPECT is much more informative than planar scintigraphy, since it allows you to more accurately determine the localization and boundaries of the area with impaired perfusion.

Contraindications

Absolute contraindications to the SPECT are in many respects similar to those for conventional planar scintigraphy. These include pregnancy and breast-feeding, since radioactive RFP material can harm the developing fetus and penetrate into breast milk. Relative contraindications to this procedure are the serious condition of the patient – fever, coma, weakness after a long illness, pronounced immunodeficiency. In such conditions, the attending physician should weigh all the advantages and disadvantages of SPECT in each case. Allergies or other intolerance reactions to most modern radiopharmaceuticals are not noted.

Preparation

As a rule, the study is not prescribed by an oncologist or other medical specialist initially – most often this procedure is preceded by another diagnostic technique (CT, MRI, scintigraphy). The basis for the appointment of single-photon emission computed tomography is the ambiguous results of the previous study. On the eve of the SPECT, special preparatory measures are usually not required – only in some cases (for example, in urology) it is necessary to adhere to the optimal water regime. Also, at the discretion of a specialist, it may be necessary to refuse certain medications – for example, affecting myocardial perfusion during cardiac SPECT.

After arriving at the medical institution, the nurse intravenously injects the patient with an RFP solution – its amount is calculated by the doctor depending on the goals of the study and the anthropological parameters of the patient. Then it is required to wait for some time – from 1 to 3 hours, necessary for the distribution of the radiopharmaceutical and its accumulation in the study area for the most reliable results of the SPECT. The duration of the distribution period also depends on many parameters – the amount of RFP injected, its type, the functional activity of the organ or tissue under study, and a number of others. By agreement with a specialist, the patient can stay in a medical institution or leave it before the time of the SPECT during the distribution of RFP throughout the body.

Methodology of conducting

Special equipment is required for the SPECT, including a table on which the patient is located, and one or more detector units capable of rotating around the table. The trajectory of this rotation can be different – on the simplest equipment for single-photon emission computed tomography, the detectors rotate in a circle, but this reduces the accuracy of the resulting image. More advanced devices for SPECT ensure the movement of detector blocks along an elliptical trajectory, close in shape to the section of the human body in the axial plane – as a result, gamma quanta travel approximately the same distance from the radiation source to the detector in all directions. The most modern equipment for performing SPECT has the ability to move detectors along a contour-adaptive trajectory – using infrared sensors, the surface of the patient’s body is determined, the detectors move around it at exactly the same distance.

In contrast to X-ray computed tomography, in most cases, the detectors at the SPECT do not move around the study area continuously – due to the low activity of isotopes in the RPF, some time is needed for signal accumulation or exposure. The duration of exposure in one projection depends on the resolution of the detector (the higher it is, the longer the accumulation of the signal will be) and the activity of the injected radiopharmaceutical. The number of such projections to obtain a full-fledged three-dimensional image of the SPECT can be more than 60. Thus, the duration of exposure and the number of projections are the main factors affecting the duration of the procedure of single-photon emission computed tomography – usually it is about 30-50 minutes, but in some cases it reaches an hour and a half. Some types of equipment for SPECT provide a mode of continuous movement of detector blocks, but at the same time the resolution of the device decreases and the diagnostic value of the technique decreases.