Radioactive isotope, gamma camera, technetium – terms that do not necessarily evoke positive associations. Wrongly so: they are important components of nuclear medicine procedures and open up numerous diagnostic and therapeutic possibilities. Scintigraphy is one of them.
Principle of scintigraphy
Scintigraphy is an examination method in which images are produced by radioactive substances, usually technetium (99mTc), introduced into the body. This can be used to assess metabolism and organ function and detect certain tissue changes.
- Radionucleides (radioisotopes) are unstable atomic nuclei of chemical elements that decay easily, releasing radioactive radiation.
- If one binds such substances to carriers (“radioactive labeling”), a radiopharmaceutical is produced that can be introduced into the organism as an injection, tablet or respiratory gas. It distributes itself in the body and then – depending on the degree of enrichment – temporarily emits radiation of varying strength. This can be registered with the help of a so-called gamma camera and converted by computer into images (scintigrams).
- Chemical compounds that are known to be incorporated into certain organs are used as carrier materials, so that these can be specifically examined. For example, pertechneat is suitable for the diagnosis of the thyroid gland, because it is absorbed by it like iodine.
Very rapidly decaying radionucleides and briskly excreted carrier substances are used, so that the duration of action of the radioactivity is limited to minutes to hours and thus the radiation exposure for the patient is very low (usually not higher than that of conventional X-rays). Nevertheless, the examination should only be performed in exceptional cases during pregnancy and lactation. The excretion of radioactive degradation products via the kidneys can be accelerated with increased fluid intake following the examination.
Types of scintigraphy
Scintigraphy is excellent for checking tissues with regard to their functionality, and even before visible changes occur there. In principle, a distinction is made between static and dynamic scintigraphy. The first can be used to assess the position, shape, size and mass of tissue and to detect abnormalities such as inflammation or tumors. The actual organ function can be assessed using dynamic scintigraphy. The techniques used for this are sequence and functional scintigraphy:
- Static scintigraphy: here, similar to a normal X-ray examination, one or more images are acquired at one time point, in some cases in two planes, to better visualize the three-dimensional distribution of the radiopharmaceutical. For functional analyses, this form is used when the state of activity distribution is stable and persists for a relatively long time. To recognize regionally either a normal, a reduced or missing activity accumulation (storage defect, “cold spots”) or an increased storage (“hot spots”).
- Sequence scintigraphy: if the distribution of radionucleides changes quite rapidly and repeatedly (e.g., when urine is excreted via the urinary tract), several images are taken at fixed time intervals (e.g., every minute) to assess the course of the process.
- Functional scintigraphy: if sequence scintigraphy is combined with computer-controlled calculation of radiation activity, conclusions can be drawn about the functional capability of entire organs or their subregions. This can be particularly useful for side-by-side comparison of blood flow or organ function (e.g. kidneys, cerebral hemispheres).
Emission computed tomography (ECT) is based on a similar principle as scintigraphy. Here, too, a radiopharmaceutical (usually fluorodeoxyglucose) is injected. The emitted radiation is then detected with the aid of rotating cameras or ring detectors and – this is the main difference – converted by the computer into cross-sectional images (computed tomography). Single-photon emission computed tomography (SPECT) also uses gamma emitters for this purpose, while positron emission tomography (PET) uses short-lived positron emitters. The latter are extremely expensive, which is why the examination is performed only in large centers.
Procedure of a scintigraphy
Whether patient preparation is necessary depends on the organ being examined and the examination method.Sometimes the patient must remain fasting, stop or take certain medications, or drink more. The examination is performed in a lying or sitting position. The most unpleasant part is the injection of the radiopharmaceutical, which is usually necessary. The gamma camera is mounted on a motor-driven tripod, moves around the patient and takes pictures at intervals of seconds or minutes. For this purpose, the patient must remain still for 10 to 30 minutes, depending on the problem and the device. Scintigraphy can take anywhere from just under fifteen minutes (for one image) to several hours.
