Positron Emission Tomography
PET[edit | edit source]
Positron Emission Tomography (PET) is a diagnostic imaging method that allows the distribution of a radiopharmaceutical within a patient’s body to be observed in cross-sectional (CT) images. It is used primarily in neurology, cardiology, and oncology.
How the device works[edit | edit source]
The patient is administered a β+ emitter with a very short half-life, typically no more than a few dozen minutes. The decaying radiopharmaceutical produces positrons, which almost immediately annihilate with electrons (the positron is the antiparticle of the electron), producing two gamma-ray photons. The resulting photons fly off in exactly the opposite direction with the same energy of 511 keV. The fact that the resulting photons travel in a straight line is utilised in detection. Only a photon for which its counterpart has also been detected on the detection ring is recorded; this is referred to as coincidence detection. The resulting tomographic image is then generated by processing a large number of such detected pairs.
Essentially, two types of detector configurations are used in the device:
- an even number of detectors rotating around the patient’s body;
- several hundred to thousands of fixed detectors arranged in the device in several rings. Opposite detectors in the same ring are connected so that they can register only those photon pairs that interact with them at the same time.
The detectors are not scintillators with conventional crystals due to the high energy of the photons; therefore, scintillators with crystals of higher density and higher atomic number are used. Examples include bismuth germanate and barium fluoride.
Three-dimensional image reconstructions are performed in a network of processors, known as transputers[1].
Positrons have a range of about 2 mm in tissue, after which annihilation occurs. This is therefore a method with very high accuracy.
Radiotracers used[edit | edit source]
The most commonly used radiotracer is the isotope 18F, which has a half-life of 110 minutes and decays into oxygen. It is most often administered in the form of [18F]-fluorodeoxyglucose (FDG), which is converted into glucose. Because fluorodeoxyglucose behaves similarly to glucose, it is taken up more in areas with more active metabolism (e.g., tumor cells). Another 18F tracer is, for example, the synthetic leucine analog [18F]-fluciclovin, which is used for the diagnosis of prostate cancer.
Other tracers include 11C, 13N, and 15O, which are biologically significant isotopes. A medical cyclotron is used on-site to produce such tracers with very short half-lives.
Process[edit | edit source]
The patient receives the radiopharmaceutical via injection or inhalation. For an FDG-PET scan, 150 to 700 MBq is administered, depending on the patient’s weight and the type of scanner (2D or 3D). After administration, the patient must wait 50 to 75 minutes to allow the radiopharmaceutical to accumulate in the relevant parts of the body. During the examination, the patient should be kept calm and warm. Otherwise, the cold may cause the patient’s body to increase sugar metabolism, which can be observed in the muscles or brown fat.
During the examination, the patient is positioned on a movable table so that the area being examined is within the range of the detectors. The axial field of view of the detectors is approximately 15–20 cm. For images extending beyond this limit, different patient positions are required. The duration of individual scans depends on the type of device, the type and dose of radiopharmaceuticals, and the patient’s weight. After two to four minutes, the device automatically moves to the next position.
Hybridy[edit | edit source]
PET is, in itself, a highly sensitive process. A drawback is that it is anatomically difficult to identify areas of higher metabolic activity, as it primarily detects metabolic processes. Added to this is the limited spatial resolution of 4–6 mm.
For this purpose, a hybrid device combining PET and CT was developed. By combining the high resolution (up to 0.35 mm) and detailed anatomical imaging of CT with the highly sensitive metabolic information from PET, higher-quality and more accurate results are achieved. Another advantage is a significant reduction in the total examination time.
Another option is the combination of PET with MRI. The advantage of integrating both into a single device is the ability to perform synchronised examinations for neurological and cardiological needs.
Advantages and disadvantages[edit | edit source]
A clear advantage of the device is its high diagnostic accuracy and spatial resolution. Modern devices have higher detection efficiency than SPECT, particularly due to the absence of collimators. Another advantage is the use of biogenic elements in metabolic imaging, which are naturally present in the body’s metabolism.
A significant disadvantage is the technical complexity of PET and, consequently, the high purchase price of the device. Furthermore, the often-necessary acquisition of a cyclotron is also costly.
