Technical Aspects of Positron-Emission-Tomography
Typically, a PET-scanner consists of a detector ring with 60 cm diameter and 20 cm length inside which the patient is positioned. For brain studies the patient bed is positioned in such a way that the entire brain is within the field of view. To investigate the entire patient the bed is moved step-by-step. The detector consists of scintillation detectors with scintillation crystals coupled to photomultipliers. The radiation, which is emitted from the patient after the injection of a weak radioactively labelled substance (radiotracer), causes light pulses within the scintillation crystals. This light is converted to electrical signals by the photomultiplier. A detector is linked to opposing detectors by a coincidence logic: if the radiation hits two opposing detectors within a time interval of, e.g. 6 ns, it is assumed that the radiation comes from a radiotracer molecule located on the line connecting the two detectors. During the PET study, which can last from a few minutes up to two or three hours, thousands of such coincidence lines are recorded. Using this data and appropriate programs images are reconstructed which show the spatial and temporal distribution of the radiotracer within the patient. In this way increased or decreased uptake in a tumour or an infarct area, respectively, can be visualised.
Today’s PET-scanners offer an image resolution of 4-5 mm. To facilitate the delineation of PET findings with the help of anatomical high resolution CT images industrial PET scanners are available only as hybrid PET/CT systems.