Basic principles and instrumentation for positron emission tomography


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Sibylle Ziegler

Technical University of Munich, Germany

Abstract

Positron emission tomography (PET) is a nuclear medical imaging technique for the quantitative measurement of physiologic parameters in vivo. PET is based on the detection of very small amounts of positron-emitter-labelled biologic molecules. Various radiotracers are available for oncological applications in the clinic and in research protocols. For example, fluorine-18 labelled Fluorodeoxyglucose (FDG), a sugar analogue, is used for tumour detection and staging, using the fact that tumour cells have an increased glucose uptake compared to normal tissue. Image generation is based on the detection of the two 511 keV gamma rays which are emitted back-to-back in coincidence during positron annihilation. Typically, small scintillation detector elements are arranged in rings, each detector element in coincidence with a number of opposing elements, covering an axial field-of-view of up to 20 cm. From these integral data, the underlying activity distribution is reconstructed using algorithms such as filtered backprojection or, more recently, iterative, statistical methods. For quantification of activity concentrations, corrections for random coincidences, scatter and attenuation need to be applied.

For this purpose advanced techniques are still being developed, including additional measurements and simulation approaches. Improving spatial resolution and sensitivity of positron tomographs is the goal of ongoing research in detector and reconstruction development. Densely packed arrays of small crystals are read out by smart light sharing systems based on photomultiplier tubes. Most recently, methods to measure and utilise the time-of-flight information in PET have been developed to further increase signal-to-noise in PET scans with short duration. In the clinical setting, the combination of PET and CT in one device, offering functional and structural information at almost the same time, has become an important tool. The combination of PET and MR is much more challenging, but has recently gained much attention.