Digital Signal and Image Processing in Jagiellonian Positron Emission Tomography

Positron Emission Tomography (PET) is a key technique in the medical imaging area, which allows for diagnosing organism functions and tracking tumour changes. In PET measurement, the patient is injected with a radiotracer containing a large number of metastable atoms of radionuclide that emits positrons. As a result of positron annihilation, two photons travelling almost in opposite directions are produced and registered by a detection system positioned to surround the patient’s body. State-of-the-art PET scanners use scintillation crystals characterised by high detection efficiency of annihilation photons.

In this context, it is worth mentioning that the Jagiellonian PET (J-PET) Collaboration developed a novel whole-body PET scanner based on plastic scintillators. They are much cheaper than crystal scintillators, providing the opportunity to reduce the high cost of PET scanners and make them more affordable. However, plastic scintillators have much lower detection efficiency of gamma quanta compared to inorganic scintillation crystals. This can be compensated by increasing the scanner field of view and improving the time resolution in measuring the time of flight of gamma quanta. The J-PET scanner consists of plastic scintillator strips read out at both ends by a pair of photomultipliers and arranged axially around a cylindrical tomograph tunnel. The axial coordinate of the annihilation photon interaction point in the scintillator strip is derived from the difference in light propagation time measured with the pair of photomultipliers.

The operational principles of the J-PET scanner are similar to conventional tomographs, except that the highly accurate time information is of paramount importance. Therefore, the J-PET scanner demands the development of novel methods at each step of the data processing. The goal of the work presented in Digital Signal and Image Processing in Jagiellonian Positron Emission Tomography is the development of signal and image processing algorithms considering the uniqueness of the J-PET detector. The proposed methods include: signal recovery based on samples of a waveform registered on photomultiplier output, reconstruction of position and time of interaction of annihilation photon in the scintillator strip, classification of PET event types, and image reconstruction that operates exclusively in the image space. Due to the dissimilarity from conventional PET scanners, the majority of the methods presented in this dissertation are innovative solutions in digital signal and image processing in tomography.