Digital PET/MRI for preclinical applications

Combining different medical imaging modalities into a single device is a promising way to attain comprehensive and complementary information. Hybrid imaging modalities thus enable new research methods, improve diagnostic accuracy, and improve clinical workflow. The seamless integration of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) will additionally lead to excellent spatial and temporal registration capabilities and will reduce the overall scan time.One of the main technological challenges is to develop a scalable PET detector architecture that is capable of supplying, controlling, cooling, synchronizing, and reading out a high number of detector channels, while at the same time keeping its interference with the MRI operation low. The approach followed in this thesis is to digitize the PET sensor signals as closely to the sensors as possible in order to maximize the PET signal-to-noise ratio. The digitization thus occurs directly within the PET detector modules, which are placed inside the bore of the MRI scanner. In a second step, the sensors measure the signals directly digitally. The downside of this approach is that it has the highest risk of influencing the MRI system. Digital electronics are known to disturb the MRI in several ways: the homogeneity of the static magnetic field becomes distorted by electronic components, the gradient fields are influenced by induced eddy currents in conductive areas, and the RF system receives spurious signals from the switching circuits.Compared to conventional invasive studies in translational research, preclinical imaging has the ability to improve the significance of preclinical studies, as well as the potential to drastically reduce the costs and the number of animals sacrificed. Preclinical imaging – form mice to rabbits – was thus chosen as a meaningful target application.This thesis covers the research on the PET hardware chains for both integration steps, with a focus on PET/MRI compatibility. The required performance parameters for a hybrid system are derived based on the fundamentals of the two single imaging modalities. Possible means of interaction between the two systems, and how the interactions degrade the performance parameters, are discussed. Methods to verify whether the requirements are met are described. They were extensively used in the development phases of all subcomponents and served to quantify the PET/MR compatibility of the final inserts. The results are discussed and compared to the stipulated requirements as well as to results of other research teams worldwide. Both systems demonstrate their image quality and their in vivo capabilities in respective measurements.The result is the world’s first digital preclinical PET scanner, which is also the world’s first digital PET/MRI system. The achieved spatial resolution in PET is higher than for all other simultaneous PET/MRI systems known, and is even similar to the best standalone PET scanners. Additionally, the accomplished PET coincidence time resolution introduces Time-Of-Flight-PET to preclinical applications. Undesired mutual influences between the two modalities are reduced to a minimum.The inserts demonstrate that the described approach of early digitization enables highest PET performance, and that the resulting MRI compatibility challenges can be solved by means of an elaborate system design.