Characterizing and reducing image distortions of hybrid PET-MRI systems
Aachen (2020) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xiii, 131 Seiten) : Illustrationen, Diagramme
The combination of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) is a powerful hybrid imaging modality that visualizes functional and anatomical information. However, combining a PET and an MRI system is technically challenging. On the one hand, MRI uses strong and time-varying magnetic fields which can disturb or even destroy the electronic components of the PET system. On the other hand, the PET components can disturb the distribution of the magnetic fields of the MRI system and thereby cause Magnetic Resonance (MR) image artifacts. Especially, MRI switches low-frequency gradient magnetic fields. The time-varying magnetic fields induce eddy currents in all conductive components which produce superimposing magnetic fields. Since the gradient fields are used for spatial encoding, a distorted gradient field is also a source of imaging artifacts. The image artifacts can be severe especially for simultaneous PET-MRI systems because a PET system typically comprises many conductive components, such as heat spreaders, cooling pipes, radio-frequency shieldings or printed circuit boards with conductive planes. In this work, a Nuclear Magnetic Resonance (NMR) probe was developed to characterize the distortion of the gradient fields caused by different components of the PET system. Due to the capability of the NMR probe to measure magnetic fields in a time-resolved manner with high sensitivity at a single position, it was shown to be superior compared to the common characterization methods. Using the NMR probe, the RF shielding of a PET system was subsequently optimized with respect to the distortion of the magnetic fields of the MRI system. Furthermore, the distortion of the gradient fields caused by a complete PET system, i.e., a preclinical PET scanner, was quantitatively characterized. For this purpose, the gradient impulse response functions of the PET system were measured. Finally, the measured distortions of the gradient fields caused by the PET system were incorporated in the MRI reconstruction to correct for imaging artifacts. Summarizing, this thesis discusses methods for minimizing MRI artifacts caused by inserted electronic and conductive components both by selecting more suitable components and by retrospective correcting of remaining artifacts during reconstruction.