ABSTRACT
Combining molecular imaging results with high-resolution structural imaging has always been an ultimate goal ever since these modalities became available. After PET/CT has ultimately proven to be of utmost benefit in many clinical situations, there are still scenarios left, where the poor soft tissue contrast of CT, additional radiation exposure and the inherently sequential acquisition PET following CT are a limitation. Therefore, combined and even simultaneous PET/MR imaging has been a goal of technical development. Following some historical, introductory and basic system design considerations, this chapter addresses the basic technical challenges that arise if PET detectors shall operate in strong static and fluctuating magnetic fields. Available solutions are exemplarily alluded to, including semiconductor (scintillation) light detectors such as avalanche photo diodes (APDs), and silicon photo multipliers (SiPM). Commercially available systems designs and types are then introduced. Then the special challenges of PET quantification in combined PET/MRI are discussed, attenuation correction being the most important to be addressed. It is discussed in light of image reconstruction techniques and of standardization of PET quantification, also for combined PET/MRI. Opportunities that arise by combining the superior soft tissue contrast and timing resolution of MRI, for instance for registration and correction of motion artefacts in PET, are discussed. Finally, the clinical applications, some of which are key applications in their diagnostic and even theranostic setting, are briefly listed and referenced. Preclinical PET/MRI systems and a reference to siting and staffing requirements to appropriately operate PET/MRI systems in the various settings conclude this chapter.
