Physics of PET/MRI Systems

This chapter provides an overview of the important physics and engineering issues related to imaging systems that perform PET and MR imaging simultaneously with a common field of view. We refer to this hereafter as simply PET/MRI, although it is sometimes referred to as hybrid, combined, integrated, or simultaneous PET/MRI. The scope includes issues that may be of interest to an audience with a primarily clinical outlook. After a summary of some of the relevant physics underlying PET and MRI, we discuss the technical challenges encountered in the integration of these technologies, how they have been addressed, and the potential impact of these issues on imaging in the clinic. Although data from scans acquired on separate PET and MRI scanners is also sometimes referred to as PET/MRI, and this approach may be sufficient for many applications, we do not address this here given that the physics of each modality are entirely independent and thoroughly addressed within its own sphere of academia and commerce. Similarly, research prototypes and animal systems are beyond the current scope. For the most part, the integration of PET and MRI for simultaneous imaging has largely retained traditional MRI approaches and geometries and requires a much greater degree of engineering on the PET side. Thus, much of this chapter pertains to the principles of PET, and MRI is included mainly in the context of understanding the requirements for a compatible PET design.