Positron emission tomography (PET) scanner provides quantitative information about various physiological and biochemical processes, such as glucose metabolism, gene expression, and drug occupancy. Additionally, insufficient morphological information in PET images can be compensated for by combining the PET scanner with morphological imaging devices, such as X-ray computed tomography and MRI. Despite the great success of PET/CT in both clinical and preclinical applications, a system combining PET and MRI has been demanded because of the advantages of MRI over CT. The technical advances made over the long development period to minimize the mutual interference between the PET and MR data acquisition processes have led to combined clinical PET/MR scanners with sequential and simultaneous imaging strategies in recent years. The major advantages of PET/MR include a smaller radiation burden than PET/CT, better soft tissue contrast when using MRI rather than CT, and possible simultaneous acquisition of images. In addition, the functional capability of MRI has been dramatically enhanced in the last two decades, which opens new opportunities for studying pathology and biochemical processes through the combinations of multi-parametric MRI and PET.

In PET scanners, scintillation detectors are used to measure the gamma rays emitted from the radiopharmaceuticals and consist of arrays of inorganic scintillation crystals and photosensors. Visible or ultraviolet photons are emitted when the gamma rays are detected by the scintillation crystal and are then measured by the photosensors. The photosensor commonly used in conventional PET (i.e., stand-alone PET or PET/CT) is the photomultiplier tube (PMT), which converts the photons into an electric current that is subsequently amplified by the cascade process of secondary emission. Although PMTs have high signal amplification gain and excellent timing properties, they are highly sensitive to both static and time-varying magnetic fields, which is a major concern in the combination of PET with MRI. A practical solution for combination of a conventional PMT based PET system with an MR scanner is to transfer patients between two separate machines using a common bed. This approach was adopted in the whole-body PET/MR system produced by Philips Medical Systems.

A simultaneously operating PET/MRI system is advantageous in reducing acquisition time, and in attaining near perfect spatial and temporal correlation between the information provided by the two imaging modalities. Therefore, several different approaches have been tried to achieve simultaneous PET and MR data acquisition with minimal mutual interference between the PET and MR data. Among these approaches, the feasibility of the use of semiconductor photosensors, such as avalanche photodiode (APD) and silicon photomultiplier (SiPM), has been demonstrated in prototype preclinical and clinical scanners and commercial PET/MRI systems based on this approach are now available.

In this lecture, the design concept and recent advances of PET/MRI scanners will be reviewed.