Solid state photomultipliers have been applied to very high resolution positron emission tomography and hybrid systems for improved quantum efficiency, size and insensitivity to magnetic fields. The multicell Geiger-mode avalanche photodiode or silicon photomultiplier (SiPM) is an ideal photodetector for high energy and space physics, astrophysics and medical imaging because it offers high gain at low bias voltage and excellent timing properties. The dynamic range of this device depends on the number of microcells per SiPM unit and the photon detection efficiency (PDE) of the device. A very high resolution PET scanner has been proposed for medical imaging of small animals employing SiPMs. A combination of PET/computerized tomography (CT) is used for acquisition of functional and structural images simultaneously. The combination of PET and MR imaging technologies has a limitation of space constraint inside the magnet, and the sensitivity of PMTs to magnetic fields.