Avalanche Photodiodes (APDs) are a solid state replacement for traditional photomultiplier tubes (PMTs) and, similar to a PMT, exhibit gain created by the impact ionization process within the device. However, unlike PMTs, APDs can detect both photons and particles while a PMT can only detect optical photons. APDs are also much thinner and more compact than PMTs, and are insensitive to magnetic fields. This insensitivity makes possible the design of hybrid PET/MRI systems. RMD's silicon APDs can achieve and sustain a high gain because they are fabricated using a deep diffused planar manufacturing process and, like other solid state devices, RMD's APDs have a high quantum detection efficiency, a wide spectral response, and are compact and rugged and are available in discrete devices, arrays, and position sensitive structures.