Positron Emission Tomography (PET) is a noninvasive medical imaging technique which provides a three dimensional image of radio pharmaceuticals disbursed in the body. It is becoming an important fundamental tool for both clinical diagnosis and medical research. The heart of this instrument is a large array of nuclear detectors which in most instances consists of bismuth germanate (BGO) scintillator crystals and photomultiplier tubes. In practice, the characteristics of photomultiplier tubes make it difficult to obtain both the required spatial resolution and desired energy resolution. A number of solutions to this problem have been investigated, but most result in serious compromises in complexity, resolution or cost. A new idea which offers a practical solution is to use groups of several BGO scintillator elements coupled to large photomultiplier tubes which provide the needed timing and energy resolution information. In addition separate solid state photodetectors would be attached to each of the BGO crystals to identify the source of the emitted light. By this means the sensitivity of the phototubes combines with the compactness of the solid state sensor to achieve a very dense high performance array. The key problem in implementing this concept is the lack of a solid state identification sensor with enough sensitivity to detect the light emitted by the scintillator. It now appears likely that silicon avalanche photodiodes offer the ideal solution. These photosensors operate with significant internal signal gain which results in a better signal-to-noise ratio than nonavalanching sensors. Our previous research on a silicon avalanche photodiode has shown that excellent performance can be obtained when they are coupled to NaI(T1) scintillator crystals. However, the sensitivity must now be substantially improved to use these promising devices with BGO. In Phase I we will focus on research to demonstrate that these devices can meet the fundamental requirements of this demanding application. In Phase II we will optimize the device and through close collaboration with PET research groups prove their suitability """"""""in the field"""""""". The benefit of this device will be improved positron emission cameras with better resolution and reduced patient dose.
