Very High Resolution Small Animal PET

Positron Emission Tomography (PET) is a leading technique in medical diagnostic imaging. PET scanners are used to detect cancerous cells through the use of a positron emitting tracer that is ingested by the patient. The positron annihilates with an electron in the body producing a pair of 511keV gamma rays emitted in opposite directions. The gamma rays are sufficiently energetic to leave the patient’s body and to reach an external detector. In conventional PET systems these gamma rays are absorbed by interacting with scintillation crystals that convert the gamma ray’s energy into small flashes of light that are collected by an array of photomultiplier tubes. This information gives the energies and positions of the pair of photons and is later used to reconstruct an image with typical system resolution of 10 mm for full body human scans and approximately 1.4mm for small animal scans. The quality of the image is limited both by the spatial resolution and the efficiency of the detectors used. Employing state-of-the-art solidstate silicon detectors developed for use in experimental particle physics significantly higher resolution images can potentially be achieved. In order to test this assumption, we have set-up in collaboration with the University of Michigan a prototype system consisting of a pair of silicon detectors each with 512 channels, 1mm thick, 1.4 mm by 1.4 mm area per pad arranged in a 32x16 array. Using this system to image a Fluorine-18 source we have already obtained a spatial resolution of 0.7 mm FWHM. Further improvements in the system will be achieved by adding an array of secondary detectors to take advantage of the photon from the Compton scattering event in the silicon, thus increasing system efficiency.