All clinical nuclear medicine imaging is done exclusively with crystal detector systems, which impose many limitations in both cost and technical performance. In 140 keV imaging, the Nal/PMT camera, the workhorse of nuclear medicine, is extremely bulky, costly, and limited in both count rate and spatial resolution. In 511 keV PET imaging, the need for exotic high Z crystals results in high cost and limited solid angle. Xenon at a density of 0.55 g/cm3 can produce a 10-fold energy resolution improvement over Nal and LSO and a time resolution comparable to LSO. Based on previous work, we propose a high pressure xenon cylindrical pulse ionization detector (20-50 mm diameter) having a pressure containing shell composed of a very low density high strength fiber composite. Through use of this composite, interaction losses with the walls of the vessel are lowered to negligible levels. Also, a highly innovative readout technique has been proposed and evaluated in which the tube interior is read out through effectively transparent walls. Pilot analytical and experimental studies have proven that a general purpose detector element can be achieved capable of both 140 keV and 511 keV imaging and having excellent 3D spatial resolution on the order of 1 mm. Experimental studies have also shown that, through use of light signals produced by both the primary interaction process and stimulated emission near the electron collection point at the anode, energy resolution approaching amplifier noise limits is possible. Thus energy resolution can be markedly improved relative to any room temperature crystal. The density of xenon employed is about 6-fold less than Nal but still affords efficient detection of 140 keV in a suitably thin detector. For 511 keV detection, the multiple interaction vertices that occur in xenon are adequately spread out among distinct tubes in an absorbing array and primary scintillation light provides coincidence time resolution of 1 ns. Thus, the proposed detector element configured in appropriate arrays can offer greatly improved performance in both of the major nuclear medicine imaging arenas. In Phase II, arrays of full sized detector elements will be developed, constructed and evaluated to predict imaging characteristics that can be achieved in very important SPECT and PET techniques.
