Evaluation of New Inorganic Scintillators for Application in a Prototype Small Animal PET Scanner

In the study of new pharmaceuticals as well as brain and genetic research, Positron Emission Tomography (PET) is a useful method. It has also recently entered the clinical domain in cardiology and particularly in oncology. Small animals such as mice, are often used to validate sophisticated models of human disease. High spatial resolution PET instrumentation is therefore necessary due to the reduced dimensions of the organs. Inorganic scintillators are employed in most of the diagnostic imaging devices. The ultimate performance of the PET scanner is tightly bound to the scintillation properties of the crystals. In the last years there has been an effort to develop new scintillating materials characterized by high light output, high detection efficiency and fast decay time. The most studied systems are mainly Ce3+-doped crystals such as LSO:Ce, YAP:Ce, LuAP:Ce, and recently also mixed Lux(RE3+)1-xAlO3:Ce crystals. These crystals are very attractive for medical application because of their high density (with the exception of YAP:Ce), short decay time, and high light output. This work deals with the development of cerium doped LuYAP crystals, with varying amount of yttrium, for application in several small animal PET scanners. In addition the scintillation properties and mechanism of lutetium based oxyorthosilicates are investigated. The results in light yield, energy resolution and decay time are presented for all the studied compounds. The light yield non-proportionality on photon energy was studied as well as the intrinsic energy resolution. Optical characteristics such as excitation, emission and absorption spectra were measured and important correlations with the scintillation properties are pointed out. Phoswich detectors are currently being considered by a number of investigators for depth of interaction (DOI) measurements in PET. The Crystal Clear Collaboration plans to construct several small animal PET scanners using LuYAP and LSO crystals. The identification of the layer of interaction is done using the pulse shape of the corresponding scintillator material. First results on the combination of LuYAP:Ce crystals with LSO:Ce crystals in phoswich geometry for the final PET scanner are presented. The experimental results show that the new DOI technique is applicable for high-resolution PET systems.