Physical characterisation and preliminary results of a pet-system using time-of-flight for quantitative studies

A first generation of Positron Emission Tomography (PET) systems using the Time of Flight (TOF) information, named TDV1, was designed and built by the LETI group (Grenoble, France) for the Orsay group, where it was recently installed. The system comprises essentially three rings of 96 CsF probes and a fourth ring of 96 BaF2 probes. Its design is aimed at whole-body quantitative, dynamic, 3 D studies. The physical characterization of TDV1 was performed as it would be for a 'conventional' non-TOF PET system, in terms of spatial transverse and longitudinal resolution along the vertical axis, sensitivity, time resolution, and inter and intra-plane uniformity. Moreover, the specific advantages of time of flight information when used in PET were studied in preliminary measurements. They consist in very fast count rate studies, elimination of random events, and improvement of the S/N ratio resulting in a so-called 'effective sensitivity gain'. Special attention was directed to the comparison of the CsF and BsF2 probes in terms of sensitivity gain, due to their resolving time of 480 psec and 380 psec on an average, for the 96 probes of each individual detection ring. Indeed, the BaF2 crystal could be an answer to most of the shortcomings of CsF; namely, its lower intrinsic efficiency than that of BGO, the limited-resolution due to the crystal size necessary and the relatively low packing fraction of the detector ring due to the hygroscopic properties of the crystal. If the most important parameters were combined into a Factor of Merit, the corresponding factor of a BaF2 scintillator 40 mm long and 20 mm in width would be more than 2 fold that of CsF. Moreover, the intrinsic spatial resolution could be substantially improved using very small parallelepipedic crystals and adapted photo multipliers tubes with fast timing capabilities. (Author)