Investigation of lanthanum scintillators for 3D PET

The main thrust for this work is the investigation and design of a PET scanner based on new Lanthanum Halide scintillators. In 3-D PET the major limitations are scanner dead-time and ability to reject randoms and scatter. Therefore, to reach the full potential of 3-D PET requires a scintillator with good timing resolution and good energy resolution. The new Lanthanum Halide scintillators have very fast decay and very high light output which leads to timing resolution and energy resolution that are both superlative. For application to PET, we have constructed pixels with dimensions 4 × 4 × 30 mm³, and have measured energy resolution of 5% (fwhm) at 511 kev and a timing resolution (fwhm) of 420ps, in coincidence with a plastic scintillator. An Anger-logic PET detector has been designed with pixels of these dimensions, coupled through a light-guide to an array of PMTs. Simulations have been performed which demonstrate very uniform energy resolution and very good pixel identification using 39 mm PMTs, although consideration will be given to larger 50 mm PMTs, as well. We have also simulated the timing resolution of this detector to be 440 ps, depending on the light-guide configuration. Using this detector as a basis of a new LaBr₃ PET scanner with 90 cm diameter and 25 cm axial extent, we predict a sensitivity of 1400 kcps/μCi/cc and a peak NEC count-rate of 120 kcps using the NEMA NU2-2001 standard. The predicted NEC is higher than existing GSO and LSO-based scanners, which has been achieved by using a very high Energy Lower Level Discriminator (ELLD = 470 keV) to take full advantage of the excellent energy resolution of LaBr₃. The relative advantage of the LaBr₃ scanner is shown to increase for a phantom with larger diameter which better simulates a heavy patient. Further, the excellent timing resolution opens the possibility of measuring time-of-flight with sufficient accuracy to reduce the noise propagation during image reconstruction, thus leading to a significant gain in signal-to-noise. Assuming a timing resolution of 500 ps, we can expect the effective NEC to increase by a factor of 3 for a thin patient (20 cm diameter) and a factor 6 for a very heavy patient (40 cm diameter). Therefore, the combination o- f excellent energy resolution and timing resolution with LaBr₃ can potentially lead to a very significant improvement in PET performance.