Hybrid Scintillator-Semiconductor Detector (HSSD) for Ultrahigh Resolution PET Imaging

1230 Objectives: In this presentation, we will discuss the design and the feasibility study of a novel hybrid semiconductor-scintillation detector (HSSD) for ultrahigh resolution PET imaging. The HSSD detector consists of a high-resolution CZT detector sitting directly on top of a scintillation detector (as shown in Fig. 1.A). The operations of the CZT and scintillation sub-detectors are electronically synchronized to pick up the complex gamma-ray interactions that could occur with this dual-layer detector structure (Fig. 1.B). Compared to conventional scintillation and CZT/CdTe detectors, the HSSD detector design provides several advantages: (1) a substantial portion (40%) of 511 keV incident gamma rays would interact at least 1-time in CZT detector of 1 cm thickness. These events, if correctly identified, offer an ultra-high (sub-250 μm) intrinsic resolution and an unprecedented energy resolution of <3 keV at 300-511 keV; (2) the overall detection efficiency approaches that of conventional scintillation detectors; (3) it provides an excellent timing resolution for a large fraction of events that interacted in the scintillation sub-detector, and (4) it allows one to use Compton kinematics on photons that scattered within the HSSD module to help reject random and scattered coincidences. Furthermore, the HSSD would allow us to harvest the vast number of single photons, and to use the Compton kinematics to derive extra imaging information and improve the overall effective sensitivity. Methods: The HSSD detector design and Monte Carlo studies. The basic HSSD detector module is shown in Fig. 1.A and B. It consists of a CZT detector of 4 cm  4 cm  1 cm with an array of anode pixels of 250 μm  250 μm or 500 μm  500 μm in size. The scintillation sub-detector consists of an LSO detector of 1 cm thickness with 40  40 discrete crystal elements, each being 1.2mm  1.2 mm  1cm. The LSO detector array is read out with a SiPM array followed by a charge divider circuitry that multiplexes the output to 4 signals [1]. We have used GATE to simulate two variations of the Siemens Inveon system, with (Fig. 1.C) and without (Fig. 1.D) the 1 cm CZT detector attached to the LSO detector array. PET images acquired with several system configurations and operating conditions will be compared in this presentation. Preliminary experimental studies. The prototype HSSD consists of a smaller (2 cm  2 cm) and thinner (0.5 cm) panel of CZT detector with 250 μm  250 μm pixels. The CZT detector is attached to a 2-D ASIC and then in close contact with an LSO crystal array. The operation of the CZT and the LSO detectors are synchronized with an existing FPGA control unit [2]. Two of the prototype HSSD detectors will be set up for coincidence measurements, also through the FPGA control logic. We use point-sources and 3-D printed resolution phantoms to evaluate the performance benefit of the HSSD detector against CZT and LSO detectors operated alone. Results: Figs. 1.E-H are experimentally acquired PET images of various resolution phantoms acquired with the UIUC CdTe PET setup [2]. In this presentation, we discuss the basic concept, the potential system implementation of a hybrid semiconductor-scintillator detector (HSSD) for ultrahigh resolution PET imaging applications, as well as the results obtained from Monte Carlo and experimental studies for evaluating the performance of the HSSD detector for potential use in ultrahigh resolution animal PET imaging applications. We compare both reconstructed images and quantitative performance measures offered by the HSSD-based PET system against similar PET system based solely on scintillation detectors and CZT detectors. Conclusions: The HSSD detector design retains the major attractions of CZT detectors, such as its excellent spatial and energy resolutions, while alleviating their limitations, such as limited timing precision, lowered sensitivity and high cost, which could provide a unique and practical solution for future ultrahigh resolution PET imaging applications.