Time-of-Flight Positron Emission Tomography (TOF-PET) scanners provide better signal-to-noise ratio (SNR) and artifact reduction compared to conventional PET systems. The performance of TOF-PET scanners improves with the timing precision of its detectors: the more accuracy in the time detection of gamma photons the better the performance. The ultimate aim of TOF-PET is to reach a 10 ps full width at half maximum (FWHM) coincidence time resolution (CTR) to resolve precisely the positron-electron annihilation point in 3 dimensions. State-of-the-art PET detectors consist of scintillation crystals coupled to silicon photomultipliers (SiPM) and show timing resolutions in the order of 100-200 ps FWHM. In this project, we focus on improving dramatically the timing properties of the SiPMs, as such improvement would have a strong impact on TOF-PET as it would improve the timing performance of most detectors that use scintillation crystals and/or Cerenkov light emitters by several-fold. State-of-the-art SiPMs are optimized for a narrow range of wavelengths (?) because of the difference in penetration depth at different wavelengths. For photons of ?=450 nm and ?=590 nm, the attenuation depth is 0.4 ?m and 2 ?m, respectively. The trade-off is to either a) to have a thicker depletion layer to absorb a wider range of wavelengths but to increase the time jitter, or b) to have a thinner depletion layer to reduce the time jitter but absorb only a narrow range of wavelengths. Therefore, there is not a state-of-the-art SiPM that provides, simultaneously, very fast time response, and high photon detection efficiency (PDE) across a wide range of wavelengths. We propose to develop an SiPM prototype with photon-trapping microstructures integrated in the depletion layer that disperses the light laterally and allows one to obtain high-detection efficiency for a wide range of wavelengths within a depletion layer of 1 ?m. With such a thin layer, the jitter in the electron drift time decreases to 10 ps and the dark current is expected to decrease as well. This new photosensor could revolutionize TOF-PET. The utilization of periodic microstructures to bend light in a perpendicular orientation and trapping photons for enhanced interaction with materials, high detection efficiency and fast response have been recently shown for wavelengths between 800-900 nm for optical communication. In this proposal, we will develop a new SiPM based on this technology. First, we will simulate the optimum layer structure to integrate the hole-trapping microstructures and an avalanche region to provide a gain of >105. Second, we will do an electronic characterization for the different type of microcells, including a gain calibration and measure quantum efficiency for different ? for each cell. Finally, we will manufacture a wafer with full-size SiPMs (3x3 mm2) and test the SiPMs with scintillation crystals and Cerenkov emitters.
Time-of-flight positron emission tomography (TOF-PET) identifies small lesions, such as tumors in early stages, with much more confidence than conventional PET. Most state-of-the-art TOF- PET detectors consist of scintillation materials coupled to silicon photomultipliers (SiPMs), and use the timing information in the detection of gamma rays to increase image quality. In this project, we pursue the development of a new concept of SiPM to increase the detection efficiency of photons and achieve a single photon timing resolution of 10 ps, several-fold better than state-of- the-art, to enhance the clinical benefits of TOF-PET.
