Després Philippe, Funk Tobias, Shah Kanai S, Hasegawa Bruce H
Physics Research Laboratory, University of California, San Francisco, 185 Berry St. suite 350, San Franciso, CA 94107, USA.
Phys Med Biol. 2007 Jun 7;52(11):3057-74. doi: 10.1088/0031-9155/52/11/009. Epub 2007 May 8.
Avalanche photodiodes (APDs), and in particular position-sensitive avalanche photodiodes (PSAPDs), are an attractive alternative to photomultiplier tubes (PMTs) for reading out scintillators for PET and SPECT. These solid-state devices offer high gain and quantum efficiency, and can potentially lead to more compact and robust imaging systems with improved spatial and energy resolution. In order to evaluate this performance improvement, we have conducted Monte Carlo simulations of gamma cameras based on avalanche photodiodes. Specifically, we investigated the relative merit of discrete and PSAPDs in a simple continuous crystal gamma camera. The simulated camera was composed of either a 4 x 4 array of four channels 8 x 8 mm2 PSAPDs or an 8 x 8 array of 4 x 4 mm2 discrete APDs. These configurations, requiring 64 channels readout each, were used to read the scintillation light from a 6 mm thick continuous CsI:Tl crystal covering the entire 3.6 x 3.6 cm2 photodiode array. The simulations, conducted with GEANT4, accounted for the optical properties of the materials, the noise characteristics of the photodiodes and the nonlinear charge division in PSAPDs. The performance of the simulated camera was evaluated in terms of spatial resolution, energy resolution and spatial uniformity at 99mTc (140 keV) and 125I ( approximately 30 keV) energies. Intrinsic spatial resolutions of 1.0 and 0.9 mm were obtained for the APD- and PSAPD-based cameras respectively for 99mTc, and corresponding values of 1.2 and 1.3 mm FWHM for 125I. The simulations yielded maximal energy resolutions of 7% and 23% for 99mTc and 125I, respectively. PSAPDs also provided better spatial uniformity than APDs in the simple system studied. These results suggest that APDs constitute an attractive technology especially suitable to build compact, small field of view gamma cameras dedicated, for example, to small animal or organ imaging.
雪崩光电二极管（APD），尤其是位置敏感雪崩光电二极管（PSAPD），对于正电子发射断层扫描（PET）和单光子发射计算机断层扫描（SPECT）中闪烁体的读出而言，是光电倍增管（PMT）颇具吸引力的替代方案。这些固态器件具有高增益和量子效率，并且有可能带来更紧凑、更坚固的成像系统，同时具备更高的空间分辨率和能量分辨率。为了评估这种性能提升，我们基于雪崩光电二极管对伽马相机进行了蒙特卡罗模拟。具体而言，我们在一个简单的连续晶体伽马相机中研究了分立型APD和PSAPD的相对优势。模拟相机由一个4×4阵列的四个通道8×8 mm²的PSAPD组成，或者由一个8×8阵列的4×4 mm²分立型APD组成。这两种配置各自需要64通道读出，用于读取来自一块6毫米厚的连续碘化铯铊（CsI:Tl）晶体的闪烁光，该晶体覆盖整个3.6×3.6 cm²的光电二极管阵列。使用GEANT4进行的模拟考虑了材料的光学特性、光电二极管的噪声特性以及PSAPD中的非线性电荷分配。在99mTc（140 keV）和125I（约30 keV）能量下，根据空间分辨率、能量分辨率和空间均匀性对模拟相机的性能进行了评估。基于APD和PSAPD的相机在99mTc下分别获得了1.0毫米和0.9毫米的本征空间分辨率，对于125I，半高宽（FWHM）的相应值分别为1.2毫米和1.3毫米。模拟结果显示，对于99mTc和125I，最大能量分辨率分别为7%和23%。在所研究的简单系统中，PSAPD的空间均匀性也比APD更好。这些结果表明，APD构成了一种颇具吸引力的技术，特别适合构建例如专用于小动物或器官成像的紧凑型、小视野伽马相机。
