Tanaka Minoru, Uehara Shuzo, Kojima Akihiro, Matsumoto Masanori
Fukuoka University Hospital, Nanakuma 7-45-1, Jonan-ku, Fukuoka 814-0180, Japan.
Phys Med Biol. 2007 Aug 7;52(15):4409-25. doi: 10.1088/0031-9155/52/15/004. Epub 2007 Jun 26.
(123)I is a radionuclide frequently used in nuclear medicine imaging. The image formed by the 159 keV photopeak includes a considerable scatter component due to high energy gamma-ray emission. In order to evaluate the fraction of scattered photons, a Monte Carlo simulation of a scintillation camera used for (123)I imaging was undertaken. The Monte Carlo code consists of two modules, the HEXAGON code modelled the collimator with a complex hexagonal geometry and the NAI code modelled the NaI detector system including the back compartment. The simulation was carried out for various types of collimators under two separate conditions of the source locations in air and in water. Energy spectra of (123)I for every pixel (matrix size = 256 x 256) were obtained by separating the unscattered from the scattered and the penetrated photons. The calculated energy spectra (cps MBq(-1) keV(-1)) agreed with the measured spectra with approximately 20% deviations for three different collimators. The difference of the sensitivities (cps MBq(-1)) for the window of 143-175 keV was less than 10% between the simulation and the experiment. The partial sensitivities for the scattered and the unscattered components were obtained. The simulated fraction of the unscattered photons to the total photons were 0.46 for LEHR, 0.54 for LEGP and 0.90 for MEGP for the 'in air' set-up, and 0.35, 0.40 and 0.68 for the 'in water' set-up, respectively. The Monte Carlo simulation presented in this work enabled us to investigate the design of a new collimator optimum for (123)I scintigraphy.
碘-123是核医学成像中常用的一种放射性核素。由159keV光电峰形成的图像由于高能γ射线发射而包含相当大的散射成分。为了评估散射光子的比例，对用于碘-123成像的闪烁相机进行了蒙特卡罗模拟。蒙特卡罗代码由两个模块组成，HEXAGON代码对具有复杂六边形几何形状的准直器进行建模，NAI代码对包括后隔层的碘化钠探测器系统进行建模。在源位于空气中和水中的两种不同条件下，对各种类型的准直器进行了模拟。通过将未散射光子与散射光子和穿透光子分离，获得了每个像素（矩阵大小=256×256）的碘-123能谱。对于三种不同的准直器，计算得到的能谱（cps MBq-1 keV-1）与测量能谱的偏差约为20%。在143-175keV窗口内，模拟和实验的灵敏度（cps MBq-1）差异小于10%。获得了散射和未散射成分的部分灵敏度。对于“空气中”设置，低能高分辨率（LEHR）准直器、低能通用（LEGP）准直器和中能通用（MEGP）准直器的未散射光子占总光子的模拟比例分别为0.46、0.54和0.90；对于“水中”设置，相应比例分别为0.35、0.40和0.68。本文所呈现的蒙特卡罗模拟使我们能够研究一种针对碘-123闪烁扫描术的新型最佳准直器的设计。
