Lee Choonik, Williams Jonathan L, Lee Choonsik, Bolch Wesley E
Department of Nuclear and Radiological Engineering, University of Florida, Gainesville, Florida 32611, USA.
Med Phys. 2005 Dec;32(12):3537-48. doi: 10.1118/1.2107067.
Two classes of anthropomorphic computational phantoms exist for use in Monte Carlo radiation transport simulations: tomographic voxel phantoms based upon three-dimensional (3D) medical images, and stylized mathematical phantoms based upon 3D surface equations for internal organ definition. Tomographic phantoms have shown distinct advantages over the stylized phantoms regarding their similarity to real human anatomy. However, while a number of adult tomographic phantoms have been developed since the early 1990s, very few pediatric tomographic phantoms are presently available to support dosimetry in pediatric diagnostic and therapy examinations. As part of a larger effort to construct a series of tomographic phantoms of pediatric patients, five phantoms of different ages (9-month male, 4-year female, 8-year female, 11-year male, and 14-year male) have been constructed from computed tomography (CT) image data of live patients using an IDL-based image segmentation tool. Lungs, bones, and adipose tissue were automatically segmented through use of window leveling of the original CT numbers. Additional organs were segmented either semiautomatically or manually with the aid of both anatomical knowledge and available image-processing techniques. Layers of skin were created by adding voxels along the exterior contour of the bodies. The phantoms were created from fused images taken from head and chest-abdomen-pelvis CT exams of the same individuals (9-month and 4-year phantoms) or of two different individuals of the same sex and similar age (8-year, 11-year, and 14-year phantoms). For each model, the resolution and slice positions of the image sets were adjusted based upon their anatomical coverage and then fused to a single head-torso image set. The resolutions of the phantoms for the 9-month, 4-year, 8-year, 11-year, and 14-year are 0.43 x 0.43 x 3.0 mm, 0.45 x 0.45 x 5.0 mm, 0.58 x 0.58 x 6.0 mm, 0.47 X 0.47 x 6.00 mm, and 0.625 x 0.625 x 6.0 mm, respectively. While organ masses can be matched to reference values in both stylized and tomographic phantoms, side-by-side comparisons of organ doses in both phantom classes indicate that organ shape and positioning are equally important parameters to consider in accurate determinations of organ absorbed dose from external photon irradiation. Preliminary studies of external photon irradiation of the 11-year phantom indicate significant departures of organ dose coefficients from that predicted by the existing stylized phantom series. Notable differences between pediatric stylized and tomographic phantoms include anterior-posterior (AP) and right lateral (RLAT) irradiation of the stomach wall, left lateral (LLAT) and right lateral (RLAT) irradiation of the thyroid, and AP and posterior-anterior (PA) irradiation of the urinary bladder.
在蒙特卡罗辐射传输模拟中，存在两类用于人体的计算体模：基于三维（3D）医学图像的断层体素体模，以及基于用于定义内部器官的3D表面方程的风格化数学体模。在与真实人体解剖结构的相似性方面，断层体模相较于风格化体模已显示出明显优势。然而，自20世纪90年代初以来虽已开发出许多成人断层体模，但目前可用于支持儿科诊断和治疗检查中剂量测定的儿科断层体模却非常少。作为构建一系列儿科患者断层体模这项更大努力的一部分，已使用基于IDL的图像分割工具，从活体患者的计算机断层扫描（CT）图像数据构建了五个不同年龄（9个月大的男性、4岁的女性、8岁的女性、11岁的男性和14岁的男性）的体模。通过对原始CT数值进行窗宽调整，自动分割出肺、骨骼和脂肪组织。借助解剖学知识和可用的图像处理技术，半自动或手动分割出其他器官。通过沿着身体外部轮廓添加体素创建皮肤层。这些体模是从同一人（9个月和4岁体模）或同性且年龄相近的两个不同人（8岁、11岁和14岁体模）的头部以及胸部 - 腹部 - 骨盆CT检查所获取的融合图像创建的。对于每个模型，根据其解剖覆盖范围调整图像集的分辨率和切片位置，然后融合成单个头部 - 躯干图像集。9个月、4岁、8岁、11岁和14岁体模的分辨率分别为0.43×0.43×3.0毫米、0.45×0.45×5.0毫米、0.58×0.58×6.0毫米、0.47×0.47×6.00毫米和0.625×0.625×6.0毫米。虽然在风格化体模和断层体模中都可以使器官质量与参考值匹配，但对这两类体模中器官剂量的并排比较表明，在准确确定外部光子照射下器官吸收剂量时，器官形状和位置同样是需要考虑的重要参数。对11岁体模进行外部光子照射的初步研究表明，器官剂量系数与现有风格化体模系列所预测的结果有显著差异。儿科风格化体模和断层体模之间的显著差异包括胃壁的前后（AP）和右侧（RLAT）照射、甲状腺的左侧（LLAT）和右侧（RLAT）照射以及膀胱的AP和后前（PA）照射。
