Hybrid computational phantoms of the male and female newborn patient: NURBS-based whole-body models
Hybrid computational phantoms of the male and female newborn patient: NURBS-based whole-body models
复制标题
DOI：
10.1088/0031-9155/52/12/001
复制
发表时间：
2007-06-21
影响因子：
3.5
通讯作者：
Bolch, Wesley E.
中科院分区：
文献类型：
--
作者：
Lee, Choonsik;Lodwick, Daniel;Bolch, Wesley E.
拟人计算体模是人体的计算机模型，用于评价由内部或外部辐射源引起的剂量分布。目前，已经开发了两类计算体模并广泛用于器官剂量评估：（1）程式化体模和（2）体素体模，其分别通过数学表面方程或3D体素矩阵来描述人体解剖结构。虽然基于数学方程的程式化幻影在改变器官位置和几何形状方面可以非常灵活，但它们在完全捕获人体内部解剖结构的解剖复杂性方面的能力有限。反过来，体素体模已经通过基于图像的分割而被开发，并且与更简单的程式化体模相比，体素体模相应地提供好得多的解剖真实性。然而，它们本身在定义磁共振或计算机断层扫描图像（体素体模构造中的两个主要来源）内以低对比度呈现的器官方面受到限制。根据定义，体素体模通常通过对轴向图像的分割来构建，因此，虽然在该观察平面中看到精细的解剖特征，但是在矢状面或冠状面中观察体素体模的解剖结构时，切片到切片的不连续性变得明显.本研究介绍了一种混合计算新生儿幻影的概念，充分利用了它的风格化和体素同行的最佳功能：幻影改变和解剖现实主义的灵活性。采用非均匀有理B样条（NURBS）曲面作为图形动画研究中的数学建模工具，取代了程式化模型中有限的数学曲面方程。利用先前开发的新生女性的全身体素体模作为混合体模构造的现实解剖框架。混合体模的构建分为三个步骤：体素体模的体素化、通过NURBS曲面的器官建模和体模体素化。两个3D图形工具，3D-DOCTOR（TM）和Rhinoceros（TM），被用于对新生体素体模进行体素化并生成NURBS表面，同时内部MATLAB（TM）代码被用于将得到的NURBS模型体素化为准备用于蒙特卡罗辐射传输计算的最终计算体模。共126个解剖器官和组织模型，包括38个骨骼部位和31个软骨部位，在混合体模内使用NURBS或多边形表面进行了描述。通过用男性特异性器官替换女性特异性器官，在女性体模发育后构建男性混合新生儿体模。调整了基于FBS的体模的外部身体轮廓和内部解剖结构，以匹配国际辐射防护委员会在其出版物89中报告的人体测量和参考新生儿数据。体素化过程旨在将NURBS模型准确转换为体积变化最小的体素体模。另外还进行了灵敏度研究，以更好地了解网格化公差和体素分辨率如何影响混合NURBS和混合体素体模之间的体积变化。构建雄性和雌性混合体- NURBS体模的方式使得所有内脏器官接近其ICRP参考质量的1%以内，皮肤（相对误差为-6.5%）和大脑（相对误差为-154%）除外。两种混合体素体模都是用0.663 mm的各向同性体素分辨率构建的-相当于ICRP 89新生儿皮肤（真皮和表皮）的参考厚度。混合-用于创建其体素对应物的NURBS体模保留了程式化体模的非均匀可伸缩性，同时保持了分割体素体模在器官形状、深度和器官间定位方面的解剖真实性。
Anthropomorphic computational phantoms are computer models of the human body for use in the evaluation of dose distributions resulting from either internal or external radiation sources. Currently, two classes of computational phantoms have been developed and widely utilized for organ dose assessment: ( 1) stylized phantoms and ( 2) voxel phantoms which describe the human anatomy via mathematical surface equations or 3D voxel matrices, respectively. Although stylized phantoms based on mathematical equations can be very flexible in regard to making changes in organ position and geometrical shape, they are limited in their ability to fully capture the anatomic complexities of human internal anatomy. In turn, voxel phantoms have been developed through image- based segmentation and correspondingly provide much better anatomical realism in comparison to simpler stylized phantoms. However, they themselves are limited in defining organs presented in low contrast within either magnetic resonance or computed tomography images-the two major sources in voxel phantom construction. By definition, voxel phantoms are typically constructed via segmentation of transaxial images, and thus while fine anatomic features are seen in this viewing plane, slice- to- slice discontinuities become apparent in viewing the anatomy of voxel phantoms in the sagittal or coronal planes. This study introduces the concept of a hybrid computational newborn phantom that takes full advantage of the best features of both its stylized and voxel counterparts: flexibility in phantom alterations and anatomic realism. Non- uniform rational B- spline ( NURBS) surfaces, a mathematical modeling tool traditionally applied to graphical animation studies, was adopted to replace the limited mathematical surface equations of stylized phantoms. A previously developed whole- body voxel phantom of the newborn female was utilized as a realistic anatomical framework for hybrid phantom construction. The construction of a hybrid phantom is performed in three steps: polygonization of the voxel phantom, organ modeling via NURBS surfaces and phantom voxelization. Two 3D graphic tools, 3D-DOCTOR (TM) and Rhinoceros (TM), were utilized to polygonize the newborn voxel phantom and generate NURBS surfaces, while an in-house MATLAB (TM) code was used to voxelize the resulting NURBS model into a final computational phantom ready for use in Monte Carlo radiation transport calculations. A total of 126 anatomical organ and tissue models, including 38 skeletal sites and 31 cartilage sites, were described within the hybrid phantom using either NURBS or polygon surfaces. A male hybrid newborn phantom was constructed following the development of the female phantom through the replacement of female- specific organs with male-specific organs. The outer body contour and internal anatomy of the NURBS-based phantoms were adjusted to match anthropometric and reference newborn data reported by the International Commission on Radiological Protection in their Publication 89. The voxelization process was designed to accurately convert NURBS models to a voxel phantom with minimum volumetric change. A sensitivity study was additionally performed to better understand how the meshing tolerance and voxel resolution would affect volumetric changes between the hybrid- NURBS and hybrid- voxel phantoms. The male and female hybrid- NURBS phantoms were constructed in a manner so that all internal organs approached their ICRP reference masses to within 1%, with the exception of the skin (- 6.5% relative error) and brain (- 154% relative error). Both hybrid- voxel phantoms were constructed with an isotropic voxel resolution of 0.663 mm - equivalent to the ICRP 89 reference thickness of the newborn skin ( dermis and epidermis). Hybrid- NURBS phantoms used to create their voxel counterpart retain the non- uniform scalability of stylized phantoms, while maintaining the anatomic realism of segmented voxel phantoms with respect to organ shape, depth and inter- organ positioning.