Segars W P, Mahesh M, Beck T J, Frey E C, Tsui B M W
Department of Radiology, The Duke University Medical Center, Durham, North Carolina 27705, USA.
Med Phys. 2008 Aug;35(8):3800-8. doi: 10.1118/1.2955743.
The authors develop a unique CT simulation tool based on the 4D extended cardiac-torso (XCAT) phantom, a whole-body computer model of the human anatomy and physiology based on NURBS surfaces. Unlike current phantoms in CT based on simple mathematical primitives, the 4D XCAT provides an accurate representation of the complex human anatomy and has the advantage, due to its design, that its organ shapes can be changed to realistically model anatomical variations and patient motion. A disadvantage to the NURBS basis of the XCAT, however, is that the mathematical complexity of the surfaces makes the calculation of line integrals through the phantom difficult. They have to be calculated using iterative procedures; therefore, the calculation of CT projections is much slower than for simpler mathematical phantoms. To overcome this limitation, the authors used efficient ray tracing techniques from computer graphics, to develop a fast analytic projection algorithm to accurately calculate CT projections directly from the surface definition of the XCAT phantom given parameters defining the CT scanner and geometry. Using this tool, realistic high-resolution 3D and 4D projection images can be simulated and reconstructed from the XCAT within a reasonable amount of time. In comparison with other simulators with geometrically defined organs, the XCAT-based algorithm was found to be only three times slower in generating a projection data set of the same anatomical structures using a single 3.2 GHz processor. To overcome this decrease in speed would, therefore, only require running the projection algorithm in parallel over three processors. With the ever decreasing cost of computers and the rise of faster processors and multi-processor systems and clusters, this slowdown is basically inconsequential, especially given the vast improvement the XCAT offers in terms of realism and the ability to generate 3D and 4D data from anatomically diverse patients. As such, the authors conclude that the efficient XCAT-based CT simulator developed in this work will have applications in a broad range of CT imaging research.
作者基于4D扩展心脏-躯干（XCAT）体模开发了一种独特的CT模拟工具，该体模是基于NURBS曲面的人体解剖学和生理学的全身计算机模型。与当前基于简单数学基元的CT体模不同，4D XCAT能精确呈现复杂的人体解剖结构，并且由于其设计特点，其器官形状可以改变，以逼真地模拟解剖变异和患者运动。然而，XCAT基于NURBS的一个缺点是，曲面的数学复杂性使得通过体模计算线积分变得困难。必须使用迭代程序进行计算；因此，CT投影的计算比简单数学体模要慢得多。为克服这一限制，作者采用了计算机图形学中的高效光线追踪技术，开发了一种快速解析投影算法，可根据定义CT扫描仪和几何形状的参数，直接从XCAT体模的表面定义精确计算CT投影。使用该工具，可以在合理的时间内从XCAT模拟并重建逼真的高分辨率3D和4D投影图像。与其他具有几何定义器官的模拟器相比，发现基于XCAT的算法在使用单个3.2 GHz处理器生成相同解剖结构的投影数据集时，速度仅慢三倍。因此，要克服这种速度下降，只需在三个处理器上并行运行投影算法即可。随着计算机成本的不断降低以及更快处理器、多处理器系统和集群的兴起，这种速度减慢基本上无关紧要，特别是考虑到XCAT在逼真度以及从解剖结构各异的患者生成3D和4D数据的能力方面有了巨大提升。因此，作者得出结论，这项工作中开发的基于XCAT的高效CT模拟器将在广泛的CT成像研究中得到应用。
