Accelerating photon transport in Monte Carlo simulation using octree structure

利用蒙特卡罗模拟，建立了光子/电子输运的高速计算方法。物体描述对于有效地进行光子传输至关重要。为了描述一个物体，已经提出了三种方法：基于形状（SV）的方法，基于体素（VB）的方法和基于实体几何的方法。然而，这三种方法都有一些缺点（如边界的准确性、计算时间、内存量、手工操作等）。针对这些问题，我们采用蒙特卡罗方法将物体的八叉树表示引入光子传输。物体的表示产生了每个介质的精确边界和光子传输的快速计算，与使用最精细体素的VB方法具有相同的精度。此外，任何对象的八叉树结构都可以系统地形成。我们的蒙特卡罗代码首先通过读取“八叉树字符串”来建立模拟几何，该字符串是在模拟前自动从对象的一组串行部分形成八叉树结构产生的，然后它在几何中传输光子。使用该代码，如果用户只是为他想要模拟光子轨迹的对象准备了一组串行部分，他可以使用由八叉树表示简化的次优几何来自动执行模拟，而无需手工形成最优几何。通过与电子伽马雨码系统（EGS4）的对比和头部模拟，验证了编码的准确性和能力。此外，通过比较VB方法的结果，我们使用Zubal的数字幻影（头部和胸部）检查了我们的代码。综上所述，该方法对任意目标的蒙特卡罗仿真都是有效的。

The study was carried out to develop a high-speed calculation method in photon/electron transportation using a Monte Carlo simulation. Object description is important in performing the photon transport efficiently. In order to describe an object, three methods have been proposed : a shape-based (SV) approach, a voxel-based (VB) approach, and a solid geometry based approach. However, these three methods have some shortcoming (e.g., accuracy of boundaries, calculation time, amount of memory, manual instructions). To the problems we introduced an octree representation of an object to the photon transport using a Monte Carlo method. The representation of the object yields accurate boundaries of each medium and fast calculation of the photon transport with the same accuracy as the VB method using the finest voxels. Moreover, the octree structure for any object can be formed systematically.Our Monte Carlo code, first, establishes the simulation geometry by reading "octree string, " which is produced by forming an octree structure from a set of serial sections for the object before the simulation automatically, and then it transports photons in the geotry. Using the code, if the user just prepares a set of serial sections for the object in which he wants to simulate photon trajectories, he can perform the simulation automatically using the suboptimal geometry simplified by the octree representation without forming the optimal geometry by handwriting. The accuracy and ability of our code were confirmed by comparison with Electron Gamma Shower code system (EGS4) and simulations using a head phantom, respectively. Moreover, we examined our code using Zubal's digital phantoms (head and thorax) by comparing the results of the VB approach. In summary, the proposed method is efficient in the Monte Carlo simulation of any object.