Custo Anna, Wells William M, Barnett Alex H, Hillman Elizabeth M C, Boas David A
Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Appl Opt. 2006 Jul 1;45(19):4747-55. doi: 10.1364/ao.45.004747.
An efficient computation of the time-dependent forward solution for photon transport in a head model is a key capability for performing accurate inversion for functional diffuse optical imaging of the brain. The diffusion approximation to photon transport is much faster to simulate than the physically correct radiative transport equation (RTE); however, it is commonly assumed that scattering lengths must be much smaller than all system dimensions and all absorption lengths for the approximation to be accurate. Neither of these conditions is satisfied in the cerebrospinal fluid (CSF). Since line-of-sight distances in the CSF are small, of the order of a few millimeters, we explore the idea that the CSF scattering coefficient may be modeled by any value from zero up to the order of the typical inverse line-of-sight distance, or approximately 0.3 mm(-1), without significantly altering the calculated detector signals or the partial path lengths relevant for functional measurements. We demonstrate this in detail by using a Monte Carlo simulation of the RTE in a three-dimensional head model based on clinical magnetic resonance imaging data, with realistic optode geometries. Our findings lead us to expect that the diffusion approximation will be valid even in the presence of the CSF, with consequences for faster solution of the inverse problem.
在头部模型中对光子输运的时间相关正向解进行高效计算，是对大脑功能扩散光学成像进行精确反演的关键能力。光子输运的扩散近似模拟比物理上正确的辐射输运方程（RTE）要快得多；然而，通常认为散射长度必须远小于所有系统尺寸和所有吸收长度，该近似才准确。在脑脊液（CSF）中，这两个条件都不满足。由于脑脊液中的视线距离很小，只有几毫米左右，我们探讨这样一种想法：脑脊液散射系数可以用从零到典型视线距离倒数量级（约0.3 mm⁻¹）的任何值来建模，而不会显著改变计算出的探测器信号或与功能测量相关的部分路径长度。我们通过基于临床磁共振成像数据、具有实际光极几何形状的三维头部模型中对RTE的蒙特卡罗模拟，详细证明了这一点。我们的研究结果使我们预期，即使存在脑脊液，扩散近似也将是有效的，这将加快反问题的求解。
