Hielscher A H, Alcouffe R E, Barbour R L
Bioscience and Biotechnology, Los Alamos National Laboratory, NM 87545, USA.
Phys Med Biol. 1998 May;43(5):1285-302. doi: 10.1088/0031-9155/43/5/017.
We analyse the limits of the diffusion approximation to the time-independent equation of radiative transfer for homogeneous and heterogeneous biological media. Analytical calculations and finite-difference simulations based on diffusion theory are compared with discrete-ordinate, finite-difference transport calculations. The influence of the ratio of absorption and transport scattering coefficient (mu(a)/mu'(s)) on the accuracy of the diffusion approximation are quantified and different definitions for the diffusion coefficient, D, are discussed. We also address effects caused by void-like heterogeneities in which absorption and scattering are very small compared with the surrounding medium. Based on results for simple homogeneous and heterogeneous systems, we analyse diffusion and transport calculation of light propagation in the human brain. For these simulations we convert density maps obtained from magnetic resonance imaging (MRI) to optical-parameter maps (mu(a) and mu'(s)) of the brain. We show that diffusion theory fails to describe accurately light propagation in highly absorbing regions, such as haematoma, and void-like spaces, such as the ventricles and the subarachnoid space.
我们分析了均匀和非均匀生物介质中与时间无关的辐射传输方程的扩散近似极限。将基于扩散理论的解析计算和有限差分模拟与离散坐标、有限差分数值传输计算进行了比较。量化了吸收与输运散射系数之比（μ(a)/μ'(s)）对扩散近似精度的影响，并讨论了扩散系数D的不同定义。我们还研究了与周围介质相比吸收和散射非常小的类空非均匀性所引起的影响。基于简单均匀和非均匀系统的结果，我们分析了光在人脑中传播的扩散和传输计算。对于这些模拟，我们将从磁共振成像（MRI）获得的密度图转换为大脑的光学参数图（μ(a)和μ'(s)）。我们表明，扩散理论无法准确描述光在高吸收区域（如血肿）和类空区域（如脑室和蛛网膜下腔）中的传播。
