Penetration depth of light re-emitted by a diffusive medium: theoretical and experimental investigation
Penetration depth of light re-emitted by a diffusive medium: theoretical and experimental investigation
复制标题
DOI：
10.1088/0031-9155/47/23/301
复制
发表时间：
2002-12-07
影响因子：
3.5
通讯作者：
Zaccanti, G
中科院分区：
文献类型：
--
作者：
Del Bianco, S;Martelli, F;Zaccanti, G
以铅笔光束照射的半无限均匀介质为研究对象，研究了光子在重新发射前穿透扩散介质的深度。利用扩散方程，得到了光子在被探测到之前穿透一定深度的概率的解析表达式，以及光子在介质各层内传播的平均路径长度。得到了连续波和时域的表达式，并找到了描述介质散射特性的简单近似标度关系。对于时间分辨测量，期望概率和平均路径长度与探测器所在光束的距离和介质的吸收系数无关。穿透深度随飞行时间的增加而增加。相比之下，对于连续波测量，概率和平均路径长度都强烈依赖于距离和吸收。穿透深度随着距离的增加或吸收的减少而增加。通过与连续波实验结果的比较，证明了解析表达式的准确性。穿透深度和平均路径长度提供了有用的信息，例如，用于测量组织氧合和肌肉和大脑的功能成像。特别是，接收到的光子到达的深度提供了实际研究组织体积的总体信息，而平均路径与吸收局部变化的灵敏度严格相关。
The depth at which photons penetrate into a diffusive medium before being re-emitted has been investigated with reference to a semi-infinite homogeneous medium illuminated by a pencil beam. By using the diffusion equation analytical expressions have been obtained for the probability that photons penetrate at a certain depth before being detected, and for the mean path length they travel inside each layer of the medium. Expressions have been obtained both for the cw and the time domain, and simple approximate scaling relationships describing the dependence on the scattering properties of the medium have been found. For time-resolved measurements both the probability and the mean path length are expected to be independent of the distance from the light beam at which the detector is placed and of the absorption coefficient of the medium. The penetration depth increases as the time of flight increases. In contrast, for cw measurements both the probability and the mean path length strongly depend on the distance and absorption. The penetration depth increases as the distance increases or absorption decreases. The accuracy of the analytical expressions has been demonstrated by comparisons with cw experimental results. The penetration depth and the mean path length provide useful information, for instance, for measurements of tissue oxygenation and for functional imaging of muscle and brain. In particular, the depth reached by received photons provides overall information on the volume of the tissue actually investigated, while the mean path is strictly related to the sensitivity to local variations of absorption.