Opt Lett. 2018 Jun 15;43(12):2756-2759. doi: 10.1364/OL.43.002756.
Diffuse correlation spectroscopy (DCS) is an optical technique that non-invasively quantifies an index of blood flow (BF) by measuring the temporal autocorrelation function of the intensity fluctuations of light diffusely remitted from the tissue. Traditional DCS measurements use continuous wave (CW) lasers with coherence lengths longer than the photon path lengths in the sample to ensure that the diffusely remitted light is coherent and generates a speckle pattern. Recently, we proposed time domain DCS (TD-DCS) to allow measurements of the speckle fluctuations for specific path lengths of light through the tissue, which has the distinct advantage of permitting an analysis of selected long path lengths of light to improve the depth sensitivity of the measurement. However, compared to CW-DCS, factors including the instrument response function (IRF), the detection gate width, and the finite coherence length need to be considered in the model analysis of the experimental data. Here we present a TD-DCS model describing how the intensity autocorrelation functions measured for different path lengths of light depend on the coherence length, pulse width of the laser, detection gate width, IRF, BF, and optical properties of the scattering sample. Predictions of the model are compared with experimental results using a homogeneous liquid phantom sample that mimics human tissue optical properties. The BFs obtained from the TD-DCS model for different path lengths of light agree with the BF obtained from CW-DCS measurements, while the standard simplified model underestimates the BF by a factor of ∼2. This Letter establishes the theoretical foundation of the TD-DCS technique and provides guidance for future BF measurements in tissue.
漫射相关光谱学（DCS）是一种光学技术，通过测量组织中漫射光强度波动的时间自相关函数，非侵入式地定量测量血流（BF）指数。传统的 DCS 测量使用连续波（CW）激光，其相干长度长于样品中的光子路径长度，以确保漫射光相干并产生散斑图案。最近，我们提出了时域 DCS（TD-DCS），允许测量光通过组织的特定路径长度的散斑波动，这具有明显的优势，可以分析选定的长路径长度的光，以提高测量的深度灵敏度。然而，与 CW-DCS 相比，在对实验数据进行模型分析时，需要考虑仪器响应函数（IRF）、检测门宽度和有限的相干长度等因素。在这里，我们提出了一个 TD-DCS 模型，描述了不同路径长度的光测量的强度自相关函数如何取决于相干长度、激光的脉冲宽度、检测门宽度、IRF、BF 和散射样品的光学性质。通过使用模拟人体组织光学性质的均匀液体荧光体样品对模型的预测与实验结果进行了比较。不同路径长度的光的 TD-DCS 模型获得的 BF 与 CW-DCS 测量获得的 BF 一致，而标准简化模型低估了 BF，因子约为 2。这封信为 TD-DCS 技术建立了理论基础，并为未来组织中的 BF 测量提供了指导。
