Department of Biomedical Engineering, Tufts University, Medford, Massachusetts, USA.
PLoS One. 2013 May 21;8(5):e64095. doi: 10.1371/journal.pone.0064095. Print 2013.
We introduce a multi-distance, frequency-domain, near-infrared spectroscopy (NIRS) method to measure the optical coefficients of two-layered media and the thickness of the top layer from diffuse reflectance measurements. This method features a direct solution based on diffusion theory and an inversion procedure based on the Levenberg-Marquardt algorithm. We have validated our method through Monte Carlo simulations, experiments on tissue-like phantoms, and measurements on the forehead of three human subjects. The Monte Carlo simulations and phantom measurements have shown that, in ideal two-layered samples, our method accurately recovers the top layer thickness (L), the absorption coefficient (µ a ) and the reduced scattering coefficient (µ' s ) of both layers with deviations that are typically less than 10% for all parameters. Our method is aimed at absolute measurements of hemoglobin concentration and saturation in cerebral and extracerebral tissue of adult human subjects, where the top layer (layer 1) represents extracerebral tissue (scalp, skull, dura mater, subarachnoid space, etc.) and the bottom layer (layer 2) represents cerebral tissue. Human subject measurements have shown a significantly greater total hemoglobin concentration in cerebral tissue (82±14 µM) with respect to extracerebral tissue (30±7 µM). By contrast, there was no significant difference between the hemoglobin saturation measured in cerebral tissue (56%±10%) and extracerebral tissue (62%±6%). To our knowledge, this is the first time that an inversion procedure in the frequency domain with six unknown parameters with no other prior knowledge is used for the retrieval of the optical coefficients and top layer thickness with high accuracy on two-layered media. Our absolute measurements of cerebral hemoglobin concentration and saturation are based on the discrimination of extracerebral and cerebral tissue layers, and they can enhance the impact of NIRS for cerebral hemodynamics and oxygenation assessment both in the research arena and clinical practice.
我们介绍了一种多距离、频域近红外光谱（NIRS）方法，用于从漫反射测量中测量两层介质的光学系数和顶层厚度。该方法的特点是基于扩散理论的直接解和基于 Levenberg-Marquardt 算法的反演过程。我们通过蒙特卡罗模拟、组织模拟体实验以及对三名人类受试者的额部进行的测量验证了我们的方法。蒙特卡罗模拟和模拟体测量表明，在理想的两层样本中，我们的方法可以准确地恢复顶层厚度（L）、两层的吸收系数（µa）和散射系数（µ' s），所有参数的偏差通常小于 10%。我们的方法旨在对成人脑内和脑外组织中血红蛋白浓度和饱和度进行绝对测量，其中顶层（第 1 层）代表脑外组织（头皮、颅骨、硬脑膜、蛛网膜下腔等），底层（第 2 层）代表脑组织。人体测量结果表明，脑组织中的总血红蛋白浓度（82±14 µM）明显高于脑外组织（30±7 µM）。相比之下，脑组织（56%±10%）和脑外组织（62%±6%）中测量的血红蛋白饱和度没有显著差异。据我们所知，这是首次在频域中使用具有六个未知参数且没有其他先验知识的反演过程，以高精度对两层介质进行光学系数和顶层厚度的反演。我们对脑内血红蛋白浓度和饱和度的绝对测量是基于对脑外和脑组织层的区分，这可以增强 NIRS 在研究和临床实践中对脑血流和氧合评估的影响。
