Significance: Numerous optical imaging and spectroscopy techniques are used to study the tissue-optical properties; the majority of them are limited in information regarding the penetration depth. A simple, safe, easily applicable diagnostic technique is required to get deeper tissue information in a multilayer structure. Aim: A fiber-based diffuse reflectance (DR) technique is used to extract and quantify the bottom layer absorption coefficients in two-layer (2L) tissue-mimicking solid phantoms. We determine the Indian black ink concentrations in a deep-hidden layer that is sandwiched between agar and silicone-based phantom layers. Approach: A fiber-based DR experiment was performed to study the optical properties of the tissue at higher penetration depth, with different fiber core diameters and a constant numerical aperture (0.5 NA). The optimal core diameter of the fiber was chosen by measuring solid phantoms. In 2L phantoms, the thickness of the top layer was kept 5.5 mm with a constant absorption and reduced scattering coefficients (μa = 0.045 mm − 1 and μs ′ = 2.622 mm − 1), whereas the absorption coefficients of the bottom layers were varied from 0.014 to 0.037 mm − 1 keeping the μs ′ the same as the top layer. A unique crossover point (Cp) was found in the DR intensity profile against distance. We examined the slope before and after the Cp. These two slopes indicate the difference between the optical properties of the top and bottom layers. Our technique got further verification, as we successfully determined the Cp with different Indian black ink concentrations, placed at the junction between the agar and silicone-based phantom layers. Results: The DR measurements were applied to 2L phantoms. Two different slopes were found in 2L phantoms compared to the one-layer (optical properties equal to the top layer of 2L). We extracted the slopes before and after the Cp in the 2L phantoms. The calculated absorption coefficients before the Cp were 0.014 ± 0.0004, 0.022 ± 0.0003, 0.028 ± 0.0003, and 0.036 ± 0.0014 mm − 1, and the absorption coefficients after the Cp were 0.019 ± 0.0013, 0.013 ± 0.0004, 0.014 ± 0.0006, and 0.031 ± 0.0001 mm − 1, respectively. The calculated absorption coefficients before the Cp were in good agreement with the optical properties of the bottom layer. The calculated absorption coefficients after the Cp were not the same as the top layer. Our DR system successfully determines the crossover points 12.14 ± 0.11 and 11.73 ± 0.15 mm for 70% and 100% ink concentrations placed at the junction of the agar and silicone layers. Conclusions: In a 2L tissue structure, the Cp depends on the absorption coefficients of top and bottom layers and the thickness of the top layer. With the help of the Cp and the absorption coefficients, one can determine the thickness of the top layer or vice versa. The slope value before the Cp in the DR profile allowed us to determine the absorption properties of the bottom layer instead of having the average behavior of the 2L phantom in the far detection range (11.0 to 17.0 mm).

中文翻译：

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基于漫反射交叉点的两层体模底层吸收系数提取

意义：大量的光学成像和光谱技术被用于研究组织光学特性；他们中的大多数在有关穿透深度的信息方面受到限制。需要一种简单、安全、易于应用的诊断技术来获取多层结构中更深层次的组织信息。目的：基于光纤的漫反射 (DR) 技术用于提取和量化两层 (2L) 组织模拟实体模型中的底层吸收系数。我们确定夹在琼脂和硅基幻影层之间的深隐藏层中的印度黑色墨水浓度。方法：进行了基于光纤的 DR 实验，以研究具有不同光纤芯直径和恒定数值孔径 (0.5 NA) 的更高穿透深度组织的光学特性。通过测量实体模型来选择光纤的最佳芯径。在 2L 体模中，顶层的厚度保持为 5.5 毫米，具有恒定的吸收和降低的散射系数（μa = 0.045 mm - 1 和 μs ' = 2.622 mm - 1），而底层的吸收系数从0.014 到 0.037 mm - 1 保持 μs ' 与顶层相同。在 DR 强度分布中发现了一个独特的交叉点 (Cp)。我们检查了 Cp 之前和之后的斜率。这两个斜率表示顶层和底层的光学特性之间的差异。我们的技术得到了进一步的验证，因为我们成功地确定了具有不同印度黑色墨水浓度的 Cp，放置在琼脂和基于硅树脂的幻影层之间的交界处。结果：DR 测量应用于 2L 体模。与一层（光学特性等于 2L 的顶层）相比，在 2L 幻影中发现了两种不同的斜率。我们提取了 2L 体模中 Cp 之前和之后的斜率。Cp前计算的吸收系数分别为0.014±0.0004、0.022±0.0003、0.028±0.0003和0.036±0.0014mm-1，Cp后的吸收系数分别为0.019±0.0.004±0.0.000.019±0.0.00.3和0.0.00.0 ± 0.0001 mm − 1，分别。在 Cp 之前计算的吸收系数与底层的光学性质非常吻合。Cp 后计算的吸收系数与顶层不同。我们的 DR 系统成功地确定了交叉点 12.14 ± 0.11 和 11.73 ± 0。70% 和 100% 的墨水浓度为 15 毫米，放置在琼脂和硅胶层的交界处。结论：在2L组织结构中，Cp取决于顶层和底层的吸收系数以及顶层的厚度。借助 Cp 和吸收系数，可以确定顶层的厚度，反之亦然。DR 剖面中 Cp 之前的斜率值使我们能够确定底层的吸收特性，而不是具有远检测范围（11.0 至 17.0 毫米）内 2L 体模的平均行为。可以确定顶层的厚度，反之亦然。DR 剖面中 Cp 之前的斜率值使我们能够确定底层的吸收特性，而不是具有远检测范围（11.0 至 17.0 毫米）内 2L 体模的平均行为。可以确定顶层的厚度，反之亦然。DR 剖面中 Cp 之前的斜率值使我们能够确定底层的吸收特性，而不是具有远检测范围（11.0 至 17.0 毫米）内 2L 体模的平均行为。