Alexandrakis G, Busch D R, Faris G W, Patterson M S
Appl Opt. 2001 Aug 1;40(22):3810-21. doi: 10.1364/ao.40.003810.
The general two-layer inverse problem in biomedical photon migration is to estimate the absorption and scattering coefficients of each layer as well as the top-layer thickness. We attempted to solve this problem, using experimental and simulated spatially resolved frequency-domain (FD) reflectance for optical properties typical of skin overlying muscle or skin overlying fat in the near infrared. Two forward models of light propagation were used: a two-layer diffusion solution [Appl. Opt. 37, 779 (1998)] and a hybrid Monte Carlo (MC) diffusion model [Appl. Opt. 37, 7401 (1998)]. MC-simulated FD reflectance data were fitted as relative measurements to the hybrid and the pure diffusion models. It was found that the hybrid model could determine all the optical properties of the two-layer media studied to ~5%. Also, the same accuracy could be achieved by means of fitting MC-simulated cw reflectance data as absolute measurements, but fitting them as relative ones is an ill-posed problem. In contrast, two-layer diffusion could not retrieve the top-layer optical properties as accurately for FD data and was ill-posed for both relative and absolute cw data. The hybrid and the pure diffusion models were also fitted to experimental FD reflectance measurements from two-layer tissue-simulating phantoms representative of skin-on-fat and skin-on-muscle baseline optical properties. Both the hybrid and the diffusion models could determine the optical properties of the lower layer. The hybrid model demonstrated its potential to retrieve quantitatively the transport scattering coefficient of skin (the upper layer), which was not possible with the pure diffusion model. Systematic discrepancies between model and experiment may compromise the accuracy of the deduced top-layer optical properties. Identifying and eliminating such discrepancies is critical to practical application of the method.
生物医学光子迁移中的一般两层逆问题是估计每层的吸收和散射系数以及顶层厚度。我们尝试解决这个问题，使用实验和模拟的空间分辨频域（FD）反射率，针对近红外中覆盖肌肉的皮肤或覆盖脂肪的皮肤的典型光学特性。使用了两种光传播的正向模型：两层扩散解[《应用光学》37, 779 (1998)]和混合蒙特卡罗（MC）扩散模型[《应用光学》37, 7401 (1998)]。将MC模拟的FD反射率数据作为相对测量值拟合到混合模型和纯扩散模型。结果发现，混合模型能够以约5%的精度确定所研究的两层介质的所有光学特性。此外，通过将MC模拟的连续波（cw）反射率数据作为绝对测量值进行拟合也能达到相同的精度，但将它们作为相对测量值进行拟合是一个不适定问题。相比之下，两层扩散模型对于FD数据无法如此准确地恢复顶层光学特性，并且对于相对和绝对cw数据都是不适定的。混合模型和纯扩散模型也被拟合到来自代表脂肪上皮肤和肌肉上皮肤基线光学特性的两层组织模拟体模的实验FD反射率测量值。混合模型和扩散模型都能够确定下层的光学特性。混合模型展示了其定量恢复皮肤（上层）传输散射系数的潜力，而这是纯扩散模型无法做到的。模型与实验之间的系统差异可能会损害推导的顶层光学特性的准确性。识别并消除此类差异对于该方法的实际应用至关重要。
