As a numerical experiment, Monte Carlo simulation (MCS) has been proven to be a credible and flexible method for predicting the distribution of light in random media. It has full control of many parameters of optical system, which may be cumbersome to obtain in a real experiment. In standard OCT system, confocal microscopy structure with different Numerical Aperture (NA) is selected to acquire superior transverse resolution and unique property of optical sectioning. But the effects of numerical aperture on the probing depth of OCT system are difficult to estimate. In this paper, a new Monte Carlo simulation model of OCT system based on confocal mode is put forward to simulate the confocal microscopy structure and focused gaussian beam. It makes up the deficiency of traditional MCS model, which can only be applied to infinity narrow beam. By applying this new model, the effects of NA on probing depth of OCT system are analyzed, and the estimation of critical probing depth of OCT system is discussed. Study indicates that a smaller numerical aperture has more advantage on the probing depth when the transverse resolution is ensured.
A novel method combining the PCA-NN algorithm established on the single-layer tissue model and the genetic algorithm based on the two-layer diffusion model has been presented to determine the optical properties of the two-layer medium from the steady-state spatially resolved diffuse reflectance. In detail, we firstly employ the PCA-NN algorithm established on the semi-infinite tissue model to extract the optical properties of the top layer from the spatially resolved reflectance that results from the photons migrating mainly within the top layer. With the knowledge of the optical properties of the top layer, the optical properties of the bottom layer are then obtained by use of the genetic algorithm for fitting the two-layer diffusion model to the reflectance data far from the source. The method was validated using the Monte Carlo generated reflectance for the two-layer medium of skin overlying fat or skin overlying muscle. And, the skin thickness was assumed to be known a priori and fixed at 5 mm. The results showed that all the optical properties of two layers can be determined by the method with the accuracy of better than 10%.
In this paper, with reference to practical applications, we investigate the accuracy of the PCA-NN method in determining the optical properties μa and μs' from the spatially resolved relative reflectance data produced by Monte Carlo simulations. To test prediction performance of PCA-NN from the reflectance data with different lengths and different measurement noises, we constructed six PCA-NNs respectively corresponding to data length = 5, 10, 15, 20, 25 and 30 mm, which were trained by higher precision reflectance produced with photons = 107. Then lower precision reflectance generated with photons = 104, 2 × 104, 5 × 104, 7 × 104, 105, 2 × 105, 5 × 105, 7 × 105 and 106 were inputted to PCA-NNs to extract μa and μs' and the accuracy of μa and μs' was calculated, respectively. The results showed that for the reflectance with the same data length, the prediction errors of μa and μs' increase as the data noise increases; but for the reflectance with the same data precision, the errors decrease as the data length becomes longer. In conclusion, the preliminary results in this paper provide a guideline for choosing appropriate measurement conditions or estimating the prediction errors in reality.
Noninvasive determination of μs' and μa is essential for clinical applications in medical diagnostics and therapeutics. Spatially resolved diffuse reflectance method is more advantageous than other techniques because of its simplicity and low-cost. The methods for solving the nonlinear inverse problem of estimates of μs' and μa from spatially resolved diffuse reflectance Rd(r) can be classified into the algorithms based on absolute or relative reflectance measurements in nature. Since absolute reflectance measurements are technically more difficult to perform than the relative one, study on the methods based on the relative reflectance has a more important meaning for real applications. Considering that there were several normalizations of Rd(r), in this paper we discussed the varieties of prediction rms errors of μs' and μa extracted from relative reflectance data of different normalization forms including Rd(r)/Rd(r)max, r2(Rd(r)/Rd(r)max), 1n(Rd(r)/Rd(r)max) and 1n(r2(Rd(r)/Rd(r)max)). Additionally, we compared the accuracies of μs' and μa determined from absolute reflectance data Rd(r) and 1n(Rd(r)) with that from relative reflectance data to study the loss of accuracy due to normalization. Rather than the traditional neural network methods, we used a new method -- PCA-NN trained with diffuse reflectance data from Monte Carlo simulations to derive μs' and μa. All the PCA-NNs were trained and tested on the space with μs' between 0.1 and 2.0 mm-1 and μa between 0.01 and 0.1 mm-1. The test results indicate that the rms errors in μs' and μa are 0.72% and 2.57% for Rd(r), 0.28% and 0.55% for 1n(Rd(r), 2.98% and 5.44% for Rd(r)/Rd(r)max, 2.22% and 3.21% for 1n(Rd(r)/Rd(r)max), 6.52% and 20.7% for r2(Rd(r)/Rd(r)max), and 2.22% and 3.21% for 1n(r2(Rd(r)/Rd(r)max)), suggesting that the normalization form 1n(Rd(r)/Rd(r)max) would be the first choice for the estimates of μs' and μa from relative reflectance data by PCA-NN. Although the loss of accuracy due to normalization is considerable, the preliminary results provide a guideline for relative reflectance measurements.
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