| Light absorption and scattering are two basic phenomena during the interaction of light with biological tissues,which are characterized by the absorption coefficient(μe)and reduced scattering coefficient(μs’).Measurement of the optical properties has been an important subject of research,because μa and us’ are related to the chemical and structural properties of biological tissues.Moreover,quantification of the tissue optical properties can help us in understanding light propagation in biological tissues,better interpreting the measured optical data,optimizing optical devices,and improving food quality and safety assessment.However,optical property measurement is prone to error due to complicated analytical models and parameter estimation procedures,which has presented a major challenge in this field.As a relatively new optical measurement technique,spatial-frequency domain imaging(SFDI)has advantages over other optical methods.SFDI can provide quantitative two or three-dimensional mapping of μab and μs’for biological materials,which can be further used to characterize spatial distribution of tissue optical properties.Moreover,spatial frequency is closely related to light penetration depth in tissues,which provides potential for accurate measurement of optical properties of two-layer or multi-layer tissues by selecting proper or optimal frequency ranges for different layers.This research is,therefore,aimed at proposing new or improved methods for estimating the optical properties of one-layer and two-layer biological tissues(liquid and solid optical phantoms,milk and horticultural products)nondestructively and accurately through Monte Carlo simulation,finite element modeling,and inverse parameter estimation algorithm optimizations by using SFDI technique.The main research topics and results are summarized as follows:(1)A SFDI system was designed and constructed for optical property measurement of a solid optical phantom and 36 apples,and the system configuration and operation procedures were established.The results showed that:1)The errors for measuring μa and μs’ of the solid phantom at the wavelengths of 460 nm,527 nm and 630 nm were 13.3%,6.4%,12.8%and 7.8%,3.4%,5.1%,respectively,compared with the reference values provided by the manufacturer.2)The measured values of μa and μs’ for the healthy apples were within an acceptable range,compared with other published literature.The,μa values of the healthy apples were much smaller than those of internal browning apples,while the differences in the μs’ values between the healthy and defective fruit were small or insignificant.The preliminary research also identified the main problems in the system development and optical property measurement,and proposed possible solutions for future research.The next research focused on effects of nine critical variables in the single integrating sphere technique on tissue scattering property measurements,SFDI system improvement and algorithm optimization for estimating optical properties of one-layer and two-layer tissues accurately using SFDI technique.(2)An improved single integrating sphere system was set up for providing reference measurements of the μa and μs’ of biological materials.Effects of nine critical variables in the single integrating sphere technique on optical property measurements were quantified by using 36 liquid samples and two solid phantoms.The errors for estimating μs’ for the liquid phantoms and pure scattering samples using the system were compared and analyzed.The results showed that:1)The entrance port diameter,detector port diameter,standard reflectivity,and anisotropy factor had a negligible effect on the estimation of μs’ with the maximum average discrepancy of less than 2.2%,while the other five variables had a significant effect on μs’ measurement.The refractive indexes and sphere wall reflectivity should be measured accurately in use.A small diameter light beam,with the illumination incident on the sample center perpendicularly,should be used in the measurement.The specular reflectance from the sample can be ignored when estimating μs’.2)Average accuracies of the measured μa and μs’ for pure absorption and pure scattering liquid samples were 15.0%and 4.7%over 550-900 nm,when the system was evaluated against the collimated transmittance method and empirical equation.The system had good reproducibility with the coefficients of variation(CV)for μa and μs’ being less than 4%.The detectable value of μa was around 0.02 cm-1,while the μs’ values for liquid samples used in this study(4 cm-1≤μs’≤ 43 cm-1)were all within the detectable range.3)The errors for measuring μs’of 12 liquid phantoms were slightly larger than those of 12 pure scattering samples,with the mean values of 5.6%and 4.5%,respectively,in 500-900 nm,and the difference was not significant(P=0.355).(3)Finite element method(FEM)was used for modeling light transfer in horticultural product tissues under structured illumination.The diffuse reflectance at the tissue surface generated by Monte Carlo(MC)simulation,FEM and analytical solution to the diffusion approximation equation were compared,which provided a theoretical basis for optical property estimation of one-layer tissues using SFDI.The results showed that:1)FEM generated reasonable diffuse reflectance,compared with the analytical method and MC simulation.2)Larger μs’/μa values resulted in smaller relative errors for diffuse reflectance estimations using FEM and analytical method,and it is suggested that values of μs’/μa should be no smaller than 10.3)FEM and analytical method both overestimated the reflectance for high μatr/fx values and underestimated the reflectance for low μatr/fx values,compared with MC simulation,and it is recommended that values of μatr/fx should be larger than 3.4)The diffuse reflectance increased nonlinearly with the rise of dimensionless scattering,while there was a nearly linear decline as the dimensionless absorption increased.When the dimensionless scattering was larger than 50,the reflectance remained stable and insensitive to μas(4)Through improvement to the SFDI system and optimization of the inverse algorithm,a two-step method was proposed for improving the estimation accuracy of optical properties for one-layer turbid media.The optical properties of solid and liquid phantoms were measured using the improved SFDI system,coupled with the optimized inverse algorithm.The results showed that:1)The improved SFDI system provided a wider detection wavelength range,a simpler system configuration,and faster and more convenient image acquisition.The system also showed good linearity(R2>0.97).2)The sensitivity coefficients of were much larger than those of μa,indicating that better estimations of μs’ can be achieved,compared to that of μa.Signals at larger spatial frequencies depended strongly on μs’,while those at smaller spatial frequencies exhibited large dependence on μa 3)The smoothing method with a span of 5 data points improved estimation accuracy.4)The optimal frequency resolution increased with μs’ and remained stable when μs’ was larger than 2 mm-1.The optimal end frequency decreased from 0.3/mfp’to 0.16lmfp’with,μs’ ’ranging from 0.4 mm-1 to 3 mm-1,while the optimal start frequency remained at 0 mm-1.5)The proposed two-step method resulted in better estimation of μs’ and μa for all 40 samples with the mean absolute errors of 4.3%and 6.5%,respectively,which represent 37.5%and 9.8%improvements over the conventional one-step method.The difference for μa was rather significant.6)The overall pattern of changes in the values of μa and μs’ estimated by the conventional one-step and two-step methods for the wavelengths of 675 nm,700 um and 715 nm was consistent with that by the two reference methods,and the proposed two-step method improved accuracy for estimating the μa and μs’ of solid and liquid phantoms than that for the conventional one-step method.(5)A stepwise method(Method-1)was proposed for estimating the μa and μs’ of two-layer turbid media,followed by validating the feasibility and determining the constraining conditions for the first layer thickness.The inverse algorithm for optical property estimation of two-layer turbid media was optimized and validated through Monte Carlo simulation,and the estimation accuracy was compared with the method of estimating μa1,μs1’c.2 and,μs2’ simultaneously(Method-2).The results showed that:1)The optical properties of two-layer turbid media and the first layer thickness could affect the sensitivity coefficients of μa1,μas1’,μa2 and μs2’.In principle,the accuracy for estimating μs1’’is the highest,followed by μa1,and estimations for μa2 and μa2’are most challenging.2)A sample-based calibration method was proposed to correct or minimize discrepancies of the reflectance between the diffusion model and Monte Carlo simulation.It was feasible to use the one-layer model for estimating μa1 and μs1’ with proper selection of frequencies.3)To achieve acceptable estimations for the optical properties of the first and the second layers,the first layer thickness should be no smaller than 0.2 mm and no greater than 2 mm.4)Frequency range optimization improved the estimation accuracy for μa1 andμs1’.Optimization-1 was based on the relationship between the values of mfp1/mfp2’ and optimal frequency range,and Optimization-2 determined the optimal frequency range differing in the values of μs1’.Optimization-1 was only applicable for estimating μa1 and μs1’,while Optimization-2 was applicable for estimating μas1’,μa2 and μsa2’.Optimization-2 resulted in better estimation accuracies for μa1 and μs1’ Optimization-1 with significant differences forμa,and Method-1 produced better estimation than Mehtod-2 with significant differences for μaa1 and fs1’.5)Frequency range optimization also improved estimation accuracies forμa2 and μs2’:Method-1 produced better estimation(11.8%and 8.9%)than Mehtod-2(24.3%and 14.0%)with significant differences for μa2 and μs2’,but the overall estimation accuracy was worse than that for μa1i and μas1’.(6)The optical properties of two-layer solid phantoms,two-layer liquid samples with different concentrations of milk as the second layer,and the skin and flesh tissues of apples(Golden Delicious,Red Rome,Delicious and Jonagold),kiwifruits and mangos were measured using the improved SFDI system,coupled with the optimized parameter estimation algorithm.Discrepancies of the measured optical properties for the skin and flesh tissues by using the SFDI technique and the single integrating sphere method were analyzed.The results showed that:1)The error values for estimating μa1,μs1’,μa2 and μs2’ of the two-layer solid phantoms were smaller than 35%,17%,26%and 27%,respectively,when the first layer thickness was within 2 mm,while the error values for estimating μa2 and μa2’ became much larger when the first layer thickness exceeded 2 mm.2)The mean error values for estimating μa1 μs1’,μa2 and μs2’of the two-layer liquid samples were smaller than 19%,7%,28%and 20%,respectively.3)An absorption peak was observed in the absorption coefficient spectra of the skin and flesh tissues for all three kinds of fruit,due to pigment constituents,and its peak value depended largely on the pigment content.The reduced scattering coefficient was related to the tissue physical properties(e.g.,firmness).The absorption and reduced scattering coefficients of the skin tissues were much larger than those of the flesh tissues,which was especially obvious for kiwifruits. |
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