Effect of light losses of sample between two integrating spheres on optical properties estimation

Real-time and accurate estimation ex vivo of four basic optical properties from thin tissue based on a cascade forward neural network

Double integrating sphere measurements obtained from thin ex vivo tissues provides more spectral information and hence allows full estimation of all basic optical properties (OPs) theoretically. However, the ill-conditioned nature of the OP determination increases excessively with the reduction in tissue thickness. Therefore, it is crucial to develop a model for thin ex vivo tissues that is robust to noise. Herein, we present a deep learning solution to precisely extract four basic OPs in real-time from thin ex vivo tissues, leveraging a dedicated cascade forward neural network (CFNN) for each OP with an additional introduced input of the refractive index of the cuvette holder. The results show that the CFNN-based model enables accurate and fast evaluation of OPs, as well as robustness to noise. Our proposed method overcomes the highly ill-conditioned restriction of OP evaluation and can distinguish the effects of slight changes in measurable quantities without any a priori knowledge.

Accuracy of retrieving optical properties from liquid tissue phantoms using a single integrating sphere

Using integrating spheres (ISs) in conjunction with the inverse adding-doubling algorithm (IAD) offers a well-established, rigorous protocol for determining optical absorption (μ_a) and reduced scattering (μs') coefficients of thin, optically homogeneous, turbid media. Here, we report the performance and use of a single IS system for experimentally retrieving optical properties in phantom media whose optical properties were well controlled. The IS system was used to measure the total reflectance and transmittance between 500 and 800 nm in liquid phantoms that were prepared to span a wide range of optical scattering and absorption coefficients. Measurements on phantoms were sequentially made using one of two broadband light sources-a halogen lamp or a supercontinuum laser. We report on the accuracy of IAD-derived optical coefficients using IS measurements made on phantoms-directly or by employing one of two previously reported correction methods. The first (sample-substitution error) correction was experimentally achieved while the second used Monte Carlo-based corrections with IAD. When experimentally calibrated reflectance and transmittance values were directly used as inputs to the IAD, mean absolute errors in recovered optical coefficients were larger than 0.4cm^-1 for absorption and more than 6cm^-1 for scattering across all phantoms and wavelengths measured. These errors reduced to 0.06-0.17cm^-1 and 0.7-2cm^-1 for μ_a and μs', respectively, with the use of corrections. Choice of light sources used, sample geometry (relative to optical coefficients), signal-to-noise of measurements, and the selection of correction methods are discussed.

Elevated-temperature-induced acceleration of PACT clearing process of mouse brain tissue

Tissue optical clearing technique shows a great potential for neural imaging with high resolution, especially for connectomics in brain. The passive clarity technique (PACT) is a relative simple clearing method based on incubation, which has a great advantage on tissue transparency, fluorescence preservation and immunostaining compatibility for imaging tissue blocks. However, this method suffers from long processing time. Previous studies indicated that increasing temperature can speed up the clearing. In this work, we aim to systematacially and quantitatively study this influence based on PACT with graded increase of temperatures. We investigated the process of optical clearing of brain tissue block at different temperatures, and found that elevated temperature could accelerate the clearing process and also had influence on the fluorescence intensity. By balancing the advantages with drawbacks, we conclude that 42-47 °C is an alternative temperature range for PACT, which can not only produce faster clearing process, but also retain the original advantages of PACT by preserving endogenous fluorescence well, achieving fine morphology maintenance and immunostaining compatibility.

Modeling changes in the hemoglobin concentration of skin with total diffuse reflectance spectroscopy

The ability to monitor changes in the concentration of hemoglobin in the blood of the skin in real time is a key component to personalized patient care. Since hemoglobin has a unique absorption spectrum in the visible light range, diffuse reflectance spectroscopy is the most common approach. Although the collection of the diffuse reflectance spectrum with an integrating sphere (IS) has several calibration challenges, this collection method is sufficiently user-friendly that it may be worth overcoming the initial difficulty. Once the spectrum is obtained, it is commonly interpreted with a log-inverse-reflectance (LIR) or “absorbance” analysis that can only accurately monitor changes in the hemoglobin concentration when there are no changes to the nonhemoglobin chromophore concentrations which is not always the case. We address the difficulties associated with collection of the diffuse reflectance spectrum with an IS and propose a model capable of retrieving relative changes in hemoglobin concentration from the visible light spectrum. The model is capable of accounting for concentration changes in the nonhemoglobin chromophores and is first characterized with theoretical spectra and liquid phantoms. The model is then used in comparison with a common LIR analysis on temporal measurements from blanched and reddened human skin.

Lookup-table-based inverse model for human skin reflectance spectroscopy: two-layered Monte Carlo simulations and experiments

Fiber reflectance spectroscopy is a non-invasive method for diagnosing skin diseases or evaluating aesthetic efficacy, but it is dependent on the inverse model validity. In this work, a lookup-table-based inverse model is developed using two-layered Monte Carlo simulations in order to extract the physiological and optical properties of skin. The melanin volume fraction and blood oxygen parameters are extracted from fiber reflectance spectra of in vivo human skin. The former indicates good coincidence with a commercial skin-melanin probe, and the latter (based on forearm venous occlusion and ischemia, and hot compress experiment) shows that the measurements are in agreement with physiological changes. These results verify the potential of this spectroscopy technique for evaluating the physiological characteristics of human skin.

Recent progress in tissue optical clearing

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical-clearing agents into tissue, such as physical methods, chemical-penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced. [Formula: see text].

Quantitative analysis of dehydration in porcine skin for assessing mechanism of optical clearing

Dehydration induced by optical clearing agents (OCAs) can improve tissue optical transmittance; however, current studies merely gave some qualitative descriptions. We develop a model to quantitatively evaluate water content with partial least-squares method based on the measurements of near-infrared reflectance spectroscopy and weight of porcine skin. Furthermore, a commercial spectrometer with an integrating sphere is used to measure the transmittance and reflectance of skin after treatment with different OCAs, and then the water content and optical properties of sample are calculated, respectively. The results show that both the reduced scattering coefficient and dehydration of skin decrease with prolongation of action of OCAs, but the relative change in former is larger than that in latter after a 60-min treatment. The absorption coefficient at 1450 nm decreases completely coincident with dehydration of skin. Further analysis illustrates that the correlation coefficient between the relative changes in the reduced scattering coefficient and dehydration is ∼1 during the 60-min treatment of agents, but there is an extremely significant difference between the two parameters for some OCAs with more hydroxyl groups, especially, glycerol or D-sorbitol, which means that the dehydration is a main mechanism of skin optical clearing, but not the only mechanism.

Different optical properties between human hepatocellular carcinoma tissues and non-tumorous hepatic tissues in vitro

There has been an ongoing search for clinically acceptable methods for the accurate, efficient and simple diagnosis and prognosis of hepatocellular carcinoma (HCC). Optical spectroscopy is a technique with potential clinical applications to diagnose cancer diseases. The purpose of this study was to obtain the optical properties of HCC tissues and non-tumorous hepatic tissues and identify the difference between them. A total of 55 tissue samples (HCC tissue, n=38; non-tumorous hepatic tissue, n=17) were surgically resected from patients with HCC. The optical parameters were measured in 10-nm steps using single-integrating-sphere system in the wavelength range of 400 to 1800 nm. It was found that the optical properties and their differences varied with the wavelength for the HCC tissue and the non-tumorous hepatic tissue in the entire wavelength range of research. The absorption coefficient of the HCC tissue (1.48±0.99, 1.46±0.88, 0.86±0.61, 2.15±0.53, 0.54±0.10, 0.79±0.15 mm(-1)) was significantly lower than that of the non-tumorous hepatic tissue (2.79±1.73, 3.13±1.47, 3.06±2.79, 2.57±0.55, 0.62±0.10, 0.93±0.16 mm(-1)) at wavelengths of 400, 410, 450, 1450, 1660 and 1800 nm, respectively (P<0.05). The reduced scattering coefficient of HCC tissue (5.28±1.70, 4.91±1.54, 1.26±0.35 mm(-1)) and non-tumorous hepatic tissue (8.14±3.70, 9.27±3.08, 2.55±0.57 mm(-1)) was significantly different at 460, 500 and 1800 nm respectively (P<0.05). These results show different pathologic liver tissues have different optical properties. It provides a better understanding of the relationship between optical parameters and physiological characteristics in human liver tissues. And it would be very useful for developing a non-invasive, real-time, simple and efficient way for medical management of HCC in the future.

Light scattering of semitransparent sintered polytetrafluoroethylene films

Polytetrafluoroethylene (PTFE) is a strongly scattering material and has been regarded to have optical properties similar to biological tissues. In the present study, the bidirectional reflectance distribution function (BRDF) and the bidirectional transmittance distribution function (BTDF) of several PTFE films, with thicknesses from 0.11 to 10 mm, are measured using a laser scatterometer at the wavelength of 635 nm. The directional-hemispherical reflectance (R) and transmittance (T) were obtained by integrating BRDF and BTDF for normal incidence. Comparison of the ratio of the measured R and T with that calculated from the adding-doubling method allows the determination of the reduced scattering coefficient. Furthermore, the effect of surface scattering is investigated by measuring the polarization-dependent BRDF and BTDF at oblique incidence. By analyzing the measurement uncertainty of BTDF in the near-normal observation angles at normal incidence, the present authors found that the scattering coefficient of PTFE should exceed 1200 cm(-1), which is much greater than that of biological tissues. On the other hand, the absorption coefficient of PTFE must be less than 0.01 cm(-1), much smaller than that of biological tissues, a necessary condition to achieve R > or =0.98 with a 10-mm-thick slab.