Absorption Coefficient Μa Research Articles

Nanoparticle-free tissue-mimicking phantoms with intrinsic scattering.

We present an alternative to the conventional approach, phantoms without scattering nanoparticles, where scattering is achieved by the material itself: spherical cavities trapped in a silicone matrix. We describe the properties and fabrication of novel optical phantoms based on a silicone elastomer polydimethylsiloxane (PDMS) and glycerol mixture. Optical properties (absorption coefficient µa , reduced scattering coefficient µs' , and anisotropy factor g) of the fabricated phantoms were retrieved from spectrophotometric measurements (in the 400-1100 nm wavelength range) using the inverse adding-doubling method. The internal structure of the phantoms was studied under a scanning electron microscope, and the chemical composition was assessed by Raman spectroscopy. Composition of the phantom material is reported along with the full characterization of the produced phantoms and ways to control their parameters.

Optical properties of drying wood studied by time-resolved near-infrared spectroscopy.

We measured the optical properties of drying wood with the moisture contents ranging from 10% to 200%. By using time-resolved near-infrared spectroscopy, the reduced scattering coefficient μs' and absorption coefficient μa were determined independent of each other, providing information on the chemical and structural changes, respectively, of wood on the nanometer scale. Scattering from dry pores dominated, which allowed us to determine the drying process of large pores during the period of constant drying rate, and the drying process of smaller pores during the period of decreasing drying rate. The surface layer and interior of the wood exhibit different moisture states, which affect the scattering properties of the wood.

Measuring absorption coefficient of scattering liquids using a tube inside an integrating sphere.

A method for measuring the absorption coefficient μa of absorbing and scattering liquid samples is presented. The sample is injected into a small transparent tube mounted through an integrating sphere. Two models for determining the absorption coefficient using the relative optical output signal are described and validated using aqueous ink absorbers of 0.5 vol.% (0.3 mm-1<μa<1.55 mm-1) and 1.0 vol.% (1.0 mm-1<μa<4.0 mm-1) concentrations with 1 vol.% (μs'≈1.4 mm-1) and 10 vol.% (μs'≈14 mm-1) Intralipid dilutions. The low concentrations give μa and μs values, which are comparable with those of biological tissues. One model assumes a uniform light distribution within the sample, which is valid for low absorption. Another model considers light attenuation that obeys Lambert-Beer's law, which may be used for relatively high absorption. Measurements with low and high scattering samples are done for the wavelength range of 400-900 nm. Measured spectra of purely absorbing samples are within 15% agreement with measurements using standard transmission spectrophotometry. For 0.5 vol.% ink absorbers and at wavelengths below 700 nm, measured μa values are higher for samples with low scattering and lower for those with high scattering. At wavelengths above 700 nm, measured μa values do not vary significantly with amount of scattering. For 1.0 vol.% ink absorbers, measured spectra do not change with low scattering. These results indicate that the method can be used for measuring absorption spectra of scattering liquid samples with optical properties similar to biological absorbers, particularly at wavelengths above 700 nm, which is difficult to accomplish with standard transmission spectrophotometry.

An improved analytic function for predicting light fluence rate in circular fields on a semi-infinite geometry.

Accurate determination of in-vivo light fluence rate is critical for preclinical and clinical studies involving photodynamic therapy (PDT). This study compares the longitudinal light fluence distribution inside biological tissue in the central axis of a 1 cm diameter circular uniform light field for a range of in-vivo tissue optical properties (absorption coefficients (μa) between 0.01 and 1 cm-1 and reduced scattering coefficients (μs') between 2 and 40 cm-1). This was done using Monte-Carlo simulations for a semi-infinite turbid medium in an air-tissue interface. The end goal is to develop an analytical expression that would fit the results from the Monte Carlo simulation for both the 1 cm diameter circular beam and the broad beam. Each of these parameters is expressed as a function of tissue optical properties. These results can then be compared against the existing expressions in the literature for broad beam for analysis in both accuracy and applicable range. Using the 6-parameter model, the range and accuracy for light transport through biological tissue is improved and may be used in the future as a guide in PDT for light fluence distribution for known tissue optical properties.

In vivo measurement of non-keratinized squamous epithelium using a spectroscopic microendoscope with multiple source-detector separations.

In the non-keratinized epithelia, dysplasia typically arises near the basement membrane and proliferates into the upper epithelial layers over time. We present a non-invasive, multimodal technique combining high-resolution fluorescence imaging and broadband sub-diffuse reflectance spectroscopy (sDRS) to monitor health at various tissue layers. This manuscript focuses on characterization of the sDRS modality, which contains two source-detector separations (SDSs) of 374 μm and 730 μm, so that it can be used to extract in vivo optical parameters from human oral mucosa at two tissue thicknesses. First, we present empirical lookup tables (LUTs) describing the relationship between reduced scattering (μs') and absorption coefficients (μa) and absolute reflectance. LUTS were shown to extract μs' and μa with accuracies of approximately 4% and 8%, respectively. We then present LUTs describing the relationship between μs', μa and sampling depth. Sampling depths range between 210-480 and 260-620 μm for the 374 and 730 μm SDSs, respectively. We then demonstrate the ability to extract in vivo μs', μa, hemoglobin concentration, bulk tissue oxygen saturation, scattering exponent, and sampling depth from the inner lip of thirteen healthy volunteers to elucidate the differences in the extracted optical parameters from each SDS (374 and 730 μm) within non-keratinized squamous epithelia.

Determination of true optical absorption and scattering coefficient of wooden cell wall substance by time-of-flight near infrared spectroscopy.

The true absorption coefficient (μa) and reduced scattering coefficient (μ´s) of the cell wall substance in Douglas fir were determined using time-of-flight near infrared spectroscopy. Samples were saturated with hexane, toluene or quinolone to minimize the multiple reflections of light on the boundary between pore-cell wall substance in wood. μ´s exhibited its minimum value when the wood was saturated with toluene because the refractive index of toluene is close to that of the wood cell wall substance. The optical parameters of the wood cell wall substance calculated were μa = 0.030 mm(-1) and μ´s= 18.4 mm(-1). Monte Carlo simulations using these values were in good agreement with the measured time-resolved transmittance profiles.

Ultrasound (US) transducer of higher operating frequency detects photoacoustic (PA) signals due to the contrast in elastic property

We report our study that shows selection in operating frequency of US-transducer used for boundary detection of PA-signals, which result due to the contrast in elastic property distribution (E(r→)) in sample material other than that of optical absorption coefficient (μa). Studies were carried out, experimentally, in tissue-mimicking Agar phantoms employing acoustic resolution photoacoustic microscopy (AR-PAM) system as an imaging unit. In the experiments, various transducers having different operating frequencies, ranging from 1MHz to 50MHz, were employed for studying frequency response of the photoacoustic signals. The study shows that, for detecting photoacoustic signals due to the contrast in elastic property, ultrasound transducer with higher operating frequency (∼50MHz) is demanded.

In vivo estimation of optical properties of rat liver using single-reflectance fiber probe during ischemia and reperfusion

To quantify the changes in optical properties of in vivo rat liver tissue, we applied diffuse reflectance spectroscopy (DRS) system using single-reflectance fiber probe during ischemia and reperfusion evoked by hepatic portal occlusion (hepatic artery, portal vein and bile duct). Changes in the reduced scattering coefficient μ s′, the absorption coefficient μ a, the tissue oxygen saturation StO2, and the oxidation of heme aa3 in cytochrome c oxidase (CcO) OHaa3 of in vivo rat liver (n = 6) were evaluated. Heme aa3 in CcO were significantly reduced (P < 0.05) during ischemia, which indicates a sign of mitochondrial energy failure induced by oxygen insufficiency of liver tissue. We found that OHaa3 obtained from the proposed method was unchanged immediately after the onset of ischemia and started gradually decreasing at 2 min after the onset of ischemia. Difference in the time course between OHaa3 and the conventional ratio metric analysis with μ a(605)/μ a(620) reported in literature demonstrates that the proposed method is effective in reduction of optical cross talk between hemoglobin and heme aa3. Our results suggest that DRS technique is applicable and useful for assessing in vivo tissue viability and hemodynamics in liver intraoperatively.

Soft tissue optical property extraction for carcinoma cell detection in diffuse optical tomography system under boundary element condition

Near infrared (NIR) light propagation through soft brain tissue extracts the optical properties of tissue. NIR light is a non-invasive technique which exhibits a high penetration depth to depict the morphological tissue characteristics. Laser sources of NIR wavelength 780nm and 850nm incident on the surface of the tissue enhances photon propagation through their profile layers. The avalanche photodiode voltage extracts the optical properties as absorption and scattering coefficients. These parameters were obtained at different depths with different tissue diameters, analyzed by simulation results. Diffuse reflectance and fluence rate measurements under time domain were carried out are provided with analytical solutions. The analytical solution for tissue structure configuration was performed at different boundary conditions as spherical or cylindrical or slab structures with photon propagation through an infinite medium. Simulation results of absorption coefficient (μa<1cm−1 for normal), scattering coefficient (μs′<50cm−1 for normal) and diffuse reflectance (Rd occurrence of reflections>4ns time for normal) differentiates the normal cells from carcinoma cells. Error percentage calculated for our system at 780nm and 850nm wavelength were minimum, as μa (780nm)=0.58%, μs′ (780nm)=0.1%, μa (850nm)=0.2%, μs′ (850nm)=0.2% when compared with existing system.

Polarization influence on reflectance measurements in the spatial frequency domain

In this work, we quantify the influence of crossed polarizers on reflectance measurements in the spatial frequency domain. The use of crossed polarizers is a very common approach for suppression of specular surface reflections. However, measurements are typically evaluated using a non-polarized scalar theory. The consequences of this discrepancy are the focus of our study, and we also quantify the related errors of the derived optical properties. We used polarized Monte Carlo simulations for forward calculation of the reflectance from different samples. The samples’ scatterers are assumed to be spherical, allowing for the calculation of the scattering functions by Mie theory. From the forward calculations, the reduced scattering coefficient and the absorption coefficient μa were derived by means of a scalar theory, as commonly used. Here, we use the analytical solution of the scalar radiative transfer equation. With this evaluation approach, which does not consider polarization, we found large errors in and μa in the range of 25% and above. Furthermore, we investigated the applicability of the use of a reference measurement to reduce these errors as suggested in literature. We found that this method is not able to generally improve the accuracy of measurements in the spatial frequency domain. Our general recommendation is to apply a polarized theory when using crossed polarizers.

Influence of the size and skin thickness of apple varieties on the retrieval of internal optical properties using Vis/NIR spectroscopy: A Monte Carlo-based study

Visible/near-infrared spectroscopy is a well-established method to measure optical properties of tissues, assuming that a light propagation model can be used to recover absorption and reduced scattering coefficients from non-invasive probing. Spectroscopic measurements have achieved success in non-destructive assessment of apple optical properties and quality attributes. However, the spectroscopy of apples must consider the size of the fruit and the presence of the thin skin layer that surrounds the flesh, to correctly read the signals acquired on the boundary. In this research, the fruit was modelled as a two layer spherical structure with various radii and finite thickness of the upper skin layer. Monte Carlo computations were performed to generate time-resolved reflectance and spatially-resolved reflectance measurements. Simulated data were then fitted using a procedure based on Levenberg–Marquardt algorithm with specific semi-infinite models. The errors in the retrieved optical properties of the flesh (absorption coefficient μa, and reduced scattering coefficient μ′s) were studied as functions of apple radius, skin thickness, and source–detector distance, for given optical parameter sets assigned to the flesh and the skin. The results suggest that the time-resolved reflectance spectroscopy may probe optical properties of the flesh regardless of the skin layer, when a sufficient source–detector distance (15mm) is used for the measurements. Similar results were found in case of using the spatially resolved spectroscopy, because measurements extend up to 15–29mm by steps of 1mm or 2mm. The computations also show that the curvature of the boundary has noticeable effect on the errors in the retrieved optical coefficients of the flesh. However, results from time-resolved spectroscopy are more influenced by the size of apples, compared with the spatially-resolved spectroscopy.

Multi-layered tissue head phantoms for noninvasive optical diagnostics

Extensive research in the area of optical sensing for medical diagnostics requires development of tissue phantoms with optical properties similar to those of living human tissues. Development and improvement of in vivo optical measurement systems requires the use of stable tissue phantoms with known characteristics, which are mainly used for calibration of such systems and testing their performance over time. Optical and mechanical properties of phantoms depend on their purpose. Nevertheless, they must accurately simulate specific tissues they are supposed to mimic. Many tissues and organs including head possess a multi-layered structure, with specific optical properties of each layer. However, such a structure is not always addressed in the present-day phantoms. In this paper, we focus on the development of a plain-parallel multi-layered phantom with optical properties (reduced scattering coefficient [Formula: see text] and absorption coefficient μa) corresponding to the human head layers, such as skin, skull, and gray and white matter of the brain tissue. The phantom is intended for use in noninvasive diffuse near-infrared spectroscopy (NIRS) of human brain. Optical parameters of the fabricated phantoms are reconstructed using spectrophotometry and inverse adding-doubling calculation method. The results show that polyvinyl chloride-plastisol (PVCP) and zinc oxide ( ZnO ) nanoparticles are suitable materials for fabrication of tissue mimicking phantoms with controlled scattering properties. Good matching was found between optical properties of phantoms and the corresponding values found in the literature.

Abstract P5-01-13: The spectroscopic feature of breast cancer

Abstract Objectives: To examine optical properties of breast cancer by time-resolved spectroscopy. Materials and Methods: We irradiated a pulsed laser of 760, 800, and 830 nm wave-length lights at multiple sites of both breasts including the site just above the cancer and detected the light transmitted through the breast with TRS-20SH (Hamamatsu Photonics K.K.). Absorption coefficient (μa), reduced scattering coefficient (μs’), total hemoglobin (tHb), and oxygen saturation (SO2) of the breast were calculated by photon diffusion equation. The clinical trial started in January 2007. A total of one hundred fifty-two breast cancer patients participated in the trial and written informed consent were obtained from all of the patients. Results: The absorption coefficient (μa) in 760, 800 and 830 nm wave-length of breast cancer tissue was significantly high compared with contra-lateral normal breast (760nm:cancer;0.078 normal;0.063 p&amp;lt;0.001. 800nm cancer:0.071, normal;0.050, p&amp;lt;0.001. 830nm: cancer;0.084, normal;0.063, p&amp;lt;0.001). The absorption coefficient and reduced scattering coefficient between breast cancer tissue and contra-lateral normal breast μa (/cm)μs'(/cm) cancernormal breastcancernormal breast760nm0.0780.0639.589.71800nm0.0710.059.239.4830nm0.0840.0639.079.22 There was no difference in reduced scattering coefficient (μs’) between breast cancer tissue and contra-lateral normal breast(760nm: cancer;9.58, normal;9.71, 800nm: cancer:9.23, normal;9.40. 830nm; cancer;9.07, normal;9.22). The tHb of breast cancer tissue was significantly high, compared with normal breast (cancer:32.3±14.6, normal breast;22.0±8.6, p=0.001). There was no difference in oxygen saturation (SO2) between breast cancer tissue and contra-lateral normal breast (cancer:73.2±4.3, normal breast;73.6±5.9, p=0.31). The tHb and SO2 between breast cancer tissue and contra-lateral normal breast cancernormal breastp-valuetHb(μM)32.3±14.622.0±8.6p&amp;lt;0.001SO2(%)73.2±4.373.6±5.9p=0.32 Conclusion: Absorption coefficient (μa) and tHb increased in breast cancer, whereas reduced scattering coefficient (μs’) and oxygen saturation did not. Citation Format: Hiroyuki Ogura, Nobuko Yoshizawa, Kenji Yoshimoto, Hatsuko Nasu, Yumiko Taki, Youko Hosokawa, Ryouichi Matsunuma, Yoshimi Ide, Etsuko Yamaki, Toshihiko Suzuki, Motoki Oda, Yukio Ueda, Yutaka Yamashita, Harumi Sakahara. The spectroscopic feature of breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-01-13.

An integrated fiber-optic probe combined with support vector regression for fast estimation of optical properties of turbid media

A fiber-optic probe system was developed to estimate the optical properties of turbid media based on spatially resolved diffuse reflectance. Because of the limitations in numerical calculation of radiative transfer equation (RTE), diffusion approximation (DA) and Monte Carlo simulations (MC), support vector regression (SVR) was introduced to model the relationship between diffuse reflectance values and optical properties. The SVR models of four collection fibers were trained by phantoms in calibration set with a wide range of optical properties which represented products of different applications, then the optical properties of phantoms in prediction set were predicted after an optimal searching on SVR models. The results indicated that the SVR model was capable of describing the relationship with little deviation in forward validation. The correlation coefficient (R) of reduced scattering coefficient μs′ and absorption coefficient μa in the prediction set were 0.9907 and 0.9980, respectively. The root mean square errors of prediction (RMSEP) of μs′ and μa in inverse validation were 0.411cm−1 and 0.338cm−1, respectively. The results indicated that the integrated fiber-optic probe system combined with SVR model were suitable for fast and accurate estimation of optical properties of turbid media based on spatially resolved diffuse reflectance.

Efficient construction of robust artificial neural networks for accurate determination of superficial sample optical properties

In general, diffuse reflectance spectroscopy (DRS) systems work with photon diffusion models to determine the absorption coefficient μa and reduced scattering coefficient μs' of turbid samples. However, in some DRS measurement scenarios, such as using short source-detector separations to investigate superficial tissues with comparable μa and μs', photon diffusion models might be invalid or might not have analytical solutions. In this study, a systematic workflow of constructing a rapid, accurate photon transport model that is valid at short source-detector separations (SDSs) and at a wide range of sample albedo is revealed. To create such a model, we first employed a GPU (Graphic Processing Unit) based Monte Carlo model to calculate the reflectance at various sample optical property combinations and established a database at high speed. The database was then utilized to train an artificial neural network (ANN) for determining the sample absorption and reduced scattering coefficients from the reflectance measured at several SDSs without applying spectral constraints. The robustness of the produced ANN model was rigorously validated. We evaluated the performance of a successfully trained ANN using tissue simulating phantoms. We also determined the 500-1000 nm absorption and reduced scattering spectra of in-vivo skin using our ANN model and found that the values agree well with those reported in several independent studies.

The effect of scattering-medium parameters on signal magnitude under acousto-optic tomography

We have experimentally studied the influence of scattering anisotropy parameter g of a medium on the magnitude of signal S (visualization parameter) at an ultrasonic frequency that is registered upon acoustooptic tomography. Aqueous solutions of mixtures of cream and skimmed milk with different ratios between them were used as scattering media. The optical properties of media (absorption coefficient μa and reduced scattering coefficient μ′S) have been measured on a spectrophotometer (Perkin-Elmer Lambda 950 UV-VIS-NIR) using the inverse adding-doubling technique. As a result of the investigation, we have found that there is a certain correlation between the value of the scattering anisotropy parameter g of aqueous solutions of investigated mixtures and the percentage of the mixture in the aqueous solution, which ensures the required small value of extinction coefficient μ of the scattering medium. An increase in signal S has been revealed with increasing anisotropy parameter g of the medium at a invariable value of extinction coefficient μ. We have concluded that, to solve an inverse problem on the acousto-optic tomography, it is necessary to take into account possible changes in the g factor in scattering media, including biological ones.

Spatially resolved diffuse reflectance in the visible and near-infrared wavelength range for non-destructive quality assessment of ‘Braeburn’ apples

Spectroscopic measurements in the visible and near infrared wavelength range have achieved success in non-destructive assessment of apple quality attributes contributed by chemical components inside the fruit such as sweetness. Nevertheless, the evaluation of quality attributes related to texture of the fruit (e.g. firmness) still remains a challenge. One of the proposed solutions is to acquire and utilize separately scattering and absorption information from spectroscopic readings for quality prediction. Since scattering is related to fruit microstructure and absorption is caused by chemical composition, construction of multivariate calibration models from these optical properties to predict corresponding quality parameters of interest could be a solution for this, and also have potential for non-destructive monitoring of fruit quality from harvest to consumption. In this research, a setup for contact spatially resolved diffuse reflectance measurements in the 500–1000nm range based on a fibre-optics probe was elaborated for the measurement of optical properties (absorption coefficient μa and reduced scattering coefficient μ′s) of ‘Braeburn’ apples. After calibration and validation of the setup on a set of liquid optical phantoms covering the relevant range of optical properties, thirty ‘Braeburn’ apples were measured before and after shelf-life storage (2 weeks at 18°C in normal atmosphere) with the setup and were analyzed for the main quality attributes (firmness and soluble solids content (SSC)). The estimated μa spectra of the apples indicated chlorophyll degradation during shelf-life storage. PLS models were investigated for apple quality prediction by using estimated optical properties spectra or diffuse reflectance spectra. These spectra covered information on chlorophyll and some carbohydrate and water absorption. The μa spectra also proved better than μ′s spectra for predicting SSC and firmness (R2-SSC=0.81; RMSEP(SSC)=0.69%; and R2-firmness=0.71; RMSEP(firmness)=9.68N). The combined μa and μ′s spectra did not improve the prediction accuracies as compared to the μa spectra alone. The diffuse reflectance spectra of the detection fibres did not provide a significantly better prediction performance for SSC, but gave slightly better firmness prediction (R2-firmness=0.73–0.83; RMSEP(firmness)=8.91–13.70N) than the μa spectra.

Optical fibre probes in the measurement of scattered light: Application for sensing turbidity

Optical fibre probes or optrodes often form the heart of multimode fibre-based measurements and sensors. An optrode usually comprises a bundle of multimode fibres, out of which one or more fibres are used for irradiating the sample, and the remaining fibres are used to collect the light reflected/scattered/fluoresced from the sample containing the measurand(s). The so-collected light carries the characteristic signature of the measurand. Here we present our work on the design and realization of optrodes for the measurement of scattered light from liquid samples. Optical properties of a solution are usually characterized by the parameters absorption coefficient μa, scattering coefficient μs, and anisotropy factor g. We have developed a simple method to determine μa, μs, and g, of a turbid medium, and a Monte–Carlo model was used to simulate the light scattering from the turbid medium. As an application, we describe the development of a turbidity sensor that has been designed and realized by employing an optrode in conjunction with a concave mirror. The estimation of turbidity is done on the basis of total interaction, by considering scattering and absorption of light from the sample solution. Details of the experiments and results are presented here.

Using a portable terahertz spectrometer to measure the optical properties ofin vivohuman skin

Terahertz (THz) time-domain spectroscopy systems permit the measurement of a tissue's hydration level. This feature makes THz spectrometers excellent tools for the noninvasive assessment of skin; however, current systems are large, heavy and not ideal for clinical settings. We previously demonstrated that a portable, compact THz spectrometer permitted measurement of porcine skin optical properties that were comparable to those collected with conventional systems. In order to move toward human use of this system, the goal for this study was to measure the absorption coefficient (μa) and index of refraction (n) of human subjects in vivo. Spectra were collected from 0.1 to 2 THz, and measurements were made from skin at three sites: the palm, ventral and dorsal forearm. Additionally, we used a multiprobe adapter system to measure each subject's skin hydration levels, transepidermal water loss, and melanin concentration. Our results suggest that the measured optical properties varied considerably for skin tissues that exhibited dissimilar hydration levels. These data provide a framework for using compact THz spectrometers for clinical applications.

Experimental and analytical comparative study of optical coefficient of fresh and frozen rat tissues

Optical properties of fresh and frozen tissues of rat heart, kidney, brain, liver, and muscle were measured in the 450- to 700-nm range. The total reflectance and transmittance were measured using a well-calibrated integral sphere set-up. Absorption coefficient μa and reduced scattering coefficient μ's were derived from the experimental measurements using the inverse adding doubling technique. The influence of cryogenic processing on optical properties was studied. Interindividual and intraindividual variations were assessed. These new data aim at filling the lack of validated optical properties in the visible range especially in the blue-green region of particular interest for fluorescence and optogenetics preclinical studies. Furthermore, we provide a unique comparison of the optical properties of different organs obtained using the same measurement set-up for fresh and frozen tissues as well as an estimate of the intraindividual and interindividual variability.