Characterization of absorption and scattering properties of small-volume biological samples using time-resolved spectroscopy

Influence of intra-abdominal pressure on the amplitude of fluctuations of cerebral hemoglobin concentration in the respiratory band

An intra-abdominal pressure (IAP) is correlated with cerebral perfusion, in a mechanism of reducing venous outflow. The elevated intra-abdominal pressure leads to an increase in the intracranial pressure and a decrease in the cerebral perfusion pressure. We studied the relationship between the IAP and the cerebral oxygenation with the use of the near infrared spectroscopy technique during a gynecological surgery. The changes in hemoglobin concentrations were analyzed in the time-frequency domain in the frequency band related to respiration. The measurements were carried out in 15 subjects who underwent laparoscopic surgery. During the laparoscopy, the intra-abdominal cavity was insufflated with CO₂, which caused a controlled increase in the IAP. It was observed that the amplitudes of respiration-related waves present in hemoglobin concentration signals show an increase of 1.5 to 8.5 times during elevation of the IAP by 15 mmHg.

Globally accelerated reconstruction algorithm for diffusion tomography with continuous-wave source in an arbitrary convex shape domain

A new numerical imaging algorithm is presented for reconstruction of optical absorption coefficients from near-infrared light data with a continuous-wave source. As a continuation of our earlier efforts in developing a series of methods called "globally convergent reconstruction methods" [J. Opt. Soc. Am. A23, 2388 (2006)], this numerical algorithm solves the inverse problem through solution of a boundary-value problem for a Volterra-type integral partial differential equation. We deal here with the particular issues in solving the inverse problems in an arbitrary convex shape domain. It is demonstrated in numerical studies that this reconstruction technique is highly efficient and stable with respect to the complex distribution of actual unknown absorption coefficients. The method is particularly useful for reconstruction from a large data set obtained from a tissue or organ of particular shape, such as the prostate. Numerical reconstructions of a simulated prostate-shaped phantom with three different settings of absorption-inclusions are presented.

Differential time course and intensity of PFC activation for men and women in response to emotional stimuli: a functional near-infrared spectroscopy (fNIRS) study

Using functional near-infrared spectroscopy (fNIRS) we recorded prefrontal cortex (PFC) activation during positive, negative and neutral film clips, based on affective ratings according to their valence and arousal, to assess gender differences in cerebral activation in 15 male and 15 female volunteers. To record PFC activation, five movie clips were presented on a 17-in. TFT screen. The recordings included a pre-stimulus 5-s local baseline and "on" and "off" segments of data, referring to fNIRS Oxy-Hb levels while stimulation (movie clip) was present and during an inter-stimulus blank screen. Our data showed gender differences in the delay period to initial PFC activation and in the course and intensity of activation produced by affective visual stimuli. During the exposure or "on" period of the stimuli we observed more pronounced overshoot and undershoot in men versus women across the range of emotions elicited. This effect was even more pronounced following stimulus cessation ("off" period). The results indicate that gender and the duration of recordings may affect the results of emotional neuroimaging studies.

Optical property measurements of turbid media in a small-volume cuvette with frequency-domain photon migration

A frequency-domain photon migration (FDPM) technique is developed for quantitative measurement of the absorption and reduced scattering coefficients of highly turbid samples in a small-volume (0.45-ml) reflective cuvette. We present both an analytical model for the FDPM cuvette and its experimental verification, using calibrated phantoms and suspensions of living cells. FDPM model fits to experimental data demonstrate that the reduced scattering (mu(s)?) and absorption (mu(a)) coefficients can be derived with accuracies of 5-10% and 10-15%, respectively. Changing the cuvette wall reflectivity alters the frequency-dependent behavior of photon density waves (PDWs). For highly reflective wall boundaries (R(eff) >/= 90-95%), PDW confinement leads to substantial enhancement in both amplitude and phase compared with identical samples in infinite media. Results from experiments on microsphere suspensions are compared with predictions from Mie theory to assess the potential of this method to interpret scattering properties in terms of scatterer size and density. Optical property measurements of biological cell suspensions are reported, and the possibility of optically monitoring cell physiology in a carefully controlled environment is demonstrated.

Frequency-domain immersion technique for accurate optical property measurements of turbid media

We demonstrate that absorption coefficient micro(a) and reduced scattering coefficient micro(s)(?) of a small turbid object can be measured to high accuracy with a frequency-domain immersion technique. For this technique the sample is immersed in a calibrated scattering medium and the optical properties are obtained from a differential measurement. Compared with conventional approaches, the immersion technique improves accuracy, minimizes variations owing to probe coupling and motion, reduces the effects of boundary conditions, and offers simple and rapid measurement once the immersion medium is calibrated. Accuracy tests of immersion-based measurements of micro(a) and micro(s)(?) agree with reference values to within 3.6% and 2.6%, respectively. These tests are limited by the accuracy of the reference samples rather than by the accuracy of the immersion medium or the precision of the immersion approach. We demonstrate the in vivo capabilities of the technique through time-resolved measurements of micro(a) and micro(s)(?) for a human hand during cuff occlusion on the upper arm.

Optical analysis of cirrhotic liver by near infrared time-resolved spectroscopy

The severity of liver cirrhosis was related with the optical properties of liver tissue. Various grades of liver cirrhosis were produced in rats by intraperitoneal injection of thioacetamide (TAA) for different periods: 4 weeks, 8 weeks, 12 weeks, and 16 weeks. Optical properties of the liver, absorption coefficient (μa) and scattering coefficient (μs'), were measured by near-infrared time-resolved spectroscopy. Histological examination confirmed cirrhotic changes in the liver, which were more severe in rats with TAA administration for longer periods. The μa increased in 4- and 8-week rats, and then decreased in 12- and 16-week rats. The μa of blood-free liver decreased as liver cirrhosis progressed. The hemoglobin content in the liver calculated from the μa values increased in 4- and 8-week rats and decreased in 12- and 16-week rats. The μs' decreased in the cirrhotic liver, probably reflecting the decrease in the mitochondria content. It was shown that μa and μs' determination is useful to assess the severity of liver cirrhosis. © 1999 Society of Photo-Optical Instrumentation Engineers.

Application of near-infrared time-resolved spectroscopy to rat liver--a preliminary report for surgical application

The applicability of near-infrared time-resolved spectroscopy to rat liver surgery was investigated. First, the technical reliability in determining the absorption coefficient (mu(a)) and reduced scattering coefficient (mu'(s)) of the liver was checked. Next, boundary effects in determining mu(a) and mu'(s) of the rat liver were examined. Finally, changes in mu(a) and mu'(s) of rat liver with ischaemia were directly measured by TRS. Our TRS system showed that the mu(a) value held a linear correlation with the ink concentration in a lipid emulsion until mu(a) reached 1.2 cm(-1), while the mu'(s) was fairly independent. The mu(a) values of blood-free rat liver and blood-containing rat liver at 780 nm were observed to be 0.43 cm(-1) and 0.67 cm(-1) by using the matching method, indicating that TRS is reliable in determining mu(a) and mu'(s) of the liver. Possible errors in mu(a) and mu'(s) determination due to the boundary effects of the rat liver were as small as 7%, when the mu(a) value was as high as observed for the liver. The oxygen saturation of haemoglobin (SO2) was changed from 64.9% to 8.0%, and the haemoglobin content (THB) from 189.1 microM to 131.6 microM by ischaemia. Mu'(s) dynamically changed in the range 7.06 cm(-1) to 11.36 cm(-1). We conclude that time-resolved measurement is applicable in the high-mu(a) region observed in the liver, and can give quantitative estimations of SO2 and THB in the liver.

Compensated transillumination

We demonstrate imaging through human tissue in vivo , using a new optical technique, compensated transillumination. Immersion in a scattering medium with absorption and scattering coefficients matched to the tissue is used for drastic improvement of image contrast. The immersion medium is composed of polymer microspheres and methylene blue dye. The optical properties of the medium are matched to those of the tissue by use of a frequency-domain measurement technique. Images of a human hand taken with this technique show the internal structure, including the outlines of bones. The mechanism for the contrast is likely the absence of blood between the bones.

Gradient-based iterative image reconstruction scheme for time-resolved optical tomography

Currently available tomographic image reconstruction schemes for optical tomography (OT) are mostly based on the limiting assumptions of small perturbations and a priori knowledge of the optical properties of a reference medium. Furthermore, these algorithms usually require the inversion of large, full, ill-conditioned Jacobian matrixes. In this work a gradient-based iterative image reconstruction (GIIR) method is presented that promises to overcome current limitations. The code consists of three major parts: 1) A finite-difference, time-resolved, diffusion forward model is used to predict detector readings based on the spatial distribution of optical properties; 2) An objective function that describes the difference between predicted and measured data; 3) An updating method that uses the gradient of the objective function in a line minimization scheme to provide subsequent guesses of the spatial distribution of the optical properties for the forward model. The reconstruction of these properties is completed, once a minimum of this objective function is found. After a presentation of the mathematical background, two- and three-dimensional reconstruction of simple heterogeneous media as well as the clinically relevant example of ventricular bleeding in the brain are discussed. Numerical studies suggest that intraventricular hemorrhages can be detected using the GIIR technique, even in the presence of a heterogeneous background.

Quantitative optical spectroscopy for tissue diagnosis

The interaction of light within tissue has been used to recognize disease since the mid-1800s. The recent developments of small light sources, detectors, and fiber optic probes provide opportunities to quantitatively measure these interactions, which yield information for diagnosis at the biochemical, structural, or (patho)physiological level within intact tissues. However, because of the strong scattering properties of tissues, the reemitted optical signal is often influenced by changes in biochemistry (as detected by these spectroscopic approaches) and by physiological and pathophysiological changes in tissue scattering. One challenge of biomedical optics is to uncouple the signals influenced by biochemistry, which themselves provide specificity for identifying diseased states, from those influenced by tissue scattering, which are typically unspecific to a pathology. In this review, we describe optical interactions pursued for biomedical applications (fluorescence, fluorescence lifetime, phosphorescence, and Raman from cells, cultures, and tissues) and then provide a descriptive framework for light interaction based upon tissue absorption and scattering properties. Finally, we review important endogenous and exogenous biological chromophores and describe current work to employ these signals for detection and diagnosis of disease.

Optical determination of fatty change of the graft liver with near-infrared time-resolved spectroscopy

A novel method for quantifying the fatty change of the graft liver by characterizing the optical property of the tissue was introduced. A wide range of lipid content in the rat liver was obtained by using different feeding regimens, with lipotropic chow (choline/methionine deficient or low chow). The liver was removed and flushed with Krebs-Ringer buffer solution with 3% albumin, and the optical properties of the liver, i.e., absorption and reduced scattering coefficients (mu(a) and mu(s)'), were measured by time-resolved spectroscopy. The fatty liver showed lower mu(a) and higher mu(s)' than the normal liver. Lower mu(a) and lower succinate dehydrogenase activity of the fatty liver suggested that the decrease in mu(a) might indicate a decrease in the mitochondrial content. The value of mu(s)' was positively correlated with the lipid content of the liver, which indicates that fat droplets inside the hepatocyte act as dominant scatterers. To subtract the contribution of the mitochondrial compartment to mu(s)', the ratio of mu(s)' to mu(a) (mu(s)':mu(a)) was useful for the assessment of the lipid content of the liver. These findings were also relevant with prediction of light scattering by the Mie theory. It was concluded that mu(a) and mu(s)' of the graft liver, measured by time-resolved spectroscopy, can be useful parameters for quantifying the fatty change of the graft liver.

A simple and novel algorithm for time-resolved multiwavelength oximetry

Using time-resolved spectroscopy, we have developed an experimental approach to obtain the absolute changes in concentration in the scattering medium of living tissues. The time-resolved Beer-Lambert equation can be applied to living tissue due to the fact that the optical attenuation by absorption can be separated from that by scattering, and the intensity of the light along the non-linear scattered optical path is exponentially attenuated by the absorption. Based on the above, the absolute concentration of haemoglobin as well as oxygen saturation in the rat head can be determined in situ under various respiratory conditions where multiwavelength measurements were performed. The optically assessed values agree with those determined directly by the gas analysis of our previous report. The present method is very simple and therefore opens up wide applications for time-resolved spectrophotometry in clinical medicine as a technique for quantitative near-infrared oxygen monitoring.

Time-resolved photon emission from layered turbid media

We present numerical and experimental results of time-resolved emission profiles from various layered turbid media. Numerical solutions determined by time-resolved Monte Carlo simulations are compared with measurements on layered-tissue phantoms made from gelatin. In particular, we show that in certain cases the effects of the upper layers can be eliminated. As a practical example, these results are used to analyze in vivo measurements on the human head. This demonstrates the influence of skin, skull, and meninges on the determination of the blood oxygenation in the brain.

A dynamic phantom brain model for near-infrared spectroscopy

This report describes the construction, fluid dynamics and optical properties of an in vitro model of the neonatal brain for testing near-infrared spectroscopy (NIRS) instruments. The brain model is a solid plastic structure containing a vascular network perfused with blood equilibrated with O2, N2 and CO2 in a closed circuit. The oxygenation state and haemoglobin concentration of the perfusate can be regulated and measured with a co-oximeter, providing a means to compare NIRS measurements of oxy-, deoxy- and total haemoglobin concentrations and haemoglobin O2 directly with a validated standard method. Fluid dynamic experiments revealed that the model's vasculature remains stable over time with minimal haemolysis. The model's optical properties were characterized by time-resolved and continuous wave NIRS between 670 and 850 nm as perfusate saturation was varied in the range 0-100%. Optical properties of the neonatal piglet brain were also determined by similar methods. No significant differences were found between the model and piglet brain in absorption coefficients, reduced scattering coefficients and optical pathlengths, indicating that the model optically simulates the piglet brain over a wide range of oxygenation states. These results demonstrate the potential utility of this dynamic phantom brain for testing NIRS instruments for accuracy and reliability.

Contribution of the mitochondrial compartment to the optical properties of the rat liver: a theoretical and practical approach

The purpose of this work was to analyze the contribution of the mitochondria to the optical properties, i.e., light absorption and scattering, of the blood-free rat liver. Firstly, a theoretical model of the reduced scattering coefficient of the liver was performed by using the Mie theory, the Rayleigh-Debye-Gans approximation, and the electron microscopy descriptions of the liver ultrastructure. Compared with the hepatocyte volume, the nucleus and the peroxisomes, the mitochondria compartment, accounting for 22% of the liver cell volume, seemed to be the predominant factor for the light scattering of the liver. Second, by using time-resolved spectroscopy and a sample substitution method, we have measured the absorption and reduced scattering coefficients of blood-free perfused rat livers, isolated hepatocyte suspensions, and isolated mitochondria suspensions. A subsequent extrapolation of the isolated mitochondria data to the in vivo mitochondrial content and a comparison with the whole liver measurements lead to the following conclusions: 1) the mitochondria account for about 50% of the liver absorption coefficient at 780 nm (mu a = 0.25 cm-1 extrapolated from isolated mitochondria vs. 0.53 +/- 0.05 cm-1 measured for the liver); and 2) the mitochondrial compartment is the primary factor for the light scattering in the rat liver (mu s' = 15.5 cm-1 extrapolated from the isolated mitochondria versus 15.9 +/- 2.4 cm-1 measured for the liver), demonstrating the relevancy of our preliminary theoretical study.

Scattering of diffuse photon density waves by spherical inhomogeneities within turbid media: analytic solution and applications

We present an analytic solution for the scattering of diffuse photon density waves by spherical inhomogeneities within turbid media. The analytic result is compared to experimental measurements. Close agreement between theory and experiment permits the use of the theory to determine the properties of unknown sphere-like objects embedded in turbid media. The analytic solution is extended to encompass several problems of practical interest in imaging, including the influence of multiple sources, multiple objects, and boundaries on the characterization of spherical inhomogeneities. We also extend the solution to encompass time-domain measurements.

Characterization of absorption and scattering properties for various yeast strains by time-resolved spectroscopy

An understanding of the optical properties of biological media and cells is essential to the development of noninvasive optical studies of tissues. Unicellular organisms offer a unique opportunity to investigate the factors affecting light propagation, since they can be manipulated in ways impossible for more complex biological samples. In this study, we examined optical absorption and scattering properties of strongly multiple scattering yeast suspensions by means of near-infrared (NIR) time-resolved spectroscopy (TRS) and a sample substitution method. We determined the critical parameters for photon migration by varying the cell organelle content, the cell ploidy, the cell size, and the concentration of suspended cells. The results indicate that the photon absorption is insensitive to cell differentiation and that the cell volume is the primary factor determining light-scattering property.