Masatsugu Niwayama

Quantitative measurement of muscle hemoglobin oxygenation using near-infrared spectroscopy with correction for the influence of a subcutaneous fat layer

The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by reflectance near-infrared spectroscopy. In this study, we investigated the influence of a fat layer on muscle oxygenation measurement by in vivo tests and Monte Carlo simulation, and we propose a method for correcting the influence. In the simulation, a three-dimensional model consisting of the epidermis, dermis, fat, and muscle layers was used. In in vivo tests, measurement sensitivity was examined by measuring oxygen consumption of the forearm muscle and the peak-to-peak variation of oxygenation in periodic exercise tests on the vastus lateralis using a newly developed multisensor type of tissue oximeter. Fat layer thickness was also measured by ultrasonography. The correction curve of measurement sensitivity against fat layer thickness was obtained from the results of simulation and in vivo tests. The values of corrected oxygen consumption were almost the same and had less variation between indivi...

Accurate NIRS measurement of muscle oxygenation by correcting the influence of a subcutaneous fat layer

Although the inhomogeneity of tissue structure affects the sensitivity of tissue oxygenation measurement by reflectance near-infrared spectroscopy, few analyses of this effect have been reported. In this study, the influence of a subcutaneous fat layer on muscle oxygenation measurement was investigated by Monte Carlo simulation and experimental studies. In the experiments, measurement sensitivity was examined by measuring the falling rate of oxygenation in occlusion tests on the forearm using a tissue oxygen monitor. The fat layer thickness was measured by ultrasonography. Results of the simulation and occlusion tests clearly showed that the presence of a fat layer greatly decreases the measurement sensitivity and increases the light intensity at the detector. The correction factors of sensitivity were obtained from this relationship and were successfully validated by experiments on 12 subjects whose fat layer thickness ranged from 3.5 to 8 mm.

Influence of a fat on muscle oxygenation measurement using near-IR spectroscopy: quantitative analysis based on two-layered phantom experiments and Monte Carlo simulation.

The influence of a subcutaneous fat layer on measurement of muscle oxygenation using near-IR spectroscopy was quantitatively investigated by two-layered phantom experiments and Monte Carlo simulations, with the aim of developing an algorithm that can correct this influence. The phantom consisted of a fat-like layer, which was a mixture of agar and titanium dioxide powder, and a muscle-like layer, which was suspension of washed bovine blood in Intralipid solution. An LED with 760 and 840 nm elements was used as an optical source, and the backscattered light was detected by photodiodes at source-detector distances of 20, 30 and 40 mm. The relationships between changes in optical density and blood concentrations were obtained at fat-like layer thicknesses of 0, 5, 10 and 15 mm under fully oxygenated and fully deoxygenated states. It was experimentally found that the change in optical density is significantly decreased and the linearity of measurement characteristics is clearly distorted by the presence of a fat layer. In the simulations, normalized light reflectance and mean optical pathlength in a muscle layer were calculated. The simulation results of the light reflectance agreed well with the experimental results. When the absorption in a muscle layer was relatively high, the mean optical pathlength in the muscle layer, or the measurement sensitivity, was not so dependent on the absorption. Therefore, the modified Beer-Lambert law can still be applied to estimate changes in muscle absorption from changes in optical density, even when a fat layer is involved. The results of simulation also suggested that the influence of a fat layer can be eliminated by correcting the measurement sensitivity using the fat layer thickness.

Quantitative measurement of muscle oxygenation by NIRS: analysis of the influences of a subcutaneous fat layer and skin

The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by reflectance NIRS. We have examined the influence of a subcutaneous fat layer on muscle oxygenation measurements. In this study, the influences of a fat layer and skin on muscle oxygenation measurement were investigated using Monte Carlo simulation and in vivo tests. Based on the experimental results, a correction curve for measurement sensitivity was determined. In the simulation, a 3-D model consisting of the epidermis, dermis, fat and muscle layers was used. In in vivo tests, measurement sensitivity was examined by measuring the falling rate of oxygenation in ischemia tests on the forearm using a newly developed multisensor type of oximeter with source-detector distances of 3-40 mm. Fat layer thickness was also measured by ultrasonography. The correction curve of measurement sensitivity against fat layer thickness was obtained from the results of simulation and in vivo tests. The measurements of oxygen consumption, calculated from the falling rates of oxygenation without correction, varied widely due to different thicknesses of fat layers. In contrast, the measurements of oxygen consumption with correction were almost the same (0.21 ±0.03 ml 100g-1 min-1). In this correction, the effect of skin on change in optical density was also taken into account using a detector with a short separation.

Quantitative muscle oxygenation measurement using NIRS with correction for the influence of a fat layer: comparison of oxygen consumption rates with measurements by other techniques

The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by near-IR spectroscopy (NIRS). We have proposed a method for correcting the influence of a subcutaneous fat layer on muscle oxygenation measurements. In this study, we validated our correction method by measuring oxygen consumption rates of the forearm muscle and comparing the measurements with those obtained by other techniques. 31P-magnetic resonance spectroscopy and positron emission tomography (PET). In NIRS, Vo2mus was obtained from the falling rate of oxygenation in ischaemia tests. The values of Vo2mos were corrected using a curve of measurement sensitivity against fat layer thicknesses, which were measured by ultrasonography. The corrected Vo2mus showed greater values and less variation between individuals than did the uncorrected one. In the 31P-NMR measurements on 10 subjects, Vo2mus was estimated from changes in phosphocreatine. The corrected Vo2mus in NIRS correlated well with the measurements by 31P-NMR compared to the uncorrected Vo2mus. This result suggested that our correction method is valid. Vo2mus was also measured using PET in one of the authors. The measured values by NIRS. 31P-NMR and PET were 0.22, 0.17, 0.24 ml 100g-1 min-1, respectively. The measurement by NIRS using our correction method was in an acceptable range.

Correction of the influences of a subcutaneous fat layer and skin in a near-infrared muscle oximeter

The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by reflectance NIRS. We have proposed a method for correcting the influence of a subcutaneous fat layer on muscle oxygenation measurement. The correction is based on an inverse relationship between the measurement sensitivity and detected light intensity. In this study, this method was validated by measuring the peak-to-peak variation of muscle oxygenation in periodic exercise tests on the vastus lateralis. A multisensor probe consisting of a light source and four photodiodes with source-detector distances of 7-40 mm was newly developed. A proximal detector with a 7-mm separation was used to eliminate the effect of skin. The fat layer thickness was also measured by ultrasonography. Results of the tests clearly showed that the presence of a fat layer greatly decreases the sensitivity of measurement and increases the light intensity at the detectors. Sensitivity correction by detected light intensity resulted in almost the same changes in muscle oxygenation due to periodic exercise among subjects regardless of different fat layer thicknesses. The proximal detector was also effective for reducing the effect of skin.

Near-infrared muscle oximeter that can correct the influence of a subcutaneous fat layer

The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by reflectance NIRS. We have proposed a method for correcting the influence of a subcutaneous fat layer on muscle oxygenation measurement. In this study, this method was validated by measuring the peak-to-peak variation of muscle oxygenation in periodic exercise tests on the vastus lateralis and the falling rate of oxygenation in ischemia tests on the forearm. A newly developed multisensor probe with source- detector distances of 7-40 mm was used. THe probe, consisting of a two-wavelength LED and four photodiodes, was connected to a 4-channel tissue oxygen monitor. The fat layer thickness was also measured by ultrasonography. Results of the tests clearly showed that the presence of a fat layer greatly decreases the sensitivity of measurement and increases the light intensity at a detector. The correction factors of sensitivity were determined from this relationship and Monte Carlo simulation. The corrected oxygenation levels were quantitatively compared among subjects in spite of different fat layer thicknesses.

Two-layered phantom experiments for characterizing the influence of a fat layer on measurement of muscle oxygenation using NIRS

Two-layered phantom experiments were performed to examine the influence of a fat layer on measurement of muscle oxygenation using near-IR spectroscopy (NIRS). The phantom consisted of a fat-like layer and a muscle-like layer which were a mixture of agar and TiO2 powder and a suspension of washed bovine blood into 0.55 percent intralipid solution. An LED including 760 and 840 nm elements was used as the optical source, and the reflectance light was detected by photodiodes at source-detector distances of 20, 30 and 40 mm. Curves of optical density changes versus blood volume ratio were obtained with fat-like layer thickness of 0, 5, 10 and 15 mm. It was found that the change in optical density is significantly decreased and that the linearity of measurement characteristics clearly deteriorated by the presence of a fat layer. This strongly suggests that a new algorithm is needed for muscle oxygenation measurement to eliminate the influence of a fat layer. In addition to the phantom experiments, Monte Carlo simulations corresponding to the experiments were performed. Although the simulations showed similar results concerning the influence of a fat layer, it was noted that the changes in optical density obtained from simulations were lower than those of the phantom experiments. This discrepancy was though to be due to the light scattering caused by blood cells.

Functional imaging of muscle oxygenation using a 200-channel cw NIRS system

Functional imaging of muscle oxygenation using NIRS is a promising technique for evaluation of the heterogeneity of muscle function and diagnosis of peripheral vascular disease or muscle injury. We have developed a 200-channel imaging system that can measure the changes in oxygenation and blood volume of muscles and that covers wider area than previously reported systems. Our system consisted of 40 probes, a multiplexer for switching signals to and from the probes, and a personal computer for obtaining images. In each probe, one two-wavelength LED (770 and 830 nm) and five photodiodes were mounted on a flexible substrate. In order to eliminate the influence of a subcutaneous fat layer, a correction method, which we previously developed, was also used in imaging. Thus, quantitative changes in concentrations of oxy- and deoxy-hemoglobin were obtained. Temporal resolution was 1.5 s and spatial resolution was about 20 mm, depending on probe separations. Exercise tests (isometric contraction of 50% MVC) on the thigh with and without arterial occlusion were conducted, and changes in muscle oxygenation were imaged using the developed system. Results showed that the heterogeneity of deoxygenation and reoxygenation during exercise and recovery periods, respectively, were clearly observed. These results suggest that optical imaging of dynamic change in muscle oxygenation using NIRS would be useful not only for basic physiological studies but also for clinical applications with respect to muscle functions.

Determination of a quantitative algorithm for the measurement of muscle oxygenation using cw near-infrared spectroscopy: mean optical pathlength without the influence of the adipose tissue

Near-infrared spectroscopy (NIRS) is a useful technique for noninvasive measurement of oxygenation of the brain and muscle. However, no accurate, quantitative algorithms for continuous wave NIRS (CW-NIRS) have yet been presented due to the following two problems. The first is that inhomogeneous tissue structure greatly affects measurement sensitivity. We previously reported on the influence of a fat layer on muscle oxygenation measurement and proposed a method for correcting the sensitivity. The second problem is that almost all algorithms for CW-NIRS have been experimentally determined, although al algorithm can be theoretically determined on the basis of diffusion theory if the mean optical pathlength in muscle in an in vivo state is known. In this study, we derived basic equations for a CW-NIRS algorithm based on diffusion theory, and we determined linear and nonlinear algorithms from mean optical pathlengths and validated them by results obtained from phantom experiments. For the determination of pathlength, the absorption and scattering coefficients of the muscle must be obtained by taking into account the influence of the fat layer. Laser pulses at 752 and 871 nm were applied to the forearms of the subjects, and the temporal point spread function (TPSF) was obtained by using a streak camera. The absorption and scattering coefficients of the muscle were determined by fitting the measured TPSF with that obtained by a Monte Carlo model consistingof skin, fat and muscle layers. From these coefficients, the mean optical pathlengths were obtained and the algorithms were determined.