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

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Abstract
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.

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