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

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Abstract
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.

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