The inhomogeneity of tissue structure because of individual differences greatly affects the sensitivity of tissue oxygenation saturation measurement by reflectance NIRS. In this study, a continue wave NIRS system was developed, which had parallel arranging one source and multi-detectors. An LED incorporated three elements of 735nm, 805nm, 850nm peak wavelengths was used for the source. There were three detectors and a source, and the detector-detector separations were 5mm, and the source-detector separations could be changed. Ischemia tests on the forearm were performed on six male and six female subjects (22-27 years of age and 45-75 kg in weigh). 80mmHg was vein ischemia test and 120mmHg press was for whole ischemia test. Meanwhile, the fat layer thickness was also measured. According to the fat thickness of the forearm, the tests are divided to three groups, the fat group, the standard group and the thin group. The results showed that the measurements of muscle oxygenation depended widely on the source-detector separations and the individual. There are less relative errors of muscle oxygenation among one of the groups than those among all of the groups. The measurements were different due to different thickness of fat layers under the same source-detector separations. The thicker the fat layer, the more obvious the muscle oxygenation changes for different tests under the same source-detector separation. Therefore, the optimal source-detector separation must consider the fat thickness for measurements of muscle oxygenation by NIRS.
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