A new measurand is proposed for use in continuous wave near-infrared spectroscopy (cw-NIRS). The conventional measurand of cw-NIRS is lc, which is the product of the change in the hemoglobin concentration (c) and the partial path lengh (l), which depends on the source-detector (SD) distance (d). The SD distance must remain constant during cw-NIRS measurements, and we cannot compare the lc value with that obtained using a different SD distance. In addition, the conventional measurand obtained using the standard measurement style sometimes includes a contribution from the human scalp. The SD distance independent (SID) measurand obtained using multi-SD distances is proportional to the product of the change in hemoglobin concentration and the derivative of the partial path length for the deep region with no scalp contribution under the assumption of a layer model. The principle of SID was validated by the layered phantom study. In order to check the limitation of assumption, a human study was conducted. The value of the SID measurand for the left side of the forehead during working memory task was approximately independent of the SD distance between 16 and 32 mm. The SID measurand and the standardized optode arrangement using flexible SD distances in a head coordinate system must be helpful for comparing the data in a population study.
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