Depth sensitivity
Definition: In the context of fNIRS, which typically uses an array of measurement optodes on the scalp to measure the brain activity, depth sensitivity refers to its ability to detect localized changes in deep tissue optical properties. ‘Depth’ is typically measured normal to the surface of the skin. Sensitivity to depth depends on multiple factors in the instrumentation (e.g., illumination strength, detector efficiency, source-detector separation, data type), and tissue optical properties (e.g. chromophore concentration). It is crucial to note here that there is no singular depth being probed by fNIRS but rather it has a continuous distribution over the probed tissue volume which can change with time and with optical properties. Typically the longer the distance between the source and detector across the surface of the measurement body (scalp, phantom etc.), the deeper the depth sensitivity.
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References: https://doi.org/10.1088/0034-4885/73/7/076701https://doi.org/10.1364/ao.36.000075
https://doi.org/10.1117/1.JBO.26.5.056003
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