Variability of human brain and muscle optical pathlength in different experimental conditions

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Abstract
Pathlength can be evaluated by measuring the time taken from a picosecond (psec) near infrared (IR) laser pulse to cross tissue. Differential pathlength factor (DPF) is calculated by dividing the mean pathlength by the inter-fiber distance. Data on DPF variability on humans are scarce. We investigated the forehead and forearm DPF in resting conditions and dynamically during brain hypoxic hypoxia, muscle ischemia and voluntary isometric exercise. At 3 cm inter optode spacing DPF at 800 nm was 4.3 +/- 0.2 (n equals 14, mean +/- SD) on the forearm, and 6.5 +/- 0.5 (n equals 8) on the forehead. Brain, muscle, and breast DPF values were almost constant over the inter optode spacing 2.5 - 4 cm. DPF was roughly constant in the central region of forehead. DPF drastically decreased under the fronto- temporal junction for the presence of muscle in the optical field. DPF decreased 5 - 10% during forearm ischemia with and without maximal voluntary contraction and during brain hypoxic hypoxia.

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