Background: A requisite for fNIRS studies of cortical blood flow is that sufficient photons are transmitted transcutaneously for the fluctuations in cerebral hemoglobin oxygenation that occur during neuronal activation to be detected. Transmission is determined by the specifications of the fNIRS device, but also influenced by the characteristics of the skin. Epidermal pigments can attenuate photon transmission; the literature states that in dark skinned subjects some NIRS devices may not achieve sufficient photon migration to monitor cortical blood flow. Hence, as fNIRS use is spreading, we describe a simple head tilt maneuver where positional redistribution of cerebral blood volume will confirm if photon transmission is sufficient. Methods: A repetitive head tilt maneuver (bending forward from a seated position, hold for 30 seconds, returning to original position X 5) performed by a pigmented (African) subject and a non-pigmented (Caucasian) subject. A 23- channel portable fNIRS system with dual wavelength (750 and 860 nm) emitters and photodiode detectors was worn over the anterior cortex, and changes in oxy, deoxy and total hemoglobin concentration measured at 50 Hz. Results: Data from both subjects were compared and found to have a comparable pattern of change in oxyhemoglobin concentration and temporal response to the effects of head tilt; clear arterial pulsations and minimal noise were also evident. Conclusion: We suggest the head tilt maneuver described as a feasible test to confirm the adequacy of transcutaneous photon transmission where fNIRS studies are to be performed in subjects with pigmented skin to detect hemodynamic change in the cortex.
Background: Prior research indicates the epidermal pigment layer of human skin (Melanin) has a significant absorption coefficient in the near infra-red (NIR) region; hence attenuation of light in vivo is a potential confounder for NIR spectroscopy (NIRS). A NIRS method developed for transcutaneous evaluation of bladder function is being investigated as a means of improving the burden of bladder disease in sub-Saharan Africa. This required development of a simple wireless NIRS device suitable for use as a screening tool in patients with pigmented skin where the NIR light emitted would penetrate through the epidermal pigment layer and return in sufficient quantity to provide effective monitoring.
Methods: Two healthy subjects, one with pigmented skin and one with fair skin, were monitored as they voided spontaneously using the prototype transcutaneous NIRS device positioned over the bladder. The device was a self-contained wireless unit with light emitting diodes (wavelengths 760 and 850 nanometres) and interoptode distance of 4cm. The raw optical data were transmitted to a laptop where graphs of chromophore change were generated with proprietary software and compared between the subjects and with prior data from asymptomatic subjects.
Results: Serial monitoring was successful in both subjects. Voiding volumes varied between 350 and 380 cc. In each subject the patterns of chromophore change, trend and magnitude of change were similar and matched the physiologic increase in total and oxygenated hemoglobin recognized to occur in normal bladder contraction during voiding.
Conclusions: Skin pigmentation does not compromise the ability of transcutaneous NIRS to interrogate physiologic change in the bladder during bladder contraction in healthy subjects.
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