Mapping of Hemodynamics on the Human Calf with Near Infrared Spectroscopy and the Influence of the Adipose Tissue Thickness

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Abstract
We investigated the influence of the adipose tissue thickness (ATT) on near-infrared spectroscopy (NIRS) measurements of the absorption coefficient (µa), the reduced scattering coefficient (µs’) and changes in concentrations of oxyhemoglobin ([O2Hb]) and deoxyhemoglobin ([HHb]). We used a frequency domain spectrometer and a special probe to generate maps of these parameters on the human calf during venous occlusion. For ATT below 6 mm to remained constant, whereas for ATT between 6 and 14 mm µa decreased quickly and became almost constant again for ATT larger than 14 mm. µs’ was not significantly altered by the ATT but the values showed a high variability between subjects. We found significantly different changes in both the [O2Hb] and the [HHb] between the proximal and distal locations of measurement. Although ATT influences the recovery of the optical properties of the underlying tissue, these differences depending on the location cannot be sufficiently explained by the ATT for the following reasons. The ATT varied little within one subject (mean difference 0.88 t 1.80 mm) The inter-subject variability was 5 times higher. For a given ATT within one subject we observed different values for changes in [O2Hb] and [HHb] depending on the measurement location. Moreover for a smaller ATT the difference between the values of A[O2Hb] and A[HHb] proximal versus distal were more distinct. The thinner the overlying tissue (ATT) the higher is the proportion of muscle tissue in the probed tissue volume. Therefore these differences are most likely coming from the muscle tissue rather than the ATT. This indicates that although the ATT has an evident influence on the measurement of opticalparameters and hemodynamics and should therefore be recognized when performing NIRS measurements, other factors will have to be considered as well.