Comparison of the spatial and temporal information retrieved from near-infrared phase and average intensity (DC) data reveals that these data types can play a complementary role in the study of the temporal and spatial features of the optical response associated with brain activation during a finger-tapping test. The optical data have been collected with a frequency-domain tissue imager at two wavelengths (690 and 830 nm) and have been analyzed using standard filtering and folding-average procedures. DC and phase data show different temporal and spatial features. A plausible explanation of the different behavior of DC and phase data has been attempted by using Monte Carlo simulations.
We report non-invasive, bilateral measurements of cerebral oxygenation performed with near-infrared spectroscopy (NIRS) on ten patients undergoing right unilateral electro-convulsive therapy (ECT). Right unilateral ECT consists of delivering an electrical current through the right brain hemisphere to induce a seizure, which is associated with significant changes in systemic and regional physiological parameters. In this work, we have examined the regional cerebral oxygenation (StO2) measured with NIRS on the right and left sides of the frontal brain region, and the systemic arterial oxygenation (SaO2) measured with pulse oximetry. On the ten patients examined, we have found that the decrease in the cerebral oxygenation on the side ipsilateral to the ECT electrical discharge (ΔStO2(ipsi)) is consistently stronger than the decrease on the contralateral side (ΔStO2(contra)). The average and standard deviation for the ipsilateral and contralateral oxygenation changes across the ten patients are ΔStO2(ipsi) = -22 ± 10% and ΔStO2(contra) = -6 ± 10%, respectively. By contrast, we observed two distinct behaviors in the arterial saturation; in five patients, SaO2 did not significantly change during the ECT procedure, and in three patients, SaO2 decreased by -16± 6%, an intermediate value with respect to the changes observed in StO2(ipsi) and StO2(contra) (we do not have the SaO2 recording in the remaining two patients for technical reasons). These results indicate that NIRS monitoring of the cerebral oxygenation during ECT has the potential of being a valuable addition to the standard monitoring of arterial saturation with pulse oximetry.
We have measured relative diffuse reflectance images on tissue-like liquid phantoms using a laser diode (784 nm, 40 mW) as a broad-beam source and a CCD camera detector. We have found that, to a good approximation, the result that the reflectance R only depends on the ratio of the absorption and reduced scattering coefficients of the medium (μa/μs') can be extended to geometrical configurations that are more general than those considered by theoretical studies. In fact, over the range 0.0028<μa/μs'<0.058 considered by us, the dependence of R on μa/μs' varies by no more than 8% by changing the incidence angle of illumination over the range 0-30°. The relative diffuse reflectance vs. μa/μs was well described by introducing an arbitrary normalization factor K in the analytic relationship for the absolute reflectance reported by S.L. Jacques [http://omlc.ogi.edu/news/may99/rd/index.html]: R=Kexp{-7.8/[3(1+μs'/μa)]}. The ratio of the relative diffuse reflectance values at two locations (for spatial changes) or at two times (for temporal changes) cancels out the unknown normalization factor K, so that this ratio can be used to obtain quantitative estimates of the corresponding change in μa/μs'. We have experimentally estimated that in our experimental conditions R is sensitive to a superficial volume about 2 mm deep and 4 mm wide.
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