Activity of the human visual cortex measured non-invasively by diffusing-wave spectroscopy.

Activity of the human visual cortex, elicited by steady-state flickering at 8Hz, is non-invasively probed by multi-speckle diffusingwave spectroscopy (DWS). Parallel detection of the intensity fluctuations of statistically equivalent, but independent speckles allows to resolve stimulation-induced changes in the field autocorrelation of multiply scattered light of less than 2%. In a group of 9 healthy subjects we find a faster decay of the field autocorrelation function during the stimulation periods for data measured with a long-distance probe (30mm source-receiver distance) at 2 positions over the occipital cortex (t-test: t(8) = -2.672, p = 0.028 < 0.05 for position 1, t(8) = -2.874, p = 0.021 < 0.05 for position 2). In contrast, no statistically significant change is seen when a short-distance probe (16mm source-receiver distance) is used (t-test: t(8) = -2.043, p = 0.075 > 0.05 for position 1, t(8) = -2.146, p = 0.064 > 0.05 for position 2). The enhanced dynamics observed with DWS is positively correlated with the functional increase of blood volume in the visual cortex, while the heartbeat rate is not affected by stimulation. Our results indicate that the DWS signal from the visual cortex is governed by the regional cerebral blood flow velocity.

Pulsation-resolved deep tissue dynamics measured with diffusing-wave spectroscopy.

We present a technique for measuring transient microscopic dynamics within deep tissue with sub-second temporal resolution, using diffusing-wave spectroscopy with gated single-photon avalanche photodiodes (APDs) combined with standard ungated multi-tau correlators. Using the temporal autocorrelation function of a reference signal allows to correct the temporal intensity autocorrelation function of the sample signal for the distortions induced by the non-constant average photon count rate. We apply this technique to pulsation-synchronized measurements of tissue dynamics in humans. Measurements on the forearm show no dependence on the pulsation phase. In contrast, the decay rate of the DWS signal measured on the wrist over the radial artery shows a pulsation-induced modulation of 60-90% consistent with pulsatile variations of arterial erythrocyte flow velocity. This might make time-resolved DWS interesting as a sensitive and fast method for investigating deep tissue perfusion, e.g. in intensive care.

Diffusing-wave spectroscopy from head-like tissue phantoms: influence of a non-scattering layer.

We investigate the influence of a non-scattering layer on the temporal field autocorrelation function of multiple scattered light from a multilayer turbid medium such as the human head. Data from Monte Carlo simulations show very good agreement with the predictions of the correlation-diffusion equation with boundary conditions taking into account non-diffusive light transport within the non-scattering layer. Field autocorrelation functions measured at the surface of a multilayer phantom including a non-scattering layer agree well with theory and simulations when the source-receiver distance is significantly larger than the depth and the thickness of the non-scattering layer. Our results show that for source-receiver distances large enough to probe the dynamics in the human cortex, the cortical diffusion coefficient obtained by analyzing field autocorrelation functions neglecting the presence of the non-scattering cerebrospinal fluid layer is underestimated by about~$40\,\%$ in situations representative of the human head.

Noninvasive detection of functional brain activity with near-infrared diffusing-wave spectroscopy.

We use near-infrared dynamic multiple scattering of light [diffusing-wave spectroscopy (DWS)] to detect the activation of the somato-motor cortex in 11 right-handed volunteers performing a finger opposition task separately with their right and left hands. Temporal autocorrelation functions g(1)(r,tau) of the scattered light field are measured during 100-s periods of motor task alternating with 100-s resting baseline periods. From an analysis of the experimental data with an analytical theory for g(1)(r,tau) from a three-layer geometry with optical and dynamical heterogeneity representing scalp, skull, and cortex, we obtain quantitative estimates of the diffusion coefficient in cortical regions. Consistent with earlier results, the measured cortical diffusion coefficient is found to be increased during the motor task, with a strong contralateral and a weaker ipsilateral increase consistent with the known brain hemispheric asymmetry for right-handed subjects. Our results support the interpretation of the increase of the cortical diffusion coefficient during finger opposition being due to the functional increase in cortical blood flow rate related to vasodilation.