Photon-density wave correlation spectroscopy detects large-scale fluctuations in turbid media

Abstract

We study the fluctuations in the photon-density wave parameters ͓average intensity ͑dc͒, modulation amplitude, and phase͔ caused by macroscopic fluctuations in the optical properties of turbid media. We present both a theoretical analysis based on diffusion theory and its experimental verification on a strongly scattering suspension containing absorbing particles ͑1-1.6 mm effective diameter͒ in turbulent motion. The photondensity waves are induced by the laser diode output ͑750 nm͒, which is intensity-modulated at 110 MHz. The dc, amplitude, and phase are acquired with an acquisition time per data point of 8 ms, which corresponds to a frequency bandwidth of 62.5 Hz. We have found that in the presence of the absorbing particles, the dc and phase average values and power spectra are in good agreement with our theoretical predictions. We have verified that our instrument can extend the measured frequency band up to the kHz region, which is appropriate for the study of fluctuations of optical parameters in biological tissues. ͓S1063-651X͑98͒01708-5͔

Toward Noninvasive 3-D Imaging of the Time Course of Cortical Activity: Investigation of the Depth of the Event-Related Optical Signal

“…The results confirmed the expectations derived from theoretical computations. By using a number of sourcedetector pairs, data such as these could be used for precise localization of optical signals (see Toronov et al, 1998, for an application of this principle to the study of the movement of absorbing objects within scattering media), with a maximum theoretical accuracy of a few millimeters. However, the situation may be more complex in dishomogeneous media, like the head.…”