Abstract
We study the fluctuations in the photon-density wave parameters [average intensity (dc), modulation amplitude, and phase] 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 diameter) in turbulent motion. The photon-density waves are induced by the laser diode output (750 nm), 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.
Highlights
In recent years the correlation spectroscopy technique was used to study optically thick media that exhibit a high degree of multiple scattering1–4͔
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. ͓S1063-651X98͒01708-5͔
Summary
In recent years the correlation spectroscopy technique was used to study optically thick media that exhibit a high degree of multiple scattering1–4͔. This technique, called diffusing wave spectroscopyDWS, relates scattered light fluctuations to the motion of scattering particles. In addition to the small-scale fluctuations due to the motion of the microscopical scatterers, biological activity in tissues may produce large-scale spatial and temporal fluctuations of tissue optical properties in the nearinfrared band. It was noted that near-infrared tissue spectroscopy has an advantage over other techniques for studying neuronal processes in that it potentially combines good temporal resolution mswith a spatial resolution of the order of 5 mm5,7͔. The analysis of large-scale fluctuations in near-infrared photon migration may provide a useful tool for the study of functional processes
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