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Phase Velocity Limit Of High-Frequency Photon Density Waves, Richard C. Haskell, Lars O. Svaasand, Steen J. Madsen, Fabio E. Rojas, Ti-Chen C. Feng, Bruce J. Tromberg

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In frequency-domain photonmigration (FDPM), two factors make high modulation frequencies desirable. First, with frequencies as high as a few GHz, the phase lag versus frequency plot has sufficient curvature to yield both the scattering and absorption coefficients of the tissue under examination. Second, because of increased attenuation, highfrequency photon density waves probe smaller volumes, an asset in small volume in vivo or in vitro studies. This trend toward higher modulation frequencies has led us to reexamine the derivation of the standard diffusion equation (SDE)from the Boltzman transport equation. We find that a second-order time-derivative term, ordinarily neglected in the derivation, …