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The estimation of optical properties of highly turbid and opaque biological tissue is a difficult task since conventional
purely optical methods rapidly loose sensitivity as the mean photon path length decreases. Photothermal
methods, such as pulsed or frequency domain photothermal radiometry (FD-PTR), on the other hand, show
remarkable sensitivity in experimental conditions that produce very feeble optical signals. Photothermal Radiometry
is primarily sensitive to absorption coefficient yielding considerably higher estimation errors on scattering
coefficients. Conversely, purely optical methods such as Local Diffuse Reflectance (LDR) depend mainly on
the scattering coefficient and yield much better estimates of this parameter. Therefore, at moderate transport
albedos, the combination of photothermal and reflectance methods can improve considerably the sensitivity of detection of tissue optical properties. The authors have recently proposed a novel method that combines FD-PTR with LDR, aimed at improving
sensitivity on the determination of both optical properties. Signal analysis was performed by global fitting the
experimental data to forward models based on Monte-Carlo simulations. Although this approach is accurate, the
associated computational burden often limits its use as a forward model. Therefore, the application of analytical
models based on the diffusion approximation offers a faster alternative. In this work, we propose the calculation
of the diffuse reflectance and the fluence rate profiles under the δ-P1 approximation. This approach is known
to approximate fluence rate expressions better close to collimated sources and boundaries than the standard
diffusion approximation (SDA). We extend this study to the calculation of the diffuse reflectance profiles. The
ability of the δ-P1 based model to provide good estimates of the absorption, scattering and anisotropy coefficients
is tested against Monte-Carlo simulations over a wide range of scattering to absorption ratios. Experimental
validation of the proposed method is accomplished by a set of measurements on solid absorbing and scattering
phantoms.
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