Abstract
Diffuse reflectance spectroscopy (DRS) based on the frequency-domain (FD) technique has been employed to investigate the optical properties of deep tissues such as breast and brain using source to detector separation up to 40 mm. Due to the modeling and system limitations, efficient and precise determination of turbid sample optical properties from the FD diffuse reflectance acquired at a source-detector separation (SDS) of around 1 mm has not been demonstrated. In this study, we revealed that at SDS of 1 mm, acquiring FD diffuse reflectance at multiple frequencies is necessary for alleviating the influence of inevitable measurement uncertainty on the optical property recovery accuracy. Furthermore, we developed artificial neural networks (ANNs) trained by Monte Carlo simulation generated databases that were capable of efficiently determining FD reflectance at multiple frequencies. The ANNs could work in conjunction with a least-square optimization algorithm to rapidly (within 1 second), accurately (within 10%) quantify the sample optical properties from FD reflectance measured at SDS of 1 mm. In addition, we demonstrated that incorporating the steady-state apparatus into the FD DRS system with 1 mm SDS would enable obtaining broadband absorption and reduced scattering spectra of turbid samples in the wavelength range from 650 to 1000 nm.
Highlights
Diffuse Reflectance Spectroscopy (DRS) is a model based technique
Our results suggest that the standard diffusion equation (SDE) is especially not suitable for predicting the amplitude demodulation introduced by samples at wavelengths higher than 1000 nm where the reduced scattering property is generally lower than 1 mm−1 and the absorption property is higher than 0.1 mm−1 due to strong water absorption above 1000 nm
We found that the key to overcome the influence of measurement uncertainties on the sample optical property recovery accuracy at short source-detector separation (SDS) is to perform FD measurements at multiple frequencies
Summary
Diffuse Reflectance Spectroscopy (DRS) is a model based technique. It could be applied to study the properties and physiological functions of biological tissues at depths from the submm range to a few cm and has been used in various clinical applications [1,2,3,4]. One of the DRS variants employs an intensity modulated light source to investigate the light amplitude demodulation and phase delay introduced by the turbid sample which can be in turn utilized to quantify the sample’s absorption (μa) and reduced scattering (μs') properties [5, 6] Such techniques are usually categorized as the frequency-domain (FD) DRS method and has been successfully used in conjunction with the standard diffusion equation (SDE) [7] to characterize the optical properties of deep tissues, such as breast and brain, with source-detector separations (SDSs) longer than 10 mm [8,9,10]. At short SDSs, the phase delay introduced by samples would become small and the optical property recovery results could be affected by the inherent random system phase fluctuation
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