Inverse Monte Carlo in a multilayered tissue model: merging diffuse reflectance spectroscopy and laser Doppler flowmetry

Abstract

Abstract. The tissue fraction of red blood cells (RBCs) and their oxygenation and speed-resolved perfusion are estimated in absolute units by combining diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LDF). The DRS spectra (450 to 850 nm) are assessed at two source-detector separations (0.4 and 1.2 mm), allowing for a relative calibration routine, whereas LDF spectra are assessed at 1.2 mm in the same fiber-optic probe. Data are analyzed using nonlinear optimization in an inverse Monte Carlo technique by applying an adaptive multilayered tissue model based on geometrical, scattering, and absorbing properties, as well as RBC flow-speed information. Simulations of 250 tissue-like models including up to 2000 individual blood vessels were used to evaluate the method. The absolute root mean square (RMS) deviation between estimated and true oxygenation was 4.1 percentage units, whereas the relative RMS deviations for the RBC tissue fraction and perfusion were 19% and 23%, respectively. Examples of in vivo measurements on forearm and foot during common provocations are presented. The method offers several advantages such as simultaneous quantification of RBC tissue fraction and oxygenation and perfusion from the same, predictable, sampling volume. The perfusion estimate is speed resolved, absolute (% RBC × mm∕s), and more accurate due to the combination with DRS. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

On the equivalence and differences between laser Doppler flowmetry and laser speckle contrast analysis

“…We have described this in an appendix to a previous paper, 23 but it is given again here for the consistency of this paper.…”

“…The average (standard deviation) estimated perfusion increase was 9.7 (0.2)%, 5.9 (0.9)%, and 8.1 (0.9)%, respectively, for LDF-perfusion, and the LASCA-perfusions calculated with 1∕K − 1 [Eq. (23)] and 1∕K 2 − 1 [Eq. (24)] when average speed was increased 10%.…”

“…23,26 The method is based on path length distributions from Monte Carlo simulations from multilayered models with given geometries and scattering properties, where the effect of absorption is added using BeerLambert's law. The optical Doppler spectrum is then calculated based on single shifted optical Doppler spectra [Eq.…”

Microcirculation assessment using an individualized model for diffuse reflectance spectroscopy and conventional laser Doppler flowmetry

“…It also penalizes a deviation from unity in the intensity relaxation factor (1 point) and nonphysiological values on the output parameters (4 points). 20 The model fitting was done in three steps. 20 First, an appropriate starting point was found for parameters mostly affecting wavelengths above 700 nm, i.e., the epidermal thickness, the three scattering parameters, the melanin fraction, the melanin exponent, and the average fraction of blood (preliminary).…”

“…20 The model fitting was done in three steps. 20 First, an appropriate starting point was found for parameters mostly affecting wavelengths above 700 nm, i.e., the epidermal thickness, the three scattering parameters, the melanin fraction, the melanin exponent, and the average fraction of blood (preliminary). In the second step, the average fraction of blood, the relative ratio between the layers, the oxygen saturation, and the vessel diameter, were obtained by fitting the whole spectra, where the parameter values obtained from step one were used as starting point.…”