Method to improve the depth sensitivity of diffuse reflectance measurements to absorption changes in optically turbid medium.

Piotr Sawosz,Adam Liebert

doi:10.1364/boe.10.005031

Abstract

We have studied the spatial distributions of the sensitivity of time-resolved near-infrared diffuse reflectance measurement. Sensitivity factors representing a change of parameters of a measured optical signal induced by absorption perturbation in a certain voxel of the medium were simulated using the diffusion equation solution. The parameters were statistical moments of measured distributions of time of flight of photons (DTOFs) i.e., the total number of photons, mean time of flight and variance. The distributions of the sensitivity of statistical moments of DTOFs to a change in absorption were generated for various source-detector separations and various optical properties of the medium. Furthermore, differential sensitivity distributions for two different source-detector separations were calculated. A measurement geometry, in which two detection spots, separated by 5 mm, in combination with two sources was proposed. For this setup differences between the signals obtained for both detectors were calculated independently for both sources and afterward summed up for both source positions. Obtained differences in moments of DTOFs assessed at two source-detector separations and summed up for different positioning of the sources allowed to shape up the sensitivity profiles. Calculated sensitivity profiles show that positive sensitivities of the mean time of flight of photons and variance of the DTOF can be obtained. These positive sensitivity areas are located just between both detection spots and cover the compartment located deeply in the medium. The sensitivity in superficial compartments of the medium is negative and much smaller in amplitude. The proposed technique can be used for improved discrimination of optical signals related to the intracerebral change in absorption which remains a serious obstacle in the application of the NIRS technique in the assessment of brain oxygenation or perfusion.

Highlights

In last decade the time-resolved measurement of diffuse reflectance became to be serious alternative to the continuous wave spectroscopy for the assessment of tissue oxygenation and perfusion [1]
It was presented in multiple studies that in some application continuous wave near infrared spectroscopy technique is insufficient because the results are strongly contaminated by an influence of extracerebral tissues [2,3,4,5,6,7]
The negative sensitivity of the mean time of flight of photons and the variance of the distributions of time of flight of photons (DTOFs) is noted for changes in the absorption coefficient appearing superficially in the medium

Summary

Introduction

In last decade the time-resolved measurement of diffuse reflectance became to be serious alternative to the continuous wave spectroscopy for the assessment of tissue oxygenation and perfusion [1]. This trend is related to the reducing costs of components as well as appearance of new technologies allowing for estimation of distributions of times of flight of photons. In measurements of the cerebral perfusion or oxygenation the technique of near infrared spectroscopy is typically used in diffuse reflectance mode It was shown in series of studies that the time-resolved technique allows to solve the problem of the contamination of the brain cortex oxygenation by components related to the extracerebral tissues. Assessment of changes in absorption coefficient as a function of depth in the tissue is possible when the DTOFs are analyzed using time-windows [11,15,16], statistical moments [17] or Mellin-Laplace moments [18]

Methods

Results

Conclusion