Spectroscopic method for determination of the absorption coefficient in brain tissue

Abstract

Abstract. I use Monte Carlo simulations and phantom measurements to characterize a probe with adjacent optical fibres for diffuse reflectance spectroscopy during stereotactic surgery in the brain. Simulations and measurements have been fitted to a modified Beer-Lambert model for light transport in order to be able to quantify chromophore content based on clinically measured spectra in brain tissue. It was found that it is important to take the impact of the light absorption into account when calculating the apparent optical path length, l p , for the photons in order to get good estimates of the absorption coefficient, a . The optical path length was found to be well fitted to the equation l p = a + b ln͑I s ͒ + c ln͑ a ͒ + d ln͑I s ͒ln͑ a ͒, where I s is the reflected light intensity for scattering alone ͑i.e., zero absorption͒. Although coefficients a -d calculated in this study are specific to the probe used here, the general form of the equation should be applicable to similar probes.

Intracerebral quantitative chromophore estimation from reflectance spectra captured during deep brain stimulation implantation

“…A previously developed method [14] was used to estimate the chromophore content from the reflectance spectra captured in the target areas, white matter and the cortex.…”

“…The coefficients a-d are probe-dependent constants that had been determined using liquid phantoms with known optical properties [14]. Based on relations found from Monte Carlo simulations, I s was estimated from the spectra by linear regression against λ for white matter, GPi, STN and ZI and against λ -1 for the cortex between 780 and 900 nm at a level 7 % above I in that interval.…”

“…Any such removed chromophore was assumed to have a concentration of 0 for that fit. A detailed description of the estimation procedure is found in [14].…”

“…Additional information, such as the chromophore content, can also be extracted from reflectance spectra recorded with DRS. For short fibre separations, this requires characterization of the probe by the use of Monte Carlo simulations or phantoms with known optical properties [13,14].…”

“…In this study DRS is used for examination of chromophore content in brain tissue. A method previously developed by Johansson [14] for estimation of these chromophores in the brain is now applied on reflectance spectra captured on patients undergoing DBS implantation. The aim of this study was to estimate and quantify chromophores from diffuse reflectance spectra captured in the cortex, white matter and target areas during intracerebral recordings in relation to implantation of DBS electrodes.…”