The Optical Effective Attenuation Coefficient as an Informative Measure of Brain Health in Aging.

Antonio M Chiarelli,Monica Fabiani,Bradley P Sutton,Kathy A Low,Chin Hong Tan,Benjamin Zimmerman,Edward L Maclin,Tania S Kong,Gabriele Gratton,Mark A Fletcher

doi:10.3390/photonics6030079

Abstract

Aging is accompanied by widespread changes in brain tissue. Here, we hypothesized that head tissue opacity to near-infrared light provides information about the health status of the brain’s cortical mantle. In diffusive media such as the head, opacity is quantified through the Effective Attenuation Coefficient (EAC), which is proportional to the geometric mean of the absorption and reduced scattering coefficients. EAC is estimated by the slope of the relationship between source–detector distance and the logarithm of the amount of light reaching the detector (optical density). We obtained EAC maps across the head in 47 adults (age range 18–75 years), using a high-density dual-wavelength optical system. We correlated regional and global EAC measures with demographic, neuropsychological, structural and functional brain data. Results indicated that EAC values averaged across wavelengths were strongly associated with age-related changes in cortical thickness, as well as functional and neuropsychological measures. This is likely because the EAC largely depends on the thickness of the sub-arachnoid cerebrospinal fluid layer, which increases with cortical atrophy. In addition, differences in EAC values between wavelengths were correlated with tissue oxygenation and cardiorespiratory fitness, indicating that information about cortical health can be derived non-invasively by quantifying the EAC.

Highlights

It is commonly accepted that, even in the absence of mild-cognitive impairment or Alzheimer’sDisease, aging is often associated with some degree of cortical atrophy, as manifested by cortical thinning [1,2,3,4], and that this atrophy can be accompanied by decrements in brain and cognitive function [5,6,7,8]
The first part provides basic statistics and reliability information about the Effective Attenuation Coefficient (EAC) data. It includes a series of analyses aimed at demonstrating that the EAC value provides information related to (a) the average thickness of the cerebrospinal fluid (CSF)-layer in a given individual, and (b) the average oxygenation of brain tissue
The second part is aimed at demonstrating that the first EAC eigen-solution (EAC1) is strongly associated with cortical thickness, whereas the second EAC eigen-solution (EAC2) is associated with cardiorespiratory fitness, providing indirect validation for its interpretation as a measure of tissue oxygenation and health

Summary

Introduction

It is commonly accepted that, even in the absence of mild-cognitive impairment or Alzheimer’sDisease, aging is often associated with some degree of cortical atrophy, as manifested by cortical thinning [1,2,3,4], and that this atrophy can be accompanied by decrements in brain and cognitive function [5,6,7,8]. DOI was proposed as a tool for studying functional changes associated with variations in oxy- and deoxy-hemoglobin concentration (functional near-infrared spectroscopy, fNIRS) [10,11] and/or in neural activity (fast optical signal and event-related optical signal, EROS [12]). Promising, these measures have remained less popular than measures based on functional magnetic-resonance imaging (fMRI) or electrophysiology (electroencephalography, EEG and event related brain potentials, ERPs). This is because of the relatively low signal-to-noise ratio (SNR) of the approach

Methods

Results

Discussion

Conclusion