Determining the optical properties of the human brain using a two-layer diffusion model for non-invasive, frequency domain optical measurements.
Pillas, Elleesse Carlisle.
The concentrations of oxyhemoglobin [HbO] and deoxyhemoglobin [Hb] in tissue and their
temporal dynamics are an important indicator of cognitive function in the brain.
Near-infrared (NIR) techniques have been employed to extract the concentration levels of
these tissue chromophores due to their non-invasive feature. An integral part of this
extraction procedure is a model of light ... read morepropagation in turbid medium such as human
tissue. In this study, we apply the diffusion model since the nature of light-tissue
interaction for tissues probed with near-infrared light is highly scatter-dominated. In
such a model of light propagation, the quantities of interest are the absorption
μa and the scattering coefficient μs. From the absorption coefficients, the
chromophore concentrations can be determined through their molar extinction
coefficients. However, the model assumes a homogeneous issue structure and inevitably
presents a limitation in the extraction process. In this study, a two-layer model of the
head is proposed to account for the heterogeneity in the tissue architecture of the head
in recovering the absorption and reduced scattering coefficients (optical properties) of
the brain. The model assumes a superficial layer of finite thickness (first layer)
situated over a semi-infinite homogeneous structure (second layer). Applied to the head,
the scalp and the skin are lumped together in the first layer and the brain comprises
the latter. Values of intensity amplitude (AC) and phase (è) are acquired in the
frequency domain at multiple source-detector distances in a reflectance geometry. The
optical properties are then recovered by performing a nonlinear regression to Monte
Carlo-generated data for the AC and phase of the detected light. It is shown that the
recovered optical properties using this two-layer model exhibits better agreement with
the simulations than those recovered from a homogeneous model. The study hopes to
provide an added avenue of refinement to a recent protocol that was developed to perform
absolute measurements of cerebral hemoglobin
Thesis (M.S.)--Tufts University, 2012.
Submitted to the Dept. of Biomedical Engineering.
Advisor: Sergio Fantini.
Committee: Angelo Sassaroli, Mark Cronin-Golomb, and Lawrence Ford.
Keyword: Biomedical engineering.read less