Modeling behavior of broadband (30 to 1000 MHz) frequency modulated near-infrared (NIR) photons through a phantom is the basis for accurate extraction of optical absorption and scattering parameters of biological turbid media. Photon dynamics in a phantom are predicted using both analytical and numerical simulation and are related to the measured insertion loss (IL) and insertion phase (IP) for a given geometry based on phantom optical parameters. Accuracy of the extracted optical parameters using finite element method (FEM) simulation is compared to baseline analytical calculations from the diffusion equation (DE) for homogenous brain phantoms. NIR spectroscopy is performed using custom-designed, broadband, free-space optical transmitter (Tx) and receiver (Rx) modules that are developed for photon migration at wavelengths of 680, 780, and 820 nm. Differential detection between two optical Rx locations separated by 0.3 cm is employed to eliminate systemic artifacts associated with interfaces of the optical Tx and Rx with the phantoms. Optical parameter extraction is achieved for four solid phantom samples using the least-square-error method in MATLAB (for DE) and COMSOL (for FEM) simulation by fitting data to measured results over broadband and narrowband frequency modulation. Confidence in numerical modeling of the photonic behavior using FEM has been established here by comparing the transmission mode’s experimental results with the predictions made by DE and FEM for known commercial solid brain phantoms.
Fiber based functional near infra-red (fNIR) spectroscopy has been considered as a cost effective imaging modality. To
achieve a better spatial resolution and greater accuracy in extraction of the optical parameters (i.e., μa and μ's), broadband
frequency modulated systems covering multi-octave frequencies of 10-1000MHz is considered. A helmet mounted
broadband free space fNIR system is considered as significant improvement over bulky commercial fiber fNIR
realizations that are inherently uncomfortable and dispersive for broadband operation. Accurate measurements of
amplitude and phase of the frequency modulated NIR signals (670nm, 795nm, and 850nm) is reported here using free
space optical transmitters and receivers realized in a small size and low cost modules. The tri-wavelength optical
transmitter is based on vertical cavity semiconductor lasers (VCSEL), whereas the sensitive optical receiver is based on
either PIN or APD photodiodes combined with transimpedance amplifiers. This paper also has considered brain
phantoms to perform optical parameter extraction experiments using broadband modulated light for separations of up to
5cm. Analytical models for predicting forward (transmittance) and backward (reflectance) scattering of modulated
photons in diffused media has been modeled using Diffusion Equation (DE). The robustness of the DE modeling and
parameter extraction algorithm was studied by experimental verification of multi-layer diffused media phantoms. In
particular, comparison between analytical and experimental models for narrow band and broadband has been performed
to analyze the advantages of our broadband fNIR system.
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