Modeling and tissue parameter extraction challenges for free space broadband fNIR brain imaging systems

Abstract

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., ȝ a and ȝ' 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.

Diagnosis of Hand Tumors using fNIR Optical System

“…Using a proper calibration of the frequency modulated tri-wavelength optical transmitter (VCSEL) and optical receiver (APD C5658 from Hamamatsu), optical tissue parameters are to be extracted using curve fitting the experimental data to √ + [5]. The diffusion of NIR photons is impacted by absorption and scattering of the inhomogeneous biological tissue of the wrist with the tumor.…”

“…The first stage is dedicated to performing functionality measurements to monitor both absorption and scattering, while the second stage is dedicated to optical parameter extraction of the ganglion cyst tissue and lipoma fatty tumor. The first stage is performed while the patients is bending his wrist to measure photon backscattering as shown in Figure 2, while the second stage is performed using transmission mode optical parameter extraction [3][4][5] after the ganglion cyst fluid and fatty tumor are aspirated. …”

“…Extracting the optical parameters is done by curve fitting the experimental broadband result of both IL and IP data to √ + . The curve fitting result was then used through the inverse problem code to extract the optical parameters [5].…”

“…Amplitude and phase of both forward and back-scattered photons out of the biological media are measured as IL and IP of the radio frequency signal and then Diffusion Equation is employed for inverse problem solving of turbid biological media. The advantages of broadband measurements are already reported [3][4][5] and are due to high accuracy in extraction of optical parameters and imaging of inhomogeneous material represented as multilayer media.…”

Advances in Red VCSEL Technology

“…Figure 13(b) illustrates the L-I-V (output power and voltage versus current) for the three wavelengths incorporated into a single package. An example of the application of the multiwavelength technology for near-infrared spectroscopic-based brain imaging has been demonstrated at Drexel University [32].…”