Oxygenation and blood flow are important biomarkers of tissue health and they play a vital role in diagnosis and monitoring of diseases in both clinical and basic science research. Diffuse optical instruments offer effective solutions for continuous monitoring of oxygen and blood flow because they are non-invasive, portable and use non-ionizing light. Traditionally, this requires use of two complementary instruments, Diffuse Optical Spectroscopy (DOS) for measuring oxygenation from tissue absorption coefficient (𝜇𝑎) and reduced scattering coefficient (μs′) and Diffuse Correlation Spectroscopy (DCS) for measuring blood flow index (F). These hybrid DOS and DCS instruments use collocated sources leading to issues like partial volume effects, increased cost, and size. Here, we propose a novel technique - Frequency Domain Diffuse Correlation Spectroscopy (FD-DCS) to overcome these issues. FD-DCS extends and generalizes DCS measurements to the frequency domain, measures a frequency dependent intensity autocorrelation function, which is fit to a frequency domain solution to the correlation diffusion equation for simultaneous estimation of static and dynamic tissue optical properties. We present experiment results validating the technique in tissue simulating liquid phantoms (intralipid + India ink + water) using a new prototype instrument. Specifically, we compare tissue optical properties of phantoms of different absorption and scattering coefficients measured with FD-DCS and commercial FD-DOS. Our results show successful estimation of 𝜇𝑎, 𝜇𝑠 and 𝐹 with minimal errors.
Significance: Quantitative measurements of cerebral hemodynamic changes due to functional activation are widely accomplished with commercial continuous wave (CW-NIRS) instruments despite the availability of the more rigorous multi-distance frequency domain (FD-NIRS) approach. A direct comparison of the two approaches to functional near-infrared spectroscopy can help in the interpretation of optical data and guide implementations of diffuse optical instruments for measuring functional activation.
Aim: We explore the differences between CW-NIRS and multi-distance FD-NIRS by comparing measurements of functional activation in the human auditory cortex.
Approach: Functional activation of the human auditory cortex was measured using a commercial frequency domain near-infrared spectroscopy instrument for 70 dB sound pressure level broadband noise and pure tone (1000 Hz) stimuli. Changes in tissue oxygenation were calculated using the modified Beer–Lambert law (CW-NIRS approach) and the photon diffusion equation (FD-NIRS approach).
Results: Changes in oxygenated hemoglobin measured with the multi-distance FD-NIRS approach were about twice as large as those measured with the CW-NIRS approach. A finite-element simulation of the functional activation problem was performed to demonstrate that tissue oxygenation changes measured with the CW-NIRS approach is more accurate than that with multi-distance FD-NIRS.
Conclusions: Multi-distance FD-NIRS approaches tend to overestimate functional activation effects, in part due to partial volume effects.
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