Two variations of a depth-selective back-projection filter for functional near-infrared spectroscopy (fNIRS) systems are introduced. The filter comprises a depth-selective algorithm that uses inverse problems applied to an optically diffusive multilayer medium. In this study, simultaneous signal reconstruction of both superficial and deep tissue from fNIRS experiments of the human forehead using a prototype of a CW-NIRS system is demonstrated.
Diffuse optical topography has excellent features as a noninvasive method that provides 2D location information of
cortical activity. However, it cannot distinguish the activation depth. We propose an image reconstruction algorithm that
suppresses undesirable effects of skin circulation. It comprises a filtering algorithm that extracts target signals from
observation data contaminated by disturbing signals and a 2D visualizing process. Computer simulations revealed its
excellent performance. We developed a depth selective diffuse optical topography system prototype and performed
phantom experiments. Our algorithm significantly suppressed the influence of the disturbing body in the shallow plane
with minimal degradation of the target signal.
Diffuse reflective optical measurement is a useful approach for monitoring the oxygen consumption of living tissue such
as brain and muscle. To improve the oxygen consumption measurement accuracy, we propose a method for suppressing
the near-surface sensitivity. Diffuse reflective light is detected at the aperture used for irradiating the light and is used as
a cancellation signal for near-field sensitivity in the conventional measurement scheme. Photon fluence density
functions and positional dependences of detected light sensitivity to change in absorbance were simulated. The
sensitivity detected at the same position (aperture) as irradiation was significantly high for the near-surface region.
With our method, the near-surface sensitivity is reduced by more than 90% while keeping target sensitivity almost
constant (only 3% deterioration). The near-surface and deep-field sensitivity was measured with a phantom with light
(785 nm) modulated at 1 kHz through an optical fiber bundle. It confirmed suppressed the near-surface sensitivity by
subtracting the light detected at the same aperture from the light detected at another aperture.
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