Crosstalk between changes in concentration of oxy-and deoxy-haemoglobin calculated by modified Lambert-Beer law
in near-infrared topography is theoretically investigated. The change in intensity detected with source-detector pairs on
the scalp caused by global or focal ahsorption change in the brain is predicted by Monte Carlo simulation. The
topographic images of changes in oxy- and deoxy-haemoglobin are obtained from the changes in intensity detected with
source-detector pairs on the scalp. The crosstalk depends on the relative position of the focal absorption change to
source-detector pairs. The crosstalk is minimised when the focal absorption change is located below a measurement
point that is the midpoint between a source and a detector.
Crosstalk between oxy- and deoxy-haemoglobin observed in near-infrared topography is investigated. The light propagation in an adult head model is predicted by Monte Carlo simulation to obtain the change in intensity detected with source-detector pairs on the scalp caused by a focal absorption change in the brain. The topographic images of changes in oxy- and deoxy-haemoglobin are obtained from the changes in intensity detected with source-detector pairs on the scalp. The crosstalk depends on the relative position of the focal absorption change to source-detector pairs. The crosstalk is minimised when the focal absorption change is located below a measurement point that is the midpoint between a source and a detector. Appropriate selection of wavelength pair is effective to reduce the crosstalk in the topographic image.
In near-IR spectroscopy, the concentration change in oxy- and deoxyhemoglobin in tissue is calculated from the change in the detected intensity of light at two wavelengths by solving the simultaneous equation based on the modified Lambert-Beer law. The wavelength-independent constant or mean optical path length is usually assigned to the term of partial optical path length in the simultaneous equation. This insufficient optical path length in the calculation causes crosstalk between the concentration change in oxy- and deoxyhemoglobin. We investigate the crosstalk in the dual-wavelength measurement of oxy- and deoxyhemoglobin theoretically by Monte Carlo simulation to discuss the optimal wavelength pair to minimize the crosstalk. The longer wavelength of the dual-wavelength measurement is fixed at 830 nm and the shorter wavelength is varied from 650 to 780 nm. The optimal wavelength range for pairing with 830 nm for the dual-wavelength measurement of oxy- and deoxyhemoglobin is from 690 to 750 nm. The mean optical path length, which can be obtained by time- and phase-resolved measurement, is effective to reduce the crosstalk in the results of dual-wavelength measurement.
Near infrared topographic imaging is an effective instrument to image brain-cortex activity. The light scattering in tissue prevents us from improving the spatial resolution of the reconstructed image; hence it is important to evaluate the effect of scattering on the spatial resolution of the image. In this study, the light propagation in the adult head model was predicted by Monte Carlo simulation to investigate the effect of fiber arrangement on the spatial resolution
of NIR topographic imaging. The image of absorbers in the topographic images obtained from the double-density arrangement of source-detector pairs was compared with that from the conventional single-density arrangement. The double density arrangement improved the spatial resolution and accuracy of the position of the absorbers in the topographic image.
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