Near Infrared Spectroscopy (NIRS) and Diffuse Optical Imaging (DOI) are finding widespread
application in the study of human brain activation, motivating further application-
specific development of the technology. NIRS and DOI offer the potential to quantify
changes in deoxy-hemoglobin (HbR) and total hemoglobin (HbT) concentration, thus
enabling distinction of oxygen consumption and blood flow changes during brain
activation. While the techniques implemented presently provide important results for
cognition and the neurosciences through their relative measures of HbR and HbT
concentrations, there is much to be done to improve sensitivity, accuracy, and
resolution. In this paper, we review the advances currently being made and issues to
consider for improving optical image quality. These include: the optimal selection of
wavelengths to minimize random and systematic error propagation in the calculation of
the hemoglobin concentrations; the filtering of systemic physiological signal clutter to
improve sensitivity to the hemodynamic response to brain activation; the implementation
of overlapping measurements to improve image spatial resolution and uniformity; and
the utilization of spatial prior information from structural and functional MRI to reduce
DOI partial volume error and improve image quantitative accuracy. An example of an
improved localized estimate of the cerebral hemodynamic response to brain activation
through filtering of systemic blood pressure variations is shown in the figure below. The
unfiltered data is shown by the blue lines in (a) showing an increased blood volume over
most regions of the head. Filtering of the first principle component (b), which is strongly
correlated with the blood pressure (c), prior to the block averaged estimate of the
hemodynamic response (see the red line in (a)) revels a more localized response to a
finger tapping task in the contra-lateral hemisphere.
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