Diffuse Optical Imaging of Brain Activation: Approaches to Optimizing Image Sensitivity, Resolution, and Accuracy
David Boas
Massachusetts General Hospital
Biomedical Imaging
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 deoxyhemoglobin (HbR) and total hemoglobin (HbT) concentration, thus enabling distinction between oxygen consumption and blood flow changes during brain activation. While the techniques presently implemented 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 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 quantitative image 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 increased blood volume over most regions of the head. Filtering of the first principal component (b), which is strongly correlated with blood pressure (c), prior to the block-averaged estimate of the hemodynamic response (see the red line in (a)), reveals a more localized response to a finger-tapping task in the contralateral hemisphere.
