Validation and visualization of two-dimensional optical spectroscopic imaging of cerebral hemodynamics

Perfusion-based functional brain imaging techniques such as fMRI and optical intrinsic signal (OIS) imaging are becoming increasingly important in both neuroscience research and intraoperative brain mapping. Recent studies have applied a spectroscopic approach to OIS imaging data, which we will call "two-dimensional optical spectroscopy" (2DOS), generating images of functional changes in hemoglobin oxygenation and blood volume. This improvement comes at the cost of several assumptions. Whereas the "gold standard" technique of fiber spectroscopy decomposes reflected light over a spectral axis, 2DOS retains both spatial dimensions by acquiring images at several wavelengths, sacrificing spectral resolution for the extra spatial dimension. Furthermore, 2DOS data are acquired interleaved within or between trials, but combined during the spectroscopic analysis as if acquired simultaneously. Thus far, the few studies employing this approach have assumed both that the reduced spectral resolution is tolerable, and that sufficient trial averaging can compensate for the temporally staggered data acquisition. To test these assumptions, we compared 2DOS results to those produced by traditional fiber spectroscopy by observing the hemodynamic response to hindpaw electrical stimulation over primary somatosensory cortex in anesthetized rats. Comparisons revealed low fitting residuals and a high level of correlation between the two, but noteworthy differences in response magnitudes. Inspection of individual timecourses revealed a lower signal-to-noise ratio for 2DOS data. For visualization and interpretation of the 2DOS images, we present a parameterized visualization strategy, in which oxy-, deoxy-, and total hemoglobin are assigned to individual color channels. The resulting composite image conveniently displays the evolution of hemodynamic responses through parenchymal and vascular compartments in space and time.