The use of depth-resolved optical imaging to separate cortical vascular compartments during functional activation

Abstract

Optical imaging of exposed cortex allows real-time observation of blood oxygenation and volume changes. These changes are most frequently visualized as 2D images, whose pixels represent superficially weighted sums of signals from deeper layers. We have developed a non-contact optical imaging system for visualizing cortical hemodynamics in 3D, to depths of >2 mm, with 100–200 micron resolution (Hillman et al, Opt Lett 29(14), 2004). The new system has been used to image rat somatosensory cortex through thinned skull during forepaw stimulation. The ability to discriminate between hemoglobin changes in superficial and deeper layers, has enabled extraction of functional time-courses that correspond to three major vascular compartments; arterial, venous and capillary (Figure shows average trends for 5 rats). It has been found that, not only can these time-courses be reliably extracted from the 3D images, they can be used to isolate the spatial distributions of the three components (figure shows masks of voxels in 3D optical image corresponding to time-course characteristics). The good correspondence between the components' locations and the true vascular architecture (see figure 1) implies that the extracted functional time-courses indeed also represent the evolution of changes in the three compartments. Features include smaller volume changes, and significantly later onset times in veins, compared to arteries and capillaries. Arterial components show larger volume changes, and much earlier returns to baseline than capillary and venous compartments. These techniques and results will be discussed, along with their implications for fMRI, 2D optical imaging and quantification of the cerebral metabolic rate of oxygen consumption.