Measurements of BOLD/CBV Ratio Show Altered fMRI Hemodynamics during Stroke Recovery in Rats

Brain responses to external stimuli after permanent and transient ischemic insults have been documented using cerebral blood volume weighted (CBVw) functional magnetic resonance imaging (fMRI) in correlation with tissue damage and neurological recovery. Here, we extend our previous studies of stroke recovery in rat models of focal cerebral ischemia by comparing blood oxygen level-dependent (BOLD) and cerebral blood volume (CBV) changes. Responses to forepaw stimulation were measured in normal rats (n=5) and stroke rats subjected to 2 h of middle cerebral artery occlusion (n=6). Functional magnetic resonance imaging was performed 2 weeks after stroke to evaluate the recovery process. After stroke, animals showed variable degrees of fMRI activation in ipsilesional cortex, the extent of which did not correlate with structural damages as measured using apparent diffusion coefficient, fractional anisotropy, blood volume, and vessel size index. While the contralesional cortex showed good overlap between BOLD and CBV-activated regions, the ipsilesional cortex showed low covariance between significantly activated voxels by BOLD and CBVw techniques. In particular, the relative activation during contralateral stimuli in the ipsilesional somatosensory cortex was significantly higher for CBVw responses than BOLD, which might be due to stroke-related alterations in fMRI hemodynamic coupling. Aberrant subcortical activations were also observed. When unaffected forelimbs were stimulated, strong bilateral responses were observed. However, little thalamic responses accompanied stimulation of affected forelimbs despite significant activation in the ipsilesional somatosensory cortex. These results suggest that stroke affects not only local hemodynamics and coupling but also other factors including neural connectivity.