Applications of NMR spectroscopy to the study of experimental stroke in vivo

Magnetic resonance spectroscopy and imaging enable us to investigate biochemical and pathophysiological changes associated with cerebral ischemia. The specific aims of these studies were to establish the relationships between energy metabolites and regional cerebral blood flow and to determine whether diffusion-weighted imaging is sensitive to the known thresholds for cerebral tissue energy failure and disturbance of transmembrane ionic gradients in gerbils.

Magnetic resonance spectroscopy measurements of energy metabolites in the gerbil brain were obtained as a function of cerebral blood flow (measured with the hydrogen clearance technique) before, during, and after unilateral or bilateral occlusion of the common carotid arteries. Diffusion-weighted and T2-weighted images were obtained in a separate series of experiments.

Major changes in brain energy metabolites were observed at flow values of 20 ml.100 g-1.min-1 and below. The cerebral blood flow threshold for maintenance of energy status was lowered in hypothermia, consistent with a protective effect. Diffusion-weighted imaging intensity increased at cerebral blood flow values of 15 to 20 ml.100 g-1.min-1 and below and increased gradually following the onset of severe global cerebral ischemia, but with a delay of about 2.5 minutes.

The spectroscopic observations suggest that the flow thresholds for electrical function and edema are a direct consequence of energy failure. Comparison of the spectroscopy and imaging data suggests that diffusion-weighted imaging is sensitive to disruption of tissue energy metabolism or to a consequence of this disruption. The possibilities arise of visualizing energy failure with the spatial resolution characteristic of magnetic resonance imaging and detecting compromised but recoverable tissue.