Comparison of EPI gradient-echo contrast changes in cat brain caused by respiratory challenges with direct simultaneous evaluation of cerebral oxygenation via a cranial window.

A gradient-echo echo planar imaging (EPI) sequence has been used to generate images of cat brain during respiratory challenges. Direct spectrophotometric measurements have been made simultaneously in order to correlate the changes in oxygen saturation as measured by spectrophotometry with the image intensity changes seen in the gradient-echo images. When blood volume remains approximately constant, as derived from the spectrophotometry data, good correlation is seen between calculated plots made of changes in the transverse relaxation rate, delta R2*, and the oxygen saturation as measured by spectrophotometry for much of the time course of the respiratory challenges of anoxia, apnea and hypercapnia. In some cases, the correlation is poorer during the recovery periods of the apnea and anoxia challenges. Those lower correlations can often be accounted for by changes in blood volume, which also affects the NMR relaxation rate. These results contribute to the understanding of the image intensity changes seen during functional brain imaging studies in humans.