MEASUREMENT OF REGIONAL BLOOD OXYGENATION AND CEREBRAL HEMODYNAMICS

An echo planar linewidth mapping technique, Shufflebutt, has allowed t emporal measurements of changes in linewidth caused by static inhomoge neities (DELTALWSI) and transverse relaxation rate (DELTAR2) in models of hypoxia and hypercapnia. We demonstrate these changes are due to i ntravascular susceptibility differences/(DELTA(chi)) between the blood and tissue. Contrast agent injections at a DELTA(chi) equivalent to t hat of deoxygenated blood showed a twofold difference between the cont rast agent and physiological anoxia values. Hypercapnia decreased both DELTALWSI and DELTAR2 consistent with an increase in blood oxygenatio n. We attribute these findings to constant oxygen extraction during an increase in blood flow, resulting in less deoxygenated venous blood a nd thus reduced DELTA(chi). For in vivo perturbations we found that DE LTAR2/DELTAR2' almost-equal-to 0.33, a ratio much different from that measured in whole blood phantoms (DELTAR2/DELTAR2' almost-equal-to 2). This demonstrates that signal changes in these studies are produced p redominantly by dephasing of extravascular protons due to field inhomo geneities produced by intravascular deoxygenated hemoglobin (deoxyHb).