MEASUREMENT OF REGIONAL BLOOD OXYGENATION AND CEREBRAL HEMODYNAMICS

An echo planar linewidth mapping technique, Shufflebutt, has allowed temporal measurements of changes in linewidth caused by static inhomogeneities (DELTALWSI) and transverse relaxation rate (DELTAR2) in models of hypoxia and hypercapnia. We demonstrate these changes are due to intravascular susceptibility differences/(DELTA(chi)) between the blood and tissue. Contrast agent injections at a DELTA(chi) equivalent to that of deoxygenated blood showed a twofold difference between the contrast agent and physiological anoxia values. Hypercapnia decreased both DELTALWSI and DELTAR2 consistent with an increase in blood oxygenation. We attribute these findings to constant oxygen extraction during an increase in blood flow, resulting in less deoxygenated venous blood and thus reduced DELTA(chi). For in vivo perturbations we found that DELTAR2/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 predominantly by dephasing of extravascular protons due to field inhomogeneities produced by intravascular deoxygenated hemoglobin (deoxyHb).