Signal changes in gradient echo images of human brain induced by hypo‐ and hyperoxia

Abstract

The effect of hypoxia (inspired oxygen fraction, FiO2 of 10% and 16%) and hyperoxia (FiO2) of 100%) on gradient echo images of the brain using long echo times was investigated in six healthy volunteers (age 24-28 years). Different flip angles were used with an FiO2 of 10% to assess the importance of saturation effects. The total cerebral blood flow was measured by a phase mapping technique during normoxia as well as hypoxia (FiO2 of 10% and 16%) and hyperoxia (FiO2 of 50% and 100%). High relative signal changes were found, independently of the flip angle, with FiO2 of 10%. With a flip angle of 40 degrees the values of delta R2* for cortical grey matter, central grey matter, white matter and the sagittal sinus were 0.79, 0.41, 0.26 and 3.00/s; with a flip angle of 10 degrees the corresponding values were 0.70, 0.37, 0.24 and 3.15/s. The total cerebral blood flow increased by 41% during inhalation of 10% O2 and decreased by 27% during 100% O2; no flow changes were seen during moderate changes in FiO2. It is concluded that flow effects play a minor role for fMRI signal strength in this application since (i) they did not abolish the signal changes caused by changes in blood oxygenation during hyper- and hypoxia, (ii) the observed signal changes were closely related to the changes in arterial oxygen saturation during hypoxia and (iii) the signal changes were little affected by changing the flip angle from 40 degrees to 10 degrees.

Noninvasive Imaging of Quantitative Cerebral Blood Flow Changes during 100% Oxygen Inhalation Using Arterial Spin-Labeling MR Imaging

“…Finally, the small CBF change measured in the current study is also at odds with the 16%-27% decrease measured with cardiac-gated VEPC MR imaging. 14,15 Because VEPC images taken of the major vessels near the circle of Willis should represent a reasonable estimate of global supratentorial CBF, 26 more investigation of this apparent discrepancy is warranted. Decreased ⌬M signals were seen in all subjects in all regions of interest during the 100% oxygen inhalation.…”

“…Two studies using cardiac-gated velocity-encoded phase contrast (VEPC) measured global CBF decreases ranging between 16% and 27%, 14,15 greater than that measured with tracer methods. Two previous ASL-based measurements have reported markedly different CBF changes, one measuring a 7% decrease, 11 the other a 33% decrease.…”

Cerebral Perfusion Response to Hyperoxia

“…An increase in the inspired fraction of oxygen (FiO 2 ) is known to be effective as a contrast agent for T 2 *-weighted magnetic resonance imaging (MRI) (Losert et al, 2002;Rostrup et al, 1995). Unlike injected MRI contrast agents, oxygen has a much faster wash-out time and fewer contraindications.…”

Quantitative Measurements of Cerebral Blood Oxygen Saturation Using Magnetic Resonance Imaging

“…Specifically, it has been demonstrated that when the concentration of deoxyhemoglobin is increased, a decrease in T2 and T2* is anticipated, resulting in a decrease in MR signal intensity in T2-and T2*-weighted images and vice versa. With animal models many investigators demonstrated that BOLD effects can be used to monitor the changes of oxygen saturation in vivo under pathophysiologic conditions, such as hypoxia (Turner et al, 1991;Prielmeier et al, 1994;Jezzard et al, 1994;Hoppel et al, 1993;Rostrup et al, 1995;Kennan et al, 1997;Lin et al, 1998a), hyper-and hypocapnia (Jezzard et al, 1994;Davis et al, 1998;Lin et al, 1999), hemodilution (Lin et al, 1998b), and ischemia (De Crespigny et al, 1992;Ono et al, 1997). However, all of the above studies only focus on relative measurements of cerebral blood oxygen saturation (Y) and little attention has been given as to how an absolute measurement of Y can be obtained with the BOLD effects.…”