Oxygen-induced frequency shifts in hyperoxia: a significant component of BOLD signal.
In comparison to the well-documented significance of intravascular deoxyhemoglobin (deoxyHgb), the effects of dissolved oxygen on the blood-oxygen-level-dependent (BOLD) signal have not been widely reported. Based on the fact that the prolonged inspiration of high oxygen fraction gas can result in up to a sixfold increase of the baseline tissue oxygenation, the current study focused on the influence of dissolved oxygen on the BOLD signal during hyperoxia. As results, our in vitro study revealed that the r1 and r2 (relaxivities) of the oxygen-treated serum were 0.22 mM(-1) · s(-1) and 0.19 mM(-1) · s(-1) , respectively. In an in vivo experiment, hyperoxic respiration induced negative BOLD contrast (i.e. signal decrease) in 18-42% of measured brain regions, voxels with accompanying significant decreases in both the T(*)2 (-12.1% to -19.4%) and T1 (-5.8% to -3.3%) relaxation times. In contrast, the T(*)2 relaxation time significantly increased (11.2% to 14.0%) for the voxels displaying positive BOLD contrast (in 41-50% of the measured brain), which reflected a hyperoxygenation-induced reduction in tissue deoxyHgb concentration. These data imply that hyperoxia-driven BOLD signal changes are primarily determined by the counteracting effects of extravascular oxygen and intravascular deoxyHgb. Oxygen-induced magnetic susceptibility was further demonstrated by the study of 1 min hypoxia, which induced BOLD signal changes opposite to those under hyperoxia. Vasoconstriction was more common in voxels with negative BOLD contrast than in voxels with positive contrast (% change of blood volume, -9.8% to -12.8% versus 2.0% to 2.2%), which further suggests that negative BOLD contrast is mainly evoked by an increase in extravascular oxygen concentration. Conclusively, frequency shifts, which are induced by the accumulation of oxygen molecules and associated magnetic field inhomogeneity, are a significant source of the negative BOLD contrast during hyperoxia.
Division of Magnetic Resonance, Korea Basic Science Institute, Cheongwon, Chungbuk, South Korea.
Youngkyu
Y
Song
Gyunggoo
G
Cho
Song-I
SI
Chun
Jin Hee
JH
Baek
HyungJoon
H
Cho
Young Ro
YR
Kim
Sung Bin
SB
Park
Jeong Kon
JK
Kim
D000818:Animals; D001784:Blood Gas Analysis; D018496:Hyperoxia / Q000097:blood* / Q000503:physiopathology; D008297:Male; D010100:Oxygen / Q000097:blood*; D017207:Rats, Sprague-Dawley; D012815:Signal Processing, Computer-Assisted; D013997:Time Factors; D014664:Vasodilation
D010100:Oxygen
MRI; blood oxygen-level dependent (BOLD); hyperoxia; oxygen; susceptibility