Optimized EPI for fMRI using a slice-dependent template-based gradient compensation method to recover local susceptibility-induced signal loss.
OBJECTIVE
Most functional magnetic resonance imaging (fMRI) experiments use gradient-echo echo planar imaging (GE EPI) to detect the blood oxygenation level-dependent (BOLD) effect. This technique may fail in the presence of anatomy-related susceptibility-induced field gradients in the human head. In this work, we present a novel 3D compensation method in combination with a template-based correction that can be optimized over particular regions of interest to recover susceptibility-induced signal loss without acquisition time penalty.


METHODS
Based on an evaluation of B(0) field maps of eight subjects, slice-dependent gradient compensation moments are derived for maximal BOLD sensitivity in two compromised regions: the orbitofrontal cortex and the amygdala areas. A modified EPI sequence uses these additional gradient moments in all three imaging directions. The method is compared to non-compensated, template-based and subject-specific correction gradients and also in a breath-holding experiment.


RESULTS
The slice-dependent gradient compensation method significantly improves signal intensity/BOLD sensitivity by about 35/43% in the orbitofrontal cortex and by 17/30% in the amygdala areas compared to a conventional acquisition. Template-based correction and subject-specific correction perform equally well. The BOLD sensitivity in the breath hold experiment is effectively increased in compensated regions.


CONCLUSIONS
The new method addresses the problem of susceptibility-induced signal loss, without compromising temporal resolution. It can be used for event-related functional experiments without requiring additional subject-specific calibration or calculation time.
Rick
Jochen
J
Department of Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany. jochen.rick@uniklinik-freiburg.de
Speck
Oliver
O
Maier
Simon
S
Tüscher
Oliver
O
Dössel
Olaf
O
Hennig
Jürgen
J
Zaitsev
Maxim
M
D000465:Algorithms; D000679:Amygdala / Q000098:blood supply; D016477:Artifacts; D001931:Brain Mapping; D002138:Calibration; D002540:Cerebral Cortex / Q000098:blood supply; D017352:Echo-Planar Imaging / Q000379:methods*; D006801:Humans; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging / Q000379:methods*; D010100:Oxygen / Q000097:blood*; D010101:Oxygen Consumption / Q000502:physiology; D012815:Signal Processing, Computer-Assisted
D010100:Oxygen