Optimized EPI for fMRI studies of the orbitofrontal cortex: compensation of susceptibility-induced gradients in the readout direction.
OBJECTIVE
Most functional magnetic resonance imaging (fMRI) studies record the blood oxygen level-dependent (BOLD) signal using gradient-echo echo-planar imaging (GE EPI). EPI can suffer from substantial BOLD sensitivity loss caused by magnetic field inhomogeneities. Here, BOLD sensitivity losses due to susceptibility- induced gradients in the readout (RO) direction are characterized and a compensation approach is developed.


METHODS
Based on a theory describing the dropout mechanism, an EPI sequence was optimized for maximal BOLD sensitivity in the orbitofrontal cortex (OFC) using a specific combination of an increased spatial resolution in the RO direction and a reduced echo time. Using measured BOLD sensitivity maps and a breath hold experiment, the model and compensation approach were tested.


RESULTS
Using typical fMRI EPI parameters, susceptibility-induced gradients in the RO direction caused dropouts in the OFC and the inferior temporal lobe. Optimizing the echo time and spatial resolution effectively reduced the dropout as predicted by the theory.


CONCLUSIONS
The model-based compensation approach effectively reduces BOLD sensitivity losses due to susceptibility-induced gradients in the RO direction. It retains the high temporal resolution of single-shot EPI and can be readily combined with methods for the compensation of susceptibility-induced field gradients in the phase-encoding and through-plane direction.
Weiskopf
Nikolaus
N
Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London, London, WC1N 3BG, UK. n.weiskopf@fil.ion.ucl.ac.uk
Hutton
Chloe
C
Josephs
Oliver
O
Turner
Robert
R
Deichmann
Ralf
R
D002540:Cerebral Cortex / Q000033:anatomy & histology*; D017352:Echo-Planar Imaging; D005625:Frontal Lobe / Q000033:anatomy & histology*; D007090:Image Interpretation, Computer-Assisted