Minimization of Nyquist ghosting for echo-planar imaging at ultra-high fields based on a "negative readout gradient" strategy.
OBJECTIVE
To improve the traditional Nyquist ghost correction approach in echo planar imaging (EPI) at high fields, via schemes based on the reversal of the EPI readout gradient polarity for every other volume throughout a functional magnetic resonance imaging (fMRI) acquisition train.


METHODS
An EPI sequence in which the readout gradient was inverted every other volume was implemented on two ultrahigh-field systems. Phantom images and fMRI data were acquired to evaluate ghost intensities and the presence of false-positive blood oxygenation level-dependent (BOLD) signal with and without ghost correction. Three different algorithms for ghost correction of alternating readout EPI were compared.


RESULTS
Irrespective of the chosen processing approach, ghosting was significantly reduced (up to 70% lower intensity) in both rat brain images acquired on a 9.4T animal scanner and human brain images acquired at 7T, resulting in a reduction of sources of false-positive activation in fMRI data.


CONCLUSIONS
It is concluded that at high B(0) fields, substantial gains in Nyquist ghost correction of echo planar time series are possible by alternating the readout gradient every other volume.
Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Switzerland. wietske.vanderzwaag@epfl.ch
Wietske
W
van der Zwaag
José P
JP
Marques
Hongxia
H
Lei
Nathalie
N
Just
Tobias
T
Kober
Rolf
R
Gruetter
D000465:Algorithms; D000818:Animals; D001921:Brain / Q000473:pathology*; D001931:Brain Mapping / Q000379:methods; D017352:Echo-Planar Imaging / Q000379:methods*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010100:Oxygen / Q000097:blood; D019047:Phantoms, Imaging; D051381:Rats
D010100:Oxygen