title: Rapid acquisition of multifrequency, multislice and multidirectional MR elastography data with a fractionally encoded gradient echo sequence.
abstract: In MR elastography (MRE), periodic tissue motion is phase encoded using motion-encoding gradients synchronized to an externally applied periodic mechanical excitation. Conventional methods result in extended scan time for quality phase images, thus limiting the broad application of MRE in the clinic. For practical scan times, researchers have been relying on one-dimensional or two-dimensional motion-encoding, low-phase sampling and a limited number of slices, and artifact-prone, single-shot, echo planar imaging (EPI) readout. Here, we introduce a rapid multislice pulse sequence capable of three-dimensional motion encoding that is also suitable for simultaneously encoding motion with multiple frequency components. This sequence is based on a gradient-recalled echo (GRE) sequence and exploits the principles of fractional encoding. This GRE MRE pulse sequence was validated as capable of acquiring full three-dimensional motion encoding of isotropic voxels in a large volume within less than a minute. This sequence is suitable for monofrequency and multifrequency MRE experiments. In homogeneous paraffin phantoms, the eXpresso sequence yielded similar storage modulus values as those obtained with conventional methods, although with markedly reduced variances (7.11 ± 0.26 kPa for GRE MRE versus 7.16 ± 1.33 kPa for the conventional spin-echo EPI sequence). The GRE MRE sequence obtained better phase-to-noise ratios than the equivalent spin-echo EPI sequence (matched for identical acquisition time) in both paraffin phantoms and in vivo data in the liver (59.62 ± 11.89 versus 27.86 ± 3.81, 61.49 ± 14.16 versus 24.78 ± 2.48 and 58.23 ± 10.39 versus 23.48 ± 2.91 in the X, Y and Z components, respectively, in the case of liver experiments). Phase-to-noise ratios were similar between GRE MRE used in monofrequency or multifrequency experiments (75.39 ± 14.93 versus 86.13 ± 18.25 at 28 Hz, 71.52 ± 24.74 versus 86.96 ± 30.53 at 56 Hz and 95.60 ± 36.96 versus 61.35 ± 26.25 at 84Hz, respectively).
authors.affiliation: Université Paris Diderot, Sorbonne Paris Cité, Clichy, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Garteiser
authors.forename: Ramin S
authors.initials: RS
authors.lastname: Sahebjavaher
authors.forename: Leon C
authors.initials: LC
authors.lastname: Ter Beek
authors.forename: Septimiu
authors.initials: S
authors.lastname: Salcudean
authors.forename: Valérie
authors.initials: V
authors.lastname: Vilgrain
authors.forename: Bernard E
authors.initials: BE
authors.lastname: Van Beers
authors.forename: Ralph
authors.initials: R
authors.lastname: Sinkus
mesh_terms: D017352:Echo-Planar Imaging; D054459:Elasticity Imaging Techniques; D006801:Humans; D008099:Liver / Q000033:anatomy & histology; D059629:Signal-To-Noise Ratio; D013997:Time Factors
keywords: MR elastography; fractional encoding; gradient echo