Purpose: In magnetic resonance elastography (MRE), a series of time-shifted images is acquired at specific phase offsets in relation to an induced mechanical excitation. To efficiently gather the set of phase offset images and to overcome limitations due to prolonged TEs and related susceptibility artifacts at low-frequency MRE, we developed an improved displacement encoding with a stimulated echoes (DENSE) method.
Methods: The proposed multiphase DENSE-MRE acquisition scheme allows full sampling of the wave propagation in 1 encoding direction during each TR using multiple readouts at specific phase offsets. With this approach, all phase offsets can be imaged in 1 TR without the need for whole sequence repetitions at time-shifted offsets relative to the excitation motion. We tested this technique in phantom experiments with 60 Hz and in the brain of 4 volunteers using 20-Hz harmonic excitation.
Results: Three-dimensional wave propagation could be acquired in 7 minutes 30 seconds. Following background phase elimination, clear wave images were obtained, showing the propagation of the waves over time. Calculated shear modulus maps of the phantom matched well to the maps obtained by conventional gradient-echo MRE. In the brain, low-frequency DENSE-MRE images were free of susceptibility-induced artifacts and the calculated maps showed a median global complex shear modulus magnitude of 0.72 kPa and phase angle of 1.03 rad across volunteers.
Conclusion: The proposed multiphase DENSE approach allows efficient low-frequency MRE with short TEs and is well-suited for low-frequency MRE of the human brain.
Keywords: brain; low-frequency MR elastography; multiphase DENSE-MRE; shear waves.
© 2019 International Society for Magnetic Resonance in Medicine.