Analysis and improvement of motion encoding in magnetic resonance elastography

Abstract

Magnetic resonance elastography (MRE) utilizes phase contrast magnetic resonance imaging (MRI), which is phase locked to externally generated mechanical vibrations, to measure the three-dimensional wave displacement field. At least four measurements with linear-independent encoding directions are necessary to correct for spurious phase contributions if effects from imaging gradients are non-negligible. In MRE, three encoding schemes have been used: unbalanced four- and six-point and balanced four-point ('tetrahedral') encoding. The first two sensitize to motion with orthogonal gradients, with the four-point method acquiring a single reference scan without motion sensitization, whereas three additional scans with inverted gradients are used with six-point encoding, leading to two-fold higher displacement-to-noise ratio (DNR) and 50% longer scan duration. Balanced four-point (tetrahedral) encoding encodes along the four diagonals of a cube, with one direction serving as a reference for the other three encoding directions, similar to four-point encoding. The objective of this work is to introduce a theoretical framework to compare different motion sensitization strategies with respect to their motion encoding efficiency in two fundamental encoding limits, the gradient strength limit and the dynamic range limit, which are both placed in relation to conventional gradient recalled echo (GRE)- and spin echo (SE)-based MRE sequences. We apply the framework to the three aforementioned schemes and show that the motion encoding efficiency of unbalanced four- and six-point encoding schemes in the gradient-limited regime can be increased by a factor of 1.5 when using all physical gradient channels concurrently. Furthermore, it is demonstrated that reversing the direction of the reference in balanced four-point (tetrahedral) encoding results in the Hadamard encoding scheme, which leads to increased DNR by 2 compared with balanced four-point encoding and 2.8 compared with unbalanced four-point encoding. As an example, we show that optimal encoding can be utilized to reduce the acquisition time of standard liver MRE in vivo from four to two breath holds.

Ristretto MRE: A generalized multi‐shot GRE‐MRE sequence

“…For 25 and 36 Hz, the standard error of the fit was slightly elevated, measuring 0.92 and 1.16 μm. The relative standard errors of the fit are within the same domain as in a previously published correlation study utilizing GRE‐MRE …”

“…The phantom scans were performed using electro‐magnetic actuation (25, 36, 40, and 60 Hz), eight wave‐phase offsets, 180 Hz bipolar MEGs, 16.6 mT/m MEG strength, motion encoding of 7.06 rad/mm, and four encoding directions following the Hadamard encoding scheme . The signal was received using a 15‐channel head coil.…”

“…The liver scan was performed with 50 Hz gravitational actuation, four wave‐phase offsets, 260 Hz MEG frequency, 18.9 mT/m MEG strength, motion encoding of 5.15 rad/mm, and Hadamard encoding . Signal was received using a 32‐channel cardiac coil.…”

“…Key to successful deployment of MRE for research and clinical applications is the reduction of scan time. While new encoding concepts such as sample interval modulation, selective spectral displacement projection, or multi‐directional reduced motion encoding show promising scan time reductions for spin‐echo (SE)‐based MRE sequences, these concepts cannot be readily transferred to gradient‐recalled echo (GRE)‐based MRE, where imaging gradient contributions need to be corrected for . While SE‐based MRE can utilize full wave encoding and typically relies on EPI readouts for fast MRE acquisition, GRE‐based MRE needs to balance signal loss due to T 2 * decay and motion sensitivity through fractional encoding .…”

Non-Invasive Characterization of Liver Disease : By Multimodal Quantitative Magnetic Resonance

“…The motion of the liver is imaged using special phase-contrast sequences [73][74][75][76]. These sequences are created through modification of basic sequences by adding motion-encoding gradients, which have to be synchronized to the vibration of the transducer.…”