Short echo time dual-frequency MR Elastography with Optimal Control RF pulses

Abstract

Magnetic Resonance Elastography (MRE) quantifies the mechanical properties of tissues, typically applying motion encoding gradients (MEG). Multifrequency results allow better characterizations of tissues using data usually acquired through sequential monofrequency experiments. High frequencies are difficult to reach due to slew rate limitations and low frequencies induce long TEs, yielding magnitude images with low SNR. We propose a novel strategy to perform simultaneous multifrequency MRE in the absence of MEGs: using RF pulses designed via the Optimal Control (OC) theory. Such pulses control the spatial distribution of the MRI magnetization phase so that the resulting transverse magnetization reproduces the phase pattern of an MRE acquisition. The pulse is applied with a constant gradient during the multifrequency mechanical excitation to simultaneously achieve slice selection and motion encoding. The phase offset sampling strategy can be adapted according to the excitation frequencies to reduce the acquisition time. Phantom experiments were run to compare the classical monofrequency MRE to the OC based dual-frequency MRE method and showed excellent agreement between the reconstructed shear storage modulus G′. Our method could be applied to simultaneously acquire low and high frequency components, which are difficult to encode with the classical MEG MRE strategy.

Wavelet MRE: Imaging propagating broadband acoustic waves with wavelet‐based motion‐encoding gradients

“…In most of the previous studies, MRE images were acquired at each frequency monochromatically. Some MRE techniques have been developed to acquire multifrequency information simultaneously 33–36 . These techniques usually make use of the fact that one MEG shape can encode more than one motion frequency 37 .…”

“…Some MRE techniques have been developed to acquire multifrequency information simultaneously. [33][34][35][36] These techniques usually make use of the fact that one MEG shape can encode more than one motion frequency. 37 However, this approach requires careful planning in terms of MEG shape and frequency as well as the arrangement of the phase offsets.…”