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

Pilar Sango-Solanas,Fabien Millioz,Kevin Tse Ve Koon,Helene Ratiney,Olivier Beuf,Eric Van Reeth,Cyrielle Caussy

doi:10.1038/s41598-022-05262-3

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.

Highlights

Magnetic Resonance Elastography (MRE) is a valuable non-invasive technique for the quantification of mechanical properties of tissues based on the measurement of the characteristics of shear waves propagating through the organ of interest[1]
The motion encoding gradients (MEG) is placed after the excitation and before signal acquisition and it oscillates at the same frequency as the mechanical excitation
The minimum echo time TE is fixed by the duration of the MEG, resulting in a degradation of the signal to noise ratio (SNR), especially when low excitation frequencies are used on tissues having short T2 values

Summary

Introduction

Magnetic Resonance Elastography (MRE) is a valuable non-invasive technique for the quantification of mechanical properties of tissues based on the measurement of the characteristics of shear waves propagating through the organ of interest[1]. Afterwards, Muthupillai et al.[1] proposed the application of oscillating motion encoding gradients (MEG) synchronized to the mechanical excitation to encode nuclear spins motion into the phase of the NMR signal This technique has been largely exploited over the last decades and has become the classical strategy to carry out MRE. Instead of using oscillating gradients to encode the oscillatory motion, the usage of radiofrequency (RF) field gradients was proposed to detect motion from spectroscopic data by relating the NMR signal as a function of the wave frequency e xcitation[24,25] This method was validated to acquire 2D magnitude images showing shear waves propagating at frequencies in the kilohertz range in phantom experiments[26]. This method was limited in terms of RF power since a very high power was needed to obtain the desired B1 gradient

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