Efficient shear wave elastography using transient acoustic radiation force excitations and MR displacement encoding

Abstract

Purpose To present a novel MR shear wave elastography (MR‐SWE) method that efficiently measures the speed of propagating wave packets generated using acoustic radiation force (ARF) impulses. Methods ARF impulses from a focused ultrasound (FUS) transducer were applied sequentially to a preselected set of positions and motion encoded MRI was used to acquire volumetric images of the propagating shear wavefront emanating from each point. The wavefront position at multiple propagation times was encoded in the MR phase image using a train of motion encoding gradient lobes. Generating a transient propagating wavefront at multiple spatial positions and sampling each at multiple time‐points allowed for shear wave speed maps to be efficiently created. MR‐SWE was evaluated in tissue mimicking phantoms and ex vivo bovine liver tissue before and after ablation. Results MR‐SWE maps, covering an in‐plane area of ~5 × 5 cm, were acquired in 12 s for a single slice and 144 s for a volumetric scan. MR‐SWE detected inclusions of differing stiffness in a phantom experiment. In bovine liver, mean shear wave speed significantly increased from 1.65 ± 0.18 m/s in normal to 2.52 ± 0.18 m/s in ablated region (n = 581 pixels; P‐value < 0.001). Conclusion MR‐SWE is an elastography technique that enables precise targeting and excitation of the desired tissue of interest. MR‐SWE may be particularly well suited for treatment planning and endpoint assessment of MR‐guided FUS procedures because the same device used for therapy can be used as an excitation source for tissue stiffness quantification.

Simultaneous fat‐referenced proton resonance frequency shift thermometry and MR elastography for the monitoring of thermal ablations

“…Finally, combining an external MRE exciter with interventional material (HIFU or percutaneous) may be particularly challenging in terms of spatial access. Translating this setup in vivo would certainly require using interventional‐specific excitation strategies such as percutaneous vibrating needles or the acoustic radiation force . However, this investigation falls beyond the scope of this paper, which aims at demonstrating the feasibility of performing simultaneous FRPRFS MR tomography and MRE with IDEAL fat‐water separation.…”

“…More recently, Hofstetter et al developed an MRE approach that relies on shear wave generation using acoustic radiation force pushes at multiple locations and composite reconstruction, allowing for elasticity mapping. This method was used successfully to assess elasticity changes following high‐intensity focused ultrasound (HIFU) ablation …”

“…This method was used successfully to assess elasticity changes following high-intensity focused ultrasound (HIFU) ablation. 19 All of these studies have used conventional PRFS thermometry so that their use in fat-containing tissues may lead to significant errors in temperature estimation. The general objective of the proposed work is to extend the interventional MRE/MR thermometry method to any type of soft tissues, including fat-containing tissues.…”

Methods of monitoring thermal ablation of soft tissue tumors – A comprehensive review

“…Here the simulations were performed offline with 1.5-3 s of computation time, but it is anticipated that this can be reduced to near real-time. The advantage of this approach over the methods used by Bour et al 187 and Hoffstetter et al 188 is that it provides a direct metric of tissue elasticity rather than just a measurement of the ARFI induced displacement or shear wave speed which are influenced by the mechanical properties of surrounding tissues, tissue heterogeneity and acoustic absorption at the focal spot. Additionally,shear wave measurements involve spatial averaging and waves must move a minimal distance to enable calculation of the wave speed.…”

Development and characterization of a tissue mimicking psyllium husk gelatin phantom for ultrasound and magnetic resonance imaging

“…Thermal dose [10] calculated from temperature change maps can be used independently or in conjunction with post-treatment contrast-enhanced imaging to evaluate the volume of the treated tissue. MR acoustic radiation force imaging (MR-ARFI) techniques [11][12][13] and MR-based elastography measurements [14][15][16] could also prove useful for HIFU treatment monitoring by providing additional information about the mechanical state of the tissue before, during, and after treatment.…”