Supersonic transient magnetic resonance elastography for quantitative assessment of tissue elasticity

Abstract

Non-invasive, quantitative methods to assess the properties of biological tissues are needed for many therapeutic and tissue engineering applications. Magnetic resonance elastography (MRE) has historically relied on external vibration to generate periodic shear waves. In order to focally assess a biomaterial or to monitor the response to ablative therapy, the interrogation of a specific region of interest by a focused beam is desirable and transient MRE (t-MRE) techniques have previously been developed to accomplish this goal. Also, strategies employing a series of discrete ultrasound pulses directed to increasing depths along a single line-of-sight have been designed to generate a quasi-planar shear wave. Such ‘supersonic’ excitations have been applied for ultrasound elasticity measurements. The resulting shear wave is higher in amplitude than that generated from a single excitation and the properties of the media are simply visualized and quantified due to the quasiplanar wave geometry and the opportunity to generate the wave at the site of interest. Here for the first time, we extend the application of supersonic methods by developing a protocol for supersonic transient magnetic resonance elastography (sst-MRE) using an MR-guided focused ultrasound system capable of therapeutic ablation. We apply the new protocol to quantify tissue elasticity in vitro using biologically-relevant inclusions and tissue-mimicking phantoms, compare the results with elasticity maps acquired with ultrasound shear wave elasticity imaging (US-SWEI), and validate both methods with mechanical testing. We found that a modified time-of-flight (TOF) method efficiently quantified shear modulus from sst-MRE data, and both the TOF and local inversion methods result in similar maps based on US-SWEI. With a three-pulse excitation, the proposed sst-MRE protocol was capable of visualizing quasi-planar shear waves propagating away from the excitation location and detecting differences in shear modulus of 1 kPa. The techniques demonstrated here have potential application in real-time in vivo lesion detection and monitoring, with particular significance for image-guided interventions.

Accuracy of real-time shear wave elastography in staging hepatic fibrosis: a meta-analysis

“…However, it is a one-dimensional imaging technology and needs special mechanical vibrating device to generate shear wave, and cannot be used for patients with ascites, obesity and stenosed rib space stenosis [38]. It cannot take two-dimensional images to avoid the nontarget structure in the liver [7,39]. SWE is a newer elastic imaging technology which is a newer variant of RTE [40].…”

Production of acoustic radiation force using ultrasound: methods and applications

“…Additionally, MR-ARFI has been used for applications in other soft tissue targets for evaluating the focus and refocusing [176, 177]. The MR-ARFI technique has also been utilized for mechanical property evaluation by measuring either induced displacements or shear wave velocities [178, 179].…”

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

“…Souchon et al showed that measuring transient shear waves in ex vivo biological tissues was also feasible . Using a small bore 7T scanner and large amplitude MEGs (140 mT/m), Liu et al demonstrated that 2D shear wave speed maps can be computed from these MR measured transient shear waves . Potential challenges to implementing this method clinically include the long acquisition time (~30 min for a 2D map) and use of MEG amplitudes that are not currently available on clinical whole‐body MRI systems.…”

“…A variety of ultrasound (US) and MR elastography (MRE) methods have been developed to measure the shear wave speed () or shear modulus () of tissues. In an ideal elastic non‐dissipative medium, the speed of a propagating shear wave is related to the shear modulus by:…”

“…Mechanical excitation is typically achieved using vibratory motion from an external oscillatory driver or the acoustic radiation force (ARF) from a focused US (FUS) transducer. 16 The use of either transient 6,[9][10][11][12] or harmonic (i.e., steady-state) 5,7,8 excitation modes have been explored. Harmonic MRE techniques are the most developed and have been used to detect liver fibrosis 17 and measure tissue mechanical properties in brain, breast, kidney, and heart.…”