Elastography Validity Criteria Definition Using Numerical Simulations and MR Acquisitions on a Low-Cost Structured Phantom

Abstract

MR Elastography is a novel technique enabling the quantification of mechanical properties in tissue with MRI. It relies on a three-step process that includes the generation of a mechanical vibration, motion capture using dedicated MR sequences, and data processing involving inversion algorithms. If not properly tuned to the targeted application, each of those steps may impact the final outcome, potentially causing diagnostic errors and thus eventually treatment mismanagement. Different approaches exist that account for acquisition or reconstruction errors, but simple tools and metrics for quality control shared by both developers and end-users are still missing. In this context, our goal is to provide an easily deployable workflow that uses generic validity criteria to assess the performance of a given MRE protocol, leveraging numerical simulations with an accessible experimental setup. Numerical simulations are used to help both determining sets of relevant acquisition parameters and assessing the data processing's robustness. Simple validity criteria were defined, and the overall pipeline was tested in a custom-built, structured phantom made of silicone-based material. The latter have the advantage of being inexpensive, easy to handle, facilitate the fabrication of complex structures which geometry resembles the anatomical structures of interest, and are longitudinally stable. In this work, we successfully tested and evaluated the overall performances of our entire MR Elastography pipeline using easy-to-implement and accessible tools that could ultimately translate in MRE standardized and cost-effective procedures.

Fast acquisition of propagating waves in humans with low-field MRI: Toward accessible MR elastography

“…The latter showed increased shear stiffness measured in phantom compartment A compared to compartment B, as expected from their composition and manual palpation. Such values are within the range observed in a previous study with similar silicone compositions ( 29 ). In vivo results, on the other hand, clearly show waves propagating along the muscle orientation with a pattern that follows the anatomical structures.…”

“…Overall, in our conditions, it is reasonable to favor acquisition acceleration by a factor of 10 and intrinsically discard high-frequency noise to obtain high SNR, which is key for reliable MRE ( 29 ), rather than to apply postprocessing filters to fully sampled scans. Filtering is quite common in MRE reconstruction pipelines, especially in those relying on spatial derivatives that are rather sensitive to noise ( 43 ).…”

“…The MRE phantom was created using a bicomponent silicone rubber (Eurosil 4 A+B, Schouten SynTec, the Netherlands) and a 3D-printed mold, following the method described in ( 29 ). The phantom (~10 cm by 11 cm by 14 cm) consisted of two 5-cm-wide compartments with different mechanical properties, obtained by varying the amount of softener (S) in the silicone mixture (1A:1B:2.5S and 1A:1B:1.5S).…”

“…The used custom vibration system, based on a loudspeaker and an air waveguide (Fig. 5D), is described in detail in ( 29 ) and is similar to the examples described in ( 13 , 48 ). In the present case, the lowest useful waveguide resonant frequencies were 49, 89, and 129 Hz.…”

Actuators for MRE: New Perspectives With Flexible Electroactive Materials

“…From an acquisition point of view, the quality of MRE data can be estimated from phase to noise ratio which in a first approximation is tied to the signal to noise ratio (SNR) and the encoded phase shift [25]. Obviously the SNR will critically rely on the acquisition parameters but also on the spin transverse relaxation time (T 2 *) of the tissue that can be really low for example in liver with iron overload [26] or in lungs [27] and lead to very low SNR.…”