The simulation of magnetic resonance elastography through atherosclerosis

Abstract

The clinical diagnosis of atherosclerosis via the measurement of stenosis size is widely acknowledged as an imperfect criterion. The vulnerability of an atherosclerotic plaque to rupture is associated with its mechanical properties. The potential to image these mechanical properties using magnetic resonance elastography (MRE) was investigated through synthetic datasets. An image of the steady state wave propagation, equivalent to the first harmonic, can be extracted directly from finite element analysis. Inversion of this displacement data yields a map of the shear modulus, known as an elastogram. The variation of plaque composition, stenosis size, Gaussian noise, filter thresholds and excitation frequency were explored. A decreasing mean shear modulus with an increasing lipid composition was identified through all stenosis sizes. However the inversion algorithm showed sensitivity to parameter variation leading to artefacts which disrupted both the elastograms and quantitative trends. As noise was increased up to a realistic level, the contrast was maintained between the fully fibrous and lipid plaques but lost between the interim compositions. Although incorporating a Butterworth filter improved the performance of the algorithm, restrictive filter thresholds resulted in a reduction of the sensitivity of the algorithm to composition and noise variation. Increasing the excitation frequency improved the techniques ability to image the magnitude of the shear modulus and identify a contrast between compositions. In conclusion, whilst the technique has the potential to image the shear modulus of atherosclerotic plaques, future research will require the integration of a heterogeneous inversion algorithm.

Finite element analysis to investigate variability of MR elastography in the human thigh

“…Of note in this regard, Thomas-Seale et. al (2016) [19] used FEA to investigate arterial stenosis, demonstrating the ability of the technique to differentiate between lipid and fibrotic plaques. Meanwhile a study by McGrath et al (2015) [20] simulated 3-D wave propagation in the human brain, raising concerns over whether MRE would be able to detect the subtle change in stiffness through dementia over the errors in MRE measurements resulting from interference patterns and heterogeneity of human tissue.…”

Magnetic Resonance Elastography through Atherosclerosis: A Feasibility Study

“…The excitation frequency was set at 100 Hz based on the successful results obtained Thomas-Seale et al [24]. With reference to Figure 1c, the motion encoding directions are defined as follows; read out (RO) along the Y axis parallel to the compression wave, phase encoding (PE) along the Z axis parallel to the shear wave and slice select (SS) along the X axis.…”

“…A synthetic data set was created using the methodology outlined in Thomas-Seale et al [24]. This technique utilises a direct solution, steady state finite element (FE) analysis (Abaqus/CAE, Dassault Systèmes Simulia Corp., Providence, Rhode Island, USA) which allows the extraction of a complex wave image, analogous to the first harmonic aligned inside the vessel wall mould, as previously discussed, and surrounded by PVA.…”

“…3. The application of this algorithm to atherosclerosis and the associated limitations are discussed in depth in Thomas-Seale et al [24,39]. Noise and compression wave components were filtered using a 2D Butterworth filter [36,40].…”

“…Recent advances in the technique have identified regional changes in the vascular wall, through phantoms and ex vivo [22]. Computational research, through synthetic data sets, has demonstrated that MRE has the potential to identify a contrast between the wave images [23] and shear moduli [24] of atherosclerotic plaque compositions.…”