Waveguide effects and implications for cardiac magnetic resonance elastography: A finite element study

Area of Science:

• Biomedical Engineering
• Medical Imaging
• Rheology

Background:

• Magnetic resonance elastography (MRE) is used to measure tissue stiffness.
• Wave propagation in small or thin structures can be affected by waveguide effects, biasing stiffness results.
• Common MRE processing methods may not adequately address these biases.

Purpose of the Study:

• To investigate biases and artifacts in MRE inversion results for small/thin structures and cardiac geometries.
• To evaluate the impact of processing techniques, including bandpass filtering and the curl operator, on MRE accuracy.
• To determine the necessity of 3D inversion and curl operator application for accurate shear modulus estimation.

Main Methods:

• Finite element models of idealized 2D/3D and realistic 3D cardiac geometries were created.
• Simulated MRE displacement data were generated across a frequency range (60-220 Hz).
• Displacement wave fields were inverted using direct inversion of the Helmholtz equation, with and without bandpass filtering and/or the curl operator.

Main Results:

• Significant biases and artifacts were observed in all geometries without the curl operator.
• Bandpass filtering alone was insufficient for accurate stiffness recovery.
• Accurate shear modulus recovery in 3D geometries required both 3D inversion and the curl operator.

Conclusions:

• Applying the curl operator to the wave field is essential for accurate MRE stiffness estimation in small structures.
• Full 3D inversion is necessary for accurate results in 3D geometries.
• These methods correct for waveguide effects and improve MRE accuracy in complex geometries like the heart.