Reverberant Magnetic Resonance Elastographic Imaging Using a Single Mechanical Driver

Irteza Enan Kabir

Supervised by Marvin M. Doyley

Computer Studies Building, Room 523

Magnetic Resonance Elastography (MRE) is a noninvasive in vivo imaging technique used to assess tissue mechanical properties. It has found success in various applications, such as breast, brain, muscles, and lungs. Researchers have developed different approaches to compute shear modulus, each with its balance of accuracy and computational efficiency. Several direct inversion algorithms have been proposed in the literature, which algebraically solve for the complex shear modulus from the Helmholtz equation. Although these direct inversion schemes are fast and accurate, they are more susceptible to noise. On the other hand, iterative inversion methods can accurately model complex, viscoelastic tissues but are computationally intensive.

Our long-term objective is to integrate MRE into our clinical workflow. More specifically, to develop methods to provide accurate MR shear modulus elastograms at the MR console when imaging the brain. In this work, we seek to estimate the local wavelength of complex wave fields using a technique known as reverberant elastography. Reverberant elastography provides fast and robust estimates of shear modulus; however, its reliance on multiple mechanical drivers hampers clinical utility. More specifically, performing MRE with multiple drivers is impractical for many clinical applications and could prove uncomfortable for patients. Our hypothesis is that, for constrained organs like the brain, reverberant elastography can generate accurate MR elastograms using just one mechanical driver.

To test this hypothesis, we initially conducted experiments on constrained phantoms with uniform and non-uniform boundaries. We used a similarity metric to quantify the extent of reverberance in different displacement com- ponents. We then extended our study to healthy volunteer brains, exploring age-related changes in brain mechanical properties. Finally, we assessed the feasibility of using a single shaker to create a reverberant field in an un- constrained medium. We designed unconstrained phantoms for this purpose. The results in this thesis highlight the effectiveness of reverberant shear wave elastography as a tool for estimating stiffness, serving as a clinical imaging biomarker.