Dynamic Elastography of Pre-stressed Material with Multi-Directional Excitation in a Table Top MRI System

There is a signi cant correlation between changes in mechanical properties and disease or injuries. For this reason were developed techniques, such as biopsy and manual palpation, to detect tissue's mechanical structure. These latter techniques are characterized by a lot of disadvantages. For instance, a biopsy is invasive and not reliable, and manual palpation is qualitative, super cial, and operator dependent. As a consequence of these drawbacks, two non-invasive techniques were developed: Ultrasound Elastography and Magnetic Resonance Elastography. This work is based on Magnetic Resonance Elastography, which can provide motion encoding simultaneously in three directions, is not depth-limited and provides a good resolution. This imaging technique utilizes standard MRI equipment and equipment and an actuator to generate vibrations transmitted to the analyzed tissue. In a method developed by researchers at UIC a decade ago, high frequency vibratory shear waves are induced and imaged in a small sample within a test tube by axially driving the test tube in the MRI system. This motion of the test tube, using a piezoelectric actuator, results in radially converging (geometrically focused) axially-polarized shear wave motion within the sample. More recently, using the same setup with a sample in the test tube, torsional vibratory motion has been induced in the test tube using a stepper motor, in order to drive torsionally polarized geometrically focused shear waves, which when compared to axially-polarized waves in the same sample, may elucidate it's anisotropy. In the present study, two innovations to this setup are considered. (1) In order to extend the frequency range of the torsional approach, which in turn improves its resolution, the stepper motor is replaced with two piezoelectric actuators that are con gured in a way to induce torsional motion. (2) In order to investigate the e ect of tensile pre-stress on shear wave motion in a sample, a new arrangement is designed, whereby the test tube is removed and the cylindricallyshaped sample hangs freely in the MRI and can be subjected to di erent known axial tensile pre-stresses while simultaneously performing MRE studies using both axially-polarized and torsionally-polarized shear waves. The measurements obtained using MRE are wave images representing the displacement eld in cross-sectional and sagittal slices within an isotropic cylindrical phantom under di erent pre-stresses. Experimental measurements are compared to computer simulations of the experiment using nite element analysis (FEA).