Quantification of the mechanical response of the spine to shear vibration and correlation to intervertebral disc disease

This dissertation investigates a novel method to non-invasively assess the mechanical response of the spine to shear vibration using magnetic resonance elastography (MRE). There are many imaging techniques to characterize disc degeneration; however, limited work has been done to assess the material properties of the intervertebral disc (IVD) within the intact spine. Also, existing imaging methods are limited in the detection of early stage changes with disc degeneration. MRE is a MRI-based technique for non-invasively mapping the mechanical properties of tissues in vivo. The general hypothesis we are testing in this work is that mechanical changes in the intervertebral disc with early disc degeneration are detectable due to abnormal shear vibration propagation across the disc space using MRE and may precede changes visible using standard anatomical imaging methods. In this work, MRE was demonstrated in isolated lumbar spinal motion segments at a range of high mechanical frequencies (1-1.5 kHz) using a standard clinical 1.5T MR scanner. Propagating waves were visualized in an axial cross-section of the IVD, which resembled wave patterns predicted using finite element modeling and were used to estimate the shear stiffness of the nucleus pulposus region of the IVD. In addition, an MRE technique was developed to visualize low frequency shear vibration (80 Hz) throughout the thoracolumbar spine in vitro. Mechanical changes induced in an animal model of early-stage disc degeneration were quantified using MRE and mechanical testing of disc laxity. MRE was able to detect altered mechanical properties from discogenic instability in the spine segments, which was verified with mechanical testing results. Further, this work demonstrates the feasibility of in vivo spine MRE in a healthy volunteer to visualize shear wave propagation throughout the thoracolumbar spine. MRE may be valuable for the diagnosis of spinal instability and early disc degeneration, as well as monitoring changes in disc health.