Magnetic resonance elastography at 3 tesla: Implementation, validation and application to a degenerative disc model.

摘要

Magnetic resonance (MR) elastography has been increasingly attractive for medical research as it provides a non-invasive method of determining the shear mechanical properties of soft tissues, and therefore offers the potential to provide a novel and sensitive measure of tissue changes related to health and disease. The present work develops a MR elastography technique to evaluate ex vivo intervertebral disc tissue samples. The implementation is described across the features of the MR scanner software, necessary experimental hardware, phantom and tissue preparation, and image post-processing.; The system was characterized across several variables, but the focus of the work shows the effect of motion encoding waveform shape as well as the number and frequency of applied motion and motion encoding cycles. The results of this evaluation found that trapezoidal motion encoding gradients perform better than sinusoidal motion encoding gradients. This result is supported by theoretical derivations. In addition, while relatively high frequencies of operation were achieved on the present system, it was found that an operating frequency of 500 Hz provided the best operational compromise.; The method was applied to quantitatively measure the shear modulus of agar gel samples and bovine intervertebral disc nucleus pulposus tissue. The agar gel measurements compared well with those from traditional compressive mechanical testing. The fitted linear regression lines for the shear modulus from the two methods against agar gel concentration were found to be equivalent. For the bovine tissue, the method was found to effectively determine the change in stiffness of the tissue due to enzymatic degradation using papain. The shear modulus was found to significantly decrease from an average value of 60 kPa before injection to 44 kPa after injection. T2 values were found to increase significantly with degradation, while T1 values remained unchanged.; The work presented here details the implementation and validation of an MR elastography system, and its novel application to the study of intervertebral disc tissue. The method is found to be sensitive to tissue changes, and provides a foundation of future work of applying the method for the detection and tracking of early degenerative changes in disc tissue in vivo.