(G-267) Rheological Validation of Magnetic Resonance Elastography-Derived Intervertebral Disc Shear Properties

Introduction:: Degeneration of the intervertebral disc (IVD) is associated with low back pain and characterized by progressive changes in the tissue’s material properties with advancing disease [1-5]. Magnetic resonance elastography (MRE) is a non-invasive imaging technique that allows in-vivo assessment of shear material properties and has previously been used to demonstrate that in-vivo IVD shear stiffness changes with degeneration [1-3]. However, while MRE provides a relative measure of changes in shear properties in the IVD, the relationship between the ‘true’ shear properties as assessed via torsional mechanical testing has yet to be determined in IVD tissue at the same frequencies. This study aims to validate MRE by creating a spectrum of hydrated agarose gels and IVD tissue to compare the MRE-derived stiffnesses to the rheologically-derived stiffnesses at multiple frequencies.

Materials and Methods:: For initial agarose validation, three batches of 2% and 4% (w/v) agarose were created to provide a spectrum of hydration. Agarose samples were cast in a ~120mm x 120mm x 30mm plastic container and scanned with a spin-echo echo-planar imaging (SE-EPI) MRE sequence at 80 and 100Hz using a 3T MR scanner (Siemens). Shear stiffness was calculated using a principal frequency analysis algorithm. Separate agarose samples (N=9/%agarose, 20mm Ø x 1.5mm plugs) that were cast simultaneously as those for MRE were used for shear mechanical testing on a rheometer (TA Instruments). The loading protocol consisted of an initial 600s preload of 0.1N, followed by a frequency sweep from 1-100Hz at a rotational strain of 0.05%. For validation on tissue, IVDs were isolated from the C1/2 or C2/3 level from 16 bovine tails, placed in dialysis tubing (cutoff 1 kDa), and loaded via equilibrium dialysis for 48 hours to create a spectrum of hydrated tissues. They were distributed into one of two different osmotic conditions (N=15/%PEG): 5% and 25% (g/mL) 20 kDa PEG in 0.15 mol/L NaCl which exert osmotic pressures of 0.027 and 0.565 MPa, respectively [7]. The IVDs were then embedded in 2% agarose for MRE analysis. After scans, the NP region was excised and samples prepared for mechanical testing (8mm Ø x 2mm thick plugs). IVD samples were preloaded and underwent a frequency sweep as described above.

Results, Conclusions, and Discussions:: The MRE-derived and rheologically-derived shear properties were assessed in agarose, providing a controllable homogenous material, and in IVD tissue in which the shear properties were altered via equilibrium dialysis. Results from agarose demonstrated that in both MRE-derived and rheologically-derived measurements of complex shear modulus, 4% samples were stiffer than 2% samples, and both techniques exhibited the expected frequency dependence with shear modulus increasing as frequency increased (Fig 1A-B). In addition, the magnitude of the measurements was similar between measurement techniques, i.e., 2% agarose @ 100Hz G^*_MRE = 26.1 ± 4.5kPa, G^*_Rheo = 29.3 ± 1.2kPa. Results from PEG-equilibrated tissue demonstrated that 25% samples were stiffer than 5% samples, and both techniques exhibited the expected frequency dependence (Fig 1C-D). However, the magnitude of the shear modulus values was different between MRE and rheology. This is likely influenced by the difference in homogeneity between agarose and NP samples and by the contribution of inertial effects during mechanical testing. A limitation of rheology is that smaller diameter samples (Ø_Agarose = 20mm / Ø_NP = 8mm) are more susceptible to inertial effects, and due to the limited size of the tissue, 8mm diameter plugs could only be extracted from the NP region. Despite this limitation, all tests occurred within the linear viscoelastic region and results demonstrated no evidence of sample slippage. In both agarose and tissue, the more hydrated samples had a significantly lower shear modulus. This hydration associated changes in shear modulus is consistent with previous studies that used either MRE [1-3] or rheologic [4,5] measurements on healthy and degenerated IVD tissue and found that healthy NP tissue, with a higher water content, exhibited lower shear stiffness values compared to degenerated NP tissue. Overall, results in both agarose and IVD tissue demonstrate that MRE and rheology can detect differences in shear properties with changing hydration and frequency. Collectively, this study validates MRE as a technique to measure the relative shear properties of IVD tissue by comparing it to values obtained by mechanical testing.


Acknowledgements (Optional): We would like to acknowledge Dr. Aleksander Skardal for the use of the rheometer and helpful technical conversations. Funded by NIAMS R01AR075062.

References (Optional): : [1]Streitberger+ MRM 2015 [2]Cortes+ MRM 2014 [3]Walter+Rad.2017 [4]Iatridis+ J.Orthop [5]Antoniou+ J.MRI [6]Bron+ J.Orthop [7]Chahine+ Biophys J 2005