Frequency‐dependent shear properties of annulus fibrosus and nucleus pulposus by magnetic resonance elastography

• Published in: NMR in biomedicine 2018-10, Vol.31 (10), p.e3918-n/a
• Main Authors: Beauchemin, P.F., Bayly, P.V., Garbow, J.R., Schmidt, J.L.S., Okamoto, R.J., Chériet, F., Périé, D.
• Format: Article
• Language: English
• Subjects: Aging; Animals; Annuli; annulus fibrosus; Annulus Fibrosus - physiology; Biological products; Biomarkers; Biomechanical Phenomena; Cattle; Degeneration; Elasticity Imaging Techniques; Frequency ranges; Humans; intervertebral disc; Intervertebral discs; Magnetic properties; magnetic resonance elastography; Magnetic Resonance Imaging; Mapping; Mechanical properties; NMR; Nuclear magnetic resonance; Nucleus pulposus; Nucleus Pulposus - physiology; Osmosis; Osmotic pressure; Piercing; Piezoelectric actuators; Piezoelectricity; Regression Analysis; Resonance; Shear modulus; Shear properties; Shear Strength; Spine; Stiffness; Vertebrae; Vibrations
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.3918

Aging and degeneration are associated with changes in mechanical properties in the intervertebral disc, generating interest in the establishment of mechanical properties as early biomarkers for the degenerative cascade. Magnetic resonance elastography (MRE) of the intervertebral disc is usually limited to the nucleus pulposus, as the annulus fibrosus is stiffer and less hydrated. The objective of this work was to adapt high‐frequency needle MRE to the characterization of the shear modulus of both the nucleus pulposus and annulus fibrosus. Bovine intervertebral discs were removed from fresh oxtails and characterized by needle MRE. The needle was inserted in the center of the disc and vibrations were generated by an amplified piezoelectric actuator. MRE acquisitions were performed on a 4.7‐T small‐animal MR scanner using a spin echo sequence with sinusoidal motion encoding gradients. Acquisitions were repeated over a frequency range of 1000–1800 Hz. The local frequency estimation inversion algorithm was used to compute the shear modulus. Stiffness maps allowed the visualization of the soft nucleus pulposus surrounded by the stiffer annulus fibrosus surrounded by the homogeneous gel. A significant difference in shear modulus between the nucleus pulposus and annulus fibrosus, and an increase in the shear modulus with excitation frequency, were observed, in agreement with the literature. This study demonstrates that global characterization of both the nucleus pulposus and annulus fibrosus of the intervertebral disc is possible with needle MRE using a preclinical magnetic resonance imaging (MRI) scanner. MRE can be a powerful method for the mapping of the complex properties of the intervertebral disc. The developed method could be adapted for in situ use by preserving adjacent vertebrae and puncturing the side of the intervertebral disc, thereby allowing an assessment of the contribution of osmotic pressure to the mechanical behavior of the intervertebral disc. This study demonstrated that global characterization of the intervertebral disc was possible with needle Magnetic Resonance Elastography (MRE) using a preclinical MRI scanner. A significant difference in shear moduli between the nucleus pulposus and annulus fibrosus and an increase of shear modulus with excitation frequency were observed. MRE can be a powerful method for mapping the complex properties of the intervertebral disc without requiring multiple tests and biopsies.