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

Abstract

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.

Overall Structure Construction of an Intervertebral Disk Based on Highly Anisotropic Wood Hydrogel Composite Materials with Mechanical Matching and Buckling Buffering

“…Besides, in Figure c, on account of the internal highly aligned fiber frame body, WF-IVD shows enhanced shear characteristics, which is consistent with the shear modulus of natural IVD. To vividly demonstrate the mechanical matching properties, the elastic modulus and shear modulus were calculated, and the modulus comparison of materials related to IVD is shown in Figure d. − The result has visually demonstrated that our work achieves the modulus matching with native IVD.…”

Magnetic Resonance Elastography of Intervertebral Discs: Spin‐Echo Echo‐Planar Imaging Sequence Validation

“…For example, Ben-Abraham et al 35 reported an average stiffness of 79 AE 15 kPa in baboon NP tissue using a spin-echo-based MRE sequence at 1000 Hz, and Cortes et al 24 reported an average stiffness value of 661 kPa for human NP tissue from normal IVDs (Pfirrmann score 1 and 2) using a gradient echo pulse sequence at 1250 Hz. Beauchemin et al 36 reported values of 56 AE 0.4 kPa and 85 AE 2 kPa in bovine NP and AF tissue, respectively, using a spin-echo sequence at 1200 Hz. Our results demonstrated lower NP stiffness values with an average of 19 AE 2 kPa and greater AF stiffness values with an average of 105 AE 15 kPa.…”

Degeneration alters structure‐function relationships at multiple length‐scales and across interfaces in human intervertebral discs

“…It will be of critical importance in future studies to parse out the time course of these functional changes to further our understanding of the pathophysiology of degeneration, and to aid in the early detection of disease. This may be facilitated by the use of techniques such as MR elastography for the in vivo non‐invasive assessment of regional tissues (Beauchemin et al, 2018; Cortes et al, 2014; Walter et al, 2017). Additionally, recent applications of high‐resolution imaging techniques, such as ultrashort echo time MRI (Berg‐Johansen et al, 2017b; Fields et al, 2015), have shown great promise in evaluating degenerative changes to the cartilage endplate and may also be of potential clinical interest given the changes we observed at this location.…”