MR diffusion is sensitive to mechanical loading in human intervertebral disks ex vivo

Abstract

Purpose: To use T2 and diffusion MR to determine the change in the mechanical function of human disks with increased degenerative state.

Materials and methods: Spatial changes in T2 and diffusion were quantified in five cadaveric human lumbar disks under compressive loads. Regression models were used to investigate the relationship between the change in MR parameters and the disk's dynamic and viscoelastic properties.

Results: Compressive loading caused a significant reduction in the disk's mean diffusivity ([11.3 versus 9.7].10(-4) .mm(2) /s, P < 0.001) but little change in T2 (P < 0.05). Diffusivity and T2 were correlated with the disk's dynamic (P < 0.01 and P < 0.05) and long-term viscoelastic (P < 0.05 and P < 0.05) stiffness. Diffusivity but not T2, was correlated with its viscoelastic dampening (r(2) = 0.45, P < 0.01) and instantaneous stiffness (r(2) = 0.44, P < 0.05). Nucleus diffusivity was significantly higher than the annulus's (-21% to -4%, P < 0.01). MR-estimated hydration was correlated with the instantaneous viscoelastic stiffness of the nucleus (r(2) = 0.35, P < 0.05) and the dynamic (r(2) = 0.44, P < 0.05) and long-term viscoelastic (r(2) = 0.42, P < 0.05) stiffness in the annulus. T2 correlated with diffusivity at low load (r(2) = 0.66, P < 0.05), but not at high load.

Conclusion: The strong correlations between diffusivity and the rheological assessments of disk mechanics suggest that MR might permit quantitative assessment of disk functional status and structural integrity.

Keywords: MR diffusivity; T2 relaxation; degeneration; intervertebral disc; mechanical competence.

Figure 1.

A diagrammatic illustration of the MR mechanical imaging chamber. The brass screw (diameter of 10 mm, 1 mm pitch) is used to apply a displacement calibrated to pro- duce a loading of 200 and 800 N to the disk.

Figure 2.

Comparison of the cyclic and time-dependent response and corresponding analysis for a grade II and 72 year old grade III disks.

Figure 3.

Comparisons of the spatial maps obtained for T2 (A) and diffusion (B) values for the Grade II and the Grade III (72 year old) disks at 200N and in response to 800 N loading states. Segmentation templates used to obtain (8*8) pixel Region of Interests for computation of estimated hydration state within the nucleus, annulus and saline regions are illustrated.

Figure 4.

The effect of loading on the change in mean and variance of T2 and ADC measurements for the disks as classified by their degenerative grade.

Figure 5.

Statistical comparisons for the effects of Loading state, Age, Functional tissue (nucleus versus annulus) and location of measurement (main effect) on the mean the coefficient of variance parameters computed for the T2 relaxation and Apparent Diffusion Coefficient values.

Figure 6.

The effect of degenerative grade and loading on the mean T2 within the nucleus and annulus tissues.

Figure 7.

Statistical comparisons for the effect of anatomical axes of measurement for the complete disk joint (main effects) and the interaction of MR axes with Loading state and tissue type (nucleus versus annulus) on the change in mean and variance measures computed for Apparent Diffusion Coefficient values.