Disc degeneration assessed by quantitative T2* (T2 star) correlated with functional lumbar mechanics

Abstract

Study design: Experimental correlation study design to quantify features of disc health, including signal intensity and distinction between the annulus fibrosus and nucleus pulposus, with T2* magnetic resonance imaging (MRI) and correlate with the functional mechanics in corresponding motion segments.

Objective: Establish the relationship between disc health assessed by quantitative T2* MRI and functional lumbar mechanics.

Summary of background data: Degeneration leads to altered biochemistry in the disc, affecting the mechanical competence. Clinical routine MRI sequences are not adequate in detecting early changes in degeneration and fails to correlate with pain or improve patient stratification. Quantitative T2* relaxation time mapping probes biochemical features and may offer more sensitivity in assessing disc degeneration.

Methods: Cadaveric lumbar spines were imaged using quantitative T2* mapping, as well as conventional T2-weighted MRI sequences. Discs were graded by the Pfirrmann scale, and features of disc health, including signal intensity (T2* intensity area) and distinction between the annulus fibrosus and nucleus pulposus (transition zone slope), were quantified by T2*. Each motion segment was subjected to pure moment bending to determine range of motion (ROM), neutral zone (NZ), and bending stiffness.

Results: T2* intensity area and transition zone slope were significantly correlated with flexion ROM (P = 0.015; P = 0.002), ratio of NZ/ROM (P = 0.010; P = 0.028), and stiffness (P = 0.044; P = 0.026), as well as lateral bending NZ/ROM (P = 0.005; P = 0.010) and stiffness (P = 0.022; P = 0.029). T2* intensity area was also correlated with lateral bending ROM (P = 0.023). Pfirrmann grade was only correlated with lateral bending NZ/ROM (P = 0.001) and stiffness (P = 0.007).

Conclusion: T2* mapping is a sensitive quantitative method capable of detecting changes associated with disc degeneration. Features of disc health quantified with T2* predicted altered functional mechanics of the lumbar spine better than traditional Pfirrmann grading. This new methodology and analysis technique may enhance the assessment of degeneration and enable greater patient stratification for therapeutic strategies.

Level of evidence: N/A.

Figure 1. Magnetic Resonance Images of Lumbar Spine and Intervertebral Disc Using T2 and Quantitative T2* Techniques

Left: T2* Map of Lumbar Spine. Top Right: Healthy Lumbar Intervertebral disc imaged with classical T2 MRI. Bottom Right: Same disc using T2* map showcasing the five regions of interest from anterior to posterior (1 to 5).

Figure 2. Quantification of Imaging Parameters, Transition Zone Slope and T2* Intensity Area, from T2* Relaxation Profile in the Coronal Plane of ROI 3

Top Left: Coronal Profile of Healthy Disc with Plot of Average T2* Relaxation Time (ms) overlaid. Top Right: Corresponding quantification of Transition Zone Slope and T2* Intensity Area of the healthy disc. Bottom Left: Coronal Profile of Degenerated Disc with Plot of Average T2* Relaxation Time (ms) overlaid. Bottom Right: Corresponding quantification of Transition Zone Slope and T2* Intensity Area of degenerated disc.

Figure 3. Representative Motion Profile

A healthy functional unit motion profile with biomechanical metrics graphically defined. These include range of motion (ROM), neutral zone (NZ), and bending stiffness.

Figure 4. Representative Correlational Plots

Top: Lateral Bending Stiffness and Bottom: Lateral Bending NZ/ROM correlated with ROI 3 Average T2* Value (ms), Transition Zone Slope (ms/mm), and T2* Intensity Area (ms). r: Pearson’s correlational coefficient and p: p-value are displayed on corresponding plot.