Lumbar intervertebral disc characterization through quantitative MRI analysis: An automatic voxel-based relaxometry approach

Purpose To develop an automated pipeline based on convolutional neural networks to segment lumbar intervertebral discs and characterize their biochemical composition using voxel-based relaxometry, and establish local associations with clinical measures of disability, muscle changes, and other symptoms of lower back pain. Methods This work proposes a new methodology using MRI (n = 31, across the spectrum of disc degeneration) that combines deep learning-based segmentation, atlas-based registration, and statistical parametric mapping for voxel-based analysis of T-1 rho and T-2 relaxation time maps to characterize disc degeneration and its associated disability. Results Across degenerative grades, the segmentation algorithm produced accurate, high-confidence segmentations of the lumbar discs in two independent data sets. Manually and automatically extracted mean disc T-1 rho and T-2 relaxation times were in high agreement for all discs with minimal bias. On a voxel-by-voxel basis, imaging-based degenerative grades were strongly negatively correlated with T-1 rho and T-2, particularly in the nucleus. Stratifying patients by disability grades revealed significant differences in the relaxation maps between minimal/moderate versus severe disability: The average T-1 rho relaxation maps from the minimal/moderate disability group showed clear annulus nucleus distinction with a visible midline, whereas the severe disability group had lower average T-1 rho values with a homogeneous distribution. Conclusion This work presented a scalable pipeline for fast, automated assessment of disc relaxation times, and voxel-based relaxometry that overcomes limitations of current region of interest-based analysis methods and may enable greater insights and associations between disc degeneration, disability, and lower back pain.