Is fractal dimension a reliable imaging biomarker for the quantitative classification of an intervertebral disk?

Abstract

This study aimed to explore the feasibility of the fractal method used in decoding disk heterogeneity, hoping to find a reliable imaging biomarker for the quantitative and continuous grading of intervertebral disks (IVDs). Totally, 180 IVDs in 65 low back pain patients (29 males, 36 female, 28-69years) were examined with MRI. Each IVD was manually segmented on axial slice (at the mid-height layer of the disk). All disks were visually evaluated regarding degeneration grade, using Pfirrmann classification, by two experienced radiologists. Fractal dimension (FD) of the IVD was calculated from the defined regions of interest and correlated with Pfirrmann grade. Fractal dimension differed significantly between any two groups (P < 0.01). The mean FDs for the four grades were as follows: Pfirrmann 1: 1.13 ± 0.02; Pfirrmann 2: 1.30 ± 0.05; Pfirrmann 3: 1.50 ± 0.05; and Pfirrmann 4: 1.65 ± 0.02. The well-hydrated IVDs displayed low fractal dimension. Degenerated IVDs displayed increased fractal dimension caused by disk heterogeneity, where the fractal dimension was shown to correlate strongly with Pfirrmann grade. Fractal dimension associated well with IVD degeneration, determined with Pfirrmann grading, suggesting that the IVD fractal analysis was a suitable detection tool for the objective and continuous classification of IVD degeneration. These slides can be retrieved under Electronic Supplementary Material.