Evaluation of human cartilage endplate composition using MRI: Spatial variation, association with adjacent disc degeneration, and in vivo repeatability

Abstract

Cartilage endplate (CEP) biochemical composition may influence disc degeneration and regeneration. However, evaluating CEP composition in patients remains a challenge. We used T2* mapping from ultrashort echo-time (UTE) magnetic resonance imaging (MRI), which is sensitive to CEP hydration, to investigate spatial variations in CEP T2* values and to determine how CEP T2* values correlate with adjacent disc degeneration. Thirteen human cadavers (56.4 ± 12.7 years) and seven volunteers (36.9 ± 10.9 years) underwent 3T MRI, including UTE and T1ρ mapping sequences. Spatial mappings of T2* values in L4-S1 CEPs were generated from UTE images and compared between subregions. In the abutting discs, mean T1ρ values in the nucleus pulposus were compared between CEPs with high vs low T2* values. To assess in vivo repeatability, precision errors in mean T2* values, and intraclass correlation coefficients (ICC) were measured from repeat scans. Results showed that CEP T2* values were highest centrally and lowest posteriorly. In the youngest individuals (<50 years), who had mild-to-moderately degenerated Pfirrmann grade II-III discs, low CEP T2* values associated with severer disc degeneration: T1ρ values were 26.7% lower in subjects with low CEP T2* values (P = .025). In older individuals, CEP T2* values did not associate with disc degeneration (P = .39-.62). Precision errors in T2* ranged from 1.7 to 2.6 ms, and reliability was good-to-excellent (ICC = 0.89-0.94). These findings suggest that deficits in CEP composition, as indicated by low T2* values, associate with severer disc degeneration during the mild-to-moderate stages. Measuring CEP T2* values with UTE MRI may clarify the role of CEP composition in patients with mild-to-moderate disc degeneration.

Keywords: T2*; cartilage endplate; intervertebral disc degeneration; low back pain; magnetic resonance imaging (MRI).

Figure 1

Diagram showing the standardized mapping of the CEP that was used to define subregions. The origin of the CEP was located at its area centroid. The central subregion had a radius that was 50% of the length of the outer CEP margin. The remaining area was divided into four subregions (anterior: 30°‐150°, left: 150°‐210°, posterior: 210°‐330°, right: ±30°). The number of voxels in each of the five subregions of the CEP was as follows: central approximately 580 to 880, anterior approximately 570 to 860, posterior approximately 530 to 820, left/right approximately 490 to 780. The number of slices that made up each subregion was as follows: central 25 to 44, anterior 40 to 71, posterior 46 to 80, left/right 13 to 23. CEP, cartilage endplate [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

A, Midsagittal UTE image from a 53‐year‐old male showed high CEP signal intensity. B, Zoomed inset showing the location of the NP ROI for investigating the association between NP T1ρ values and CEP T2* values. ROIs for the NP had between 160 and 230 voxels. T1ρ values were averaged for NP voxels in the caudal half of the disc bordering the central CEP subregion. CEP, cartilage endplate; NP, nucleus pulposus; UTE, ultrashort echo‐time [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

A, Mean T2* values for each subregion of n = 16 cadaveric CEPs imaged in situ. Mean CEP T2* values were highest centrally and lowest posteriorly. B, Pairwise differences in T2* values between CEP subregions. P values show the result of paired t tests between subregions. T2* values in the posterior subregion were significantly lower than those in the central, left, and right subregions. Pairwise differences between T2* values in the other subregions were not significant. CEP, cartilage endplate [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Scatterplots show relationships between NP T1ρ, CEP T2*, and age for n = 23 discs pooled. A, Age was inversely associated with mean T1ρ values in the NP. B, Age was inversely associated with mean T2* values in the central CEP. C, Mean NP T1ρ values were not significantly associated with mean T2* values in the central CEP. CEP, cartilage endplate; NP, nucleus pulposus [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

A, Discs were split into three similarly sized age groups (n = 8, 7, 8 discs). In the youngest age group (<50 years), discs that were adjacent to CEPs with the shortest T2* values had lower mean T1ρ values (P = .025). In the older age groups, mean NP T1ρ values were similar between discs with the highest vs lowest CEP T2* values (51‐60 years: P = .62; >60 years: P = .39). B, Representative CEP T2* maps and NP T1ρ maps (three midsagittal slices) from each age group. For ease of 2D visualization, the T2* maps were projected onto the z‐x plane. In the youngest age group (age <50 years), compared with the NP of M27, NP of F38, adjacent to CEP with shorter mean T2* relaxation time, had lower mean T1ρ value. In the older age groups, NP T1ρ values were similar for discs adjacent to CEPs with low vs high T2* values. CEP, cartilage endplate; NP, nucleus pulposus [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Overlapping T2* histograms show low inter‐test variability relative to inter‐subject variability. In each histogram, the amount of tissue is calculated by dividing the number of voxels with T2* values within a given range by the total number of voxels in that particular CEP. CEP, cartilage endplate [Color figure can be viewed at wileyonlinelibrary.com]