Associations between vertebral body fat fraction and intervertebral disc biochemical composition as assessed by quantitative MRI

Abstract

Background: There is an interplay between the intervertebral disc (IVD) and the adjacent bone marrow that may play a role in the development of IVD degeneration and might influence chronic lower back pain (CLBP).

Purpose: To apply novel quantitative MRI techniques to assess the relationship between vertebral bone marrow fat (BMF) and biochemical changes in the adjacent IVD.

Study type: Prospective.

Subjects: Forty-six subjects (26 female and 20 male) with a mean age of 47.3 ± 12.0 years.

Field strength/sequence: 3 T MRI; a combined T_1ρ and T₂ mapping pulse sequence and a 3D spoiled gradient recalled sequence with six echoes and iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) reconstruction algorithm.

Assessment: Using quantitative MRI, the vertebral BMF fraction was measured as well as the biochemical composition (proteoglycan and collagen content) of the IVD. Furthermore, clinical Pfirrmann grading, Oswestry disability index (ODI), and visual analog scale (VAS) was assessed.

Statistical tests: Mixed random effects models accounting for multiple measurements per subject were used to assess the relationships between disc measurements and BMF.

Results: The relationships between BMF (mean) and T_1ρ /T₂ (mean and SD) were significant, with P < 0.05. Significant associations (P < 0.001) were found between clinical scores (Pfirrmann, ODI, and VAS) with T_1ρ /T₂ (mean and SD). BMF mean was significantly related to ODI (P = 0.037) and VAS (P = 0.043), but not with Pfirrmann (P = 0.451). In contrast, BMF SD was significantly related to Pfirrmann (P = 0.000) but not to ODI (P = 0.064) and VAS (P = 0.13).

Data conclusion: Our study demonstrates significant associations between BMF and biochemical changes in the adjacent IVD, both assessed by quantitative MRI; this may suggest that the conversion of hematopoietic bone marrow to fatty bone marrow impairs the supply of available nutrients to cells in the IVD and may thereby accelerate disc degeneration.

Level of evidence: 2 Technical Efficacy Stage: 3 J. Magn. Reson. Imaging 2019;50:1219-1226.

Keywords: bone marrow fat; chronic lower back pain; intervertebral disc degeneration; spine.

Figure 1:

Shown is an example of T_1ρ and T₂ data fitting. Four data points were acquired for each method (4 TSL times for T_1ρ and 4 TE times for T₂). Using the equations shown, a mono-exponential signal decay is fitted to the acquired data for each pixel. The resulting T_1ρ and T₂ values from a 38 year old subject without LBP and Pfirrmann grade 1 for all discs are depicted on the color maps on the right.

Figure 2:

Shown are T₁-weighted FSE images along with the corresponding BMF fat fraction maps of a subject with low Pfirrmann grading (a and b) and a subject with high Pfirrmann grading (c and d). The left image a) had grade 1 for all discs whereas the right image c) had grade 2,2,3,3, and 5 from L1 to L5. Absolute BMF content was assessed by IDEAL MRI and is reflected by the color maps in b and d. These color-coded maps reflect the fat fraction in the bone marrow on a pixel-by-pixel basis. Higher absolute BMF content as well as larger BMF variations can be appreciated in d compared to b. Although absolute fat content cannot be assessed by clinical T₁-weighted FSE, a larger variation in signal intensity that reflects BMF variations can also be qualitatively determined in c compared to a.

Figure 3:

This figure shows the mean and SD of BMF fat fraction for each lumbar level separately. A T₁-weighted FSE image is shown on the left to illustrate the different lumbar levels. A significant (p<0.05) increase in BMF was found between each level going from 38% fat fraction in L1 up to 44% fat fraction in L5.

Figure 4:

The graph shows the associations between the vertebral body BMF mean and SD with T_1ρ / T₂ mean for each lumbar level. An inverse relationship is clearly demonstrated for each level. Only at L5/S1 the T_1ρ / T₂ mean values are low (degenerated disc) for different vertebral fat fractions. However, T_1ρ / T₂ varied much more for different BMF variations (SD).

Figure 5:

This figure shows T₂-weighted FSE images, T_1ρ and T₂ relaxation maps as well as corresponding BMF fact fraction maps of a subject with low Pfirrmann grading (a, b, c, and d) as well as a subject with high Pfirrmann grading (e, f, g, and h). The subjects with low Pfirrmann grading shows higher T_1ρ and T₂ values as well as lower BMF fat fraction content compared to the subject with high Pfirrmann grading. These result reflect the inverse relationships found between T_1ρ and T₂ values and BMF as well as the associations between Pfirrmann and BMF as well as Pfirrmann and T_1ρ and T₂.