Noninvasive quantification of human nucleus pulposus pressure with use of T1rho-weighted magnetic resonance imaging

Abstract

Background: Early diagnosis is a challenge in the treatment of degenerative disc disease. A noninvasive biomarker detecting functional mechanics of the disc is needed. T1rho-weighted imaging, a spin-lock magnetic resonance imaging technique, has shown promise for meeting this need in in vivo studies demonstrating the clinical feasibility of evaluating both intervertebral discs and articular cartilage. The objectives of the present study were (1) to quantitatively determine the relationship between T1rho relaxation time and measures of nucleus pulposus mechanics, and (2) to evaluate whether the quantitative relationship of T1rho relaxation time with the degenerative grade and glycosaminoglycan content extend to more severe degeneration. It was hypothesized that the isometric swelling pressure and compressive modulus would be directly correlated with the T1rho relaxation time and the apparent permeability would be inversely correlated with the T1rho relaxation time.

Methods: Eight cadaver human lumbar spines were imaged to measure T1rho relaxation times. The nucleus pulposus tissue from the L1 disc through the S1 disc was tested in confined compression to determine the swelling pressure, compressive modulus, and permeability. The glycosaminoglycan and water contents were measured in adjacent tissue. Linear regression analyses were performed to examine the correlation between the T1rho relaxation time and the other measured variables. Mechanical properties and biochemical content were evaluated for differences associated with degeneration.

Results: A positive linear correlation was observed between the T1rho relaxation time on the images of the nucleus pulposus and the swelling pressure (r = 0.59), glycosaminoglycan content per dry weight (r = 0.69), glycosaminoglycan per wet weight (r = 0.49), and water content (r = 0.53). No significant correlations were observed between the T1rho relaxation time and the modulus or permeability. Similarly, the T1rho relaxation time, swelling pressure, glycosaminoglycan content per dry weight, and water content were significantly altered with degeneration, whereas the modulus and permeability were not.

Conclusions: T1rho-weighted magnetic resonance imaging has a strong potential as a quantitative biomarker of the mechanical function of the nucleus pulposus and of disc degeneration.

Fig. 1

Representative T2-weighted (left) and T1ρ-weighted (right) images of the spine of a fifteen-year-old male donor. The average degenerative grade is 1.43. The 5-mm-diameter circular regions of interest where the T1ρ relaxation times were determined are denoted by the black circles. Scale bar (small, horizontal white bar) = 10 mm. The numbers on the right of the red bar indicate the T1ρ relaxation time.

Fig. 2

Correlation between T1ρ relaxation time and degenerative grade (A) and between T1ρ relaxation time and age (B). The asterisks denote significance (p < 0.05). The T1ρ relaxation times in this in vitro study (filled triangles) were within the range of values in a previous in vivo study of ten asymptomatic forty to sixty-year-old subjects (open circles).

Fig. 3

Comparison of T1ρ relaxation time (A), swelling pressure (P_sw) (B), and sulfated-glycosaminoglycan content (s-GAG) per dry weight between non-degenerated and degenerated discs (C). The asterisks denote significance (p < 0.05). The biochemistry and T1ρ data for sixteen of the thirty-three discs were previously reported.

Fig. 4

Correlation between T1ρ relaxation time and swelling pressure (P_sw) (A) and between swelling pressure and sulfated-glycosaminoglycan content (s-GAG) per dry weight (B). The asterisks denote significance (p < 0.05).

Fig. 5

Correlation between T1ρ relaxation time and sulfated-glycosaminoglycan content (s-GAG) per dry weight (A) and between T1ρ relaxation time and water content (B). The asterisks denote significance (p < 0.05).