Introduction

Low back pain (LBP) is one of the main causes leading to disability worldwide, resulting in a great burden on global health care system [1, 2]. A number of studies have reported that intervertebral disc degeneration (IDD) is a significant contributor of nonspecific LBP, with a lifetime prevalence of over 80% [3, 4]. Early stages of IDD are mainly in the form of biochemical changes, including glycosaminoglycan decreasing. It will lead to a decrease of hydrostatic pressure, resulting in nucleus pulposus (NP) dehydration as well as loss of structural and mechanical properties of intervertebral discs (IVDs). Detecting early changes in IDD is important for developing preventative strategies or reestablishing degenerated IVDs, such as nucleus replacement, cell therapy, and growth factor therapy [5]. Conventional T2 mapping is sensitive to changes of tissues with relatively long T2 relaxations, but limited in identifying early biochemical changes of IVDs [5]. Ultrashort echo time (UTE) imaging could catch very short T2/T2* signals (< 10 ms) [6-9], and has been confirmed to be sensitive to deep tissue matrix changes of subtle and preclinical degeneration [10]. Additionally, previous studies showed that T1rho mapping is a promising technique for diagnosis of biochemical degeneration [11-13]. This preliminary study aims to assess whether human lumbar IDD can be detected using these quantitative approaches, including regular R2* mapping, UTE-T2* mapping and T1rho mapping. Furthermore, we systematically compared the efficacy of R2*, UTE-T2* and T1rho values in the diagnosis of early IDD.

Materials and Methods

Seventy-six patients with nonspecific LBP were enrolled in this study. Subjects were divided into three age groups: 19-39 y, 40-60 y, and 60-85 y. All data were acquired on a 3.0T MRI scanner (Discovery MR750w, GE Healthcare, Milwaukee, WI). Patients underwent MRI examinations including: (1) sagittal T2WI(2) R2* mapping (3) UTE-T2* mapping; (4) T1rho mapping. T2WI were used for Modified 5-level Pfirrmann grading. Disc degenerations were also divided into three categories: normal (Pfirrmann grade I), early stage degeneration (Pfirrmann grade II-III), and advanced stage degeneration (Pfirrmann grade V). R2*, UTE-T2*, and T1rho maps were calculated by mono-exponential fitting using AW4.6 GE Workstation or a custom code in MATLAB (MathWorks, Natick, MA). Five circular regions of interest (ROIs) with equal size were manually drawn on T2WI images from anterior to posterior (Figure 1), including the anterior annulus fibrosus (AAF; ROI 1), the nucleus pulposus (NP; ROI 3), and posterior annulus fibrosus (PAF; ROI 5). The mean value of ROI 1-5 was treated as the averaged measurement of the whole disk. Statistical analysis was conducted using SPSS 22.0 software (IBM, Armonk, NY). Correlations of quantitative values with Pfirrmann grades and age were analyzed using Spearman's rank correlation. Receiver operating characteristic (ROC) analysis was performed and area under the curve (AUC), sensitivity and specificity were obtained to assess the diagnostic efficacy of each quantitative parameter in differentiating normal IVDs from early stage degeneration, and early stage degeneration from advanced stage degeneration. P value less than 0.05 was considered statistically significant.

Results

We found that R2*, UTE-T2* and T1rho values in NP showed moderate correlations with Pfirrmann grade (r = -0.419, 0.480, -0.614, respectively; P < 0.001). No significant correlation was observed in R2* and UTE-T2* values in AAF with Pfirrmann grade. R2*, UTE-T2* and T1rho values in PAF showed low correlations with Pfirrmann grade (r = -0.210、0.218、-0.398, respectively; P < 0.001). The AUC values of the metrics for differentiating the normal and early stage degeneration IVDs ranked as follows: T1rho > UTE-T2* > R2*. For differentiating the early and advanced stage degeneration IVDs, the rank of the diagnostic efficacy was UTE-T2* > R2* > T1rho. Comparing the quantitative parameters in different regions, diagnostic performance of NP was the highest, and that of AAF was the lowest.

Conclusion

The current study demonstrated the feasibility of quantitative evaluation of human IDD in vivo by R2*, UTE-T2* and T1rho mapping. We found that T1rho value is superior to UTE-T2* and R2* values for differential diagnosis of the early IVD, while UTE-T2* value is optimal for differential diagnosis of advanced IVD. Furthermore, the diagnostic performance of these quantitative parameters was highest in NP. Overall, we concluded that R2* and UTE-T2* mapping provides another promising method for quantitatively evaluated lumbar IDD, and T1rho mapping can be considered an effective tool for distinguishing IDD at the earlier stage of the degenerative process.

References

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