Local Analysis of T1ρ, T2, and R2–R1ρ Compositional MR Imaging in Patients with ACL Injury Using Voxel-Based Relaxometry
Colin Russell1, Valentina Pedoia1, Keiko Amano1, and Sharmila Majumdar1

1Radiology and Biomedical Imaging, University of California, San Francsico, San Francisco, CA, United States

Synopsis

This multicenter study employs VBR as a novel technique to analyze patients with ACL tears at the time of injury and 6 months after ACL reconstruction. T and T2 analysis, correlation, and dispersion difference (R2–R) are three methods employed to highlight significant cartilage changes. The most posterior region of the posterior lateral tibia and the patella indicated partial cartilage recovery 6 months after reconstruction, demonstrated by decreasing T and T2, decreased T T2 correlation baseline to 6 months, and dispersion differences (R2–R). The trochlea displayed symptoms of cartilage degeneration, such as elevated T and T2 and dispersion differences.

Introduction

The association between anterior cruciate ligament (ACL) tears and early osteoarthritis (OA) despite ACL reconstruction (ACLR) is a well-studied phenomenon 1,3. T and T2 relaxation mapping are noninvasive methods to quantitatively characterize cartilage macromolecular changes in early OA 2,4. As changes in cartilage composition occur before radiographic evidence is observable, voxel-based relaxometry (VBR) is a novel technique that could provide additional information on localized changes 3. In this study, VBR is used to analyze cartilage of patients with ACL tears at the time of injury and 6 months after ACLR using T and T2 analysis, correlation, and dispersion differences (R2–R) to highlight cartilage changes. This study is the first application of VBR analyzing dispersion differences, a novel method to better assess chemical and diffusive exchange, extending the work of Wang et al., who demonstrated R (1/T) could characterize cartilage composition more accurately than traditional T and T2 analyses by removing the experimental parameter-dependence of T 4. Such information can advance the current understanding of chemical exchange in cartilage degeneration.

Methods

64 patients with no previous history of knee trauma or disease, and sustaining acute, unilateral ACL tears (28 Female; Age=28.7±2.9 years; BMI=24.5±3.1 kg/m2) were scanned using a 3T MR (GE Healthcare). 2 of these patients had previous ACLR in the contralateral knee, and 2 patients did not receive ACLR. Bilateral scans were performed with an 8-channel phased array knee coil (Invivo Inc.) at the time of injury (baseline). 56 patients (24 Female; Age=29.3±12.7 years; BMI=24.7±3.0 kg/m2) were imaged 6 months after ACLR. MRI protocol included quantitative combined T/T2 (T TSL= 0/10/40/80 ms, spin-lock frequency=500 Hz, FOV 14 cm, 256×128 matrix, slice thickness 4 mm, T2 preparation TE=0/12.87/25.69/51.39ms). Non-rigid registration was applied to allineate images onto a template image, allowing VBR to determine relaxation time differences, R2–R dispersion differences and T T2 correlations at the voxel level, obtaining Statistical Parametric Maps (SPM).

Results

Significant elevations in T and T2 were observed in the injured knee compared to the contralateral knee at baseline. Elevations were particularly noted in the most posterior aspect of the lateral tibia (LT T volume significantly different=39.6%; average p-value in overall compartment=0.01, Fig.1). After 6 months, elevated T and T2 were still observed in the injured knee (LT T volume significantly different=47.3%, average p-value in overall compartment=0.01). The analyses of p-value and percent-difference SPMs reveal a more diffuse and anteriorly shifted elevation of T and T2 in the overall pLT at 6 months, no longer focused in the most posterior aspect of pLT as at baseline (Fig.1). Longitudinal comparisons show a significant increase in T and T2 in the trochlea and a decrease in T and T2 in the patella after 6 months. Local correlation analysis between T and T2 is reported in Figure 2. The patella compartment exhibited higher T T2 correlations in the injured side than contralateral at baseline (R=0.91 and R=0.78 respectively, Fig.2A-B), while correlations between injured and contralateral patellar cartilage were more similar at 6 months (R=0.90 and R=0.87, respectively, Fig.2C-D). In dispersion difference analysis, the posterior region of the trochlea remained fairly similar in the injured and uninjured knees at baseline. However, 6-month injured dispersion differences were lower than contralateral (Fig.3). At baseline, the injured patella displayed lower dispersion differences than the contralateral patella. These differences were unobserved at 6 months. A similar trend was observed in the pLT.

Discussion and Conclusions

The trochlea, patella and pLT are three regions demonstrating significant changes after ACL injury. Beyond elevated T and T2 in baseline injured pLT, dispersion difference analysis suggests T and T2 values are more similar at 6 months than baseline, which may suggest a partial recovery following reconstruction. Future experiments can further elaborate on dispersion difference analysis with various TSLs, isolating the pH-dependent factor, as demonstrated in the simplified Chopra model 4,5: $$$R_{1\rho}=R_2+p_{ex}{\large[}R_{2ex}+\frac{{k_{ex}\Delta w_0^2}}{k_{ex}^2+\Delta w_0^2+w_1^2}\large]$$$. Similar findings in patellar cartilage from relaxation time analysis and dispersion differences reinforce this observed reversible degeneration seen in pLT. Reduction in correlation between T and T2 in the injured patella at 6 months compared to baseline indicates greater similarity between injured and uninjured knee relaxation times after 6 months, which may be due to surgical restoration of normal tibiae translation (Fig.2). The trochlea shows greater relaxation times at 6 months than baseline, indicating cartilage degeneration 6 months post-reconstruction. Dispersion differences echo this finding between injured and contralateral knees at 6 months, compared to similar values at baseline (Fig.3). Findings from this study emphasize the need for local and multi-parametric approaches for more sensitive analyses of early degeneration after injury.

Acknowledgements

This study was possible thanks to the combined efforts of the AF-ACL Consortium. This study was funded by the Arthritis Foundation and General Electric Healthcare.

References

[1] Barenius B et al. Am J Sports Med. 2014;42(5):1049-57. [2] Xi L et al. Osteoarthritis Cartilage. 2007 Jul;15(7):789-97. [3] Pedoia V et al. J Magn Reson Imaging. 2015 Oct 7. [4] Wang et al. Magn Reson Imaging. 2014 [5] Chopra et al. J Magn Reson. 1969;59(3):361-372.

Figures

Fig.1: Average cross-sectional T comparison visualization maps of all patients overlaid onto registered image. P-value maps (C,F) show significant differences between injured and contralateral knees at both time points. The pLT exhibits a highly focused region with elevated T values (A-C), becoming more diffuse and anterior after 6 months (D-F).

Fig.2: Average cross-sectional T T2 correlation maps of all patients overlaid onto registered image. Higher T T2 correlation suggests injury presence, as seen comparing injured (A) and uninjured (B) patellar cartilage. At 6 months, injured (C) and uninjured (D) patellar cartilage is more similar, suggesting improvement in cartilage following ACLR.

Fig.3: Average cross-sectional R2–R maps of all patients overlaid onto registered image. Side-to-side changes, baseline to 6 months, indicate regions of cartilage degeneration. In the trochlea, insignificant baseline differences (A,C) followed by lower injured differences after 6 months (B,D), suggest degeneration, as T in healthy cartilage is greater than T2.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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