Longitudinal sodium MRI of cartilage in patients with knee osteoarthritis: Baseline vs. 16 months follow-up
Guillaume Madelin1, Ding Xia1, Gregory Chang1, Svetlana Krasnokutsky2, Steven B Abramson2, and Ravinder R Regatte1

1Department of Radiology, New York University Langone Medical Center, New York, NY, United States, 2Department of Rheumatology, New York University Langone Medical Center, New York, NY, United States

Synopsis

In this longitudinal study, we measured the sodium concentration in knee cartilage in 12 patients with osteoarthritis (OA) with quantitative 23Na MRI at 7 T. Sodium measurements were performed at baseline and 16 months follow-up (on average), with and without fluid suppression by inversion recovery (IR). We show that only fluid-suppressed measurements show a significant decrease of mean [Na+] in different regions of cartilage over 16 months follow-up in OA patients. Quantitative 23Na IR-MRI could therefore be a useful imaging biomarker to monitor cartilage degradation over time, and help assess the efficiency of potential disease modifying OA drugs.

Purpose

To evaluate the potential utility of sodium (23Na) MRI of cartilage for longitudinal studies in patients with knee osteoarthritis (OA). OA is the most common form of arthritis in synovial joints and a leading cause of chronic disability in the elderly population. Osteoarthritis is a degenerative disease of the articular cartilage that can be associated with a reduction in glycosaminoglycan (GAG) concentration, changes in the size and organization of collagen fibers1,2. Sodium MRI has been shown to strongly correlate with the GAG concentration in cartilage3,4 and is therefore a good candidate as an imaging biomarker to assess loss of GAG and cartilage degradation over time in patients with OA.

Methods

MRI scans: Twelve symptomatic subjects (mean age = 67±11 yr) with knee OA and Kellgren-Lawrence grades 1-3 (7 KL=1, 3 KL=2, 2 KL=3) were scanned at 7 T (Siemens) with a dual-tuned 1H/23Na knee coil (birdcage transmit, 8-channel receive) and with 2 sequences: (1) Radial 3D (R3D), with TE 0.4 ms, TR 100 ms, flip angle 90°, FOV 200 mm isotropic, 10,000 projections, dwell time 80 μs, real resolution 3.3 mm isotropic, nominal (reconstructed) resolution 2 mm, TA 16:44 min; (2) Inversion recovery (IR) with adiabatic WURST pulse5,6 (IRW) for synovial fluid suppression, with the same acquisitions parameters as R3D except TR 140 ms, TI 24 ms, WURST pulse 240 Hz/10 ms, TA 23:25 min. All images were reconstructed offline in Matlab with standard 3D regridding. All subjects were scanned at baseline (scan 1) and 16 months follow-up (scan 2) (mean delay = 478±40 days).

Quantification: Sodium concentration [Na+] quantification was calculated from linear regression of the signal intensity of gel phantoms (4% Agar with 100, 150, 200, 250, 300 mM [Na+])4,6. The image of an uniform solution phantom (45 mM NaCl) filling the whole volume inside the coil was acquired with R3D, normalized, and used to correct the cartilage data prior to sodium quantification processing (to correct the receive inhomogeneity from the 8-channel array7). Sodium maps were also corrected using T1 and T2* from cartilage8 to increase accuracy to cartilage sodium (note that the phantoms [Na+] on the maps will therefore be inaccurate).

Measurements: Three regions-of-interest (ROIs) of 40 pixels were drawn on the patellar (PAT), femoro-tibial medial (MED) and femoro-tibial lateral (LAT) cartilage, over 8 consecutive slices. All sodium maps (baseline and follow-up) for each subject were co-registered prior to ROI measurements. The mean and standard deviation (std) over all pixels were then calculated for each ROI (in mM) in each subject.

Statistics: The rank sum test was applied to the summary statistics of means and std in all subjects to assess the significance of the difference of the measures between the baseline and 16 months follow-up scans.

Results

Fig. 1 and 2 show examples of [Na+] maps in PAT, MED and LAT, from 2 different OA patients. Sodium maps calculated from R3D (no fluid suppression) present little difference in the ROIs, while data from IRW (fluid suppression that removes partial volume effect from synovial fluid) present significant visual difference in the ROIs.

Fig. 3 shows the boxplots of mean and std [Na+] measured in PAT, MED and LAT for each subject and each sequence, at baseline (scan 1) and 16 months follow-up (scan 2). We can observe that, for most of the subjects, mean and std values measured from the follow-up scans were lower than from the baseline scan, and that only measurements from IRW were significantly different between the 2 scans (see Table 1 for the p values). The average decrease of mean [Na+] was 70±50 mM over all patients.

Discussion

Sodium MRI with synovial fluid suppression by IR is more sensitive to the sodium content within cartilage4 and allows us to estimate the rate of loss of GAG over time in patients with OA. Due to the small number of subjects, no correlation was found between the decrease in mean and std [Na+] and KL grade. More subjects with KL 1 to 4 need to be scanned to assess if the proposed measurements can not only estimate accurately the degree of cartilage degradation, but also predict the rate of degradation.

Conclusion

Quantitative sodium MRI with synovial fluid suppression by IR at 7 T could be a potential useful imaging biomarker for follow-up studies of cartilage degradation due to OA, and for assessing the efficiency of disease modifying OD drugs (DMOADs). Future work will involve increasing the acquisition speed and resolution with improved 3D non-Cartesian sequences9, and assess the repeatability of the follow-up study on asymptomatic controls10.

Acknowledgements

This work was supported by the Center for Advanced Imaging Innovation and Research (CAI2R), a NIBIB Biomedical Technology Resource Center (NIH P41 EB017183). Other grant support: R01 AR056260, R01 AR060238, R01 AR067156, and R03 AR065763.

References

1. Borthakur A, Mellon E, Niyogi S, et al. Sodium and T1rho MRI for molecular and diagnostic imaging of articular cartilage. NMR Biomed 19(7), 781–821, 2006.

2. Wheaton AJ, Borthakur A, Shapiro EM, et al. Proteoglycan loss in human knee cartilage: quantitation with sodium MR imaging—feasibility study. Radiology 231(3), 900–905, 2004.

3. Shapiro EM, Borthakur A, Gougoutas A, Reddy R. 23Na MRI accurately measures fixed charge density in articular cartilage. Magn Reson Med 47(2), 284–291, 2002.

4. Madelin G, Babb J, Xia D, et al. Articular cartilage: Evaluation with fluid-suppressed 7.0-T sodium MR imaging in subjects with and subjects without osteoarthritis. Radiology 268(2), 481-491, 2013.

5. Kupce E, Freeman R. Adiabatic pulses for wide-band inversion and broadband pulse. J Magn Reson A 115(2), 273–276, 1995.

6. Madelin G, Lee J-S, Inati S, et al. Sodium Inversion recovery MRI in the knee joint in vivo at 7T. J Magn Reson 207, 42-52, 2010.

7. Brown R, Madelin G, Lattanzi R, et al. Design of a nested eight-channel sodium and four-channel proton coil for 7T knee imaging. Magn Reson Med 70, 259-268, 2013.

8. Madelin G, Jerschow A, Regatte RR. Sodium relaxation times in the knee joint in vivo at 7T. NMR Biomed 25(4), 530–537, 2012.

9. Pipe JG, Zwart NR, Aboussouan EA, et al. A new design and rationale for 3D orthogonally oversampled k-space trajectories. Magn Reson Med 66, 1303–1311, 2011.

10. Madelin G, Babb J, Xia D, et al. Reproducibility and repeatability of quantitative sodium magnetic resonance imaging in vivo in articular cartilage at 3T and 7T. Magn Reson Med 68, 841-849, 2012.

Figures

Fig. 1. Examples of [Na+] maps calculated from radial 3D (R3D, no fluid suppression) and IR WURST (IRW, fluid suppression) acquisitions. Representative axial slices of the knee of an OA patient (KL=1, scan delay = 458 days). Notice the decrease of [Na+] in PAT detected with IRW.

Fig. 2. Examples of [Na+] maps calculated from radial 3D (R3D, no fluid suppression) and IR WURST (IRW, fluid suppression) acquisitions. Representative coronal slices of the knee of an OA patient (KL=3, scan delay = 483 days). Notice the decrease of [Na+] in MED and LAT detected with IRW.

Fig 3. Boxplots of mean and std [Na+] measured in PAT, MED and LAT in 12 OA subjects (KL=1-3), at baseline and 16 months follow-up, acquired with radial 3D (R3D) and IR WURST (IRW). Only fluid-suppressed data (IRW) show statistically significant difference (*). See Table 1 for p values.

Table 1. P values to assess statistical significance in the decrease of [Na+] from R3D and IRW in different cartilage regions at baseline and after 16 months follow-up (see Fig. 3). SEQ = sequence, PAT = patellar, MED = femoro-tibial medial, LAT = femoro-tibial lateral, STD = standard deviation.



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