Introduction

Cartilage and meniscus degeneration are hallmarks of knee osteoarthritis (OA). MR T2 and T1ρ mapping of cartilage showed its value for the early detection and prediction of future progression of OA ^[1-3]. Moreover, T2 mapping of meniscus was correlated with the severity of OA degeneration ^[4]. Although 7T MRI offers higher signal-to-noise ratio and acceleration factors compared to 3T, shorter RF wavelengths often lead to substantial B1⁺ inhomogeneities resulting in signal loss and biased quantification at 7T ^[5,6]. Parallel transmit, post-processing algorithms, and high-permittivity dielectric padding ^[7,8] were proposed to correct for B1⁺ inhomogeneities. Dielectric pads placed near the region with low B1⁺ can increase the local B1⁺ magnitude at the cost of global B1^{+ [9,10]}.
The aim of this 7T study is to: i) evaluate the effect of dielectric pads on the B1⁺ field distribution and the quantification of T2 and T1ρ in knee cartilage and meniscus; and ii) calculate the scan-rescan T2 and T1ρ repeatability with and without dielectric padding.

Methods

Twelve subjects were (age range, 18-59 years; BMI range, 21.6-34.5 kg/m²; 5 women; 5 right knees) were enrolled in this ongoing IRB-approved study. All subjects were imaged at 7T MRI (Magnetom Terra, Siemens) using a 1-Tx/28-Rx knee coil (Quality Electrodynamics). Morphological turbo-spin echo (TSE), DESS, B0, B1^{+ [11]}, MAPSS T2 and T1ρ ^[12], and multi-echo spin echo (MESE) T2 mapping sequences were acquired (Figure 1). Each subject left the MRI room after first scan and was repositioned for the second scan of the same knee. Repeated scans were measured: i) without dielectric pads (N=4); ii) with dielectric pads (N=3); or iii) one scan with and other scan without dielectric padding (N=5).
Two in-house made dielectric pads (155×160 mm², CaTiO₃ suspended in D₂O ^[9]) were placed anteriorly over tibia and over either side of the knee. DESS images from both scans were non-linearly registered in ANTs ^[13]. Nine cartilage and meniscus compartments were automatically segmented on DESS images using an in-house deep learning algorithm ^[14], followed by manual corrections in ITK-SNAP as needed. In order to evaluate the same knee location with respect to its position in the magnet, medial compartments in right knees were assigned as left, and medial compartments in left knees were assigned as right (i.e., medial meniscus=right meniscus). B1⁺ maps were calculated as the ratio of measured and prescribed flip angles. All T2 and T1ρ maps were calculated voxel-wise by fitting a mono-exponential decay to the data using a two-parameter least-square fitting routine. Mean B0, B1⁺, T2 and T1ρ values were calculated in each compartment. Coefficients of variation (CVs) and Bland-Altman plots were used to evaluate scan-rescan repeatability.

Results

The effect of dielectric pads on morphological TSE images and relative B1⁺ maps is presented in Figure 2. Application of dielectric pads increased relative B1⁺ in most regions with statistically significant increase in tibia, meniscus, trochlea and femoral condyle on the right side of the knee coil (Figure 3). Dielectric padding resulted in overall decrease of relaxation times (means: MESE-T2, -1.41 ms; MAPSS-T2, -1.14 ms; MAPSS-T1ρ, -0.71 ms), and in statistically significant decrease in right meniscus (MAPSS) and right tibia (MESE-T2). B0 fields were not significantly affected by padding.
Scan-rescan T2 and T1ρ maps acquired without dielectric pads are shown in Figure 4. Scan-rescan CVs for MAPSS T2 and T1ρ were higher in measurements without (range, 1.9-7.0%) than with (range, 0.8-4.7%) dielectric pads (Figure 5). Bland-Altman plots showed more narrow limits of agreement, and thus improved repeatability, with dielectric pads (MAPSS-T2, 3.96 ms; MAPSS-T1ρ, 7.73 ms) than without dielectric pads (MAPSS-T2, 7.23 ms; MAPSS-T1ρ, 8.06 ms) (Figure 5).

Discussion

Dielectric pads positioned over tibia inside the RF knee coil showed improved B1⁺ homogeneity. Our results agree with the findings of Fagan and colleagues ^[8]. Insufficient B1⁺ can lead to imprecise RF pulses and low signal-to-noise ratio resulting in biased T2 and T1ρ quantification. Tibia, meniscus and trochlea compartments on the right side of knee RF coil showed poor scan-rescan T2 and T1ρ repeatability without dielectric padding. Dielectric padding seems to improve the reproducibility of T2 and T1ρ quantification in cartilage and meniscus at 7T, resulting in CVs similar to the previously reported 1.4-4.1% for MAPSS-T2 and 1.6-3.9% for MAPSS-T1ρ in healthy cartilage at 3T ^[15].

Conclusion

Application of dielectric pads reduced B1⁺ inhomogeneities and improved the quality of morphological images and the reproducibility of T2 and T1ρ quantification in cartilage and meniscus at 7T. Increased reproducibility of quantitative 7T MRI with dielectric padding could help improve patient’s MRI follow-up and the evaluation of new prevention and treatment strategies for OA.

Acknowledgements

This study was partially supported by Siemens Healthineers.

References

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