Introduction

To date, cartilage T₂ and T_1ρ changes in response to mechanical loading have only been investigated for the tibiofemoral but not for the patellofemoral joint compartment ^1-3, mainly because of strong susceptibility to motion. In this work, robust comparative T₂ and T_1ρ mapping under in situ loading is demonstrated with prospective motion correction and T₂ and T_1ρ changes in response to loading are evaluated quantitatively in a pilot study.

Methods

All experiments were performed on a Magnetom Trio 3T system (Siemens Healthineers, Germany), using an 8-channel multipurpose coil (NORAS MRI products, Germany) for signal reception. Knee loading was realized with a MR-compatible pneumatic loading device and prospective motion correction was performed with a moiré phase tracking (MPT) system (Metria Innovation Inc., Milwaukee, US) ^4,5. For MRI of the patellofemoral joint under loading, the subject was positioned on the scanner bed with a knee flexion angle of 40°-50° (Fig. 1). Transverse T₂ and T_1ρ maps were acquired with fat suppression and with equal spatial resolution (0.6 × 0.6 × 3.6 mm³) and field of view. Both sequences were augmented with regular position updates and inter-scan position locking. T₂ mapping was conducted with a 2D multiple spin-echo sequence (TE = 13.8/27.6/41.4/55.2/69.0/82.8 ms) while T_1ρ mapping was performed with a 3D fast low-angle shot (FLASH) sequence with a variable spin-lock preparation time (τ = 0/10/20/30/40 ms) ⁶. T₂ maps were calculated from the acquired data using the Matlab-based StimFit algorithm ⁷ while T_1ρ maps were determined with a simple single-exponential decay model. Manual cartilage segmentation and quantitative evaluation was performed with the MIPAV software. A slice in the center of the cartilage contact area was selected and mean T₂ and T_1ρ values were determined within a region of interest (ROI) covering the predominantly load-bearing lateral patellar facet. Furthermore, the ROI was divided into two equally thick layers: a superficial and a deep cartilage zone. A pilot study with combined T₂ and T_1ρ measurements under different loading conditions (0/20/40 kg) was performed in ten healthy subjects (male, age: (27 ± 2) years)

Results and Discussion

Figure 2 shows T₂ and T_1ρ maps of the patellofemoral cartilage under different loading conditions for a healthy subject. Patellar T₂ and T_1ρ are higher in superficial than in deep cartilage layers and exhibit a substantial load-induced decrease for the superficial cartilage layer. In contrast, the deep patellar cartilage layer shows a substantial T₂ increase and a less pronounced T_1ρ increase. Interestingly, the non-load-bearing zone of the medial patellar facet shows a T₂ as well as T_1ρ increase with loading. These findings suggest a fluid transport from superficial to deep cartilage and to non-weight-bearing medial regions during loading as already observed for the tibiofemoral joint ³. Repeated measurements on two subjects revealed a higher reproducibility for T_1ρ mapping than for T₂ mapping (Fig. 3). Out of the ten subjects measured in the pilot study, one subject had to be excluded due to excessive residual motion artifacts. Results from the statistical evaluation of the subject study are presented in Fig. 4. The average relative spatial standard deviation over the ROI was significantly lower for T_1ρ than for T₂ maps, demonstrating that T_1ρ shows less spatial variation in patellar cartilage than T₂, which is in line with ex vivo studies on human cartilage specimen ⁸. While no significant T₂ differences for the three loading conditions were observed in the statistical evaluation, patellar T_1ρ was significantly reduced under loading. Separate evaluations of superficial and deep cartilage did not show any significant load-induced T₂ changes. Significant T_1ρ changes in response to loading were only observed for the superficial but not for the deep cartilage. These findings suggest that T_1ρ is a more sensitive marker for load-induced changes than T₂, which is in line with results obtained for the tibiofemoral joint in a larger subject cohort ³. The different T₂ and T_1ρ response for the superficial and deep cartilage zones can be explained with structural differences (superficial cartilage: tangential fiber orientation, deep cartilage: radial fiber orientation) ⁹. However, it should be noted that the separation into superficial and deep cartilage layers in this work was purely based on geometry but not on microscopic structure. Furthermore, only the patellar cartilage was evaluated quantitatively since the femoral cartilage was not rigidly coupled to the tracking marker on the knee cap, which gave rise to load-induced slice offsets hampering a proper comparison. The quantitative results from this pilot study need to be validated in larger subject cohorts, using more sophisticated cartilage segmentation and registration methods.

Acknowledgements

We thank Dr. Ari Borthakur for providing the source code of the T_1ρ mapping sequence. This work was funded in part by the NIH grant 2R01DA021146, in part by the Helmholtz Alliance ICEMED - Imaging and Curing Environmental Metabolic Diseases, and in part by the German Research Foundation (DFG), grant number LA 3353/2-1.