Knee MRI with in situ loading is strongly hampered by subject motion. However, the resulting artifacts can be efficiently suppressed by prospective motion correction. The functional principle of prospective motion correction based on optical tracking is explained and its use for knee MRI with in situ loading is demonstrated. Challenges, limitations and pitfalls of the technique are addressed. Results of anatomical and relaxometric cartilage MRI with prospective motion correction are presented. Non-rigid body motion and tracking marker fixation are the main limitations for most orthopedic applications. The ease of implementation and the correction efficacy of prospective motion correction are sequence-dependent.
Objectives
This presentation will explain the functional principle of prospective motion correction based on optical tracking and demonstrate its use for knee MRI with in situ loading. Challenges, limitations and pitfalls of the technique will be addressed. Results of anatomical and relaxometric cartilage MRI measurements with in situ loading will be presented.Background
Load-induced knee cartilage deformation has been extensively studied in cadaver joints, using pressure-sensitive films (1) as well as MRI (2–5). To study the impact of cartilage loading in vivo, quantitative MRI has been conducted after various activities (e.g., running, squatting or cycling), comparing pre- and post-exercise values of cartilage thickness and relaxation parameters (6–9). These studies underline the need for measurements with in situ loading to gain insight into the biomechanics of the joint. In vivo cartilage deformation under in situ loading has been studied with biplane fluoroscopy (13) and MRI (14, 15). Furthermore, contact area changes as well as tibiofemoral and patellofemoral kinematics under loading have been investigated (15–19). In situ loading can be realized either with open MRI systems enabling knee imaging in an upright position with natural weight-bearing (16, 17) or with MRI-compatible loading devices, which fit into the scanner bore. In contrast to scanners with a horizontal bore, open MRI systems enable more physiological loading in a standing position, albeit at the expense of reduced image quality since such open MRI scanners are typically low-field systems (B0 ≤ 0.5 T). For high-field MRI in a horizontal bore, dedicated pulley systems with a foot plate and a weight suspended outside of the scanner bore have been proposed (14, 18–21). In a recent work demonstrating T2 and T1ρ cartilage mapping with in situ loading, a custom-built pneumatic loading device has been applied (22).Knee MRI with prospective motion correction
Knee MRI experiments with in situ loading are strongly hampered by subject motion, which is typically controlled through joint immobilization during the scan. While the tibiofemoral joint can be loaded without knee flexion and sufficiently stabilized in this position, loading of the patellofemoral joint requires knee flexion and consequently thigh muscle activation for stabilization, which is typically associated with joint shaking. This gives rise to considerable motion artifacts, which encumber high-quality MRI with submillimeter resolution. As a suitable countermeasure, prospective motion correction (PMC) (23) based on a moiré phase tracking (MPT) system has recently been proposed (24). The MPT system consists of a single in-bore camera and a single tracking marker. The tracking system reports translational as well as rotational marker motion with a frame rate update of 80 frames/s. Using the optically tracked motion data, a position update of the MRI measurement volume can be performed in real time during the scan. Additionally, correction of inter-scan subject motion (position locking) ensures that all imaging scans of a session are performed with the initially planned field of view (FOV). It has been demonstrated that PMC can effectively suppress motion artifacts arising from small involuntary knee motion during loaded MRI scans of the knee joint (22, 25).MRI Methods
Anatomical as well as relaxometric knee MRI with in situ loading were performed on a Magnetom Trio 3T system, using PMC for motion artifact suppression. Anatomical MRI was conducted with a 3D turbo-spin echo sequence and a dedicated spoiled 3D gradient-echo sequence for cartilage morphometry. The latter was used for quantitative assessment of load-induced patellofemoral cartilage deformation and contact area changes. 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 T1ρ mapping was performed using a 3D fast low-angle shot (FLASH) sequence augmented with a variable spin-lock preparation (duration: 0/10/20/30/40 ms).Conclusion
Prospective motion correction enables robust knee MRI under flexion and substantial in situ loading via strong suppression of motion artifacts. Non-rigid body motion and tracking marker fixation are the main limitations for most applications. The ease of implementation and the correction efficacy of PMC are sequence-dependent. While sequences with short repetitions times and simple coherence pathways can usually be corrected very efficiently, PMC for sequences based on long or multiple coherence pathways is more challenging and often requires more advanced correction strategies.1. Csintalan RP, Schulz MM, Woo J, McMahon PJ, Lee TQ: Gender differences in patellofemoral joint biomechanics. Clin Orthop 2002:260–269.
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