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Abstract PurposeTo compare T1 and T2 measurements across commercial and prototype 0.55T MRI systems in both phantom and healthy participants using the same vendor‐neutral pulse sequences, reconstruction, and analysis methods. MethodsStandard spin echo measurements and abbreviated protocol measurements of T1, B1, and T2 were made on two prototype 0.55 T systems and two commercial 0.55T systems using an ISMRM/NIST system phantom. Additionally, five healthy participants were imaged at each system using the abbreviated protocol for T1, B1, and T2 measurement. The phantom measurements were compared to NMR‐based reference measurements to determine accuracy, and both phantom and in vivo measurements were compared to assess reproducibility and differences between the prototype and commercial systems. ResultsVendor‐neutral sequences were implemented across all four systems, and the code for pulse sequences and reconstruction is freely available. For participants, there was no difference in the mean T1 and T2 relaxation times between the prototype and commercial systems. In the phantom, there were no significant differences between the prototype and commercial systems for T1 and T2 measurements using the abbreviated protocol. ConclusionQuantitative T1 and T2 measurements at 0.55T in phantom and healthy participants are not statistically different across the prototype and commercial systems. 

ABSTRACT Background0.55T systems offer unique advantages and may support expanded access to cardiac MRI. PurposeTo assess the feasibility of 0.55T cardiac MR Fingerprinting (MRF), leveraging a deep image prior reconstruction to mitigate noise. Study TypePhantom and prospective in vivo assessment. PopulationISMRM/NIST MRI system phantom and 18 healthy subjects (11 female; ages 28 ± 8 years). Field Strength and SequencesMRF, modified Look‐Locker inversion recovery (MOLLI), and T₂‐prepared balanced steady state free precession (T₂‐bSSFP) at 0.55T. AssessmentMRF T₁and T₂maps were reconstructed using (1) a low‐rank technique with sparse and locally low‐rank regularization (SLLR‐MRF) and (2) a deep image prior (DIP‐MRF). Accuracy and precision of MRF and conventional sequences were evaluated in a phantom. In vivo performance of MRF was evaluated in the 18 healthy subjects, with 7 subjects also undergoing conventional mapping. Myocardial T₁and T₂values were compared among methods and image quality scored by three readers (2, 3, and 4 years of experience) on a 5‐point scale. Statistical TestsLinear regression, Bland–Altman, intraclass correlation coefficient, and one‐way ANOVA withp < 0.05 considered significant. ResultsMean measurements in the left ventricular septum were 671 ± 31 ms (MOLLI), 761 ± 147 ms (SLLR‐MRF), and 686 ± 39 ms (DIP‐MRF) for T₁, and 63.5 ± 5.7 ms (T₂‐bSSFP), 47.5 ± 12.7 ms (SLLR‐MRF), and 45.2 ± 4.5 ms (DIP‐MRF) for T₂. Compared to conventional mapping, DIP‐MRF exhibited significantly lower T₂but no differences in T₁(p > 0.99). Standard deviations within the myocardium were significantly lower with DIP‐MRF compared to SLLR‐MRF (39 vs. 147 ms for T₁and 4.5 vs. 12.7 ms for T₂). Overall image quality ratings were significantly lower for SLLR‐MRF (T₁: 2.3, T₂: 2.9), which were significantly lower compared to conventional mapping methods (T₁: 3.4, T₂: 3.9), and DIP‐MRF (T₁: 3.8, T₂: 4.1) received higher scores. Data ConclusionThis study demonstrated the feasibility of cardiac MRF on a commercial 0.55T system, enabled by a deep image prior reconstruction for denoising. Evidence Level2. Stage of Technical Efficacy1. 

This paper presents an iterative Jacobian-based inverse kinematics method for an MRI-guided magnetically-actuated steerable intravascular catheter system. The catheter is directly actuated by magnetic torques generated on a set of current-carrying micro-coils embedded on the catheter tip, by the magnetic field of the magnetic resonance imaging (MRI) scanner. The Jacobian matrix relating changes of the currents through the coils to changes of the tip position is derived using a three dimensional kinematic model of the catheter deflection. The inverse kinematics is numerically computed by iteratively applying the inverse of the Jacobian matrix. The damped least square method is implemented to avoid numerical instability issues that exist during the computation of the inverse of the Jacobian matrix. The performance of the proposed inverse kinematics approach is validated using a prototype of the robotic catheter by comparing the actual trajectories of the catheter tip obtained via open-loop control with the desired trajectories. The results of reproducibility and accuracy evaluations demonstrate that the proposed Jacobian-based inverse kinematics method can be used to actuate the catheter in open-loop to successfully perform complex ablation trajectories required in atrial fibrillation ablation procedures. This study paves the way for effective and accurate closed-loop control of the robotic catheter with real-time feedback from MRI guidance in subsequent research.