Breast Coil Optimization for Low Field MRI and Future MR-Guided Proton Therapy

Objective: Ultra-low field (ULF) magnetic resonance imaging of the breast and chest wall was conducted using optimized radiofrequency (RF) coil designs, and to explore future applications in MR-guided proton therapy (MRgPT), simulation studies of a pencil proton beam trajectory in low and ULFs were conducted. Methods: The focus in this study is the RF coil design, which is approached as a multi-objective optimization (MOO) problem and solved using a defined performance metric and Pareto optimality formalism. Our winding pattern algorithm was developed to simultaneously optimize the coil's magnetic field homogeneity, signal-to-noise ratio (SNR), and magnetic field drop-off into the chest wall for improved imaging of organs-at-risk. Five single-channel, transmit/receive breast-shaped RF coils were constructed. 3D phantom MRI was performed at 6.5 mT to evaluate field homogeneity and SNR. The first in vivo breast imaging at 6.5 mT was conducted. Monte Carlo simulations were employed to investigate the effects of copper wire and static field strengths on proton beam deflection. Results: The MOO method was validated - the observed field homogeneity, the relative ratio of SNRs, and the MRI chest signal agreed for all coils with the simulations. In vivo imaging revealed good breast tissue contrast and clear heart visualization. Our findings indicated that a ∼50 mT field with ∼10 mT/m gradient strengths can minimize proton beam deflection, achieving ∼2 × 2 × 5 mm³ spatial resolution within minutes, supporting the clinical feasibility of MRgPT. Conclusion: Our MOO method can be used to design breast coils that meet a wide range of imaging requirements..