Breast Coil Optimization for Low Field MRI and future MR-guided Proton Therapy

Objective: Magnetic resonance imaging of the breast and the chest wall imaging was performed at ultra-low field (ULF) using optimal radiofrequency (RF) coil designs and, with an eye toward future application in MR-guided proton therapy (MRgPT), simulation studies of a pencil proton beam trajectory in low and ultra-low magnetic fields were performed. Methods: The primary focus in this study is the RF coil design, which is approached as a multi-objective optimization (MOO) problem and effectively solved using a Pareto optimality formalism. Our coil winding pattern algorithm simultaneously optimizes the magnetic field homogeneity of the coil, the attainable MRI signal-to-noise ratio (SNR), and the magnetic field drop-off into the chest wall for improved OAR imaging. Five transmit/receive single channel breast-shaped RF coils were constructed, and 3D phantom MRI was performed at 6.5 mT. The RF magnetic field homogeneity across the imaging volume and SNR were analyzed among the different coils. In vivo breast imaging was performed for the first time at 6.5 mT. Monte Carlo simulations were implemented to study the influence of copper wire on a proton beam, as well as the beam deflection at various static field strengths. Results: Our MOO method was validated - the observed field homogeneity, the relative ratio of attainable SNRs for all five breast coils, and the MRI signal from the chest agree with the simulations. In vivo breast and chest wall images acquired at ULF reveal good breast tissue contrast and can also visualize the heart. Finally, the results of this work indicate that a field strength of ∼50 mT with gradient strengths of ∼10 mT/m to minimize proton beam deflection, and the ability to achieve a ∼2 ´ 2 ´ 5 mm3 spatial resolution within few minutes are essential for making MRgPT clinically feasible. Conclusion: Our MOO method can be used to design breast coils that meet a wide range of imaging requirements.