Design and realization of a multi‐coil array for B₀ field control in a compact 1.5T head‐only MRI scanner

Purpose To design and implement a multi‐coil (MC) array for B 0 field generation for image encoding and simultaneous advanced shimming in a novel 1.5T head‐only MRI scanner. Methods A 31‐channel MC array was designed following the unique constraints of this scanner design: The vertically oriented magnet is very short, stopping shortly above the shoulders of a sitting subject, and includes a window for the subject to see through. Key characteristics of the MC hardware, the B 0 field generation capabilities, and thermal behavior, were optimized in simulations prior to its construction. The unit was characterized via bench testing. B 0 field generation capabilities were validated on a human 4T MR scanner by analysis of experimental B 0 fields and by comparing images for several MRI sequences acquired with the MC array to those acquired with the system's linear gradients. Results The MC system was designed to produce a multitude of linear and nonlinear magnetic fields including linear gradients of up to 10 kHz/cm (23.5 mT/m) with MC currents of 5 A per channel. With water cooling it can be driven with a duty cycle of up to 74% and ramp times of 500 μs. MR imaging experiments encoded with the developed multi‐coil hardware were largely artifact‐free; residual imperfections were predictable, and correctable. Conclusion The presented compact multi‐coil array is capable of generating image encoding fields with amplitudes and quality comparable to clinical systems at very high duty cycles, while additionally enabling high‐order B 0 shimming capabilities and the potential for nonlinear encoding fields.