Design and realization of a multi-coil array for B field control in a compact 1.5T head-only MRI scanner.

Purpose: To design and implement a multi-coil (MC) array for B 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 field generation capabilities, and thermal behavior, were optimized in simulations prior to its construction. The unit was characterized via bench testing. B field generation capabilities were validated on a human 4T MR scanner by analysis of experimental B 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 shimming capabilities and the potential for nonlinear encoding fields.