Goodbye wires and formers: 3-D additive manufacturing and fractal cooling applied to construction of MRI gradient coils

The high pulse frequencies employed in MRI gradient and RF coils call for the use of dedicated construction techniques involving special wires and cooling systems. These requirements are needed because conventional (e.g., solid-core) wires exhibit skin effects at frequencies above 10 kHz, which effectively concentrate all the current in the periphery of the wire, leading to heating losses due to high resistance. To mitigate the resistance problem due to skin-depth, many gradient coils (and some RF coils) employ cords of twisted and/or woven thin insulated wires (e.g., Litz wires) that force currents to traverse the entire wire cross-section. Litz wires are hard to configure into the complex designs required for gradient coils, due to a minimum turning radius of several millimeters and the asymmetric bending forces required for winding the wires onto formers. Another challenge in MRI gradient coil manufacturing is the ability to cool RF and gradient coils, especially at high pulse rates. Our approach to this problem has been to replace traditional wire-coil construction methodology with multi-layer additive manufacturing methods, which lend themselves to design and manufacture automation. Additive manufacturing can enable dramatic (i.e., nearly three-fold) improvement in cooling efficiency, through the use of bio-mimetic fractal approaches. Building gradient and/or RF coils layer by layer, we have added conductive, insulating and cooling elements with appropriate interconnects as necessary. A prototype multi-layer Litz wire structure was developed, with fractal cooling, which showed superior performance (in terms of 80% reduced resistive losses at high frequency) to the comparable non-Litz wire configuration.