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

Abstract

The high p ulse frequencies em p loyed in MRI gradient and RF coils call for the use of dedicated construction techniques involving s p ecial 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 p eri p hery of the wire, leading to heating losses due to high resistance.To mitigate the resistance p roblem due to skin-de p th, many gradient coils (and some RF coils) em p loy 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 p ulse rates. Our a pp roach to this p roblem has been to re p lace 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 a pp ro p riate interconnects as necessary. A p rototy p e multi-layer Litz wire structure was develo p ed, with fractal cooling, which showed su p erior p erformance (in terms of 80% reduced resistive losses at high frequency) to the com p arable non-Litz wire configuration.

Applicability and integration of concrete additive manufacturing in construction industry: A case study

“…Geometric flexibility and architectural freedom AM offers more customization possibilities and geometric flexibility. 9,14,27 This geometric flexibility could also be the starting point for the introduction of new, optimally designed housing forms. Indeed, classic building products are usually rectangular (or simple shapes) and are not optimally designed.…”

Direct digital additive manufacturing technologies: Path towards hybrid integration

“…The inkjet printing technology will play an increasingly dominant role in shaping the printed electronics which is emerging as an estimated $300 billion market [4]. The additive printed electronics technology in combination with additive manufacturing technologies is opening up new possibilities for conformal electronic circuits and systems embedded in advanced 3D structures [5][6][7]. The 3D additive manufacturing (AM) technologies are advancing at a rapid pace since their introduction more than 20 years ago.…”

Gradient coil design and intra-coil eddy currents in MRI systems

“…The use of litz wire is one of the possible solutions used to lessen the severity of the skin effect and consequently lower the effective resistance and resistive losses. However, gradient coils manufactured using litz wires can be an expensive process as grooves typically need to be machined in fibre glass tubes to position the wires and this can be particularly difficult for some transverse coil designs [131].…”

“…To reduce intra-coil eddy currents, litz wires may be used, however they are difficult to configure in transverse split-gradient coils with complex geometries and prove costly to machine into fiberglass tubes [131]. Some other solutions have been previously proposed to reduce the intra-coil effects, for example, by either reducing the coil cross-section, which is effective but causes the Ohmic resistance increase or by varying the conductor cross section only where the eddy currents are stronger [68,148].…”