An Analytical Approach to the Design of Quiet Cylindrical Asymmetric Gradient Coils in MRI

Abstract

We consider the design of asymmetric gradient coils in a conventional cylindrical bore magnetic resonance imaging system. The gradient coils are switched on and off during the scan, and are therefore subject to Lorentz forces that cause them to buckle during the switching process. This in turn creates a pressure wave within the coil, and that gives rise to acoustic noise. We present a simplified linearized model for the deflection of the coil due to electromagnetic forces, which is amenable to solution using analytical methods. Closed-form solutions for the coil deflection and the pressure pulse and noise level within the coil are obtained. These are used to design new coil winding patterns so as to reduce the acoustic noise. Sample results are shown both for unshielded and shielded gradient coils. Extensions of this model are indicated, although it is suggested that the advantages of the present closed-form solutions might then not be available, and fully numerical solutions may be needed instead.