Advanced modelling and optimization of gradient coils and their physical behaviour in traditional and paired MRI systems

Abstract

Gradient Coils form a critical part of magnetic resonance imaging (MRI) systems. By providing orthogonal magnetic fields, superimposed atop the main magnetic field it is possible to form a spatial encoding within an imaging volume. This allows physiological information obtained by RF coils to be spatially localised. Although gradient coils have been a critical part of MRI systems since their inception the growing focus on asymmetric and hybrid imaging systems has presented a need for new and improved design methods. Asymmetric and split systems such as MRI-Linac are geometrically constrained which often leads to reduced coil performance. These additional constraints, if not considered, lead to sub optimal designs. This work looks to improve the performance of coils in these scenarios. Work investigates the performance of asymmetric coils, in regards to their shielding performance, with an aim to reduce secondary field caused by eddy currents. Asymmetric coils typically perform worse than their symmetric counterparts and many of the methods used to design gradient coils implicitly favour symmetry which is not present in asymmetric designs.This continues into an investigation into optimised cooling systems for power and coupled energy optimised coils and how sufficient cooling is an important part of design. With proper cooling a coil optimised to reduced stored energy can perform on par with a power loss optimised coil with the added benefit of reduced coupled energy.Building on these ideas this work concludes with an investigation into a new method of coil design.Moving away from the traditional Cartesian gradients the Coil Array method provides means to design a large number of orthogonal fields which may vary spatially within the imaging region. This method allows moving gradients within the imaging volume, allowing faster high resolution imaging with a reduced risk of peripheral nerve stimulation.iii