Superelliptical Insert Gradient Coil with Field Modifying Layers for Breast MRI

INTRODUCTION With higher gradient strength and slew rate, local gradient coils such as planar insert gradients can attain higher spatial and temporal resolution than the body gradients. Many imaging tasks, such as dynamic contrast enhanced (DCE) MRI for breast lesion characterization, can benefit from high spatial and temporal resolution. However, the homogeneous gradient volume (HGV) of the planar gradient is relatively small due to its inherent geometry and for applications such as breast imaging, the HGV may be too small to cover both breasts. Cylindrical gradient coils with a vertical bore have been designed to image one breast [1]. The magnetic field and gradient strength of uni-planar gradient systems falls off nearly exponentially with distance from the coil surface. As a result, the usable linear gradient region is narrow in the direction of each gradient. In this work, to create a wider HGV, the planar insert gradient geometry was stretched laterally and the edges were bent vertically using a superelliptical curvature on the left and right sides, resulting in a superellipse former (see Fig 1). Distribution of the wire patterns on the vertical edges increases the gradient uniformity in the left/right and anterior/posterior directions. Furthermore, to increase the gradient strength and to further improve the gradient homogeneity in all directions, an extra outer layer or Field Modifying (FM) layer of current windings was added. We present the design and optimization methods, performance analysis, and force/torque calculations for a multi-layered superelliptical local insert gradient designed for breast imaging with an HGV that is extended in all directions. This design overcomes flat planar gradient limitations [2] and can accommodate both breasts. METHODS First, wire patterns were created on a cylinder by using stream functions (SF), ( , ) ( ) im m m S z h z v e φ φ ∞