Gradient coil with active endcap shielding

Shielded gradient coil assemblies produce eddy currents in surrounding metal structures. Such eddy currents can degrade image quality and produce acoustic noise. These effects are magnified for high‐field MRI scanners because of increased Lorentz forces. Conventional actively shielded gradient assemblies consist of primary and secondary coils in the shape of cylindrical shells surrounding the imaging volume. One of the principal regions of field leakage is at the ends of the gradient structure, and these fields are responsible for substantial eddy current generation. We present here a new shielded gradient coil design with an endcap current distribution having both azimuthal and radial components. Our active endcap design significantly reduces the fringe field at the cryostat inner bore. A peak fringe field reduction near the end of the gradient coil of 42% is obtained for the Z gradient relative to a traditional design without active endcaps. The reduction of peak fringe field near the end of the coil for transverse gradients (X and Y) is approximately 40%. We discuss the degree to which the suppression of peak fringe fields corresponds to a reduction in the acoustic noise generated near the end of the warm bore. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 31B: 12–23, 2007