Winding patterns for actively shielded shim coils with asymmetric target-fields

• Published in: Measurement science & technology Vol. 14; no. 4; pp. 484 - 493
• Main Authors: Brideson, M A, Forbes, L K, Crozier, S
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.04.2003; Institute of Physics
• Subjects: Exact sciences and technology; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Magnetic components, instruments and techniques; Magnetic shielding in instruments; Physics; Asymmetric configuration; Stream function; Magnetic resonance imaging; Windings; Compensation; Theoretical study; Shielding; Fourier series
• ISSN: 0957-0233; 1361-6501
• DOI: 10.1088/0957-0233/14/4/312

A method is presented for calculating the currents and winding patterns required to design independent zonal and tesseral shim coils for magnetic resonance imaging. Both actively shielded and unshielded configurations are considered. Streamline, target field and Fourier series methods are utilized. The desired target field is specified at 2 cylindrical radii, on and inside a circular conducting cylinder. The specification is over some asymmetric portion of the coil's length. Arbitrary functions are used in the outer sections to ensure continuity of the magnetic field across the entire length of the coil. The entire field is then periodically extended as a half range cosine Fourier series about either end of the coil. The resultant Fourier coefficients are then substituted into the Fourier series expressions for the internal and external magnetic fields, and current densities and stream functions on both the primary coil and shield. A contour plot of the stream function directly gives the required coil winding patterns. Spherical harmonic analysis and shielding analysis on field calculations from a ZX shim coil indicate that example designs and theory are well matched.