Design and Characterization of Self-Shielded MRI Gradient Coils With Finite Track Width

Abstract

Gradient coils are essential for the performance of magnetic resonance imaging systems. Usually, coils are designed assuming thin wire tracks. Here, we design an MR gradient coil set using a more general approach considering the exact track width using the discrete wire approach. The effect of track width on the DC current density distribution and resultant magnetic fields using both loop and Golay coils are first demonstrated. Both, self-shielded X and Z gradient coils of definite width/thickness are designed and optimized. The resistance and inductance of the coils are calculated using the stream functions approach. Track current distribution was used to compute the magnetic fields over the desired volume, and at the cryostat. The linearity of the magnetic field over the volume, the figure of power, and the shielding ratio of the coil are used as parameters in the optimization process. The DC characteristics of the designed coils with definite (small) track width and thickness were compared for verification to that of the corresponding thin wire design where they were found to have approximately similar characteristics. Using our design methodology, the coils’ frequency-dependent resistances and inductances were directly/efficiently calculated. The harmonic and transient eddy current interactions between the longitudinal and transverse gradient coils were computed where track slitting was employed to reduce such interactions. This work stresses the importance of considering coil track width in the design process particularly for wide tracks as well as computing the coil’s figure of merit, harmonic and transient coil characteristics/interactions.

Keywords

• MRI gradient coils
• transverse and longitudinal gradient coils
• stream functions
• eddy currents
• harmonics
• transient analysis