A Method for Reducing Secondary Field Effects in Asymmetric MRI Gradient Coil Design

Goal: This research introduces an original method for the design of MRI gradient coils that reduces secondary field effects created by eddy current coupling. The method is able to deal with asymmetric coils and provides a new way to ensure a reduction in the magnitude of the eddy current induced fields. Methods: New constraints are introduced at the surface of passive objects to bind the normal field component below a given value. This value is determined by first treating the passive surface as an active surface, and then, calculating the ideal stream function on that surface to produce the desired secondary field. Two coils were designed, one to image the knee and the other to image the head and neck. Results: The secondary field was analyzed using linear regression and was found to improve the secondary field from 10.41 to 0.498 mT/m and from 7.84 to 0.286 mT/m in the examples used. The power loss in the passive structure also decreased to below 1% of the original value using the new method. Conclusion: The method shows the ability to constrain the field to values below the minimum seen under the traditional approaches. Significance: This will allow the design of asymmetric systems with highly linear, reduced magnitude of secondary fields and may lead to better image quality.