Reverse‐engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry‐application in exposure evaluations

Abstract

For many MRI installations, the coil pattern that generates pulsed magnetic fields produced by gradient coils is not provided by the manufacturer. This has implications for accurate assessments of MRI worker exposures, which is currently an important topic of research. To correctly model the level of exposure, a full three-dimensional distribution of the magnetic field in the locality of the magnet end is required, which can be difficult to obtain by experimental measurements. This research presents one possible approach, in which the prediction of a current distribution that generates an approximately identical magnetic field profile is constrained by a small number of experimentally measured magnetic field sample points within a plane outside the gradient set. The presented methodology may take into consideration other important descriptors such as field uniformity in the imaging volume, gradient coil geometry/dimension, driving current, central field strength, etc. To exemplify the application of the proposed approach, the current density and matching magnetic field distributions of x-and z-axis gradient coils are approximated without preknowledge of the gradient coil patterns for a MRI system, followed by exposure evaluations of a tissue-equivalent numerical worker model in the vicinity of said gradients.

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

“…In this study, we combined the phantom measurement and a designated inverse electromagnetic (EM) method to characterize the gradient field both inside and outside of the DSV for the Australian MRI‐Linac system. A grid phantom was first designed to measure a small number of gradient field samples by using the three‐dimensional (3D) Prewitt operator .…”

A Finite Difference Method for the Design of Gradient Coils in MRI—An Initial Framework

“…For example, the eddy currents generated in the cryostat and RF structure (volume resonator, array, shielding) due to gradient switching can be evaluated while patient's specific absorption rate levels are calculated. These lead to a better understanding of the fields within patients and general temporal field behavior during an MRI scan, thus offering insight into fundamental EM problems related to MRI [33], [34]. Also acoustic noise induced by the fast switching of the gradient coils has been considered in many studies [35]- [37] and an FD framework is suitable to integrate such studies into coil design.…”

An electromagnetic reverse method of coil sensitivity mapping for parallel MRI – Theoretical framework

“…Similar to our recent work on exposure evaluations by reverseengineering of gradient coils [20], the geometry of a typical radio frequency (RF) coil can be inversely determined from the measured magnetic field information. In low field cases, this reverse method is made possible by the reasons that the RF coil can be represented by a mathematical model using a few model descriptors.…”