Coupled circuit numerical analysis of eddy currents in an open MRI system

Abstract

We performed a new coupled circuit numerical simulation of eddy currents in an open compact magnetic resonance imaging (MRI) system. Following the coupled circuit approach, the conducting structures were divided into subdomains along the length (or width) and the thickness, and by implementing coupled circuit concepts we have simulated transient responses of eddy currents for subdomains in different locations. We implemented the Eigen matrix technique to solve the network of coupled differential equations to speed up our simulation program. On the other hand, to compute the coupling relations between the biplanar gradient coil and any other conducting structure, we implemented the solid angle form of Ampere's law. We have also calculated the solid angle for three dimensions to compute inductive couplings in any subdomain of the conducting structures. Details of the temporal and spatial distribution of the eddy currents were then implemented in the secondary magnetic field calculation by the Biot-Savart law. In a desktop computer (Programming platform: Wolfram Mathematica 8.0®, Processor: Intel(R) Core(TM)2 Duo E7500 @ 2.93GHz; OS: Windows 7 Professional; Memory (RAM): 4.00GB), it took less than 3min to simulate the entire calculation of eddy currents and fields, and approximately 6min for X-gradient coil. The results are given in the time-space domain for both the direct and the cross-terms of the eddy current magnetic fields generated by the Z-gradient coil. We have also conducted free induction decay (FID) experiments of eddy fields using a nuclear magnetic resonance (NMR) probe to verify our simulation results. The simulation results were found to be in good agreement with the experimental results. In this study we have also conducted simulations for transient and spatial responses of secondary magnetic field induced by X-gradient coil. Our approach is fast and has much less computational complexity than the conventional electromagnetic numerical simulation methods.

Study on the radiofrequency transparency of electrically floating and ground PET inserts in a 3 T clinical MRI system

“…However, the safety issue can also be possible to handled well for the case of floating PET configuration with proper electric isolation of the whole instrument from the patient and from the PET electronics. The other important consideration considering electric grounding is that, the currents induced in the shield in the form of the eddy current should be transferred to the ground as fast as possible, so that the secondary unwanted fields and the increase in temperature generated 37 from these eddy currents (e.g., RF eddy currents and gradient eddy currents) can have a negligible or short‐time effect on the PET electronics or in the imaging volume. For the case of electric floating, the decay of the eddy current depends solely on the shielding materials 39 (e.g., highly conductive, and a thin shield is preferrable), and on the optimal shield design 29,39 (e.g., slotted shield design).…”

“…Moreover, in terms of electric safety, electric grounding of the RF shield is always preferable 30 . However, the combined detector and cable RF shield is a large electric load to the MRI RF coil that absorbs relatively large RF power 23,35 and reduce the image SNR, 35,36 and can also work as a large conductor to induce high‐gradient eddy currents with long decay time‐constant 37 that would further increase image artifacts, 23,24 like the ghosting artifact in the EPI image. On the other hand, the RF floating of the RF shield of detector modules from the cable RF shield would reduce the electric loading of the RF coil, and hence is good for high SNR imaging with reduced artifacts.…”

Eddy currents analysis methods for an MRI longitudinal gradient coil

“…The magnetic fields are solved inside the elements and from which the eddy currents are calculated. However, dealing with a problem in a large space domain is computationally expensive where large memory and time are consumed 36,45,47 …”

“…Many numerical methods have been used to calculate and analyze eddy currents: finite difference time domain (FDTD) method, [33][34][35][36] finite element method (FEM), [37][38][39][40][41] and network analysis (NA) method. 16,[42][43][44][45][46] FDTD and FEM methods are traditional numerical computational techniques for eddy currents analysis in which the Maxwell equations are solved. In both methods, the source coils, conducting structures, and surrounding space are discretized into small elements (e.g., Yee cells for FDTD, and tetrahedral for FEM).…”

Fast analytical calculation method for eddy current induced by gradient fields in an MRI system

“…Md. Shahadat Hossain Akram performed a novel coupled-circuit numerical simulation of eddy currents in an open, compact MRI system (Akram et al , 2014). The conducting structures were divided into subdomains along their length and thickness by a coupled circuit method.…”