Dual optimization method of radiofrequency and quasistatic field simulations for reduction of eddy currents generated on 7T radiofrequency coil shielding

Abstract

Purpose: To optimize the design of radiofrequency (RF) shielding of transmit coils at 7T and reduce eddy currents generated on the RF shielding when imaging with rapid gradient waveforms.

Methods: One set of a four-element, 2 × 2 Tic-Tac-Toe head coil structure was selected and constructed to study eddy currents on the RF coil shielding. The generated eddy currents were quantitatively studied in the time and frequency domains. The RF characteristics were studied using the finite difference time domain method. Five different kinds of RF shielding were tested on a 7T MRI scanner with phantoms and in vivo human subjects.

Results: The eddy current simulation method was verified by the measurement results. Eddy currents induced by solid/intact and simple-structured slotted RF shielding significantly distorted the gradient fields. Echo-planar images, B1+ maps, and S matrix measurements verified that the proposed slot pattern suppressed the eddy currents while maintaining the RF characteristics of the transmit coil.

Conclusion: The presented dual-optimization method could be used to design RF shielding and reduce the gradient field-induced eddy currents while maintaining the RF characteristics of the transmit coil.

Keywords: 7T MRI; RF shielding; Tic-Tac-Toe RF coil; echo-planar imaging; eddy current simulation; full wave RF simulation; gradient coil model.

Figure 1

Schematics of the (a1) Tic-Tac-Toe transmit/receive elements and (a2) RF shielding. (a3) Copper shielding components. Schematic diagrams of (b1) X gradient coil; (b2) Y gradient coil; (b3) Z gradient coil with the copper shielding to show the coil relative position. Red and blue colors indicate opposite current directions. The Y=0 plane is represented by the light blue color plane.

Figure 2

Time domain: (a1) Normalized measured gradient field Gz at different positions along the Z direction. The curve for the “Simple Loop” was obtained using a simple RF loop-array coil without any RF shielding (used to represent the ideal gradient field in the measurements). (a2) Normalized simulated gradient field Gz at different positions along the Z direction. The “Ideal” Gz is calculated when the TTT coil structure is not present. (b1–b2) Eddy current pulse response function “H(t,z)” as a function of time and position (obtained by Eqs. 1 and 2.) Frequency domain: Six different cases have been used to study the top panel and copper thickness influence for X-gradient and Z-gradient fields. Positive “Z” positions are towards the top panel. Simulated gradient field distribution at the Y=0 plane are shown for 6 cases: 1) distribution with no RF copper shielding 2) distribution with intact 4 sides 18 µm and no top copper shielding 3) distribution with intact 5 sides 18 µm copper shielding 4) distribution with intact 4 sides 4 µm and no top copper shielding 5) distribution with intact 5 sides 4 µm copper shielding and 6) distribution with 5 sides 4 µm copper shielding that includes the proposed slots. (c1–c6) X-gradient field distributions at 10KHz and (c7–c12) Z-gradient field distributions at 10KHz.

Figure 3

Four different copper shielding comparisons. (a1) intact double 4 µm (2×4 µm) copper shielding, (a2) intact single 4 µm copper shielding, (a3) 18 longitudinal slots in the double 4 µm (2×4 µm) copper shielding and (a4) proposed copper slots in the double 4 µm (2×4 µm) copper shielding. The inner copper layer slots are based on the RF current distribution patterns and external copper layer slots are based on the eddy current simulations. (b1–b4) represents 11 slices of EPI images for the above-mentioned 4 copper shielding. (c1–c4) Reflection coefficients (measured and simulated using FDTD) for the transmit coil with the above-mentioned 4 copper shielding. (d1–d3) RF currents on the copper shielding with four different excitation modes (uniform phase, quadrature, 180° phase shift between adjacent channels and one arbitrary set of 4 phases). The RF current distribution maps are presented at 297 MHz; plotted on the top of the density maps are the instantaneous current vectors. (e1–e3) Overlaid instantaneous RF current vectors of the four different excitation modes. The current vectors inside the red dashed box are zoomed in to show the vector patterns. (d1) and (e1) are for intact 4 µm single/double layer copper shielding, (d2) and (e2) are for the simple slots and (d3) and (e3) are for the proposed copper slots.

Figure 4

In-vivo EPI images (22 slices to cover the whole brain) with (a) the proposed slots in double 4µm (2×4 µm) copper shielding and with (b) 18 µm intact/no-slots copper shielding.

Figure 5

Ghosting ratio comparisons (measured with EPI scans) between 5 tested/discussed copper shielding methods. The curves represent the ratio between maximum of the background intensity and the image signal intensity. Amongst the different shielding, the proposed shielding shows minimum ghosting.

Figure 6

In-vivo EPI images with the modified slots in the double 4µm (2×4 µm) copper shielding using the 20-ch Tx coil with 32-ch Rx insert. The 5 top slices inside (a) are scaled by 10 times and shown in (b) to show the noise and eddy current ghosting distortion.