Improved 7 Tesla transmit field homogeneity with reduced electromagnetic power deposition using coupled Tic Tac Toe antennas

Abstract

Recently cleared by the FDA, 7 Tesla (7 T) MRI is a rapidly growing technology that can provide higher resolution and enhanced contrast in human MRI images. However, the increased operational frequency (~ 297 MHz) hinders its full potential since it causes inhomogeneities in the images and increases the power deposition in the tissues. This work describes the optimization of an innovative radiofrequency (RF) head coil coupled design, named Tic Tac Toe, currently used in large scale human MRI scanning at 7 T; to date, this device was used in more than 1,300 neuro 7 T MRI scans. Electromagnetic simulations of the coil were performed using the finite-difference time-domain method. Numerical optimizations were used to combine the calculated electromagnetic fields produced by these antennas, based on the superposition principle, resulting in homogeneous magnetic field distributions at low levels of power deposition in the tissues. The simulations were validated in-vivo using the Tic Tac Toe RF head coil system on a 7 T MRI scanner.

Figure 1

The 16-channel Tic Tac Toe (TTT) Tx head coil FDTD model and experimental implementation. In (a), the coil geometry with the locations of the 16 channels of the transmit head coil, which are divided into 4 levels in Z-direction; In (b), the assembled 16-channel TTT Tx coil and its dimensions; In (c), the region of interest (white dashed line) plotted over the relative permittivity map of the Duke model at ~ 297.2 MHz (7 T proton frequency). In (d), the ports, matching rods, and tuning rods of a representative panel of the coil. In (e), the RF power splitting configuration—exclusively using 2-way and 4-way Wilkinson power dividers—to drive the 16-channel transmit coil using the system’s sTx or pTx modes. The Tx channels associated with the coil’s levels 1, 2, 3, and 4 (shown in (a)) experience normalized voltage amplitudes equal to 1, 0.5, 0.5, and 1/√2, respectively.

Figure 2

S-parameter comparison between simulations and experiments of the Tic Tac Toe 16-channel Tx head coil. In (a), the FDTD simulated s-matrix using a transmission line model mechanism; in (b), the experimentally measured s-matrix of the constructed head coil. The color scale limits were modified to compensate for the electrical losses in the constructed coil (not included in FDTD model).

Figure 3

Phase-only B₁⁺ RF shim cases for the 16-channel Tic Tac Toe RF coil. This analysis investigates the performance of several configurations using 2-way and 4-way splitters for implementation on the sTx mode. In (a, b), the phase-only RF shim cases, with the amplitude scheme derived from the eigenmodes of the RF coil (described in Fig. 1b) and in reference, were compared with RF shimming optimizations where the amplitudes of the Tx channels were randomly permutated but can only take on normalized values = 1, 1/√2, or 0.5. Approximately 300,000 optimizations were performed, presented as the colored dots. The cost functions for the RF shimming optimizations were the ${\mathrm{CV}}_{{\mathrm{B}}_1^{+}}$, ${\mathrm{CV}}_{{\mathrm{B}}_1^{+}}/{\min }_{{\mathrm{B}}_1^{+}}$ , and ${\max }_{{\mathrm{B}}_1^{+}}/{\min }_{{\mathrm{B}}_1^{+}}$, following the flowchart in (c). The region of interest for the B₁⁺ field stats is the entire head from cerebellum excluding the nasal cavities and the ears (Fig. 1c). The black arrows point to the case selected as initial condition for the next optimizations, which was chosen due to having a combination of low ${\mathrm{CV}}_{{\mathrm{B}}_1^{+}}$, low ${\max }_{{\mathrm{B}}_1^{+}}/{\min }_{{\mathrm{B}}_1^{+}}$, and high ${\min }_{{\mathrm{B}}_1^{+}}$. The circles represent the RF shim cases with the best ${\mathrm{CV}}_{{\mathrm{B}}_1^{+}}$ and the asterisks are the cases with the best ${\max }_{{\mathrm{B}}_1^{+}}/{\min }_{{\mathrm{B}}_1^{+}}$ for each cost function.

Figure 4

SAR and B₁⁺ phase-only RF shimming of the 16-channel Tic Tac Toe RF coil. The B₁⁺ homogeneity parameters (${\mathrm{CV}}_{{\mathrm{B}}_1^{+}}$ and ${\max }_{{\mathrm{B}}_1^{+}}/{\min }_{{\mathrm{B}}_1^{+}}$ in X and Y axes of the plot, respectively) are compared with the power efficiency (a) and SAR efficiency (b). Each point corresponds to an RF shim case using a cost function (Eq. 1) that includes ${\mathrm{CV}}_{{\mathrm{B}}_1^{+}}$, ${\max }_{{\mathrm{B}}_1^{+}}/{\min }_{{\mathrm{B}}_1^{+}}$, and average SAR. Average B₁⁺ constraints were also included. Two RF shim cases were chosen for experimental implementation: (1) power and SAR efficient shim and (2) B₁⁺ homogeneous shim.

Figure 5

Simulated and experimental B₁⁺ maps comparisons for 2 RF shim cases. In (a), simulated and in-vivo B₁⁺ maps and statistics of the power and SAR efficient RF shim case. In (b), simulated and in-vivo B₁⁺ maps for the B₁⁺ homogeneous RF shim case. The statistics from the simulations were calculated over the region of interest shown in Fig. 1c. The yellow arrows point to regions of low B₁⁺ that were mitigated with the B₁⁺ homogeneous RF shim case. In (c), the central slice profiles from the simulated and experimental B₁⁺ maps [white dashed lines in (a) and (b)]. The differences observed in the mean B₁⁺ are due to the electrical losses in the coil structure/cables/plugs/ports/connections/splitters. With a linear system and superposition principle, the overall loss is included as part of the simulation data (“With losses” colorbar). Since the overall electrical loss does not affect B₁⁺ distribution (represents a fixed drop in the intensity), the calculated CV values for the simulations and experiments are comparable for the two RF shim cases (20% vs. 21% and 17% vs. 18%). After including the overall measured electrical loss (27.4% in voltage as described in “Methods” section), the maximum and average values for the simulations and experiments are also comparable, 0.38 vs. 0.37 and 0.32 vs. 0.32 $\mu \mathrm{T}/\sqrt{\mathrm{W}}$ for maximum B₁⁺ field, and 0.22 vs. 0.22 and 0.20 vs. 0.20 $\mu \mathrm{T}/\sqrt{\mathrm{W}}$ for the average B₁⁺ field.

Figure 6

Axial slices of the FLAIR images acquired at resolution of 0.7 × 0.7 × 2 mm³. The images show the differences between the power and SAR efficient (a) and B₁⁺ homogeneous (b) shim cases. The yellow arrows point to the regions where the dropout was mitigated by the B₁⁺ homogeneous case. The parameters of the acquisition were: TE/TI/TR = 103/2900/13,500 ms, acceleration factor 2, BW = 230 Hz/Px, transversal acquisition of 64 slices, field of view 224 × 176.4 mm² in axial plane, and acquisition time = 7:36 min.

Figure 7

Sagittal slices of the 3D-SPACE acquired at 0.6 mm isotropic resolution, showing full brain and cerebellum coverage in a volunteer with large head size (~ 205 mm in anterior–posterior direction measured from the forehead). The images were obtained using the B₁⁺ homogeneous RF shim case on the sTx mode. The parameters of the acquisition were: TE/TR = 369/3400 ms, acceleration factor 3, BW = 488 Hz/Px, transversal acquisition of 224 slices, field of view 192 × 165.6 mm² in axial plane, and acquisition time = 8:11 min.