1. Purpose

Developing radio frequency (RF) coils for high field MRI is challenging because of higher RF frequencies than standard fields, and thus shorter wavelengths [1], [2], [3]. Consequently, destructive and constructive interference leads to dark and bright areas in the image, as well as to specific absorption rate (SAR) hotspots. Penetration depth also decreases. In recent years elements adapted from antennas have been introduced to high-field MRI to mitigate these effects and improve efficiency at higher frequencies. For example, dipoles have shown advantages over conventional loops especially for deeper targets [4], [5]. Other designs consist mostly of variations of the dipole antenna such as bowties [6] and combined loop-dipole elements [7]. This abstract introduces the TEM horn antenna [8] as an efficient element for high-field imaging (>3 T).

1. Methods

The horn antenna is shown in Figure 1 (a) and is designed to image body regions at 4.7 T. It is fed using coaxial cable that excites a parallel-plate waveguide, and is matched at 200.4 MHz. To operate at this frequency both waveguide and horn sections are filled with deionized water, and a 19-mm-thick water spacer is placed between horn and phantom. The phantom is a 40×40×30 cm³ plastic container filled with a solution (isopropyl alcohol, deionized water and sodium chloride) to mimic the average electrical properties of the body (ε_r=34, σ=0.4 S/m) [9]. The horn is simulated using High Frequency Structure Simulator (HFSS V.15, Ansys, USA) in the presence of the phantom and fabricated as a hollow 3-D printed acrylic (PMMA) watertight container. Scattering parameters were measured on the bench using an Agilent 4395A VNA. Images were acquired on a 4.7 T whole-body MRI system (Unity Inova; Varian, Palo Alto, California). The acquisition parameters are TE=7 ms, TR=1000 ms or longer, 420×420×300 mm FOV and 2 × 2 × 10 mm resolution.

1. Results

Figure 1 (b) shows the measured S₁₁ of the antenna in presence of the load showing excellent tune and match (-20dB) at 200MHz. The simulated average sensitivity of the coil (B₁⁺/√W) is 0.056 µT/√W which compares favorably to that of a dipole (0.050 µT/√W ) . Figure 2 (a) and (b) show the simulated B₁⁺ field for coronal and sagittal slices respectively. The coronal plane is located at a depth of 5 cm from the antenna aperture. The B₁⁺ field on the sagittal slice show the significant penetrating depth of the B₁⁺ field of the proposed antenna. Acquired signal in coronal and sagittal slices are shown in Figure 3 (a) and (b) respectively. The acquired images are consistent with the simulation results.

Conclusion

The proposed pyramid horn shows promising ability to image deep body targets at high fields. It is readily fabricated and with this setup has not required laborious tuning and matching. Future work includes assembling multiple TEM horns into an array to measure coupling and perform imaging in vivo. The horn and spacer will be filled with D₂O to eliminate the corresponding signal.

Acknowledgements

The authors gratefully acknowledge funding from the Natural Sciences and Engineering Research Council (Canada) and thank Messrs. Karim Damji and Peter Šereš for assistance with the MRI measurements. We acknowledge Dancam Design and Mr. Ken Hennig for helping in fabrication. Software access through the Canadian Microelectronics Corporation, and scholarships from the University of Alberta are also acknowledged.

References

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