Introduction

Imaging of a human body at ultrahigh fields (7 T and higher) requires advanced coil arrays. To achieve maximum B1 in a region of interest (ROI) in transmit and maximum SNR in receive, antenna elements of these arrays should preferably create circularly polarizaed RF magnetic field (i.e. operate in the quadrature regime). To enhance performance of body imaging at such high static fields, especially for deeply located ROIs, on should design array elements as radiative antennas that enable wave propagation inside tissues. In ultrahigh field MRI dipole antennas have proven themselves as coils creating maximum B1 in the center of a human pelvis normalized by an accepted Tx power and by maximum local SAR [1]. So far, the best performance has been shown by FDAs (Fractioned Dipole Antennas) [2]. The FDA is a resonant dipole antenna, miniaturized using multiple quasi-lumped inductors. To create a quadrature coil the FDA was later equipped with a loop antenna fed by a separate orthogonal channel [3] (so called dipole-loop antenna). This dual-channel array element configuration allows to achieve better B1 performance with RF-shimming and reduce maximum local SAR in complex RF-shimming procedures. In this work, we present an alternative radiative array element with two orthogonal channels, where two identical coupled dipoles are combined. By independent excitation of the even and the odd modes of the coupled dipoles we can create two different field distributions in the ROI very similar to ones created by a dipole-loop antenna. In this work, the performance of the proposed array element was investigated by numerical simulations and on-bench experiments including magnetic field measurements.

Methods

The experimental dual-dipole array element was built of two identical fractioned dipole antennas both made on RO4003 PCB with 0.803 mm thickness. To excite the modes independently, the inputs of the dipoles were connected to the outputs of a ratrace coupler. The latter is a four-port device, with two input and two output ports. If one input port is fed, the signal is splitted between the output ports with zero phase shift and equal amplitudes. If the other input port is fed, the signal is splitted with equal amplitudes, but a 180-degrees phase shift. Outputs of the coupler are fully isolated, which allow the two modes of the dipoles to be excited independently. A schematic view of the coil model made in CST Microwave Studio is presented in Fig. 1a. The proposed dual-dipole coil was compared to the dipole-loop coil from [3] taken as a reference. The CST model of the reference coil is shown in Fig. 1b. Frequency-domain FEM simulation was performed to calculate magnetic fields and SAR distributions for a given transmit power applied to both channels. To prove the simulation results we measured H-field distributions in an anechoic chamber using a near-field scanner at the depth of 6 cm inside a water-salt phantom. The fractionated dipoles in the proposed coil were similar to one in the dipole-loop coil and had the length of 30 cm with 5-cm fractions all made on a common PCB. In the experiment both channels of the proposed coil were matched at 298 MHz. The same was done for the reference coil. In the field measurements the same power was applied to both the compared coils.

Results

The simulated H-fields both the proposed dual-dipole coil (even and odd mode channels) and for the reference dipole-loop coil (dipole and loop channels) are presented in Fig. 2. The measurement results are also presented in Fig. 2 with markers. The field distributions were normalized to the maximum field created by the proposed antenna. As follows from Fig. 2, the proposed coil (using the even mode) provides 25% higher RF magnetic field level for the given power than the dipole-loop coil (the dipole is driven). The odd mode provides practically the same magnetic field for the given power at the depth of 6 cm. The measured coupling between the ports of the proposed coil was -23 dB. The simulated B1+ per square root of maximum SAR level was the same for the compared coils. The measurement results are in good agreement with simulations.

Conclusion

A new array element suitable for Tx/Rx with possibility of a quadrature excitation for body imaging at 7 T has been proposed and demonstrated. The proposed design based on two coupled dipoles exhibited similar RF-fields as the dipole-loop coil, but with 25% higher transmit efficiency of the even mode due to higher radiation resistance of a double dipole. The future work consists in testing the proposed design in an array configuration under RF-shimming.

Acknowledgements

This suppored by European Union’s Horizon 2020 research and innovation programme under grant agreement No 736937.