Recent advances in RF transmission for MRI suggested that at high field (7 T or higher), electric antennas could be used as an alternative transmitters. In this work, we propose general concepts of “Multi-pole antenna array”, which combines monopole, dipole antennas, and their variations to form a transmit array for various purpose. The general concept was first explained. For demonstration purpose, several examples of second order array, i.e. quadrature antenna array, were proposed. In this study one example, the Trident antenna, was built and tested both for spine imaging and cortex imaging on human mimicking phantoms at 7 T.

The multi-pole array can be composed by monopoles, dipoles, and their combinations. Figure 1 shows some of these examples, among which monopole and “bident” (a bent dipole) have roughly half of the field-of-view (FOV) in comparison with a dipole. These basic building blocks can be used to build higher order array, based on the number of available transmit ports and target transmit FOV (as shown in Figure 1).

Quadrature antenna arrays for second-order multi-pole implementations on a single transmit scanner are shown in Figure 2. The effectiveness of dipole array implementation has already been shown previously for spine imaging¹ at 7 T. The other arrays, which have roughly half of FOVs, have the potential to be used for brain imaging particularly at 7 T or higher fields. In all implementations, the scanner transmit power is split by a single hybrid to drive two antennas; additional phase delay can be added in one of the ports to realize field steering.

For demonstration purpose, two trident antennas, one for spine and one for posterior cortex, were implemented as a transmit receive antenna and tested on gel phantoms² mimicking dielectric properties of target tissues at 7 T (Figure 3), the length of the monopole was about 240 mm, whereas each arm of the dipole is about 225 mm. The reflection coefficient, S₁₁, for the trident was better than -24 dB.

MR experiments were performed on a Siemens Magnetom 7T-830-AS scanner. To demonstrate the field steering capability, cables with different phase delays were inserted between the balun and the 90-deg port of the hybrid while axial gradient-echo (GRE) images were acquired at the center of the trident.

GRE images from the dipole spine array as well as the two tridents are shown in Figure 4, with all arrays used as transmit-receive coils. Images with three different phase delays were selected for each array: one has minimum intensity in the middle of the array (left), one has maximum intensity at that location (right), and one phase delay in between (middle). Field steering capability is clearly demonstrated.

Similar to the circular dipole³, the bident antenna shares some similarities with circular dipole or even loop coil to some extent, e.g. the possibility of overlapping for decoupling, and the transmit field twisting observed. This fact can be taken into consideration when designing higher order arrays for reducing inter-channel coupling. Although it has be shown that circular dipole could possibly achieve similar performance with a dipole at the center³, similar investigation for the bident should be performed in the future.

Inter-channel coupling could be another issue for antenna array in general. In principle it can be reduced to some extent with careful geometric layout. In addition, the heavy loading could improve the directionality of the antenna thus may help to reduce the issue. Given the transmitters at high field are usually low-Q, other tricks like slightly detuning could further reduce the coupling. Additional benchtop testing or simulation could be needed for further investigation. Nevertheless, it has been shown previously¹ that moderate coupling is tolerable in some situation.

In general, construction of a dipole or its variant is simpler than building a monopole. It was noticed in practice and simulation that monopole seems to be sensitive to the design of the ground plane. Besides the examples presented here, other antenna designs, e.g. circular dipole array can be easily integrated within the proposed framework.

A general framework of multi-pole was proposed for antenna array design. Several quadrature array designs using second order multi-poles were proposed. Trident antennas, a special case of these arrays, were built for human spine and human posterior cortex imaging, with its field steering capability demonstrated on phantom at 7 T. The proposed framework can be used as guidance when designing new antenna arrays.