People interested in electromagnetic field analysis with specific interest in RF array and gradient coil designs for high field MRI.To improve high field MRI imaging quality by mitigating the deleterious effects of eddy currents which were generated on the RF array coil when imaging with rapid gradient pulse and sequences in high field MRI.In MRI, eddy currents are inevitably induced in the electrically conductive surroundings including the magnet cryostat vessel, RF coil, RF shield and other peripheral metallic structures. This is due to the time-varying magnetic fields generated by the switching of gradient coils. The eddy currents sequentially distort the spatiotemporal qualities of the primary gradient fields within the imaging region. This results in image distortion and artefacts. In order to control eddy currents on the magnet cryostat vessel, many studies have introduced constrains into gradient coil design [1]. Some studies have also discussed the issue of eddy currents on RF shields and the deleterious effect from RF shields has been demonstrated through imaging experiments [2]. However, analysis of eddy current effects on RF array coils has had little discussion to date. In high field MRI RF coil design, micro-strip structured RF coils are well utilised as they provide obvious advantages [3]. The micro-strip RF coil structures however generally have a wide strip conductor and a large patch of copper ground, which can potentially create the issue of unwanted eddy currents. In this work, the eddy currents on a 12 channel micro-strip RF array head coil for 7T (300MHz) MRI were analysed and compared with that of a slotted, double-sided copper RF shield. A preliminary approach is proposed to reduce the eddy current effect, without compromising RF performance.

In order to study the eddy currents on the RF array coil, a 12 element micro-strip array head coil (fig.1d) was modelled with a 300mm inner diameter. Each RF coil element (Fig.1a) is 200mm long and constructed with a 20mm wide strip conductor and a 60mm wide ground panel. The equivalent circuit diagram is shown in Fig.1b and the magnetic field (H field) is calculated and displayed in Fig.1c.

A slotted RF coil shield with double-sided copper was modelled to compare the eddy currents on the RF array coil. The RF shield was designed with a 300mm inner diameter, 24 slots and 0.2mm thickness between two sides. Fig.2 illustrates the orientation of the RF shield (a), the RF head array coil (b) and the gradient coil. Only half of the X-gradient coil is illustrated for ease of viewing.

The X-gradient coil was energized with a 476Amp current at a frequency of 1 kHz. The eddy current distribution on the RF shield and RF coil was then calculated, shown in Fig.3a and b (only half quarter of subjects were plotted here). We found that the eddy currents distribute evenly on the RF shield but can be concentrated at some points on the RF array coil. As illustrated in Fig.3d, the maximum deviations of eddy current induced gradient fields are 2.23 µT and 1.15µT for the RF shield and the RF array coil respectively. The numerical simulation results demonstrate that the eddy current effect on the micro-strip array coil is an important issue to be considered.

In order to reduce gradient field-induced eddy currents on the RF array coil, slots cut on the ground of the micro-strip element were utilised. As shown in Fig. 4b, the coil ground panel was cut along the longitude direction, with three 1mm width slots which were180mm in length, the micro-strip conductor (Fig. 4a) was kept the same. The modified RF coil was then modelled and simulated. The H-field was calculated and shown in Fig. 4c-d. Compared to that of the initial coil structure without slots, there is no visible change in the RF field pattern, strength and S-parameters. The eddy current distribution and gradient field distortion were then calculated and shown in Fig. 4c-d. The maximum field deviation is significantly reduced by 90% and down to 0.13 µT.

In this work, the eddy current issue for high field RF array coil was analysed. The numerical simulations indicate that this issue is worth considering in relation to high field RF coil design, particularly for micro-strip type array coils. A ground slotting method was proposed which shows promising potential for RF coil eddy current control. Experimental demonstrations will be carried out in future work.