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Design of large-size, high-frequency, loop-type surface coil using dipole antennas for ultrahigh filed MR imaging: a feasibility study at 400MHz and 600MHz
Shasha Yue1 and Xiaoliang Zhang2,3

1Institute of Biophysics, Chinese Academy of Sciences, Beijing, China, 2Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, United States, 3UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco, CA, United States

### Synopsis

Due to the parasitic capacitance and increased inductance, building large-size high frequency RF coils, including surface coils for human MR imaging applications at ultrahigh fields, is a challenge. By taking the advantage of high frequency capability and large size of dipole antennas, in this work, we propose a novel surface coil design method using the proposed V-shaped dipole antennas for large-size, high-frequency, loop-type surface coils in ultrahigh field MR applications in humans. The results show that the proposed large size loop-type dipole surface coils (~20cm or ~16cm diameter) have the potential to generate common B1 field distributions of traditional loop-type surface coils at 9.4T (400MHz) and 14T (600MHz). This opens up the possibility of constructing high frequency large size surface coils for ultrahigh field MR imaging in humans.

### Introduction

In all coil categories, surface coils offer the highest sensitivity. In human MR, imaging of some body parts, e.g. the abdomen or the liver, often requires large size surface coils in order to attain more complete images. Due to the parasitic capacitance and increased inductance, building large-size high frequency RF coils, including surface coils for human MR imaging applications at ultrahigh fields, is a challenge. By taking the advantage of high frequency capability and large size of dipole antennas 1,2,3,4, in this work, we propose a novel surface coil design method using the proposed V-shaped dipole antennas for large size high frequency, loop-type surface coils in ultrahigh field MR applications in humans. The results show that the proposed large size loop-type dipole surface coils (~20cm diameter at 400MHz and ~16cm at 600MHz) have the potential to generate common B1 field distributions of traditional loop-type surface coils at 9.4T (400MHz) and 14T (600MHz). This opens up the possibility of constructing high frequency large size loop-type surface coils for ultrahigh field MR imaging in humans.

### Methods

After the two conductive rods are rotated by a certain angle $\alpha$ , the V-shaped structure is formed as showed in figure 1(a). The surface coils composed of the several V-shaped dipole antennas are showed in figure 1(b), figure 1(c) and figure 5(a). For the surface coil which is combined with 4/6/8 V-shaped dipole antennas, $\alpha=90^{\circ}/120^{\circ}/135^{\circ}$. It can be seen that the antenna is placed equidistantly on the circle. For 400MHz, the length of rod is 119.5mm, the diameter of the surface coil is 200mm. For 600MHz, the length of rod is 75.8mm, the diameter of the surface coil is 160mm. Only one V-shaped dipole antenna is excited. A serious of simulations are performed with CST MICROWAVE STUDIO to evaluate the performance of the large size surface coil. The cylindrical phantom ( $\sigma=0.47 \text{S/m}, \epsilon_{r}=34, \rho=1445.0 \text{kg/m}^{-3}$ )with a diameter of 60mm and height of 30mm is placed 5mm away from the surface coil, showed in figure 4(a).

### Results

The simulated E-field, B1+(1.0W acc), B1-(1.0W acc) field distributions of the designed surface coil on the center horizontal xy plane and the center vertical xz/yz planes are showed in figure 2 and figure 3, respectively for the 400MHz of 9.4T and the 600MHz of 14T. Unlike the field distribution of single dipole antenna, a uniform B1 field is excited in the central area surrounded by these antennas.The B1+ and the B1- field distributions of the designed surface coil are similar to that of the traditional surface coil. The magnetic field near the metal conductor area is stronger than the center area. That is why this kind of structure is called surface coil. The simulated SAR maps for 400MHz and 600MHz are showed in figure 4(b), from which it can be seen that the 600MHz coil have a higher SAR value for the same phantom.

### Discussion/Conclusion

The B1+ and B1- distributions and also the electric field distribution of the proposed surface coils with dipole antennas suggest that it is feasible to construct the high frequency loop-type surface coils with a large size of up to ~20cm or ~16cm diameter for MR imaging at the ultrahigh fields of 9.4T and 14T. Although only one V-shaped dipole in the coil is excited, the rest of dipoles are also excited due to the sufficient electromagnetic coupling among the dipole antennas.

### Acknowledgements

We would like to thank Chao Luo and Zhe Wang for their technical assistant in the simulation software and calculation. This work was supported in part by a 100-talent plan A-class award from Chinese Academy of Sciences, a Pengcheng Scholar Award, and a grant from National Major Scientific Equipment Research and Developmental Project (ZDYZ2010-2).

### References

[1] Raaijmakers A J E, Italiaander M, Voogt I J, et al. The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla[J]. Magnetic resonance in medicine, 2016, 75(3): 1366-1374.

[2] Yan X, Zhang X, Wei L, et al. Design and test of magnetic wall decoupling for dipole transmit/receive array for MR imaging at the ultrahigh field of 7T[J]. Applied magnetic resonance, 2015, 46(1): 59-66.

[3] Shasha Y, Chao L, Nan L, et al. Feasibility exploration of constructing volume RF coil with coupled dipole antennas at 9.4T[C]//Proc. 26th Annu. Meeting ISMRM. 2018.

[4] Raaijmakers A J E, Ipek O, Klomp D W J, et al. Design of a radiative surface coil array element at 7 T: the single‐side adapted dipole antenna[J]. Magnetic resonance in medicine, 2011, 66(5): 1488-1497.

### Figures

Figure1. (a) schematic of the V-shaped dipole antenna (b) schematic of the surface coil composed by 4 V-shaped dipole antennas (c) schematic of the surface coil composed by 8 V-shaped dipole antennas

Figure2. The simulated eclectic field distributions and B1+ and B1- distributions of the surface coil on the xy/xz/yz planes for 400MHz. The surface coil is composed by 4 V-shaped dipole antennas.

Figure3. The simulated eclectic field distributions and B1+ and B1- distributions of the surface coil on the xy/xz/yz planes for 600MHz. The surface coil is composed by 4 V-shaped dipole antennas.

Figure4. The simulated SAR map

Figure5. The simulated B1-field distribution of the surface coil composed by 6 V-shaped dipole antennas

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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