Introduction

Design and construction of non-array volume coils at ultrahigh fields are often technically challenging because of the high operating frequency ^1,2. The dipole antenna offers high frequency capabilities which are beneficial to the design of high frequency RF coils for ultrahigh field MR applications where the conventional lumped design shows the limitation. Novel applications utilizing dipole antennas for RF coil designs have been explored. Previous study demonstrated that it is feasible to design non-array volume coils using the coupled dipole antennas ³. Due to the characteristics of dipole antennas, the electromagnetic coupling between standard dipoles is relatively weak comparing with conventional lumped element loop resonators. To form a non-array volume coil, massive and dense dipole antennas have to be employed. At the commonly used MR field strengths, including the ultrahigh fields of 9.4T and 11.7T, the dipole antenna has to be long enough to achieve the desired resonant frequency, resulting in over-length volume coil which are often not efficient. In this work, we aim to alleviate these two issues of weak coupling and over-length in the non-array dipole volume coil by using modified dipole antennas with right-angled end-section. The results show that electromagnetic coupling among the dipoles is augmented and the volume coils with multiple resonance modes and uniform B1 fields can be formed with much less number of dipoles. This treatment on dipole antennas also provides certain flexibility in coil length control. The proposed method improves the practicality of constructing the high frequency RF volume coil with the dipole antennas for MR imaging at high and ultrahigh fields.

Methods

The modified dipole antennas and the volume RF coil constructed with them are investigated by electromagnetic simulation method with the time domain solver of CST studio. The target resonant frequency of the volume coil was set to 400MHz and 500MHz, the corresponding proton Larmor frequencies of 9.4T and 11.7T. B1 fields of the volume coil on three orthogonal planes were numerically mapped. The distribution of linear field and quadrature field was analyzed. The modified dipole antenna is bent at the end part of the straight half-wave dipole antenna at right angle, as showed in figure 1(a). Four right-angled-end dipole antennas are equidistantly placed in parallel to form the volume coil and the bend parts are staggered a bit to avoid the unwanted connection, as showed in figure 1(b). The material of the arms of the dipole antennas is set as the annealed copper and the cross section is circle with radius of 1mm. The size of gap between two arms where the feeding line is connected to is 2mm. For 400MHz,the diameter of volume coil is 120mm and the height of the volume is 191.2mm. The arm length of single antenna is 162.4mm. For 500MHz, the diameter of volume coil is 120mm and the height of the volume is 62.4mm. The arm length of single antenna is 99mm.

Results

The simulated electric and B1 field maps for the regular half-wave dipole antenna and the right-angled-end dipole antenna are showed in figure 2. The B1 field distribution of the volume RF coils for 400MHz and 500MHz on the center horizontal xy plane and the center vertical xz/yz plane are showed in figure 3 and figure 4 respectively, from both it can be known that a uniform B1 field is generated in the center area of volume coil. Although only one dipole antenna in the volume coil is excited, the rest of antennas are also excited due to the sufficient electromagnetic coupling among them. The number of dipole antenna is much smaller than 32 which is the number mentioned in the previous abstract ³. The quadrature magnetic field distributions of the volume coil generated by driving two orthogonal ports are showed in figure 5. The relatively uniform distribution of the quadrature field exhibits that the dipole volume coil can also be used in quadrature.

Discussion/Conclusion

A new design method of non-array volume coils using modified dipole antennas for ultrahigh field MR applications is proposed and investigated. The design is implemented at 400 MHz and 500MHz, the corresponding frequencies of proton at 9.4T and 11.7T, showing the capability of having homogeneous B1 fields and quadrature operation in that frequency range. With the modified resonant structure of the right-angled-enddipole, the non-array volume RF coils can be built with a much reduced number of dipole antennas and also the coil length can be controlled to a certain extent to achieve a practical length for more efficient signal excitation and reception. The arrangement of the modified dipole antenna to form the volume RF coil as shown in figure 1(b) is convenient to implement and use in practice.

Acknowledgements

This work was supported in part by a 100-talent plan A-class award from Chinese Academy of Sciences, a NSFC grant under Grant No. 61571433, and a Pengcheng Scholar Award, and a grant from National Major Scientific Equipment Research and Developmental Project (ZDYZ2010-2).