Introduction

Arrays consisting of transmit antennas have the ability to mitigate B1 non-uniformity through optimal combination of phase/amplitude and radio frequency (RF) excitation waveform [1]. At ultrahigh frequencies such arrays are essentially required to achieve acceptable B1+ field uniformity and optimized transmit efficiency [2, 3]. Particularly radiative dipole arrays support the desired high penetration at ultra-high field (UHF), and have been explored successfully particularly for UHF body applications [4-6]. For better field control, transmit efficiency, and specific absorption rate (SAR) it is often desirable to have a greater number of transmit channels. In UHF body applications, 16 dipoles can be arranged around the body with ease. For UHF head applications, however, this is difficult to achieve without suffering unacceptably high inter-element coupling. A sleeve monopole concept has been previously described [7]. We explored this concept for MRI and designed and built a 10.5T 16-CH sleeve monopole array and compared it to an 8-CH end-loaded dipole array and an 8-CH monopole array [8, 9].

Methods

Figure 1 shows single element drawings of the dipole, monopole and sleeve monopole (a-c) and coil arrays constructed from these elements (d-i). The 8-CH dipole and monopole array dimensions are: 24cm x 24cm2 and the length of antenna was 16cm for dipole and 12 cm for monopole. The 16-CH sleeve monopole array dimensions are: 20cm x 22cm2 with sixteen equally spaced elements and the antenna length was 20 cm. Floating cable traps were used to suppress the cable sheath currents for all array elements [10]. The floating cable traps are 5 cm long PETG pipe structures 1.2 cm in diameter with a 0.5 cm diameter hole. Four capacitors are used to adjust the frequency. A floating cable trap is positioned at the feed-side of the monopole antenna, and functions similarly to an explicitly grounded sleeve in a sleeve monopole and act as a ground. The current distribution falls to zero on the amplifier side of the traps. All input reflections and coupling coefficients were measured in bench measurements using a 16-CH network analyzer (ZNBT8, Rohde & Schwarz, Munich, Germany). The values for the loaded condition were measured and summarized in Fig.2. Simulated B1+ efficiency and 10g averaged SAR were calculated using XFdtd (REMCOM, State College, PA) with 2mm isotropic resolution as shown in Fig. 3. Experiments were performed at 10.5T and B1+ fields were obtained using an actual flip angle imaging (AFI) sequence for a cylindrical phantom (17 cm diameter and 30.5 cm long) with uniform electrical properties (σ= 0.6 S/m and εr = 49) [11]. B1+ fields were calculated in MATLAB (Mathworks, Inc., Natick, MA, USA) and were normalized to 1 W for B1+ efficiency (µT/√W). Noise correlation matrices were obtained to evaluate crosstalk between elements (Fig.2) [12]. T1- and T2-weighted images of a 37 kg deceased porcine head were acquired with the 16-CH sleeve monopole, Fig.5.

Results and Discussion

The 16-CH sleeve monopole array supports tight 2.8 ± 0.65 cm spacing between adjacent elements with acceptable S21 values (from -8.7 dB to -19.6 dB in Fig. 2) without any split resonance peaks. The physical spacing required to achieve similar decoupling values was 9 cm for the 8-CH end-loaded dipole antenna array and 8-CH monopole array. For this reason, it was not practical to construct 16CH versions of dipole or monopole head coils. The proposed sleeve monopole antenna array shows 47% lower B1+ efficiency in simulation and 52% lower B1+ efficiency in experiment compared to the 8-CH monopole antenna array. Similar B1+ efficiency for simulation (9% higher) and experiment (2% higher) was measured compared to an 8-CH end-loaded dipole array (Fig. 3). The B1+ coverage was ~ 10 cm for the 8 CH monopole array, and increased to 21 cm for both the 8-CH end-loaded dipole array and 16-CH sleeve monopole arrays. Compared to the 16-CH sleeve monopole antenna array, the 8-CH monopole antenna array had up to 3 times higher peak 10g SAR values. The 8-CH end-loaded dipole antenna array had 20% lower peak SAR compared to the 16 channel sleeve monopole. The 8-CH monopole antenna array shows the highest B1+ efficiency due to an increased directivity compared to 8-CH end-loaded dipole and 16-CH sleeve monopole antenna arrays. However, the highest SAR at the top of the target limits the usability of the monopole antenna array for human head application at 10.5T.

Conclusion

The main advantages of the sleeve monopole antenna array are an increased longitudinal field of view compared to the original monopole antenna array and reduced coupling between elements.

Acknowledgements

Support provided by NIH-U01-EB025144, NIH-S10-RR026783, NIH-BTRC-P41-EB015894, NIH-P30-NS076408 and the WM Keck Foundation.

References

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