Purpose

Transmission line resonators (TLRs) have been applied to develop mechanically flexible arrays for high field MRI combining the advantages of parallel imaging with the possibility of form-fitting¹. The aim of our work is to develop a flexible transceiver array based on TLRs for cardiac MRI at 7 T. One of the major technical challenges is the mutual decoupling between individual elements. Efficient decoupling has been demonstrated using coil annexes¹ with the limitation of interfering with the coil’s geometry. Therefore, we pursue a decoupling ring-based technique^2,3 for TLR arrays. In this work, we evaluate the performance of decoupling rings, including decoupling efficiency and effects on the transmit field of the TLR, using electromagnetic simulations (EMS) and MR measurements at 7 T.

Methods

Figures 1.a,b show the schematic of TLR designs with and without decoupling ring. Each element is composed of two conductors (84 mm outer diameter, 2 mm width) deposited on both sides of a 150 µm thick polyimide substrate. In array configuration, decoupling rings of nearest neighbors are deposited on opposite sides of the substrate for overlapping (Figs. 1c,d). Decoupling optimization was done by analytically calculating the shared magnetic flux as function of the decoupling ring overlap distance³. Initially, the performance of the decoupling rings for a single element TLR, 4-element and 12-element array of TLRs was evaluated in 3D EMS (XFdtd 7.5, Remcom, USA) and circuit co-simulation⁴ (ADS, Keysight Technologies, USA)⁵. For the single element, the transmit efficiency ($$$B_1^+/\sqrt{P_{input}}$$$) with and without decoupling ring was simulated and validated by the following experiment: two TLRs with and without decoupling rings (Figs. 2a,b) were fabricated (db electronic Daniel Böck GmbH, Bregenz, Austria). Each element was tuned and matched to 297.2 MHz (1H @ 7T) using the circuit diagram shown in Fig. 2c. For MR experiments, a 160x160x70 mm³ box filled with polyacrylic acid gel with tissue-like properties was used as load (Fig. 2d). The distance between the TLRs and the phantom was 10 mm. MRI experiments were carried out on a whole body MRI scanner (Magnetom 7T MRI, Siemens Healthineers, Erlangen, Germany). Flip angle maps were acquired using the Saturated Turbo FLASH method⁵, applying a slice-selective saturation pulse (2 ms duration, 20 V amplitude) for both TLRs with and without decoupling ring. $$$B_1^+$$$ maps normalized to the input power were calculated using the measured flip angle maps considering an insertion loss of -1.5 dB of the coil cables and the T/R switch, which was measured on the bench. Finally, a 3D gradient echo sequence (TR/TE = 14/6.75 ms, Tacq = 2:25 min, 1x1x1 mm³ nominal resolution, 96x192x192 mm³ FOV) was used to acquire MR images of the phantom. To test decoupling efficiency, S-parameters were simulated in a 4-element array. With the results from the single TLR and the 4-element array, the final 12-element array was designed and S-parameters were simulated using 3D EMS.

Results

Fig. 3 shows transversal transmit efficiency maps calculated from measured flip angle maps for single TLR elements with and without decoupling ring. It was observed that transmit efficiency is slightly increased on the central TLR axis close to the TLR. Measurements are in agreement with simulation results, as shown for central axis profiles in Fig. 3c. 3D gradient echo images are shown in Fig. 4. No significant artifacts were observed for the images acquired with the TLR containing the decoupling ring. Simulated S-parameters for the 4-element and 12-element TLR arrays are shown in Fig. 5 (matching ≤ -38 dB, coupling ≤ -21 dB).

Discussion

The increase in transmit efficiency close to the coil for TLRs with decoupling ring can be interpreted as a field concentration effect caused by the decoupling ring. At larger distances the profile is comparable for both TLRs with and without decoupling ring. The effect can also be observed in the acquired gradient echo images (Fig. 4): the TLR with decoupling ring has higher signal close to the coil in comparison to the TLR without decoupling ring.

Conclusion

Efficient decoupling by decoupling rings was demonstrated using 3D EM simulation. MR experiments have successfully demonstrated that the presence of the decoupling ring does not degrade the transmit performance of the TLR. Also, image quality is not affected by the presence of the decoupling ring. As a next step, the 12-element flexible array of TLRs for cardiac MRI at 7 T decoupled by overlapped decoupling rings will be physically implemented in order to validate simulations and evaluate the transmit efficiency of the array.

Acknowledgements

Austrian/French FWF/ANR programme Blanc grant, Nr. I1371-B24, “FLEXAR7” Austrian/French OeAD/MAEE projects WTZ/PHC Amadée FR10/2015