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An 8-channel transmit 32-channel receive 7T head coil for 1Tx and pTx scanner modes1University of Glasgow, Glasgow, United Kingdom, 2Department of Clinical Physics and Bioengineering, NHS Greater Glasgow and Clyde, Glasgow, United Kingdom
Synopsis
In this work, a nested array of 8-transmit elements was developed and combined with a 32-channel receive array for imaging the human brain at 7T. This setup can be used in the single channel and parallel transmit mode of the Siemens 7T scanner. The transmit array was designed to make the setup less claustrophobic and provide an open feel, benefitting fMRI and patient studies. The transmit performance of this setup is presented here.
Introduction
Multi-channel RF transmit arrays are an essential tool for
imaging at ultra-high field (UHF≥7T). A variety of transmit array
designs based on loops, microstrips, dipoles, as well as single and dual-row
designs are being used for imaging the human brain at 7T 1,2. In
this work, we present a shielded 8-channel loop array which can be interfaced
to the 7T scanner in both single-channel and parallel transmit modes. Furthermore,
two large cutouts were provided in the shield to achieve an open-face feel
which will improve patient acceptance in a clinical environment. Methods
The transmit array design was optimized using CST Studio Suite (CST, Darmstadt, Germany). The main design criteria were to provide an open-face design whilst maximizing the transmit efficiency. The 8-channel transmit array was designed on a fiberglass tube of 28-cm inner diameter and extended 21-cm along the z-direction. The outer tube consisted of a slotted double-layered 7µm shield at a distance of 24mm from the coil. The coil model is shown in figure 1. Each loop consisted of 12 capacitors in series to tune the element to 297.2MHz. A combination of overlap, inductive decoupling and shared conductor decoupling methods are used. The adjacent elements are overlapped and inductive decoupling is used between the next neighboring elements.
Two large cutouts of 80x145mm were made in the shield to create the open-face design. The two elements surrounding the cutouts were decoupled using a shared conductor to minimize the width of the bridge above the nose. This also allowed us to include large mirrors in the coil setup similar to 3T clinical product coils.
The transmit array is combined with a 32-channel receive array, which is a retuned version of the 9.4T coil in Ref. 3. Active detuning of the transmit array is achieved by adding a diode in series. The internal PIN bias lines were shared in a way to allow the 8Tx32Rx setup to be interfaced to the 7T Siemens Magnetom scanner in both single and pTx modes. For single channel mode operation, we used a custom built 1x8 power splitter with in-built transmit phase. A picture of the full setup is shown in figure 2.
Results
The coil setup is tuned and matched using a head and shoulder phantom filled with tissue equivalent solution (εr = 52.1 and s = 0.49S/m). The S-parameters of the transmit array, measured in the presence of the actively detuned receive array, are shown in figure 3. Decoupling between adjacent elements was better than -18dB. Coupling between the second-neighboring elements was reduced to less than 18dB by nesting an inductor between them.
The scanner measurements presented here were collected in the single-channel mode. The comparison between simulated and measured field maps is shown in figure 4. CP mode phase with 45° phase offset produced a left-right asymmetry largely due to the asymmetry introduced in the shield due to the cutouts. A ‘CP-like’ phase configuration was calculated and realized in the power splitter. The central peak in the simulated field map at the coil feed point is 109nT/v and the measured value is 96nT/v.
In TxRx mode (without the Rx array) the transmit efficiency of this setup was within 5% of the widely used Nova head coil. Our next steps includes characterizing the transmit and receive performance and safety validation of the coil as per Ref. 4.
Discussion
Our aim was to design an 8Tx32Rx 7T head coil with features that are more acceptable in a clinical environment. The large cutouts provide an open feel, which allows us to incorporate a mirror system similar to 3T head coils. This pTx array can also be used in single channel mode using the splitter. The asymmetry introduced by opening the shield and the different coil geometry around the cutouts was compensated by adjusting the transmit phase in the splitter. The estimated loss due to the cutouts in the shield and the varying coil geometry was around 10%. The single channel mode will have an additional 1.1dB loss introduced by the splitter compared to the pTx mode.Conclusion
An 8-channel transmit array was designed and optimized in the numerical domain. The performance of the constructed transmit array was in close agreement with the simulated results. This setup was aimed to impove patient comfort whilst keeping the losses to minimum. The final setup will also include a 32-channel receive array. The transmit and receive performance of the full setup will be characterized and compared in 1Tx and pTx modes in the near future.Acknowledgements
No acknowledgement found.References
1. Adriany G, Van de Moortele PF, Wiesinger F, et al. Transmit and receive transmission line arrays for 7T parallel imaging. Magn Reson Med 2005;53:434–445
2. Avdievich NI, Pan JW, Hetherington HP. Improved longitudinal coverage for human brain at 7T: A 16 element transceiver array. In Proceedings of the 19th Annual Meeting of ISMRM, Montreal, Canada, 2011. p 328
3. Shajan G, Kozlov M, Hoffmann J, Turner R, Scheffler K, Pohmann R. A 16-channel dual-row transmit array in combination with a 31-element receive array for human brain imaging at 9.4 T. Magn.Reson. Med. 2014; 71: 870–879
4. Hoffmann J, Henning A, Giapitzakis IA, Scheffler K, Shajan G, Pohmann R and Avdievich NI. Safety testing and operational procedures for self-developed radiofrequency coils NMR in Biomedicine 29(9) 1131–1144.
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