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Development of a low cost multi-channel tune and match device for transceiver arrays at high magnetic fields

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Avdievich,  Nikolai
Research Group MR Spectroscopy and Ultra-High Field Methodology, Max Planck Institute for Biological Cybernetics, Max Planck Society;
Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Walzog,  J
Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Steffen,  T
Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Henning,  Anke
Research Group MR Spectroscopy and Ultra-High Field Methodology, Max Planck Institute for Biological Cybernetics, Max Planck Society;
Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Citation

Avdievich, N., Walzog, J., Steffen, T., & Henning, A. (2015). Development of a low cost multi-channel tune and match device for transceiver arrays at high magnetic fields. Poster presented at 32nd Annual Scientific Meeting ESMRMB 2015, Edinburgh, UK.


Cite as: http://hdl.handle.net/21.11116/0000-0001-21A9-B
Abstract
Purpose/Introduction: Phased arrays improve B1 homogeneity for head imaging up to 9.4T (1). Tight fit arrays further improve transmit performance (2). However, due to close proximity loading of the elements depends on the head size. Therefore, tuning and matching are necessary before each experiment. Automatic tuning/matching (3, 4) is a valid but complicated approach. In case of well decoupled arrays and moderate subject motion a multi-channel mechanical switch can be used (5) for tuning and matching. These switches, however, often contain magnetic components and are expensive. In this work we have developed a low cost electronic tuning/matching selector box using solid state integrated switches (Mini-circuits, USA) for an 8-channel 9.4T (400 MHz) phased array. Subjects and Methods: A major component of the tuning/matching selector box (Fig. 1), i.e. an 8-way absorptive switch, consists of two 4-way absorptive switches (GSWA-4-30DR+) and a 2-way reflective switch (SWM-2-50DR+). Control electronics in combination with a rotary channel selector allows easy and quick connection of the RF input to one of the 8 outputs of the ODU socket (Fig. 1). Figure 2 shows the final setup for array tuning/matching. All 8 array channels are combined together using an ODU connector, which is plugged into the box. An RF sweeper is connected to the RF input and can be sequentially connected to each of the array elements using the channel selector. The only magnetic component, the power supply providing +5 V, -5 V (Fig. 1a), has to be placed outside of the magnet room and connected by the extension cord. Results: Using this setup the 8-channel transceiver array can be tuned and matched within 2 min with the subject present directly at the scanner. Usually it takes less since some elements doesn’t have to be readjusted. In comparison to mechanical switches solid state switches are lossy, which does not cause any issue since their VSWRs are sufficiently good. In our case the tuning/matching box introduced -2.1 dB loss (from the ODU plug to the BNC RF input) with input and output VSWRs measuring 1.2 and 1.13, respectively. Discussion/Conclusion: As an alternative to a complicated automatic tuning/matching of multi-channel transceiver arrays we have developed a low cost 8-channel tune/match selector, which allows quick adjustment of arrays inside the magnet room. The design allows an easy extension to a larger number of channels.