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High-field magnetic resonance imagers (MRI) give better signal-to-noise ratio (SNR) and image contrast. However clinical MRIs are currently limited to 3 Tesla (T) magnetic field strength. To create an uncommon 15 T scanner for research use, we evaluated several low-cost, intended for wireless communication, GaAs enhancement-mode pseudomorphic high electron mobility transistors (ePHEMT) in the critical preamplifier slot. This paper reports the experimental results that were obtained at both module and system levels.When evaluated in our prototype 15 T scanner front-end's preamplifier slot, the candidate devices' sub 1dB noise figures enabled image SNR ~ 110 in a water phantom (test object) with a standard pulse sequence. Crisp and artifact-free images could be obtained with these candidate devices. In conclusion, high performance pre-amplifiers for 15 T MRIs can be realized using low cost ePHEMTs originally marketed for wireless communications.

The main cause of loss in the PIN-Schottky limiter is the diodes' parasitic capacitances. Techniques to counter the parasitic capacitances include using bare chip, air cavity packaging, diode stacking, mesa construction, isolating the Schottky diode from the signal path and connecting the diodes to a low impedance node. But the aforementioned techniques either sacrifice cost, manufacturability, size, performances or thermal ruggedness. To reduce loss in the PIN-Schottky limiter, we re-configured its parasitics into a low pass ladder network. This paper reports on the new configuration's changed large signal and transient performances. We observed improved isolation at 0.9 and 2.4 GHz, and a beneficial reduction in the initial energy that slips through the limiter before limiting begins. In conclusion, this configuration simultaneously improves insertion loss, matching, isolation and spike leakage suppression. It has the potential to improve limiting performances in wideband receivers for wireless communication and medical imaging.

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