In this work, we demonstrate a low power Q spoiling circuit that uses Gallium Nitride (GaN) depletion mode HEMT devices at 3T field strength. In contrast to conventional Q-spoiling using PIN diodes, a depletion mode FET Q spoiling circuit is in the blocking mode when the FET is unpowered, providing a useful safety feature. Moreover, conventional PIN diode Q-spoiling requires 50-100 mA forward bias current, but GaN FET switches require negligible bias current. As coil array counts increase, the substantial reduction in drive currents becomes an important advantage, and in particular if wireless receive arrays are to be developed.

In the past, various groups have examined and implemented FET devices ^1-3 or MEMs ^4-5 for switching. While MEMs devices are promising, they need a high control voltage and are less commercially available. The use of GaN FET switches has been discussed and explored ^1-3 in MRI. In our prior work ³, we demonstrated Q-spoiling at 1.5T for a Gradient Echo (GRE) 60° flip angle sequence using GaN power switches with high off-state capacitance and a low on-resistance. Here we report performance improvements and SNR for operation to 3T and with higher power Fast Spin Echo imaging, both of which increase the voltage and current stress on the devices.

We constructed four 11 cm by 14 cm coils with different Q-spoiling circuits, and tuned them to ω₀=127.7 MHz (3T). All of the Q-spoiling networks used ~140 nH inductance across ~10 pF capacitance. One coil employed conventional Q spoiling, in which we used a nonmagnetic MAP7441F-1091 PIN diode (Figure 1a). We constructed three FET based coils (Figure 1b) using 25W, 40W and 75W GaN HEMT devices as outlined in Table 1. These devices varied in on-resistance and off-capacitance to achieve a range of blocking impedances given a chosen inductor and capacitor. We designed the circuit such that during receive mode, the scanner imposes a negative voltage onto the FET gate deactivating the Q-spoiling. During transmit mode, a positive 0.6V is imposed on the FET gate, turning on the device to Q-spoil the coil. To avoid magnetic packaging, we acquired the GaN HEMT in die form and gold wire bonded it to a PCB board (Figure 2). We measured S₁₁ to determine the blocking network impedances with an HP E5071C network analyzer. We tested the final coils in a General Electric 3T MR 750 scanner. Each of the four receive coils was tested individually on a loaded Nickel Chloride doped phantom with a fast spin echo (FSE) sequence (TE=10 ms, TR=500 ms, FOV=24 cm x 24 cm, 4 echoes).

We measured the GaN switch blocking impedance circuits to be 5.8kΩ, 5.9kΩ, 6.5kΩ using the 25W, 40W, and 75W FET devices respectively (Table 2). This makes sense since the higher power parts have smaller on-resistance (but larger off capacitance). The PIN diode required 34mA, 40mA, and 76mA of bias current to achieve the blocking impedance of the three previously mentioned FET coils respectively. The MRI scanner provides a bias current of 100mA - the PIN diode blocking impedance at this current was 6.7kΩ.

The coils were matched so that |S₁₁|<= -20dB. In the scanner, great care was made so that the location of each coil and the phantom was the same for each scan. Even slight differences in position as small as a centimeter could potentially cause ~1dB difference in SNR measurements. After performing a fast spin echo sequence and examining the images, we found that the SNR quantities for the PIN diode coil and the three power GaN FET coils are similar (PIN=43.3dB, 25W=43.4dB, 40W=43.9dB, 75W=43.7dB) (Figure 3). The agreement of the SNR quantities implies that additional noise sources intrinsic to the FET devices (ie shot noise) do not appear to have a substantive effect on image quality.

We successfully demonstrated a range of different power depletion mode GaN FETs for low power Q-spoiling switches at 3T and with a fast spin echo (FSE) sequence. FSE sequences at 3T induce larger voltages on receive coils, which convert to large circulating currents in the blocking circuit. This necessitates microwave RF power FETs capable of supporting high currents but have low off-capacitance. We achieved SNR comparable with conventional PIN diode Q-spoiling with each device. Depletion mode GaN switches, because of their default on-state, enable RF safe arrays under loss of power. In particular, the negligible supply current needs will be highly compatible with low-power on-coil electronics and wireless coil array development.