As interest in wireless MRI increases, we questioned how to detect the state of the MRI scanner (ie receive or transmit mode) with minimal additional hardware components while having no access to the scanner hardware. Such a task could eventually be used as part of a micro-controller approach to wirelessly detune an MRI receive coil. We have seen the use of low power FET based Q-spoiling techniques for coil detuning using Gallium Nitride FET devices in the MRI scanner ^1-2. However, we hope to complement this low power Q-spoiling method with a means to wirelessly trigger these switches. We want to extract information without having access to the hardware of the commercial MRI scanner by detecting the transmit pulse externally. We do this by adding a probe in the scanner which can relay detected RF pulses as an interrupt to a microcontroller³. Here, we explore two different setup links involving simple hardware, to route the probed information to detect the state of the machine.

For a magnetic probe, we made up a small 3.5cm by 4.5cm surface loop and tuned it to 63.88MHz, placing it 8 centimeters away from the imaging coil so as not to affect the image. This probe was placed inside the scanner and was unconnected to any scanner hardware, making it wireless from the system. In our first setup, we routed this probe signal using a coaxial cable leading to a LTC 5536 peak detector, which would then trigger an interrupt on a microcontroller in the console room (Fig 1a, 2). In our second setup, we instead routed this probe signal optically using an IE-E97 LED and a photo-logic detector IF-D96F in order to trigger the interrupt instead (Fig 1b, 3).

As a proxy for the RF transmit mode detection, the microcontroller will switch on an LED to visually indicate RF pulse detection during the scanner transmit mode. This digital logic can easily be attached to the gate of a FET switch, for example, to activate Q-spoiling or to trigger a readout.

We used an Arduino Mega 2560 microcontroller board, which is centered on the ATmega2560 8-bit microcontroller with an AVR-based architecture. We ran both an SPGR gradient echo sequence (GRE) (TE=25ms, TR=50ms, flip angle=60°, resolution=1.04mmx1.04mm) and a fast spin echo pulse (FSE) (TE=20ms, TR=500ms, echo train=4, resolution=1.04mmx1.04mm) on a General Electric 1.5T scanner.

Both our peak detector setup, and our optical link setup functioned properly, and we were able to successfully observe an LED flashing in sync with the scanner in transmit mode. Both setups, involving the peak detector or optical link, were connected by a 5V supply at the receive end of the probe. On the microcontroller, we had to set up the necessary registers (EIMSK and EICRB in the AVR architecture) so that the microcontroller will detect external hardware interrupt of either a rising edge or a falling edge (Fig 4 for code snippet).

We can visualize from the LED indicator attached to the microcontroller, that the probe was successfully reading the transmit signals and relaying it to the Arduino. (Fig 5 animation). Both methods implemented were successful. One caveat we had to take into account was the induced EMF. Using our sniffer loop area, and assuming 0.2 G maximum RF strength, we calculated the induced EMF to be 12.6V. Our loop has only 2 capacitors on each side, resulting in ~6.3V across each of the capacitors. Therefore, we made sure that the LTC5536 peak detector and the IE-E97 LED can handle input voltages of ~6.3V.

From the datasheet, the IE-E97 LED had a maximum reverse breakdown voltage of~6V and so we added in an antiparallel protection diode. In fact, when we removed the protection diode from the circuit to rerun a fast spin echo sequence, the LED was damaged after the scan, further validating our suspicions.

We successfully demonstrated two means of RF transmit peak detection while having limited access to the scanner hardware. This can potentially be useful as a quick and easy way to activate and deactivate the readout of auxiliary electronics in the bore, or even actively spoiled MRI receive coils. Interestingly, θ,-θ pulses (0 degree NOOP) would trigger low-data bit rates for setting microcontroller states under pulse sequence control. These approaches may provide useful tools for prototyping wireless MRI systems.