A human-compatible gradient coil for visualizing ultrasound propagation

In pursuit of a magnetic resonance imaging (MRI) based means to tailor transcranial focused ultrasound neuromodulation to a patient's unique skull morphology, this study presents a specialized gradient coil that sensitizes MRI images to ultrasonic vibrations at depths equivalent to the human cortex in a technique called the magnetic resonance hydrophone. The coil comprises a 60 mm diameter, pancake-style design that encodes acoustic displacements into MR images at the cost of an inhomogeneous encoding field. The coil was coupled with a 500 kHz, custom built, ultrasonic transducer. Both the magnetic field gradient of the coil and the acoustic field of the transducer were characterized in benchtop experiments. Acoustic standing waves were estimated in silico. Resulting MR images displayed a sinusoidal phase pattern modulated by both the transducer's acoustic field and the coil's magnetic field gradient. Acoustic pressures were estimated from the resulting images and compared to hydrophone measurements. The pancake-style coil produced a pressure measurement uncertainty pattern due to electronic noise that increased exponentially with depth. Uncertainty at locations between 0 and 30 mm of depth within a region approximately 10 mm wide scaled between approximately 20 kPa and 100 kPa. On average, the MRH underestimated the hydrophone by 12 kPa with the difference between the two following a standard deviation of 21 kPa.