Mimicking focused ultrasound with a loop coil in acoustic radiation force imaging

Purpose To enable development of MR‐acoustic radiation force imaging (MR‐ARFI) methods for targeting ultrasound in human subjects without the regulatory, acoustic, or hardware challenges associated with actual transcranial ultrasound setups. Methods MR‐ARFI is a phase‐contrast imaging method that measures focal tissue displacement produced by an ultrasound transducer, when the transducer is pulsed simultaneously with a motion encoding gradient. The ultrasound‐induced focal phase shift can be mimicked with a small loop coil that is driven by a DC pulse to produce a resonance frequency offset at the same time as the ultrasound pulse in an MR‐ARFI pulse sequence. A coil was designed and built for use in MR‐ARFI. Its focus size was characterized, its field map was measured, and volunteer experiments were performed to demonstrate its function in transcranial phase‐contrast and magnetization‐prepared MR‐ARFI. Results Off‐resonance field maps measured with the constructed loop coil were within 0.87% of simulations in a slice 15 mm from the coil's surface. Its “focus” further had a full‐width‐at‐half‐maximum of 22.9 mm in simulation versus 22.7 mm in the field map. In vivo results showed that the same coil driven with 13.7 mA current produced a phase shift corresponding to a realistic effective displacement of 3.5 m in a slice 19 mm from the coil in MR‐ARFI. Conclusion A pulsed DC loop coil can mimic ARF‐induced displacements in vivo, facilitating development of MR‐ARFI methods in vivo.