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

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.

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.

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 $\mu$ m in a slice 19 mm from the coil in MR-ARFI.

A pulsed DC loop coil can mimic ARF-induced displacements in vivo, facilitating development of MR-ARFI methods in vivo.