A human-compatible gradient coil for visualizing ultrasound propagation.

Area of Science:

• Biomedical Engineering
• Medical Imaging
• Acoustics

Background:

• Transcranial focused ultrasound (tFUS) neuromodulation requires precise targeting based on individual skull anatomy.
• Current methods lack real-time MRI-based feedback to guide tFUS delivery.
• Acoustic monitoring techniques are needed to ensure safety and efficacy of tFUS.

Purpose of the Study:

• To develop and characterize an MRI-based system for monitoring ultrasonic vibrations during tFUS.
• To create a specialized gradient coil for the magnetic resonance hydrophone (MRH) technique.
• To assess the feasibility of tailoring tFUS neuromodulation using patient-specific skull morphology.

Main Methods:

• A 60 mm diameter, pancake-style gradient coil was designed to sensitize MRI images to ultrasonic vibrations.
• The coil was integrated with a 500 kHz custom ultrasonic transducer.
• Benchtop experiments characterized the magnetic field gradient and acoustic field; in silico methods estimated acoustic standing waves.
• Resulting MR images were analyzed to estimate acoustic pressures and compared to conventional hydrophone measurements.

Main Results:

• The MRH system successfully visualized acoustic displacements as sinusoidal phase patterns in MR images.
• Acoustic pressure estimations from MR images showed modulation by both acoustic and magnetic fields.
• Pressure measurement uncertainty due to electronic noise increased exponentially with depth, ranging from 20-100 kPa between 0-30 mm depth.
• The MRH system underestimated hydrophone measurements by an average of 12 kPa (SD 21 kPa).

Conclusions:

• The developed gradient coil enables MRI-based monitoring of ultrasonic vibrations for tFUS applications.
• The MRH technique shows potential for real-time feedback in tFUS neuromodulation.
• Further optimization is needed to reduce depth-dependent uncertainty and improve pressure estimation accuracy.