Authors: Davi Cavinatto
Mentors: Steven Allen
Insitution: Brigham Young University
Previous work indicates that magnetic gradients oscillating at the same frequency and direction as ultrasound (US) longitudinal displacement can encode particle movement into the complex phase of a magnetic resonance (MR) image. Until now, the coil configuration (Helmholtz) used to generate this oscillating magnetic gradient has constrained the use of this technique to small imaging volumes. Here, we explore the feasibility of using a single coil configuration to improve the versatility of the apparatus, making it possible to visualize US waves as they propagate through tissue that was previously inaccessible through the technique, such as the human brain. This novel approach to the visualization of US waves could potentially be used to establish the missing correlation between the results of neuromodulation treatments and their mechanism of action, thus improving the scientific rigor of this field of research.
Wolfram’s Mathematica and COMSOL’s Multiphysics were used for developing a single-coil configuration in silico. The coil design was constrained by the minimum imaging distance from the coil (20mm), gradient needed for an image with signal-to-noise ratio of approximately 10 (0.4 T/m), minimum coil inner radius for fitting the US transducer (20mm), maximum peak current at the coil (20A), and frequency of operation (500kHz). Using Biot-Savart’s Law and Mathematica, we estimated the number of turns needed and the total length of the wire. In order to reduce the skin effect and proximity effect due to the frequency of operation (500kHz), a specific Litz wire configuration was chosen for the windings. Plots for the magnetic gradient over the central axis of the coil were created and compared on both programs to confirm the accuracy of the model.
Plots of both the mathematical and in-silicon models matched and proved the high efficiency of the coil system at the frequency of application. The two magnetic field gradient plots corroborate the feasibility of the proposed single coil system for imaging US waves and verification of the functioning of neuromodulation in the extension of the cerebral cortex.