Magnetic coil design for quantitative ultrasound imaging of magnetic nanoparticle density

Magnetic nanoparticles residing in living tissues under external magnetic field may induce tissue displacement that can be measured by ultrasound imaging. We propose a method to take magnetic force images which is proportional to the magnetic nanoparticle density under the assumption of mechanical coupling of the nanoparticles with background tissues. After measuring the axial displacement component by ultrasound imaging, we take Laplacian of the displacement component to get magnetic force images. Using FEM analysis, we have found that average Laplacian image intensity is proportional to the magnetic force at the magnetic-particle-containing region in spite of some artifacts in the images. Based on this observation, we propose a coil system, consisting of a Helmholtz coil and a Maxwell coil, to apply spatially uniform magnetic force to the magnetic nanoparticles. Through FEM analysis of the displacement and magnetic field analysis of the coil system, we have estimated coil currents that can induce measurable displacement in soft tissues.