Normalization of magnetic field effects: Towards quantitative magnetomotive ultrasound imaging

In magnetomotive ultrasound (MMUS) imaging superparamagnetic iron oxide nanoparticles (NP) are used as contrast agents and a time-varying external magnetic field acts to move the particles and thereby the nanoparticle-laden tissue. Recently we proposed a frequency and phase sensitive algorithm to reduce motion artifacts. However, the method is not quantitative as the particle movement induced is dependent not only on the field strength, but also on the field gradient, plus material parameters. Here we assess the measured nanoparticle movement across the image plane in comparison with simulations of the magnetic force, to evaluate the potential for image normalization of magnetic field effects. We found that the movement decreased with distance to the iron core tip, from which the magnetic field was extending, and approaches zero at the transducer face. This finding did not coincide with the simulation and may make it difficult to enable quantification. The coefficient of variation between measurements on the homogeneous phantom was typically in the order of 15% for all frequencies, indicating the expected accuracy for quantitative measurements.