Revolving Permanent Magnet for Magnetomotive Ultrasound

In magnetomotive ultrasound (MMUS), solenoids have been extensively used to generate varying magnetic fields, that act to move nanoparticles and their surrounding tissue. Here we show, compare and contrast two magnetic devices, one based on a solenoid, another containing a rotating permanent magnet. Suitable field characteristics for this application include high flux density and gradient, and uniform field laterally in the image plane, and the corresponding design features are highlighted. Field measurements show that both magnets generate suitable fields. The magnetic force potentially exerted by the permanent magnet even exceeds that of the solenoid up to a depth of 4 mm. Considering the substantial heat generation in high inductance electromagnets, a permanent magnet would be preferable as it eliminates the need for cooling. Thus, we suggest the use of permanent magnets in MMUS devices to reduce the size, while maintaining a high field and gradient required to move nanoparticles.