Increased Displacement in Magnetomotive Ultrasound Imaging by Adding a Homogeneous Magnetic Field
Abstract: Magnetomotive Ultrasound, MMUS, is an imaging modality used to reveal a magnetic contrast agent using an external time-varying inhomogeneous magnetic field. By this, the particles are set in motion, and the motion is detected with ultrasound. The technique has applications in cancer detection but is limited in penetration as the magnetic field decreases rapidly with distance. Instead of increasing the size of the conventionally used magnetic probe which would lead to heating and clinical practicality problems, adding a homogeneous magnetic field has been suggested to increase the force and thereby displacement. Since the magnetic force is proportional to both the magnetic field strength and the field gradient, the second homogeneous field will increase the force. The force is also proportional to the magnetic susceptibility of the contrast agent which is increased by this second field (pre-magnetization). A homogenous magnetic field was generated using a Helmholtz coil (2x150 turns) driven by an AC current (5 Hz, 4 A peak to peak) synchronized in phase with the rotating neodymium magnet generating the inhomogeneous field (also 5 Hz). The fields set in motion magnetic particles embedded in a tissue-mimicking material and the tissue motion was imaged using an ultrasound scanner (Visualsonics F2). The images were processed using the previously published algorithm to measure the axial component of the tissue motion. The homogeneous field reached a peak value of 6 mT at the center of the coil, this in addition to the magnetic field produced by the rotating neodymium magnet. The displacement in each image pixel, averaged over a region of interest encompassing the nanoparticle insert, versus distance from the face of the rotating magnet is greater when the coil is added. Similarly, the displacement increases as a function of the homogeneous magnetic field in both directions of the circular motion. Adding the coil with a homogeneous field does increase the detected magneto-motion, in accordance with the theoretical model. This points to a possible way to increase the sensitivity in MMUS by adding a homogeneous magnetic field.
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