Increased Displacement in Magnetomotive Ultrasound Imaging by Adding a Homogeneous Magnetic Field

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 homogenous magnetic field was generated using a Helmholtz coil (2×150 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 a previously published algorithm to measure the axial component of the tissue motion. Adding the coil with a homogeneous field does increase the detected magneto-motion, in accordance with the theoretical model. A 40% increase was measured at a depth of 53mm for an additional 6mT magnetic field. This points to a possible way to increase the sensitivity in MMUS by adding a homogeneous magnetic field.

I. Introduction

Magnetomotive ultrasound (MMUS) is an imaging modality used to locate the presence of iron -oxide nanoparticles using an external time-varying inhomogeneous magnetic field and ultrasound detection. The magnetic field is commonly gener-ated using electromagnets but a rotating permanent magnet was shown to be more suitable for some applications [1]. However, this solution comes with the drawback of a shorter magnetic force range. New strategies to generate the magnetic field are necessary and it was suggested that a combination of magnetic fields could be used. Wang et al. [2] showed that the addition of a permanent magnet prior to applying the time-varying magnetic field to magnetize the nanoparticles results in a higher displacement and a higher detection sensitivity. Following the idea of adding an extra magnetic field, the use of a Helmoltz coil instead of a permanent magnet emerged as the coil is a relatively easy way to generate a magnetic field over a large area. The generated field is homogeneous and in such a way will not create any force on the particles alone but combined with the inhomogeneous field, it will increase the displacement of the particles and the image sensitivity in MMUS. In this study is this combination of fields investigated both theoretically and in a corresponding experimental model.