2K-3 Magnetically Vibrated Brachytherapy Seeds: Ferromagnetic Core Models and Image Reconstruction Methods

Magnetically Induced Motion Imaging (MIMI) uses an oscillating magnetic field and ultrasonic motion-tracking techniques to vibrate and identify brachytherapy seeds in situ. The efficacy of the technique relies on the ability to generate and detect seed vibration, and distinguish this vibration signal from other motion sources. The vibration of the seed depends on the torque generated by a ferromagnetic core in the seed. A design goal is to maximize the torque for the limited amount of core material that can be placed within a seed. We have developed 3D finite-element models for two seed core geometries, an ellipsoid and a rod capped by two semi-hemispheres. Both seed cores have identical volumes (7.4times10^-10m³ ), length (4mm), and permeability (mu_r=4000). Calculation by the Maxwell Stress Tensor method yields a torque for the rod 1.4 times that of the ellipsoidal core, demonstrating the substantial sensitivity of torque on core geometry. The oscillating seeds act as dipole shear wave sources, with maximum vibration amplitude at the ends of the seed and a vibration minimum at the center of length. This gives rise to a characteristic vibration amplitude distribution in the surrounding tissue, with two lobes per seed. By taking advantage of the opposing phase of the seed ends, we demonstrate a method that links these lobes. A compounding technique for suppressing ring-down artifact is demonstrated. These methods are demonstrated on RF data acquired from seeds in beef muscle tissue. 3D vibration isosurface maps of seed vibration amplitude are presented and found to be in good agreement with previously reported simulations