Ferromagnetic brachytherapy seed motion in soft tissue: models, measurements and ultrasound detection

We have previously reported a technique for identifying modified prostate brachytherapy seeds. In this method, an alternating magnetic field is used to vibrate the seeds in situ with micron amplitudes. These vibrations are detected with Power Doppler or other ultrasound signal processing techniques to highlight the seeds in the image. Here we report results of finite-element simulations of seed and tissue motion in response to the magnetically induced forces. Good agreement with measured displacements in a tissue-mimicking phantom is found. Simulated (Field II) Power Doppler ultrasound images based on the finite-element simulations show good agreement with in vitro Power Doppler images. Images of multiple seeds in a single field of view are presented and demonstrate good resolution with the technique.