Phantom Characterization Using the Mechanical Resonance of a Tissue Embedded Magnetic Sphere
Thomas Ersepke, Tim C. Kranemann, Georg Schmitz
2016 IEEE Int. Ultrasonics Symp. (IUS), Tours, Sep. 2016
Characterization of viscoelastic properties of phantom materials has been an important subject to validate methods for ultrasound elastography. The frequency response of the motion of a rigid sphere, embedded in a viscoelastic medium, that is excited by an acoustic radiation force (ARF), has been utilized to estimate the viscoelastic properties of the medium. This response is commonly tracked with ultrasound based methods, which have to use push and tracking pulses alternately. As an alternative optical tracking methods allow continuous measurements but are only applicable for optically transparent phantom materials. We present a new method to acquire the velocity response of an embedded, magnetic sphere by measuring the voltage induced in a receiving coil. Magnetized chrome-plated NdFeB spheres with a diameter of 4 mm (257 mg) are embedded in tissue mimicking phantoms made of gelatin, comprising different degrees of stiffness. The embedded magnetic sphere is exposed to an ultrasound beam of a focused transducer and is excited by an amplitude-modulated acoustic radiation force. By varying the modulation frequency (30-400 Hz), the velocity response of the magnetic sphere is measured with a self-made receiving coil, positioned around the phantom. To validate the experimental setup, the velocity response of the embedded sphere was additionally measured with a laser vibrometer. The proposed measurement method shows a high accuracy with a mean absolute error (MAE) of 0.4 Hz concerning the resonance frequencies of the acquired frequency responses of both methods. Repeated measurements of a single sample revealed a standard deviation of 1.3 Hz, indicating a high measurement precision. The proposed method is suitable for opaque phantoms and allows tracking the sphere motion with high resolution in a simple and fault-tolerant setup.