Frequency response of soft tissue displacements induced by the force on magnetic nanoparticles

In Magnetomotive Ultrasound (MMUS), tissue embedded magnetic nanoparticles (NP) are mechanically excited by a magnetic field and the resulting motion of the surrounding tissue is tracked to image the NP distribution. This mechanism of excitation is of particular interest for elastography applications and offers an alternative to the acoustic radiation force excitation in order to estimate the tissue's mechanical properties. Measuring the frequency-dependent, tissue properties is relevant for several elastography techniques but common MMUS setups utilize solenoid/yoke configurations with excitation frequencies limited up to a few Hz. In this work, we present an MMUS setup which allows to measure the entire frequency range, relevant in elastography, by decoupling the gradient field from the excitation field. A cylindrical inclusion containing 90% nanoparticles and 10% gelatin was embedded in phantoms of different gelatin concentrations (10%, 15% w/w). A static, magnetic field was generated by two permanent magnets arranged opposite each other, producing a constant gradient of 3.4 T/m along the displacement direction. A solenoid generated a superimposed, sinusoidal excitation field of 18 mT at frequencies between 50-1000 Hz. For validation, the on-axis displacement of the inclusion was tracked by a laser vibrometer at its surface due to reflections at a thin slice of glass powder. The frequency responses of the displacement measurements showed resonance frequencies of 310 Hz for the soft (10%) and 500 Hz for the stiff (15%) phantom, respectively. The proposed setup proved capable of tracking the mechanical tissue dynamics precisely beyond the frequency limit of standard MMUS setups.