Acoustic Manipulation of Macroscopic Objects

Abstract

Acoustic waves can apply radiation forces to trap and levitate objects. Most recently, three-dimensional acoustic traps have been created using single-sided multi-element arrays. Almost all the work accomplished in this field is concerned with the trapping of light objects in air or microscopic objects in liquids. The purpose of this work is to investigate acoustic manipulation of macroscopic objects such as kidney stones specifically for non-invasive medical treatments. Toward this end, a 1.5 MHz 256-element focused ultrasound array was used. A characterization of the array output was performed to equalize the complex vibrational output of each element and produce uniform vortex beams. Vortex beams were generated and compared to simulation of the fields from an idealized array. Measurements show improvement of beam symmetry up to 9%. The lateral trapping strength of different acoustic beams was measured to examine their ability to trap and manipulate large dense objects. This was the first effort to quantify the lateral forces on large objects and compare them to theoretical predictions. Vortex beams and two other beam shapes having a toroidal pressure field in the focal plane were used to measure the lateral trapping strength. Spherical targets of various sizes were placed in the acoustic traps and held in the focal plane of the array by a frame mounted to the array. The array-frame setup was rotated until the bead fell out of the acoustic trap. Good agreement between measurements and theory was achieved. Lateral acoustic trapping and steering of large targets was demonstrated along two-dimensional path against a flat surface in water. Three-dimensional traps with the aid of gravitational pull were synthesized and millimeter-sized glass spheres were levitated and manipulated in water above a tissue mimicking phantom. The stability of the acoustic off-axis steered traps was shown to be most stable for a ratio of bead to beam width less than unity. Finally, a 3-mm spherical glass bead, a kidney stone mimicking target, was successfully manipulated and steered in three-dimensional path inside a live pig with ultrasound imaging.