Field Characterization and Compensation of Vibrational Nonuniformity for a 256-Element Focused Ultrasound Phased Array

Abstract

Multielement focused ultrasound phased arrays have been used in therapeutic applications to treat large tissue volumes by electronic steering of the focus, to target multiple simultaneous foci, and to correct aberration caused by inhomogeneous tissue pathways. There is an increasing interest in using arrays to generate more complex beam shapes and corresponding acoustic radiation force patterns for manipulation of particles such as kidney stones. Toward this end, experimental and computational tools are needed to enable accurate delivery of desired transducer vibrations and corresponding ultrasound fields. The purpose of this paper was to characterize the vibrations of a 256-element array at 1.5 MHz, implement strategies to compensate for variability, and test the ability to generate specified vortex beams that are relevant to particle manipulation. The characterization of the array output was performed in water using both element-by-element measurements at the focus of the array and holography measurements for which all the elements were excited simultaneously. Both methods were used to quantify each element's output so that the power of each element could be equalized. Vortex beams generated using both compensation strategies were measured and compared to the Rayleigh integral simulations of fields generated by an idealized array based on the manufacturer's specifications. Although both approaches improved beam axisymmetry, compensation based on holography measurements had half the error relative to the simulation results in comparison to the element-by-element method.

Noninvasive acoustic manipulation of objects in a living body

“…All array elements project into a region where their harmonic waves or multicycle bursts interfere and are positioned on a concave spherical surface to concentrate the wave energy within a region around the center of curvature, the focus. The elements were tuned to transmit acoustic waves equal in amplitude and with no relative phase delay between elements when focused to the center of curvature of the array (32). A specific field structure or beam is created by judicious phase delays among the waves transmitted by the elements.…”

“…The goal of the work presented herein was to transcutaneously manipulate an object within a living animal body using the beam from a single transducer. We developed a system (22,32) and methods to produce 3D manipulation of a millimeter-sized object chosen to mimic a kidney stone. We successfully demonstrate both in a water bath and in live pigs, the ability to execute complex motion to remotely move an object along a path entirely controlled by the acoustic field under ultrasound image guidance.…”

Experimental Validation of k-Wave: Nonlinear Wave Propagation in Layered, Absorbing Fluid Media

“…Both the input plane and the validation measurements were made with the same hydrophone, and since the field is linear, the effect of uncertainty in the hydrophone frequency response should be limited (the short pulse has some bandwidth). It is possible that the differing impact of the directional response of the hydrophone on the input plane and the validation measurements leads to underestimation of the focal pressure [35]; this will be investigated in future work.…”

Innovations in Ultrasound Technology in the Management of Kidney Stones

“…This may enable directed capture and navigation of a stone through the calyceal system. 58 In recent years, tractor beam technology has been shown to have the capability to move targets > 1 cm in size. 59 Integration with current ultrasonic propulsion technology may allow for more directed, non-invasive stone movements.…”