Field characterization of therapeutic ultrasound phased arrays through forward and backward planar projection

Abstract

Spatial planar projection techniques propagate field measurements from a single plane in front of a transmitter to arbitrary new planes closer to or further away from the source. A linear wave vector frequency-domain projection algorithm is applied to the acoustic fields measured from several focused transducer arrays designed for ultrasound therapy. A polyvinylidene difluoride hydrophone is first scanned in a water tank over a plane using a three-dimensional positioning system to measure the complex pressure field as a function of position. The field is then projected to a series of new planes using the algorithm. Results of the projected fields are compared with direct measurements taken at corresponding distances. Excellent correlation is found between the projected and measured data. The method is shown to be accurate for use with phase-controlled field patterns, providing a rapid and accurate method for obtaining field information over a large spatial volume. This method can significantly simplify the characterization procedure required for phased-array application used for therapy. Most significantly, the wavefront propagated back to a phased array can be used to predict the field produced by different phase and amplitude settings of the array elements. A field back-projected to the source could be used as an improved source function in acoustic modeling.

A Novel Device for Total Acoustic Output Measurement of High Power Transducers

“…Under such conditions it may be possible to "back-project" the field measured via hydrophone scanning at the exit from the target [5]. This opens up the possibility of simultaneous field characterization for parameters such as focal diameters and intensity, and perhaps even side-lobes.…”

Evaluation of a wave vector frequency domain method for nonlinear wave propagation.

“…A single plane also has the potential to characterize the radiative behavior of its source. 2 This could be particularly useful in the case of high power transducers, where the source itself commonly radiates higher harmonics of the fundamental. In this case, the backprojection could serve to separate source signals from medium-dependent acoustic nonlinear effects.…”

Investigating the effect of thickness and frequency spacing on multi-frequency acoustic kinoforms

“…1. A uniform incident pressure was assumed and the angular spectrum approach (ASA) was then used to propagate the field to the target depth [8]. However, for kinoforms possessing narrow fringe spacing (high spatial frequencies) and substantial fringe heights, the thin element approximation can break down.…”