Measurement of Angle of Ultrasound Propagation from Phase

Abstract

Acoustic field calibrations often use Fast Fourier Transforms (FFT) to quantify the spectral amplitude components of detected acoustic waveforms. The ability of FFTs to provide phase information is often overlooked. This phase data may be useful in determining the angle of propagation of the ultrasound beam. The angle of propagation at the focal peak (and any other point in the field) can be calculated easily and quickly without additional measurement, and may be the quickest and most accurate method of aligning the sound axis with respect to the beamplotting system's co-ordinates.Acoustic fields have been measured experimentally using a system capable of waveform acquisition. Measurements were made using a fibre-optic hydrophone (Precision Acoustics, UK) which provided spatial resolution of <100 μm. Two operating configurations of a 10 strip array HIFU (high intensity focused ultrasound) transducer were tested, as was a single element HIFU device. Theoretical pressure and phase distributions for these transducers were predicted using a linear acoustic field model. Results show that for the single element, radially symmetric device, beam alignment measurements using phase data at the focal peak are in agreement with the more conventional method based on finding the on-axis peak positions. In the case of a transducer with a number of elements de-activated to produce an asymmetric ultrasound source, the angle of propagation at the focal peak was altered, indicating a change in performance of the transducer which otherwise might not have been detected using the "on-axis peaks" method. Simulations agreed with the experimental data.