On the Bandwidth and Beam Profile Characteristics of a Simple Low-Frequency Collimated Ultrasound Beam Source

J Greenhall,V K Chillara,D N Sinha,C Pantea

doi:10.1115/1.4050851

Abstract

Abstract We numerically investigate the bandwidth and collimation characteristics of ultrasound beams generated by a simple collimated ultrasound beam source that consists of a piezoelectric disk operated near its radial mode resonances. We simulate the ultrasound beam generated in a fluid medium as a function of the excitation frequency for two cases: (1) free piezoelectric disk that corresponds to zero-traction along the lateral edge and (2) fixed piezoelectric disk that corresponds to zero-displacement along the lateral edge. We present and discuss the physical mechanism underpinning the frequency-dependent collimation and bandwidth properties of the ultrasound beams. We observe that the collimated beam generated by the free disk repeatedly lengthens/shortens and also extends/retracts sidelobes with increasing frequency. Alternatively, fixing the piezoelectric disk results in a consistent beam profile shape across a broad range of frequencies. This facilitates generating broadband signals such as a Gaussian pulse or chirp, which are common in ultrasound imaging. Thus, the fixed piezoelectric disk finds application as a collimated ultrasound beam source in a wide range of applications including medical ultrasound imaging, scanning acoustic microscopy, sonar detection, and other nondestructive ultrasound inspection techniques.

Highlights

Collimated beams are attractive to the optics and acoustics communities because they are able to propagate long distances with little diffraction and demonstrate focusing regions that are significantly larger than traditional beams [1]
Collimated beams have the potential to benefit a range of acoustic applications including nano- and micro-particle manipulation [2,3] and acoustic imaging [4], where the narrow beamwidth improves lateral resolution and the large focusing region reduces spherical aberrations in the image
Chillara et al demonstrated a simple technique for generating a lowfrequency collimated ultrasound beam (LF-CUB) by exciting the radial modes of a thin piezoelectric disk, which resulted in the generation of a beam with an approximately Bessel profile [12,13,14]

Summary

Introduction

Collimated beams are attractive to the optics and acoustics communities because they are able to propagate long distances with little diffraction and demonstrate focusing regions that are significantly larger than traditional beams [1]. Chillara et al demonstrated a simple technique for generating a lowfrequency collimated ultrasound beam (LF-CUB) by exciting the radial modes of a thin piezoelectric disk, which resulted in the generation of a beam with an approximately Bessel profile [12,13,14]. It was found that clamping the lateral edges of the disk significantly reduces the amplitude of the Bessel-like side lobes at the radial mode frequencies, which narrowed the width and extended the length of the LF-CUB. This LF-CUB source has been investigated at discrete radial mode frequencies, but it has not been studied for its behavior at off-resonance frequencies. We model the LF-CUB source numerically to (1) characterize the frequency-dependent surface vibration characteristics of the source with and without lateral constraint and (2) study the corresponding acoustic beam profile characteristics in water

Simulation Setup

Findings

Low-Frequency Collimated Ultrasound Beam