Nonlinear propagation in ultrasonic fields: measurements, modelling and harmonic imaging

Area of Science:

• Acoustics
• Medical Physics
• Nonlinear Acoustics

Background:

• High amplitude ultrasound fields exhibit nonlinear propagation, leading to waveform distortion and harmonic generation.
• Diffraction effects from the transducer's nearfield complicate nonlinear propagation analysis.

Purpose of the Study:

• To investigate nonlinear acoustic propagation and harmonic generation in the fields of circular, focused, and rectangular transducers.
• To compare experimental results with numerical predictions from the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation.
• To explore the implications of nonlinear propagation and harmonic generation for medical ultrasound imaging and therapeutic applications.

Main Methods:

• Experimental studies of nonlinear acoustic fields generated by different transducer geometries.
• Numerical simulations using a finite difference solution of the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation.
• Analysis of harmonic beam characteristics, including beamwidth and side-lobe levels.

Main Results:

• Nonlinear propagation effects and harmonic generation were observed and quantified for various transducer types.
• Experimental results were found to be in good agreement with KZK equation predictions.
• Harmonic beams exhibit narrower widths and reduced side-lobe levels compared to fundamental beams.

Conclusions:

• Nonlinear propagation is a significant factor in high amplitude ultrasound fields.
• The generation and characteristics of ultrasonic harmonics have important implications for medical ultrasound.
• Harmonic imaging offers potential for enhanced diagnostic resolution in medical ultrasound applications.