Nonlinear acoustics in tissue harmonic imaging

Abstract

In recent years the interest in nonlinear acoustics has dramatically increased in diagnostic ultrasound. There are two main areas where nonlinear acoustics is used in medical imaging: tissue harmonic imaging (THI) and imaging of ultrasound contrast agents. Although similar approaches are used in both of these areas, they are very different in that THI is based on nonlinear propagation of sound in tissue, whereas contrast imaging is based on the nonlinear scattering from resonant microbubbles. The clinical benefits of THI are reduced phase aberration artifacts and overall clutter, improved border delineation, and increased contrast resolution. The basic principles of nonlinear propagation of sound beams in tissue are discussed here. Theoretical and experimental results are used to demonstrate some of the properties of nonlinear propagation and their relation to imaging. To a large extent the clinical benefits of THI are explained with the nonlinear propagation properties. Imaging considerations and techniques like pulse inversion and power modulation are discussed. These techniques are also used in imaging contrast agents (microbubbles) and an effort is made to separate the various issues in these two imaging areas and explain their differences.