New and old approaches for nonlinear-contrast agents imaging

Abstract

Microbubble, nonlinear-contrast agents are used in nonlinear ultrasound imaging to increase the contrast between these agents and the surrounding tissue. Similar approaches were developed in Russia in the 1980s for bubble measurements at sea. Two methods can enhance nonlinear imaging. (1) Sum-frequency nonlinear imaging has several advantages over second harmonic imaging due to the absence of nonlinear distortion in electronic circuits and emitters. This method allows one to use significantly less ultrasound amplitude than in the method of second harmonic imaging. This method was used for bubble detection in sea sediments. (2) The nonlinear difference frequency Doppler effect is based on moving bubbles that are insonified by two acoustic beams of different frequencies directed from different directions. Difference-frequency scattering provides significant enhancement of the Doppler frequency shift. This method was tested in laboratory conditions. Nonlinear scattering by microbubbles could also lead to remote, direct blood-pressure measurements. This method could be based on measurements of microbubble resonance frequencies by using nonlinear scattering. The resonance frequency depends on the ambient pressure; nonlinear scattering provides high-resolution measurements of the resonance frequencies and avoids the influence of scattered signals reflected by different inhomogeneities in the human body. [Work was supported by Stevens.]