Effects of nonlinear ultrasound propagation through varying contrast-agent concentrations

Several methods are being developed for quantitative contrast echography. These methods aim at the assessment of perfusion defects or alterations, caused for instance by the presence of ischemic or cancerous tissue [1, 2]. In order to extract quantitative data from contrast echography, the concentration of the diluted ultrasound contrast agent (UCA) must be accurately measured. To this end, the sensitivity to UCA of ultrasound (US) imaging techniques has been increasingly improved in the past decade by the introduction of several dedicated contrast-enhancement imaging modes, often referred to as harmonic modes, which exploit the nonlinear behaviour of the UCA microbubbles [3]. A common measure for the performance of these imaging modes is the contrast-to-tissue ratio (CTR), which is the ratio between the acoustic backscatter coming from UCA and tissue [4]. The determination of the CTR is usually based on independent measurements of the backscatter from UCA and tissue, while the interaction between the two systems is not considered. In this study, the nonlinear distortion of US propagating through varying concentrations of Definity UCA (Bristol-Myers Squibb) is measured and modeled. The implications on contrast imaging are also discussed and quantified.