Measurement of the contrast agent intrinsic and native harmonic response with single transducer pulse waved ultrasound systems

Ultrasound contrast agents, i.e., small gas filled microbubbles, enhance th e echogenicity of blood and have the potential to be used for tissue perfus ion assessment. The contrast agents scatter ultrasound in a nonlinear manne r and thereby introduce harmonics in the ultrasound signal. This property i s exploited in new ultrasound techniques like harmonic imaging, which aims to display only the contrast agent presence. Much attention has already bee n given to the physical properties of the contrast agent. The present study focuses on practical aspects of the measurement of the intrinsic harmonic response of ultrasound contrast agents with single transducer pulse waved u ltrasound systems. Furthermore, the consequences of two other sources of ha rmonics are discussed. These sources are the nonlinear distortion of ultras ound in a medium generating native harmonics, and the emitted signal itself which might contain contaminating harmonics. It is demonstrated conceptual ly and by experiments that optimization of the contrast agent harmonic resp onse measured with a single transducer is governed by the transducer spectr al sensitivity distribution rather than the resonance properties of the con trast agent. Both native and contaminating harmonics may be of considerable strength and can be misinterpreted as intrinsic harmonics of the contrast: agent. practical difficulties to filter out the harmonic component selecti vely, without deteriorating the image, may cause misinterpretation of the f undamental as a harmonic. (C) 1999 Biomedical Engineering Society. [S0090-6 964(99)01005-X].