Second harmonic imaging and harmonic Doppler measurements withAlbunex

Gaseous ultrasound contrast agents, such as Albunex, have been shown to be valuable clinically for the better identification of certain anatomic structures and for quantitating organ perfusion. However, the application of a contrast agent sometimes also reduces the image contrast between the blood and the surrounding tissues. This paper presents experimental results to show that this problem may be alleviated by using harmonic signals generated by nonlinear backscattering of these microbubbles. The results show that the second harmonic signal produced by Albunex is significant and that harmonic images in which backscattered echoes from solid tissues were filtered out, but not those from Albunex, can be obtained. In addition, a continuous wave Doppler arrangement has been implemented. It permits quantitative harmonic Doppler measurements at different frequencies, concentrations, and transmitted acoustic pressures. When Albunex was compared to polystyrene microsphere suspensions which possess exclusively linear behavior, Doppler power at second and subharmonic frequencies could only be detected from these microbubbles. A comparison of results on Doppler powers measured at the first harmonic from polystyrene spheres and Albunex indicates that at a transmitting frequency of 2.5 MHz, the Doppler power from Albunex is 12 dB stronger than the polystyrene spheres, whereas at 5 MHz, the Doppler power from Albunex is 3 dB weaker than that from the polystyrene spheres