Imaging techniques for ultrasound contrast agents

Abstract

Contrast agents are used today to enhance and aid organ (myocardium, liver, brain, etc.) perfusion studies. From the early days of contrast imaging it was understood that microbubble specific imaging modalities would have to be invented to utilize the contrast effect. The two main characteristics of contrast microbubbles that have defined today’s ultrasound modalities are (1) microbubbles are nonlinear scatterers that generate harmonic signals and (2) they exhibit a transient response, i.e., they are destroyed by ultrasound. Harmonic Doppler and imaging techniques were introduced at first to help with the detection of microbubbles in small vessels and capillaries. The harmonic response of bubbles and their Doppler signals increase with increasing acoustic amplitudes (mechanical index, MI), which results in fast bubble destruction. Triggered (intermittent) imaging, insonifying every n cardiac cycles (where n=1,2,3,...), was established to minimize destruction. Triggered imaging is a cumbersome technique. Real-time imaging techniques are now available. They are made possible by low-amplitude (MI) scanning and by more sensitive low MI techniques like pulse inversion and amplitude modulation. Both triggered and real-time techniques will be discussed in this presentation and clinical examples will be presented.