Ultrasound contrast agents: present but not seen

The presence of ultrasound contrast agents (UCAs) in the body is normally determined by the echo intensity displayed on an imaging system. In diagnostic ultrasound, failing to visualize the contrast agent usually means the end of its usefulness. However, the authors show that UCAs, when disrupted by an ultrasound field, still leave an environment rich of nuclei for inertial cavitation. Therefore, fragments of UCAs are still present in the body although invisible to the imaging system. In the present work, the authors developed an experimental arrangement to measure the threshold at which the echo-contrast loses its property as an effective scatterer (P1), and the threshold at which inertial cavitation develops (P2). P1 and P2 were measured as a function of the concentration, pulse repetition frequency (PRF) and number of acoustic cycles. The results measured for Albunex(R) in Isoton II solution, show that P1 was directly proportional to the concentration and inversely proportional to the PRF. P2 was inversely proportional to the concentration and independent of PRF. Both P1 and P2 were relatively independent of the number of acoustic cycles.