Effect of ultrasound transmit power on liver enhancement with imagent® US, a PFC-stabilized microbubble contrast agent

The pressure of the ultrasound wave may limit the longevity of microbubble-based contrast agents. This study evaluated liver enhancement over time as a function of transmit power after the administration of AFO145 (Imagent® US; Alliance Pharmaceutical Corp., San Diego, CA). Eight rabbits with an avascular liver lesion created by percutaneous injection of 1.0 ml of ethyl alcohol 7 days prior to scanning were imaged with an Acuson 128XP/10 at 7 MHz before and after four separate intravenous injections of 0.25 ml of AFO145 spaced at least 1 h apart. The avascular lesion served as an internal standard against which liver enhancement could be compared. After contrast injection, scanning over the same plane was either continuous at (a) maximum or (b) minimum transmit power (9 dB below maximum), or intermittent at (c) minimum power for 5 s every 15 s, or (d) for 5 s every 60 s. Each session was terminated after 15 min or when contrast was no longer visible in the hepatic parenchyma and blood vessels. Videodensitometry was used to assess liver-to-lesion intensity difference over time. Both the degree and duration of liver enhancement were dependent on the transmit power. Liver enhancement with imaging at minimum power for 5 s/min was nearly two times greater and persisted nearly eight times longer (P < 0.01) than at maximum power and continuous insonation. Ultrasound transmit power affects both the peak and duration of liver enhancement. A lower power and shorter insonation time after AFO145 administration dramatically lengthens the imaging window for liver lesion detection.