Effect of acquisition rate on liver and portal vein enhancement with microbubble contrast

We showed that tissue enhancement with microbubbles is dependent upon transmit power. Because intermittent imaging decreases bubble exposure to ultrasound, and also decreases the ability of the sonographer to maintain anatomic orientation, we aimed to determine the optimum frame rate that maximizes enhancement and allows for continued anatomic orientation. Seven rabbits with an avascular liver lesion created by percutaneous injection of 1 mL ethyl alcohol 7 days earlier were imaged with an Acuson 128XP/10 using a 7-MHz sector transducer at fixed transmit power. Each rabbit was imaged 5 times in random order, at 1 frame/30 s, 1frame/5 s, 1frame/s, 4 frames/s, and 28 frames/s. The same plane was imaged at all frame rates from before to 15 min after the bolus injection of 0.3-mL (0.1-0.12 mL/kg) of AF0150 (Imagent, Alliance Pharmaceutical Corp., San Diego, CA). Liver and portal vein videointensity relative to the lesion were evaluated over time. In this study, liver enhancement progressively increased as the frame rate was reduced (p<0.001). Peak, duration, and area under the time-intensity curve were all greater at the lower frame rates (1 fr/30 s, 1 fr/5 s, and 1 fr/s) than at 28 fr/s (p<0.05). Anatomic orientation was maintained at 1 frame/s rate at which peak enhancement was 44% greater and duration was 100% longer than at 28 frames/s (p<.05). Portal vein enhancement was not affected by frame rate. In conclusion, with intermittent imaging, enhancement was dependent upon frame rate and the ability of the region being imaged to replenish its bubbles between consecutive acquisitions. The 1 frame/s allowed for anatomic orientation and adequate tissue contrast.