Abstract
Recent research has found that contrast-enhanced diagnostic ultrasound (CEDUS) has the potential to induce localized injury in the liver, with clearly observable effects for contrast agent doses higher than the recommended dose and maximal mechanical index values. This study was undertaken to assess effects with intermittent exposure at lower contrast doses of infusion and at reduced output to determine thresholds. In addition, microbubble (MB) suspensions with enhanced content of larger MBs were tested. Exposure from a phased array probe (GE Vivid 7 Dimension, GE Vingmed Ultrasound, Horten, Norway) was applied at 1.6 MHz and 1-s intermittent frame trigger for 10 min with infusion of MB suspension with normal (1.8 µm), medium (3.1 µm) and large (5.3 µm) mean MB diameters. The bio-effect endpoint was the count of hepatocytes stained with Evans blue dye in frozen sections. For the normal MBs, the count increased for clinically relevant infusion dosages, but leveled off above 20 µL/kg/min. The evidence of injury declined with time from 30 min to 4 h and was lacking at 24 h. The exposure thresholds in terms of peak rarefactional pressure amplitude, divided by the square root of frequency (in situ mechanical index) were 1.7, 1.3 and 1.2 for the normal-, medium- and large-sized MB suspensions. The enhanced efficacy for larger MBs lends support to the two-criterion model for cavitational microvascular injury during CEDUS. Overall, CEDUS in liver appears to have markedly less potential for induction of tissue injury than has been reported in other tissues, which indicates a satisfactory safety profile for CEDUS using recommended parameters in normal liver.