Acoustic attenuation by contrast agent microbubbles in superficial tissue markedly diminishes petechiae bioeffects in deep tissue.

OBJECTIVE: To measure how ultrasound attenuation by contrast agent microbubbles (MBs) in superficial tissue affects petechiae creation in underlying deep tissue.
MATERIALS AND METHODS: Studies using Sprague-Dawley rats were approved by the Animal Care and Use Committee. MBs were injected intravenously, and 12 ultrasound pulses (100 sinusoids of 1 MHz ultrasound per pulse) were applied through the skin overlying the hindlimb adductors at intervals of 10 or 60 seconds. In some groups, the skin was resected and immediately returned without re-establishing vascular connections. Muscle petechiae were counted.
RESULTS: Applying ultrasound through unperfused skin after bolus and continuous intravenous MB injection yielded, respectively, 30-fold and 3.5-fold more petechiae than for perfused skin. Surprisingly, petechiae/mm2 decreased with a higher MB dosage [0.12 +/- 0.05 (1 x 10 MBs/g) vs. 0.04 +/- 0.02 (3 x 10 MBs/g)] when ultrasound was applied through perfused skin. In contrast, petechiae/mm2 was approximately proportional to MB dosage for unperfused skin [0.17 +/- 0.10(5) (1 x 10 MBs/g) vs. 0.42 + 0.14 (3 x 10(5) MBs/g)]. In comparison to MB-free controls, MB solutions in this concentration range reduced the peak-negative pressure of ultrasound by 65% to 85%.
CONCLUSIONS: Acoustic attenuation by MBs in skin markedly reduces petechiae creation in deep muscle. Petechiae inhibition is dependent on [MB]2.1 and, therefore, dominates the otherwise proportional relationship between petechiae and [MB] in muscle. The drop of peak-negative pressure below a critical microvessel rupturing threshold is the probable mechanism for petechiae inhibition. These results indicate that high MB doses could, paradoxically, reduce the potential for petechiae creation and may have important bearing on the design of contrast ultrasound-based therapeutics.