On the usefulness of the mechanical index displayed on clinical ultrasound scanners for predicting contrast microbubble destruction

Objective. The purpose of this study was to evaluate the mechanical index (MI) displayed on clinical ultrasound scanners as a predictor of exposure conditions related to the destruction of sonographic microbubble contrast agents. Methods. Sonazoid (GE Healthcare, Oslo, Norway) and Optison (GE Healthcare, Princeton, NJ) microbubbles were injected into a tissue-mimicking flow phantom. Gray scale imaging was performed with 4 different scanners and 3 different transducers (3.5 MHz curved linear, 2.5 MHz convex, and 7.5 MHz linear array), and the MI displayed by the scanner was varied from 0.2 to 1.5 by changing the system output power. All other scanning parameters were kept constant. Downstream changes in echogenicity were monitored with a PowerVision 7000 scanner (Toshiba America Medical Systems, Tustin, CA) as an indirect measure of bubble destruction. Video intensity changes within the flow tube were determined as a function of MI for the different scanner/transducer combinations, and the best linear fit was determined. Results. At a displayed MI of 0.7, different scanner/transducer combinations exhibited a range in video intensity from +16% to -3% of baseline for Sonazoid and from +8% to -71% for Optison. At an MI of 0.3, reductions in video intensity of up to 32% were produced. These results indicate a wide range in bubble destruction at identical MI values. Likewise, regression analysis found no linear fits for all scanner/transducer combinations (r(2) < 0.046). Conclusions. The MI displayed on clinical ultrasound scanners does not predict the degree of microbubble destruction and should not be used by itself to define exposure conditions for destruction of microbubble contrast agents.