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On the Usefulness of the Mechanical Index Displayed on Clinical Ultrasound Scanners for Predicting Contrast Microbubble Destruction

Department of Radiology, Thomas Jefferson University, Philadelphia, Pennsylvania USA. Dr Shi is now with Philips Research, Briarcliff Manor, New York USA. Dr Dai is now with the Department of Diagnostic Ultrasound, Peking Union Medical College Hospital, Beijing, China.

Address correspondence and reprint requests to Flemming Forsberg, PhD, Department of Radiology, Thomas Jefferson University, Suite 763J, Main Building, 132 S 10th St, Philadelphia, PA 19107 USA. E-mail: flemming.forsberg{at}jefferson.edu

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 (r² < 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.

Key Words: acoustic field conditions • mechanical index • microbubble destruction • sonographic contrast media

Abbreviations: MI, mechanical index

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