Frequency dependence of thresholds for lethal cardiomyocyte injury in myocardial contrast echocardiography

Contrast enhanced diagnostic ultrasound employs microbubble activation for microvascular imaging; however, the on-screen Mechanical Index is a poor parameter for safety guidance. More research is needed on microvascular bioeffects, particularly their variation with frequency. A GE Vivid 7 with an S3 probe operated at 1.6 MHz, and an S5 probe operated at 2.5 and 3.5 MHz was used for myocardial contrast echocardiography of rats mounted in a water bath. Power settings were varied in 2 dB steps for determination of the thresholds for cardiomyocyte injury. The contrast agent was made to duplicate the properties of the clinical agent Definity. The scans were intermittently triggered each 4 heartbeats from the ECG signal. The cardiomyocyte death was assessed using Evans blue vital staining. Thresholds were defined as the mean of the lowest exposure with a statistically significant cardiomyocyte death and the next lower exposure level. Thresholds were 1.2 MPa, 1.7 MPa, and 2.7 MPa peak rarefactional pressure amplitude (derated for 1 dB/cm/MHz attenuation) for 1.6, 2.5, and 3.5 MHz, respectively. Linear regression showed that the thresholds were essentially proportional to frequency (0.72 f1.02, r2 = 0.97). Theoretical analysis is under development to explain this dependence and develop a contrast-specific safety parameter.