Transient myocardial contrast after initial exposure to diagnostic ultrasound pressures with minute doses of intravenously injected microbubbles: Demonstration and potential mechanisms

Background: We have observed a transient but significant increase in myocardial contrast intensity with intravenously injected perfluorocarbon- exposed sonicated dextrose albumin (PESDA) microbubbles that occurs on initial exposure to pulsed ultrasound (transient-response imaging). The characteristics and magnitude of this response were examined in the present study. Methods and Results: In 14 dogs, the myocardial contrast intensity produced by transient-response imaging (TRI) was compared with conventional 30-Hz imaging (CI) after a 0.005 to 0.030 mL/kg intravenous injection of PESDA. TRI was produced either by measuring myocardial contrast during triggered (1 pulse per cardiac cycle) ultrasound or by withholding real time ultrasound transmission until after microbubbles had entered the myocardium after intravenous injection. Both first-harmonic imaging (2.0 to 3.5 MHz) and second-harmonic imaging (2.0 to 2.5 MHz fundamental, 4.0 to 5.0 MHz received) were used. TRI produced over three times the anterior myocardial contrast intensity of CI (36 ± 12 U TRI versus 11 ± 11 U CI; P<.01), with visually better anterior and posterior myocardial contrast. The spatial extent of myocardial ischemia was easily visualized after intravenous PESDA by use of TRI and correlated closely with risk area as measured with Monastral blue (r = .99, P = .002). Conclusions: TRI produces significantly greater myocardial contrast than CI and may dramatically enhance the ability of intravenous ultrasound contrast agents to identify myocardial perfusion abnormalities.