Benefits of reducing the cardiac cycle-triggering frequency of ultrasound imaging to increase myocardial opacification with FSO69 during fundamental and second harmonic imaging

Background: Recent work has shown significant enhancement in myocardial contrast intensity with brief ultrasound pulsing gated to a discrete portion of the cardiac cycle over conventional 30 Hz imaging. We hypothesized that limiting ultrasound imaging to less than every cardiac cycle would further intensify the myocardial echo-contrast effect. We therefore sought to determine the best pulsing frequency for ultrasound imaging to achieve optimal myocardial perfusion after the intravenous administration of FSO69 using fundamental and second harmonic imaging. Methods and Results: In 13 male mongrel dogs, myocardial contrast opacification was determined while varying the cardiac cycle-triggering frequency of ultrasound imaging after intravenous injections of FSO69. Resulting myocardial echo-contrast intensities with a cardiac cycle-triggering frequency of every beat during end-diastole were compared with those with a cardiac cycle-triggering frequency of every third and fifth beat. Myocardial opaci-fication, measured by background-subtracted peak intensity and visual scoring, was significantly greater when ultrasound imaging was triggered to every third and fifth beats compared with every beat. These benefits were seen with imaging in both the fundamental and second harmonic modes. Optimal myocardial opacification with FSO69 was achieved with injections as low as 0.1 ml, a dose that produced significant acoustic shadowing in only 24% of the injections. The degree of myocardial opacification was not significantly affected when the images were acquired during end-systole or end-diastole. Conclusions: Electrocardiogram-gated ultrasound imaging to every third or fifth cardiac cycle greatly improves myocardial opacification compared with imaging each cardiac cycle. This benefit was increased twofold to threefold with the use of second harmonic imaging as compared with fundamen-tal imaging.