RELATION OF CONTRAST ECHO INTENSITY AND FLOW VELOCITY TO THE AMPLIFICATION OF CONTRAST OPACIFICATION PRODUCED BY INTERMITTENT ULTRASOUND TRANSMISSION

Intermittent ultrasound transmission during contrast echocardiography, so-called transient response imaging (TRI), amplifies contrast intens ity. This effect of TRI is attributed is decreased microbubble destruc tion by reduced exposure time to ultrasound energy. The present study examined the hypothesis that the signal amplification produced by TRI is related to the baseline intensity present in the image and the velo city of Flow. We performed second harmonic (2.5/5.0 MHz) imaging durin g both continuous (frame rate 55 Hz) and electrocardiogram-triggered T RI mode. Contrast images produced by perfluorohexane microbubbles (AF0 150) in a steady Row model were obtained every minute throughout the d ecay phase at transit velocities of 8.1, 6.2, 3.4, 1.9, and 0.7 cm/sec . The decay of videointensity over time could be Fitted to a sigmoid c urve for both imaging modes with r > 0.99. For individual velocities. The intensity with TRI was greeter than that with continuous imaging ( CI) at any time and velocity. The mean increase in intensity between m odes throughout decoy was 8.2 +/- 3.7, 12.8 +/- 4.2, 25.7 +/- 5.8, 49. 5 +/- 8.0, and 64.0 +/- 14.4 gray levels For the respective velocity l evels studied (p < 0.0001). Although varying with baseline intensity a t early and late phases, the TRI amplification plateaued during middec ay, and within the intensity range of 16 to 143 gray levels For CI and 67 to 186 gray levels for TRI, it showed no overlap among the differe nt velocity levels. Thus the ability of TRI to enhance contrast opacif ication is much greater at low flow velocities, which has implications regarding the mechanism of TRI effect and preferential visualization of intramyocardial coronary arteries by this agent. Although this effe ct was influenced by the baseline intensity, ii was relatively constan t for each velocity level within an optimal intensity range during mid decay, providing the basis for flow velocity measurement by contrast e cho.