Second Harmonic Imaging of Echocardiographic Contrast Agents: Unique Features and Promising Characteristics

Echocardiographic contrast microbubbles expand and contract rhythmically when ultrasound energy is applied to them. These oscillations generate sound waves not only at the primary or input frequency, but also at higher harmonic frequencies. Solid tissues do not generally produce strong harmonic signals, thus the detection of higher harmonic frequencies is relatively specific for echocardiographic contrast agents We therefore examined the imaging characteristics of various contrast agents using novel echocardiographic system producing images from second harmonic signals (Contrast Specific Imaging, Acuson). As anticipated, unenhanced tissues were poorly seen in this imaging mode, while contrast containing structures were clearly visualized. Agent-specific differences in image appearance and signal strength following contrast administration were identified. Second harmonic imaging improved detection of small concentrations of contrast. Small contrast containing structures were more clearly resolved than with conventional imaging. Intracavitary blood flow patterns were observed. Time intensity curves generated during administration of contrast medi were similar for conventional and second harmonic imaging. Cardiac cycle-dependent changes in second harmonic signal intensity were noted. Conclusions: Second harmonic imaging provides excellent visualization of structures containing ultrasonic contrast agents. The intensity and appearance of the contrast effect observed with the Contrast Specific Imaging system 3s dependent upon the size and composition of the microbubbles. Tuning of the second harmonic imaging system to the specific contrast agent may be useful to optimize contrast detection.