Second harmonic imaging of an intravenously administered echocardiographic contrast agent: Visualization of coronary arteries and measurement of coronary blood flow.

Objectives: This study sought to evaluate the potential of second harmonic contrast echocardiography to assess coronary vasculature.

Background: Newer transpulmonary ultrasound contrast agents capable of resonance phenomena detected by harmonic imaging may theoretically be able to demonstrate blood flow in the myocardium.

Methods: Transthoracic B-mode images and Doppler were obtained using a prototype second harmonic ultrasound system after femoral vein injection of AF0145 (10 to 40 mg) in 13 closed chest dogs (mean weight 25.6 kg). Coronary Doppler flow was simultaneously invasively measured using an intracoronary flow wire and visually compared with transthoracic Doppler flow. "Noninvasive" coronary vasodilator reserve was determined by measuring the ratio of the Doppler time velocity integral after adenosine to the baseline value and compared with the "invasive" intracoronary determination.

Results: Harmonic imaging showed heterogeneous opacification of the myocardium characterized by linear branching structures consistent with intramyocardial coronary arteries, which were not clearly visible during conventional ultrasound imaging. In nine dogs, transthoracic Doppler was performed, and characteristic coronary Doppler flow was observed, identical to the simultaneously observed intracoronary Doppler flow. Intracoronary adenosine (120 to 150 microgram) equally increased intracoronary and transthoracic Doppler flow velocities. The calculated "noninvasive" and "invasive" coronary vasodilator reserve ratios were similar ([mean +/- SD] 3.3 +/- 1.0 and 3.6 +/- 1.2, p = NS), with excellent correlation (r = 0.95, p = 0.0012).

Conclusions: These findings indicate that noninvasive assessment of intramyocardial coronary vasculature and measurement of coronary blood flow reserve are possible using second harmonic contrast echocardiography.