Contrast Harmonic Intravascular Ultrasound

Objective: We sought to investigate feasibility of vasa vasorum imaging using the novel technique of contrast harmonic intravascular ultrasound. Methods: Prototype intravascular ultrasound (IVUS) instrumentation was developed for the sensitive detection of microbubble contrast agents. The technique, "harmonic" imaging, involves transmitting ultrasound at 20 MHz (fundamental) and detecting contrast signals at 40 MHz (second harmonic). Phantom experiments were conducted to investigate the detection of a small vessel in the wall surrounding a larger vessel. In vivo experiments were conducted in atherosclerotic rabbit abdominal aortas. Results: The phantom experiments showed improved small vessel detection in harmonic mode relative to fundamental mode. For the in vivo experiments, harmonic imaging enabled the visualization of contrast agent outside the aortic lumen through a statistically significant (P < 0.001) enhancement of image power, consistent with the detection of adventitial microvessels. These microvessels were not detected in fundamental imaging mode. Conclusions: These results indicate the feasibility of contrast harmonic intravascular ultrasound as a new technique for vasa vasorum imaging.