Detection of Luminal-Intimal Border and Coronary Wall Enhancement in Intravascular Ultrasound Imaging After Injection of Microbubbles and Simultaneous Sonication With Transthoracic Echocardiography

A 61-year-old man presented with unstable angina (Braunwald class 2B). Coronary angiography revealed a mild lesion on the very proximal segment of the left anterior descending coronary artery (LAD) and a significant stenosis (80%) in the mid-segment. Intracoronary ultrasound was used to further evaluate proximal coronary artery stenosis. It was found to be a soft plaque without significant luminal stenosis but without clear definition of the luminal-intimal boundary.

Intravenous injection of gas-filled microbubble ultrasound contrast agents have been used for endocardial border detection, especially when they are sonicated by acoustic power and produce harmonics. We performed continuous intracoronary ultrasound recordings (EndoSonics; 20 MHz) in the proximal left anterior descending coronary artery before (Figure 1, A), during (Figure 1, B) and after injection (Figure 1, C) of 4 mL of SonoVue (ultrasound contrast agent with lyophilized capsule filled with sulfurhexafluoride). Simultaneously with contrast injection, ultrasound acoustic power of 0.6 mechanical index was delivered via a transthoracic transducer (2.5 MHz) toward the left main to sonicate the delivered microbubbles. Immediately after the passage of the microbubble contrast agent, which was clearly detected by the intracoronary ultrasound probe, an enhancement of the entire plaque and adventitia was seen. The luminal-intimal boundary appeared to show a ring-like enhancement, which clearly defined the inner borders of the coronary arterial wall (Figure 2). The precise mechanism of this observation is not clearly defined, although the adhesion of microbubbles to inflamed endothelial cells was reported previously. Sonication of the microbubbles by the external acoustic energy may facilitate adherence to endothelium for a short time, and then may be washed out by the forthcoming blood. Acute coronary syndromes are the result of plaque rupture or endothelial erosion in the majority of cases. Therefore, techniques that can help define the integrity of the luminal-intimal border and intraplaque leakage of blood through the vasa vasorum or plaque cap could be of major importance for detecting vulnerable plaque and understanding the pathophysiology of acute coronary syndromes.