Intracoronary imaging modalities for vulnerable plaques

The concept of vulnerable plaque (VP) has been widely accepted as the primary cause of acute coronary syndrome (ACS) and sudden cardiac death. ACS is thought to result from sudden disruption of a VP with subsequent occlusive thrombosis. VP typically consists of several components; a large necrotic core, thin fibrous cap, increase in macrophage activity, increase in vaso vasorum, and positive remodeling. In recent years, invasive or non-invasive diagnostic imaging modalities have been developed for indentifying VP. VP has been recognized in various modalities not only by visualization of cross sectional images by high-resolution imaging modalities, such as virtual histology intravascular ultrasound (VH-IVUS), integrated backscatter (IB) IVUS, and optical coherence tomography (OCT), but also by direct visualization by intracoronary angioscopy. VH-IVUS uses advanced radiofrequency signal analysis of ultrasound signals and allows detailed qualitative and quantitative assessment of plaque composition, while IB-IVUS analyzes the radiofrequency signal by applying a fast Fourier transformation of the component of the backscattered signals. Different tissue components reflect the radiofrequency signaling at different power levels, which could be used to differentiate various tissue components. Angioscopy allows direct visualization of internal surface of the lumen, providing the detailed information of characteristics of plaque and thrombus. Optical coherence tomography (OCT) is an analog of IVUS, but uses light instead of sound. OCT has a 10-fold higher image resolution (10-15 μm) compared to conventional IVUS, therefore it is able to provide superior image quality. The commercially available versions of the technology used time-domain (TD) OCT (M2, M3, Lightlab, Westford, MA, USA) and fourier-domain (FD) OCT (C7XR, Lightlab, Westford, MA, USA). OCT is the only imaging modality with high enough resolution to measure fibrous cap thickness and neovascularization. Moreover OCT has a unique ability of detecting macrophages. In this review, we attempted to summarize the advantages and limitations of the currently available intravascular modalities.