Table 1. Thin Cap Fibroatheroma Features
Histology61
Cap thickness <65 μm (based on results by Burke et al62 acquired from ruptured coronary plaques in male cadavers)
Large necrotic core
Increased macrophage infiltration
Virtual histology IVUS63, 64
Focal lesion, containing necrotic core (≥10% of total plaque area) in direct contact with the lumen (cap cannot be visualized)
Percent atheroma volume ≥40%
Optical coherence tomography65, 66
Wide lipid arc (>90 degrees), suggesting increased lipid content. Lipid arc is defined the widest arc demarcating a signal poor region with diffuse borders65, 67
Necrotic lipid pools presence (signal‐poor regions poorly delineated, underlying a signal rich cap), quantified based on number of quadrants occupied
Superficial microcalcifications (subtending a <90‐degree arc and with a border with lumen <100 μm thick)
Cap thickness <65 μm—although different limits have been proposed, with studies suggesting that OCT‐derived in vivo thin cap limit should be increased (postmortem/histological preparation—related alterations in previous studies)68, 69
When virtual histology IVUS is used concomitantly to assess deeper plaque layers, OCT fibroatheromas are confirmed to have a high lipid content and low levels of fibrosis and exhibit more‐expansive remodeling.70
  • Modality‐specific definitions are necessary, given that no single approach can accurately visualize all aspects of histology, attributed to poor resolution for IVUS and poor penetrance for OCT. Given complimentary features of all methods, the best possible assessment of intrinsic instability (see Figure 1) would currently entail application of multiple modalities on a single plaque or, alternatively, combination of modalities into multifunctional systems. IVUS indicates intravascular ultrasound; OCT, optical coherence tomography.