Journal of Invasive Cardiology

Assessing the clinical importance of angiographically intermediate coronary artery stenoses (> 40% and < 70%) at the time of cardiac catheterization remains a challenge. Since not all intermediate stenoses are hemodynamically significant, determining fractional flow reserve (FFR) by pressure wire measurement is a validated method for determining the physiologic import of an intermediate lesion. FFR represents maximum achievable blood flow to the myocardium supplied by a stenotic artery as a fraction of normal maximum flow. Its normal value is 1.0, and a value of < 0.75 has been reported to identify stenoses associated with inducible ischemia. Both retrospective and prospective studies have shown that deferral of angioplasty in patients with FFR > 0.75 results in low target stenosis event rates.^1–3
Intravascular ultrasound (IVUS) has demonstrated that the extent of coronary atherosclerosis is underestimated by coronary angiography, and IVUS permits direct measurements of the lumen, coronary artery wall and components of atherosclerotic plaques.^4–6 IVUS-derived minimum lumen area (MLA) < 4.0 mm² and/or minimum lumen diameter (MLD) < 2.0 mm have been shown to predict the probability of an adverse clinical event in follow up.^7,8 Qualitative assessment of atheroma morphology has been shown to have clinical relevance in predicting vulnerable coronary plaques that may result in an acute coronary syndrome, although these studies have been limited by inadequate characterization of plaque components by traditional ultrasound technology.
Tissue characterization of atherosclerotic plaques by IVUS can be enhanced by analysis of the back-scattered radiofrequency (RF) ultrasound signal, which contains amplitude and frequency.^9–12 In standard IVUS grayscale images, calcified regions of plaque and dense fibrous components generally reflect ultrasound energy well, and thus appear bright and homogenous, whereas regions of low echo-reflectance in IVUS images are usually labeled “soft” or “mixed” plaque. Unfortunately, utilizing such grayscale images to differentiate and quantify plaque components is technically difficult. Spectral analysis of the RF IVUS backscatter data offers an in vivo opportunity to easily assess plaque composition.¹³ The analyzed RF information is used to reconstruct tissue maps to provide “virtual histology” intravascular ultrasound (VH-IVUS) information about the vessel wall and atherosclerotic plaque. Spectral analysis of the RF ultrasound backscatter signals has been shown to have 80–92% accuracy in identifying the four possible basic tissue types in an atherosclerotic plaque: fibrous, fibrolipidic, calcified and lipid necrotic (core).^13,14
The purpose of this pilot study was to examine the VH-IVUS morphologic characteristics of intermediate coronary artery stenoses and adjacent vessel segments with an FFR > 0.75. Since VH-IVUS is now widely available, the findings reported are relevant to the practicing interventional cardiologist.