Intravascular ultrasound elastography: an overview

Abstract

The composition and morphology of the atherosclerotic lesion are currently considered more important determinants of acute coronary ischemic syndromes than the degree of stenosis. When a lesion is unstable, it may rupture and cause an acute thrombotic reaction. A rupture prone plaque contains a large lipid pool covered by a thin fibrous cap. The stress in the cap increased with decreasing thickness. Additionally, it may be weakened by macrophage infiltration. Intravascular ultrasound elastography might be an ideal technique to assess the presence of lipid pools and identify high stress regions. Elastography is a technique to assess local mechanical properties of tissue. The underlying principle is that the deformation of tissue by a mechanical excitation is a function of its mechanical properties. The deformation of the tissue is determined using ultrasound. For intravascular purposes, the intraluminal pressure is used as the excitation force. The radial strain in the tissue is obtained by cross-correlation techniques on the radio frequency (rf) signal. The strain is colour-coded and plotted as a complimentary image to the IVUS echogram. Elastography was validated in vitro using diseased human coronary and femoral arteries. After the ultrasound experiments, the specimens were processed for routine histology to counterstain collagen, smooth-muscle cells, and macrophage activity. Regions were segmented in the elastograms based on their strain values. Next, the dominant plaque type (fibrous, fibro-fatty or fatty) was defined by observers blinded to the elastographic result. These experiments demonstrate that the strain in the three plaque types is different (Kruskall–Wallis p<0.001). Especially between fibrous and fatty tissue, a highly significant difference (Wilcoxon p<0.001) was found. In vivo, the technique is validated in an atherosclerotic Yucatan mini-pig animal model. High-resolution echo frames (30 frames per second) were acquired near end-diastole. In this phase of the pressure cycle, catheter motion was minimal. Frames with a pressure difference of approx. 5 mm Hg were taken to determine the elastograms. This in vivo validation study in Yucatan mini-pigs revealed higher strain values in fatty material (ANOVA p<0.001). All cross-sections with a fatty plaque were identified with the elastogram by the presence of high strain values. Additionally, data are acquired in patients referred for Percutaneous Transluminal Coronary Angioplasty with the same set-up as tested in the animal study. Ultrasound data of soft, fibrous, calcified and stented plaques are acquired near end-diastole. The elastogram of soft plaques, as identified from the deformation during the pressure cycle, reveals strain values of 1% with increased strain up to 2% at the shoulders of the plaque. Calcified material, as identified from the echogram, shows low strain values of 0–0.2%. The elastogram of stented plaques reveals very low strain values, except for two regions: these are between the stent struts and at the shoulders of the plaque. In conclusion, intravascular elastography appears to be a unique tool to determine local mechanical properties in atherosclerotic lesions to identify fibrous and fatty tissue. Experiments have demonstrated the feasibility of this technique to be applied in vivo.