Relationship Between Palpography and Virtual Histology in Patients With Acute Coronary Syndromes

Invasive coronary imaging of inflammation to further characterize high-risk lesions: what options do we have?

Coronary atherosclerosis remains a leading cause of morbidity and mortality worldwide. The underlying pathophysiology includes a complex interplay of endothelial dysfunction, lipid accumulation and inflammatory pathways. Multiple structural and inflammatory features of the atherosclerotic lesions have become targets to identify high-risk lesions. Various intracoronary imaging devices have been developed to assess the morphological, biocompositional and molecular profile of the intracoronary atheromata. These techniques guide interventional and therapeutical management and allow the identification and stratification of atherosclerotic lesions. We sought to provide an overview of the inflammatory pathobiology of atherosclerosis, distinct high-risk plaque features and the ability to visualize this process with contemporary intracoronary imaging techniques.

Intravascular imaging assessment of pharmacotherapies targeting atherosclerosis: advantages and limitations in predicting their prognostic implications

Intravascular imaging has been often used over the recent years to examine the efficacy of emerging therapies targeting plaque evolution. Serial intravascular ultrasound, optical coherence tomography, or near-infrared spectroscopy-intravascular ultrasound studies have allowed us to evaluate the effects of different therapies on plaque burden and morphology, providing unique mechanistic insights about the mode of action of these treatments. Plaque burden reduction, a decrease in necrotic core component or macrophage accumulation-which has been associated with inflammation-and an increase in fibrous cap thickness over fibroatheromas have been used as surrogate endpoints to assess the value of several drugs in inhibiting plaque evolution and improving clinical outcomes. However, some reports have demonstrated weak associations between the effects of novel treatments on coronary atheroma and composition and their prognostic implications. This review examines the value of invasive imaging in assessing pharmacotherapies targeting atherosclerosis. It summarizes the findings of serial intravascular imaging studies assessing the effects of different drugs on atheroma burden and morphology and compares them with the results of large-scale trials evaluating their impact on clinical outcome. Furthermore, it highlights the limited efficacy of established intravascular imaging surrogate endpoints in predicting the prognostic value of these pharmacotherapies and introduces alternative imaging endpoints based on multimodality/hybrid intravascular imaging that may enable more accurate assessment of the athero-protective and prognostic effects of emerging therapies.

Relationship between tissue characteristics and mechanical properties of coronary plaques: a comparison between integrated backscatter intravascular ultrasound (IVUS) and speckle-tracking IVUS

High-risk coronary plaques have certain morphological characteristics. Thus, comprehensive assessment is needed for the risk stratification of plaques in patients with coronary artery disease. Integrated backscatter intravascular ultrasound (IB-IVUS) has been used successfully used to evaluate the tissue characteristics of coronary plaques; however, the mechanical properties of plaques have been rarely assessed. Therefore, we developed Speckle-tracking IVUS (ST-IVUS) to evaluate the mechanical properties of coronary plaque. This study aimed to evaluate the relation between the tissue characteristics of coronary plaques using IB-IVUS and their mechanical properties using ST-IVUS. We evaluated 95 non-targeted plaques in 95 patients undergoing elective percutaneous coronary intervention to the left anterior descending artery. We set regions of interest (ROIs) in the cross-sectional images of coronary plaques where we divided 120 degree plaques into four quadrants (every 30 degrees), with the center at the area of maximum atheroma thickness. We measured relative calcification area (%CA, relative fibrous area (%FI) and relative lipid pool area (%LP) in a total of 380 ROIs. In ST-IVUS analysis, we measured strain in the circumferential direction of the lumen area (LA strain: %), the external elastic membrane area strain (EEM strain: %), and strain in the radial direction (radial strain: %). On global cross-sectional area IB-IVUS analysis, the %CA was 1.2 ± 1.2%; the %FI was 49.0 ± 15.9%, and the %LP was 49.7 ± 16.5%. In ST-IVUS analysis, the LA strain was 0.67 ± 0.43%; the EEM strain was 0.49 ± 0.33%, and the radial strain was 2.02 ± 1.66%. On regional analysis, the %LP was not associated with the LA strain (r = - 0.002 p = 0.97), the EEM strain (r = - 0.05 p = 0.35), or with the radial strain (r = - 0.04 p = 0.45). These trends were seen between the %FI and the LA strain (r = 0.02 p = 0.74), the %FI and the EEM strain (r = 0.05 p = 0.35), and the %FI and the radial strain (r = 0.04 p = 0.50). A significant correlation was only observed between the %CA and the LA strain (r = - 0.15 p = 0.0038). Our findings indicate that the associations between mechanical properties and tissue characteristics lacked statistical significance, more often than not, and that it is necessary to evaluate the mechanical properties as well as plaque characteristics for risk stratification of coronary plaques.

Observation of plaque behavior and tissue characterization of coronary plaque using speckle tracking intravascular ultrasound (ST-IVUS) and iMap imaging system

Tissue characterization plays an important role in the development of acute coronary syndromes. iMap is an intravascular ultrasound (IVUS) tissue characterization system that provides useful information by reconstructing color-coded maps. Mechanical properties due to dynamic mechanical stress during a cardiac cycle may also trigger vulnerable plaque. Speckle tracking IVUS (ST-IVUS) has been introduced to observe plaque behavior in relation to mechanical properties. We report the case of an 84-year-old woman with stable coronary artery disease who underwent percutaneous coronary intervention, at which time IVUS demonstrated mainly three low echoic areas like lipid pools with thick fibrous caps. Pathological evaluation with iMap revealed that one low echoic area was occupied with necrotic tissue and that the other two areas occupied fibrotic. Although those tissue characterizations were different, they showed similar stretching behavior at systole by ST-IVUS which depicted plaque behavior from IVUS images using a color mapping. The mechanical properties of individual coronary plaques may differ depending on the tissue disposition. It is necessary to consider mechanical properties using ST-IVUS as well as to evaluate tissue characterization in plaque risk stratification.

Plasma biomarkers and plaque strain predict long-term cardiovascular events in patients with acute coronary syndrome

To test whether circulating and intracoronary biomarkers and coronary plaque strain have additive values to Global Registry of Acute Coronary Events (GRACE) score for predicting long-term cardiovascular events in ACS patients. One hundred ACS patients were enrolled and the GRACE score and plasma levels and intracoronary gradients of a number of biomarkers were measured. Coronary plaque burden and morphology in non-critical stenotic plaques were determined by intravascular ultrasound (IVUS) technique, and the maximal shear strain (SS_max) and maximal area strain (AS_max) were determined by intravascular ultrasound elastography (IVUSE) technique. Patients were followed for cardiovascular events and the predictive values of clinical characteristics, plasma biomarkers and plaque parameters were compared with GRACE score, and the incremental values of these measurements to the GRACE score were assessed. GRACE score, plasma biomarkers and plaque strain were independent predictors of cardiovascular events. Combination of GRACE score, plasma biomarkers and plaque strains significantly improved the predictive value of the GRACE score alone with the receiver-operating characteristic area increased from 0.457 to 0.667 (P=0.014). The combination of circulating and intracoronary biomarkers, plaque strain and GRACE score provides a better predictive tool than GRACE score alone in patients with ACS.

Validation of Noninvasive In Vivo Compound Ultrasound Strain Imaging Using Histologic Plaque Vulnerability Features

BACKGROUND AND PURPOSE

Carotid plaque rupture is a major cause of stroke. Key issue for risk stratification is early identification of rupture-prone plaques. A noninvasive technique, compound ultrasound strain imaging, was developed providing high-resolution radial deformation/strain images of atherosclerotic plaques. This study aims at in vivo validation of compound ultrasound strain imaging in patients by relating the measured strains to typical features of vulnerable plaques derived from histology after carotid endarterectomy.

MATERIALS AND METHODS

Strains were measured in 34 severely stenotic (>70%) carotid arteries at the culprit lesion site within 48 hours before carotid endarterectomy. In all cases, the lumen-wall boundary was identifiable on B-mode ultrasound, and the imaged cross-section did not move out of the imaging plane from systole to diastole. After endarterectomy, the plaques were processed using a validated histology analysis technique.

RESULTS

Locally elevated strain values were observed in regions containing predominantly components related to plaque vulnerability, whereas lower values were observed in fibrous, collagen-rich plaques. The median strain of the inner plaque layer (1 mm thickness) was significantly higher (P<0.01) for (fibro)atheromatous (n=20, strain=0.27%) than that for fibrous plaques (n=14, strain=-0.75%). Also, a significantly larger area percentage of the inner layer revealed strains above 0.5% for (fibro)atheromatous (45.30%) compared with fibrous plaques (31.59%). (Fibro)atheromatous plaques were detected with a sensitivity, specificity, positive predictive value, and negative predictive value of 75%, 86%, 88%, and 71%, respectively. Strain did not significantly correlate with fibrous cap thickness, smooth muscle cell, or macrophage concentration.

CONCLUSIONS

Compound ultrasound strain imaging allows differentiating (fibro)atheromatous from fibrous carotid artery plaques.

A Review of Intravascular Ultrasound-based Multimodal Intravascular Imaging: The Synergistic Approach to Characterizing Vulnerable Plaques

Catheter-based intravascular imaging modalities are being developed to visualize pathologies in coronary arteries, such as high-risk vulnerable atherosclerotic plaques known as thin-cap fibroatheroma, to guide therapeutic strategy at preventing heart attacks. Mounting evidences have shown three distinctive histopathological features-the presence of a thin fibrous cap, a lipid-rich necrotic core, and numerous infiltrating macrophages-are key markers of increased vulnerability in atherosclerotic plaques. To visualize these changes, the majority of catheter-based imaging modalities used intravascular ultrasound (IVUS) as the technical foundation and integrated emerging intravascular imaging techniques to enhance the characterization of vulnerable plaques. However, no current imaging technology is the unequivocal "gold standard" for the diagnosis of vulnerable atherosclerotic plaques. Each intravascular imaging technology possesses its own unique features that yield valuable information although encumbered by inherent limitations not seen in other modalities. In this context, the aim of this review is to discuss current scientific innovations, technical challenges, and prospective strategies in the development of IVUS-based multi-modality intravascular imaging systems aimed at assessing atherosclerotic plaque vulnerability.

Endovascular shear strain elastography for the detection and characterization of the severity of atherosclerotic plaques: in vitro validation and in vivo evaluation

This work explores the potential of shear strain elastograms to identify vulnerable atherosclerotic plaques. The Lagrangian speckle model estimator (LSME) elasticity imaging method was further developed to estimate shear strain elasticity (SSE). Three polyvinyl alcohol cryogel vessel phantoms were imaged with an intravascular ultrasound (IVUS) scanner. The estimated SSE maps were validated against finite-element results. Atherosclerosis was induced in carotid arteries of eight Sinclair mini-pigs using a combination of surgical techniques, diabetes and a high-fat diet. IVUS images were acquired in vivo in 14 plaques before euthanasia and histology. All plaques were characterized by high magnitudes in SSE maps that correlated with American Heart Association atherosclerosis stage classifications (r = 0.97, p < 0.001): the worse the plaque condition the higher was the absolute value of SSE, i.e. |SSE| (e.g., mean |SSE| was 3.70 ± 0.40% in Type V plaques, whereas it was reduced to 0.11 ± 0.01% in normal walls). This study indicates the feasibility of using SSE to highlight atherosclerotic plaque vulnerability characteristics.

Imaging atherosclerosis and risk of plaque rupture

Atherosclerosis imaging strategies can delineate characteristics of plaques at risk of rupture and thrombosis. Structural plaque imaging identifies high-risk plaque features, including lipid pools, thin fibrous caps, and intraplaque hemorrhage. New molecular imaging techniques complement structural imaging approaches by illuminating important features of plaque biology, with a prominent focus on detecting inflammation as a high-risk phenotype. As we unravel the molecular and structural characteristics underlying thrombosis-prone plaques, there is significant promise for eventual early identification and prediction of atherosclerotic plaque complications before they occur. Here we focus on recent imaging insights into high-risk arterial plaques, the etiologic agent of acute myocardial infarction, stroke, and sudden cardiac death.

Mechanical analysis of arterial plaques in native geometry with OCT wall motion analysis

The mechanical behavior of an atherosclerotic plaque may encode information about the type, composition, and vulnerability to rupture. Human arterial segments with varying plaque burden were analyzed ex vivo with optical coherence tomography (OCT) to determine plaque type and to determine compliance during pulsatile inflation in their native geometry. Calcifications and lipid filled plaques showed markedly different compliance when analyzed with OCT wall motion analysis. There was also a trend towards increased circumferential variation in arterial compliance with increasing plaque burden.