Intravascular ultrasound elastography: assessment and imaging of elastic properties of diseased arteries and vulnerable plaque.

OBJECTIVE: Intravascular elastography is concerned with methods for measuring the local elastic properties using intravascular ultrasound (IVUS). The elastic properties of the vessel wall and atheroma can be measured through the strain. Knowledge of these mechanical properties is useful for guiding interventional procedures (balloon dilatation, ablation) and detection of plaque vulnerability. METHODS: Elastograms and palpograms (images of strain) were constructed using the relative local displacements between IVUS images acquired at two levels of intravascular pressure with a 30-MHz echo catheter. These displacements were estimated from the time shift between gated radio-frequency echo signals using cross-correlation algorithms with interpolation around the peak. RESULTS: Experiments on gel-based phantoms mimicking atherosclerotic vessels demonstrated the capability of elastography to identify soft and hard plaques independently of the echogenicity contrast. In vitro experiments on human arteries have demonstrated the potential of intravascular elastography to identify different plaque types based on the mechanical properties. These plaques could not be identified using the IVUS image alone. Regions with elevated mechanical stress could also be detected. These stress concentrations are related to plaque fracture. CONCLUSION: Intravascular elastography provides information that is frequently unavailable or inconclusive from the IVUS image and therefore may assist in the diagnosis and treatment of atherosclerotic disease.

B-mode ultrasound assessment of pravastatin treatment effect on carotid and femoral artery walls and its correlations with coronary arteriographic findings: a report of the Regression Growth Evaluation Statin Study (REGRESS).

OBJECTIVES: In this B-mode ultrasound study we assessed pravastatin treatment effects on carotid and femoral artery walls and investigated the correlations between the state and evolution of peripheral and coronary atherosclerosis. BACKGROUND: The Regression Growth Evaluation Statin Study (REGRESS) was an 11-center, 2-year, double-blind, placebo-controlled, prospective study of 885 men with coronary artery disease (CAD) (total cholesterol 4 to 8 mmol/liter). The study primarily investigated pravastatin treatment effects on the coronary lumen. This report focuses on the 255 patients who participated in the REGRESS ultrasound study. METHODS: Carotid and femoral artery walls were imaged at baseline and at 6, 12, 18 and 24 months. Pravastatin treatment effect was defined as the difference in progression of the combined intima-media thicknesses (IMT) between treatment groups. RESULTS: Pravastatin treatment effects were highly significant (combined IMT: p = 0.0085; combined far wall IMT: p < 0.0001; common femoral artery far wall IMT: p = 0.004). Correlations between the IMTs of the arterial wall segments ranged from -0.17 to 0.81. Baseline correlations between IMT and percent coronary lumen stenoses ranged from 0.23 to 0.36. Baseline IMT correlated with the mean coronary segment diameter (r = -0.32, p = 0.001) and minimal coronary obstruction diameter (r = -0.27, p = 0.005). There were no individual correlations between IMT and coronary lumen variables (p > 0.30). CONCLUSIONS: Pravastatin treatment effects on carotid and femoral artery walls were observed. B-mode ultrasound imaging studies of peripheral arterial walls could not describe the state and evolution of the coronary lumen in the individual patient, but proved to be a highly suitable tool for the assessment of antiatherosclerotic properties of agents.

Blood flow assessment with intravascular ultrasound catheters: the ideal tool for simultaneous assessment of the coronary haemodynamics and vessel wall?

We present the potentials of a novel method of intracoronary flow visualization and quantification that is based on conventional intravascular ultrasound (IVUS) imaging catheters. The quantification of flow is obtained from analysis of the rate of decorrelation of digitized radiofrequency ultrasound echo signals. Flow information is superimposed on the IVUS image using a colour scale. Integration of the blood velocity components normal to the scan plane permits calculation of the volume flow. Validation using IVUS and electromagnetic (EM) flowmeter recordings were obtained in vivo from instrumented pigs. IVUS flow (IVUS(f)) compared favourably to EM flow (EM(f)): IVUS(f)=1.0 EM(f)+5.72 cc/min, r2=0.98. Clinical results for the first five patients investigated are reported. A Doppler wire was used to measure the flow in four coronary arteries and one renal artery in baseline and hyperaemia conditions. IVUS flow and derived coronary flow reserve (CFR) demonstrated a very good agreement with the data derived from the combination of quantitative angiography and velocity when measured with the Doppler wire (DOP(f)): IVUS(f)=1.01 DOP(f)-20 cc/min, r2=0.90 and IVUS(cfr)=1.03 DOP(cfr)-0.03, r2=0.93. This demonstrates that simultaneous morphological and physiological assessment of coronary or peripheral arteries with one IVUS catheter is feasible. This method should be very useful for the evaluation of intermediate coronary stenoses or the results of revascularization procedures.

Assessment of medial thinning in atherosclerosis by intravascular ultrasound.

This study investigated the in vitro (40 MHz) and in vivo (30 MHz) feasibility of intravascular ultrasound to document the influence of atherosclerotic lesions on the typical 3-layered appearance of muscular arteries. The in vitro images of 39 arteries were compared with the corresponding histologic sections. Media and lesion thickness were measured at the areas of minimal and maximal lesion thickness. The median media thickness was 0.8 mm in the absence of a lesion, decreasing to 0.3 mm in the area of maximal atherosclerosis. The ultrasonic data correlated closely with histologic measurements (0.6 and 0.3 mm, respectively). The in vivo study was performed in 29 patients undergoing coronary or peripheral vascular procedures. A total of 150 still-frames were selected for quantitative analysis. The median media thickness was 0.6 mm in the absence of a lesion, decreasing to 0.1 mm in the area with maximal atherosclerosis. This study revealed that intravascular ultrasound imaging accurately determines that media thickness of muscular arteries is inversely related to lesion thickness. In vitro data, verified with histology, can be translated to humans in vivo.

Assessment of arterial disease and arterial reconstructions by intravascular ultrasound.

Clinical application of intravascular ultrasound to assess arterial atherosclerotic disease was introduced in humans after extensive in vitro and in vivo animal studies. Real-time images, obtained with a 30 MHz element mounted on a 5 F catheter, consistently confirmed angiographic images, up till now considered to be the gold standard. In addition to these data, ultrasonic cross-sectional imaging provided information on the composition of atheroselectic lesions and the size and shape of the lumen. Based on the experimentally derived criteria for tissue characterization, a better insight into arterial morphology could be obtained, allowing improved planning of interventional or reconstructional procedures. Moreover intravascular ultrasound has proved valuable as a post-interventional procedure to monitor and assess the quality of interventional results. The ultrasound images are clearly superior to angiographic studies, albeit the ultrasonic information is an adjunct to angiography and, as yet, not a substitute. We present our initial experience with intravascular ultrasound obtained in patients with substantial peripheral arterial disease.

Intravascular echographic assessment of vessel wall characteristics: a correlation with histology.

In vivo application of intravascular high frequency ultrasonic imaging for peripheral and coronary artery disease is a promising technique for vascular surgeons, radiologists and cardiologists. This report demonstrates in vitro results obtained with a high frequency imaging catheter (40 MHz) in 70 human specimens including arteries with and without atherosclerosis, veins, coronary artery bypass grafts and vascular prosthetic material. Correlation between the ultrasonic images and the histologic characteristics of the corresponding vessel wall tissue and lumen geometry was established. In addition, the effect of intervention techniques i.e. balloon angioplasty, spark erosion and laser were studied with ultrasound and histology. It is anticipated that development of such a catheter imaging technique has potential for diagnostic imaging and for combination with therapeutic systems.

The role of intraoperative two-dimensional echocardiography in the assessment of thoracic aorta pathology.

The role of intraoperative two-dimensional echocardiography is discussed in 15 consecutive patients with thoracic aorta pathology undergoing cardiac surgery. A 5 MHz mechanical scanner was used before and immediately after cardiopulmonary bypass. In 5 patients intraoperative two-dimensional studies revealed crucial morphologic information which, consequently, had a marked influence on their planned surgical procedure. In 3 patients the findings provided additional information whereas in the remaining patients the intraoperative echocardiographic findings confirmed the preoperative diagnosis. Following surgery the adequacy of cardiac repair was assessed and, in one patient, epicardial echocardiography indicated the necessity for reoperation. The application of intraoperative two-dimensional echocardiography leads to a better understanding of the pathology involved and facilitates a more appropriate decision concerning the surgical procedure.

Left ventricular performance in unstable angina: assessment with radionuclide techniques.

In a clinical trial we studied left ventricular performance at rest in 50 patients with unstable angina by radionuclide techniques. Thallium-201 scintigraphy was performed on admission in 38 patients and repeated after 48 hr in 32 patients. Also dynamic blood pool scintigraphy was performed in 37 patients on admission and in 45 patients after 48 hr. Of the 50 patients, 27 (54%) had no recurrent episodes of myocardial ischemia, but 23 (46%) patients showed recurrent ischemic episodes of whom 11 (22%) patients developed a myocardial infarction. The thallium-201 images showed perfusion defects in 27 (71%) of the 38 patients studied on admission and in 20 (63%) of the 32 patients studied at 48 hr. No relation between clinical outcome and presence or absence of defects was observed. Left ventricular ejection fraction was abnormal (less than 50%) in 19 (51%) of the 37 patients on admission and in 29 (64%) of the 45 patients studied at 48 hr. As with thallium-201, no relation could be established between clinical outcome and left ventricular ejection fraction. It is concluded that radionuclide techniques are useful to provide insight into the pathophysiological mechanism of unstable angina, but they are not helpful for the short-time assessment of clinical outcome.