Quantification of atherosclerotic plaque composition in cholesterol-fed rabbits with 50-MHz acoustic microscopy

Echolucency of carotid plaque is useful for selecting high-risk patients with chronic coronary artery disease who benefit from intensive lipid-lowering therapy

BACKGROUND

Ultrasound assessment of the carotid artery provides prognostic information on coronary events. This study examined whether ultrasound assessments of plaque echolucency of the carotid artery are useful for identifying patients with coronary artery disease (CAD) who are at high risk but could benefit from lipid-lowering therapy for secondary prevention.

METHODS

Ultrasound assessment of carotid plaque echolucency with integrated backscatter (IBS) analysis was performed in 393 chronic CAD patients with low-density lipoprotein cholesterol (LDL-C) levels <100 mg/dL on statin therapy. All patients were prospectively followed up for a maximum of 96 months or until the occurrence of one of the following coronary events: cardiac death, nonfatal myocardial infarction, or unstable angina pectoris requiring unplanned revascularization.

RESULTS

During the follow-up period, 45 coronary events occurred. Patients were stratified by IBS (≤-16.3 or >-16.3 dB, median value) and LDL-C level (<70 or 70-99 mg/dL). Multivariate Cox proportional hazards analysis showed that patients with lower IBS and LDL-C 70-99 mg/dL had significantly higher probabilities of coronary events compared with those with higher IBS and LDL-C <70 mg/dL, after adjustment for a baseline model of risk factors (hazard ratio 5.15; 95% confidence interval 1.21-22.0, p = 0.03). In contrast, patients with lower IBS and LDL-C <70 mg/dL had an improved prognosis comparable with those with higher IBS. Addition of LDL-C levels to the baseline model of risk factors improved net reclassification improvement (NRI) and integrated discrimination improvement (IDI) in patients with lower IBS (NRI, 0.44, p = 0.04; and IDI, 0.035, p < 0.01), but not in those with higher IBS.

CONCLUSIONS

Evaluation of echolucency of the carotid artery was useful for selecting CAD patients at high risk of secondary coronary events but who could benefit from lipid-lowering therapy.

A pilot study of scanning acoustic microscopy as a tool for measuring arterial stiffness in aortic biopsies

This study explores the use of scanning acoustic microscopy (SAM) as a potential tool for characterisation of arterial stiffness using aortic biopsies. SAM data is presented for human tissue collected during aortic bypass graft surgery for multi-vessel coronary artery disease. Acoustic wave speed as determined by SAM was compared to clinical data for the patients namely, pulse wave velocity (PWV), blood pressure, cholesterol and glucose levels. There was no obvious trend relating acoustic wave speed to PWV values, and an inverse relationship was found between systolic and diastolic blood pressure and acoustic wave speed. However, in patients with a higher cholesterol or glucose level, the acoustic wave speed increased. A more detailed investigation is needed to relate SAM data to clinical measurements.

•

Scanning acoustic microscopy (SAM) is a potential tool for arterial stiffness.

•

SAM provides a measure of the acoustic wave speed.

•

In this pilot study, no clear trend was observed with pulse wave velocity.

•

Blood pressure was inversely related with acoustic wave speed.

•

Trends observed with other clinical markers such as glucose and total cholesterol.

Rapid stabilization of vulnerable carotid plaque within 1 month of pitavastatin treatment in patients with acute coronary syndrome

We determined time course of stabilization of echolucent carotid plaques by statin therapy in patients with acute coronary syndrome (ACS). Treatment with 4 mg/d pitavastatin (n = 33) or placebo (n = 32) was initiated within 3 days after onset of ACS in 65 patients with echolucent carotid plaque. Vulnerable carotid plaques were assessed by measuring plaque echolucency using carotid ultrasound with integrated backscatter (IBS) analysis before and 1 month after treatment in all patients. The calibrated IBS value (intima-media IBS value minus adventia IBS) of vulnerable carotid plaques favorably changed at 1 month after treatment in both groups, but the echolucency at 1 month improved more in the pitavastatin than in the placebo group (pitavastatin group: -18.7 +/- 3.3 dB at pretreatment versus -12.7 +/- 2.3 dB at 1 month *P < 0.001; placebo: -19.0 +/- 3.5 dB versus -16.9 +/- 3.2 dB, P < 0.05, *P < 0.01 versus the value at 1 month in placebo group). Levels of CRP, VEGF, and TNFalpha at 1 month were significantly lower in pitavastatin than placebo group. In conclusion, pitavastatin improved carotid plaque echolucency within 1 month of therapy in patients with ACS, in association with decrease in the inflammatory biomarkers related to vulnerable plaques.

Echolucent carotid plaques predict future coronary events in patients with coronary artery disease

OBJECTIVES

The purpose of this study was to examine whether echolucent carotid plaques predict future coronary events in patients with clinically stable coronary artery disease (CAD).

BACKGROUND

Although rupture of coronary plaques is considered a major cause of acute coronary syndromes (ACS), the clinical estimation of coronary vulnerability still remains inconclusive. Ultrasound evaluation of carotid plaques with integrated backscatter (IBS) analysis can indicate the consistency/structure of the plaques. Lipid-rich lesions known as "unstable plaques" appear as echolucent plaques with low IBS values using this technique.

METHODS

We investigated the echogenicity of carotid plaques using ultrasound with IBS in 286 consecutive CAD patients (71 with ACS and 215 with stable CAD). Coronary plaque complexity was also determined angiographically in stable CAD patients followed up for 30 months or until the occurrence of coronary events.

RESULTS

The calibrated IBS values of carotid plaques in ACS patients were significantly lower than those in stable CAD patients (p < 0.01). Echolucent carotid plaques accurately predicted the existence of complex coronary plaques (predictive power of 83%). Kaplan-Meier analysis demonstrated a significantly higher probability of coronary events developing in patients with echolucent carotid plaques than in patients without this type of plaque (p < 0.001). The presence of echolucent carotid plaques in stable CAD patients predicted future coronary events independent of other risk factors (odds ratio 7.0, 95% confidence interval 2.3 to 21.4; p < 0.001).

CONCLUSIONS

Echolucent carotid plaques with low IBS values predicted coronary plaque complexity and the development of future coronary complications in patients with stable CAD. Qualitative evaluation of carotid plaques using ultrasound with IBS is a clinically useful procedure for risk assessment of CAD patients.

Ultrasonic delineation of aortic microstructure: the relative contribution of elastin and collagen to aortic elasticity

Aortic elasticity is an important factor in hemodynamic health, and compromised aortic compliance affects not only arterial dynamics but also myocardial function. A variety of pathologic processes (e.g., diabetes, Marfan's syndrome, hypertension) can affect aortic elasticity by altering the microstructure and composition of the elastin and collagen fiber networks within the tunica media. Ultrasound tissue characterization techniques can be used to obtain direct measurements of the stiffness coefficients of aorta by measurement of the speed of sound in specific directions. In this study we sought to define the contributions of elastin and collagen to the mechanical properties of aortic media by measuring the magnitude and directional dependence of the speed of sound before and after selective isolation of either the collagen or elastin fiber matrix. Formalin-fixed porcine aortas were sectioned for insonification in the circumferential, longitudinal, or radial direction and examined using high-frequency (50 MHz) ultrasound microscopy. Isolation of the collagen or elastin fiber matrices was accomplished through treatment with NaOH or formic acid, respectively. The results suggest that elastin is the primary contributor to aortic medial stiffness in the unloaded state, and that there is relatively little anisotropy in the speed of sound or stiffness in the aortic wall.

Characterization of aortic microstructure with ultrasound: implications for mechanisms of aortic function and dissection

Specific ultrasonic tissue characterization parameters were correlated with the three-dimensional architecture and material properties (density, compressibility, size, and orientation) of aortic elastic elements at the microscopic level. The medial layer of 10 samples of normal canine aorta were insonified in vitro utilizing acoustic microscopy from 30 to 44 MHz. The following quantitative indexes exhibited substantial anisotropic elastic behavior in radial (R), circumferential (C), and longitudinal (L) directions: backscatter coefficient (R:0.9 +/- 0.2; C:0.008 +/- 0.0008; LL:0.0077 +/- 0.0008 sr(-1) cm(-1)); frequency dependence of backscatter (R:3.3; C:1.4; L:1.5); attenuation coefficients 1(R:105 +/- 22; L:135 +/- 13; C:131 +/- 14 dB/cm). Thus, the ultrasonic indexes are anisotropic: equivalent in the C and L directions, but markedly different in the R direction. These data are indicative of an aortic microstructure that interacts with ultrasonic waves as thin sheet-like elastic layers instead of independent elastin fibers. This specific sheet-like organization of elastin microfibers may function to limit shear injury to concentric aortic lamellae and prevent aortic dissection. The marked anisotropic behavior of normal aortas suggests that ultrasound may be useful for nondestructive characterization of vascular integrity.

Measurement of the ultrasonic properties of human coronary arteries in vitro with a 50-MHz acoustic microscope

Ultrasonic attenuation coefficient, wave propagation speed and integrated backscatter coefficient (IBC) of human coronary arteries were measured in vitro over the -6 dB frequency bandwidth (36 to 67 MHz) of a focused ultrasound transducer (50 MHz, focal distance 5.7 mm, f/number 1.7). Corrections were made for diffraction effects. Normal and diseased coronary artery sub-samples (N = 38) were obtained from 10 individuals at autopsy. The measured mean +/- SD of the wave speed (average over the entire vessel wall thickness) was 1581.04 +/- 53.88 m/s. At 50 MHz, the average attenuation coefficient was 4.99 +/- 1.33 dB/mm with a frequency dependence term of 1.55 +/- 0.18 determined over the 36- to 67-MHz frequency range. The IBC values were: 17.42 +/- 13.02 (sr.m)-1 for thickened intima, 11.35 +/- 6.54 (sr.m)-1 for fibrotic intima, 39.93 +/- 50.95 (sr.m)-1 for plaque, 4.26 +/- 2.34 (sr.m)-1 for foam cells, 5.12 +/- 5.85 (sr.m)-1 for media and 21.26 +/- 31.77 (sr.m)-1 for adventitia layers. The IBC results indicate the possibility for ultrasound characterization of human coronary artery wall tissue layer, including the situations of diseased arteries with the presence of thickened intima, fibrotic intima and plaque. The mean IBC normalized with respect to the mean IBC of the media layer seems promising for use as a parameter to differentiate a plaque or a thickened intima from a fibrotic intima.

Noninvasive quantitative tissue characterization and two-dimensional color-coded map of human atherosclerotic lesions using ultrasound integrated backscatter: comparison between histology and integrated backscatter images

OBJECTIVES

The purpose of the present study was to define clinicopathologically whether integrated backscatter (IB) combined with conventional two-dimensional echo (2DE) can differentiate the tissue characteristics of calcification (CL), fibrosis (FI), lipid pool (LP) with fibrous cap, intimal hyperplasia (IH) and thrombus (TH) and can construct two-dimensional tissue plaque structure in vivo.

BACKGROUND

It is difficult to characterize the components of plaque using conventional 2DE techniques.

METHODS

Integrated backscatter values of plaques were measured in the right common carotid and femoral arteries (total 24 segments) both during life and after autopsy in 12 patients (age 68 to 84 years, 10 men and two women). Integrated backscatter values were determined using a 5-12 MHz multifrequency transducer, setting the region of interests (ROIs) (11 x 11 pixels) on the echo tomography of the entire arterial wall (55 +/- 10 ROI/segment) and comparing it with histologic features in the autopsied arterial specimens.

RESULTS

Corrected IB values obtained before death and at autopsy were significantly correlated (r = 0.93, p < 0.01). Corresponding to the histologic features, corrected IB values on the rectangle ROIs obtained during life were divided into five categories: category 1 (TH) 4 < IB < or = 6; category 2 (media and IH or LP in the intima) 7 < IB < or = 13; category 3 (FI) 13 < IB < or = 18, category 4 (mixed lesion) 18 < IB < or = 27 and category 5 (CL) 28 < IB < or = 33. In category 2, media and intima were differentiated using conventional 2DE. Under the above procedures, color-coded maps constructed with IB-2DE obtained during life precisely reflected the histologic features of media and intima.

CONCLUSIONS

Integrated backscatter with 2DE represents a useful noninvasive tool for evaluating the tissue structure of human plaque.

Age-related alterations of cardiac tissue microstructure and material properties in Fischer 344 rats

The cardiac aging process is accompanied by global mechanical dysfunction that reflects increased myocardial stiffness. Accordingly, age-related changes in microscopic material properties of myocardium were delineated with high-frequency ultrasound (US) (30 to 44 MHz) tissue characterization methods for aging Fischer 344 rats at 6 (adult), 18 (aged), and 24 (senescent) months of age. The excised lateral wall of the left ventricle of rats (n = 10 per group) was insonified with a 50-MHz acoustic microscope for determination of integrated backscatter, backscatter coefficient and attenuation coefficient. Histological and biochemical analyses for collagen content and cardiac myocyte diameter were performed. Collagen concentration increased progressively with age, with the greatest increments occurring from 6 to 18 months (38.0 +/- 6.3 to 53.0 +/- 7.1 mg/g dry wt), and leveling off at 24 months (60.0 +/- 7.4 mg/g dry wt). Tissue microscopic material properties also changed progressively from 6 to 24 months of age, as determined by US methods: integrated backscatter increased (-44.7 +/- 1.8 vs. -40.8 +/- 1.9 dB, p < 0.05), attenuation increased (47.1 +/- 5.9 to 65.3 +/- 7.8 dB/cm, p < 0.05), and the backscatter coefficient increased (0.73 +/- 0.16 x 10(-5) to 3.76 +/- 1.6 x 10(-5) cm(-1), p < 0.05), from 6 to 24 months of age in each case. Age-related alterations in indices of cardiac microscopic material properties were closely correlated with the changes in cardiac microstructure. Ultrasonic tissue characterization may prove to be a sensitive tool to monitor changes in the cardiac microstructure, such as increased collagen deposition, that occur within age-related diastolic dysfunction.

Ultrasonic integrated backscatter coefficient profiling of human coronary arteries in vitro

A theoretical formulation for the profile of the integrated backscatter coefficient (IBC) is derived. This new formulation is based on a theoretical treatment by Chen et al. [1]. It includes correction for the diffraction of the ultrasonic beam and correction for the non-ideal nature of the reference signal. The inclusion of these correction factors permits accurate quantitative profiling of the IBC over the transducer focal zone. Experimental measurements are first performed on well-calibrated vessel-equivalent phantom materials and subsequently on human coronary arteries in vitro. A spherically focused 50.0 MHz f/1.83 transducer is used. IBC profiles are shown for three samples that are representative of early, mid, and advanced atherosclerotic coronary disease. The IBC profiles clearly differentiate the arterial tissues. However, variation between samples with histologically confirmed intimal thickening (N = 24) was large. The mean IBC (+/- 1 standard deviation), in (Sr.mm)-1, for media, adventitia, and thickened intima were 3.86 x 10(-3), 1.53 x 10(-2), and 2.24 x 10(-2), respectively. The mean IBC of thickened intima is larger than previous measurements obtained from femoral arteries, and the mean IBC for media and adventitia layers are lower, reflecting differences in tissue composition between coronary and femoral vessels.

Delineation of the extracellular determinants of ultrasonic scattering from elastic arteries

Elastic arteries consist of three primary components: elastin fibers, extracellular collagen matrix and smooth muscle cells. However, the relative contribution of elastin and collagen fibers to overall ultrasonic scattering from an intact arterial wall is poorly understood. To define the principal source of extracellular scattering from the medial layer of elastic arteries, canine ascending aortas (n = 10) were excised, fixed and sectioned for insonification. Subsequently, aortic specimens were restudied after treatment to dissolve all tissue components except extracellular collagen matrix (n = 5) and elastin fibers (n = 5). Histological staining revealed very few elastin fibers and sparse intact collagen in collagen-isolated and elastin-isolated tissues, respectively. Integrated backscatter, attenuation and backscatter coefficients differentiated these two treated tissues. The backscatter coefficient for elastin-isolated tissue demonstrated a fivefold increase over collagen-isolated tissue, suggesting that elastin fibers represent a primary scattering component within elastic arteries, and the collagen fibers may provide a secondary component of scattering.

In vitro characterization of a novel, tissue-targeted ultrasonic contrast system with acoustic microscopy

Targeted ultrasonic contrast systems are designed to enhance the reflectivity of selected tissues in vivo [Lanza et al., Circulation 94, 3334 (1996)]. In particular, these agents hold promise for the minimally invasive diagnosis and treatment of a wide array of pathologies, most notably tumors, thromboses, and inflamed tissues. In the present study, acoustic microscopy was used to assess the efficacy of a novel, perfluorocarbon based contrast agent to enhance the inherent acoustic reflectivity of biological and synthetic substrates. Data from these experiments were used to postulate a simple model describing the observed enhancements. Frequency averaged reflectivity (30-55 MHz) was shown to increase 7.0 +/- 1.1 dB for nitrocellulose membranes with targeted contrast. Enhancements of 36.0 +/- 2.3 dB and 8.5 +/- 0.9 dB for plasma and whole blood clots, respectively, were measured between 20 and 35 MHz. A proposed acoustic transmission line model predicted the targeted contrast system would increase the acoustic reflectivity of the nitrocellulose membrane, whole blood clot, and fibrin plasma clot by 2.6, 8.0, and 31.8 dB, respectively. These predictions were in reasonable agreement with the experimental results of this paper. In conclusion, acoustic microscopy provides a rapid and sensitive approach for in vitro chracterization, development, and testing of mathematical models of targeted contrast systems. Given the current demand for targeted contrast systems for medical diagnostic and therapeutic use, the use of acoustic microscopy may provide a useful tool in the development of these agents.

Parametric (integrated backscatter and attenuation) images constructed using backscattered radio frequency signals (25-56 MHz) from human aortae in vitro

Quantitative ultrasonic tissue characterization using backscattered high-frequency intravascular ultrasound could provide a basis for the objective identification of lesions in vivo. Representation of local measurements of quantitative ultrasonic parameters in a conventional image format should facilitate their interpretation and thus increase their clinical utility. Toward this goal, the apparent integrated backscatter, the slope of attenuation (25-56 MHz) and the value of the attenuation on the linear fit at 37.5 MHz were measured using the backscattered radio frequency signals from in vitro human aortae. Local estimations of these ultrasonic parameters from both normal and atherosclerotic aortic segments were displayed in a B-scan format. The morphological features of these parametric images corresponded well to features of histological images of the same regions. The attenuation from 25-56 MHz of seven segments of the medial layer (both with and without overlying atheroma) were measured using the multinarrow-band backscatter method. The average attenuation in the media at 24 degrees C +/- 3 degrees C was 45 +/- 16 dB/cm at 25 MHz and 102 +/- 13 dB/cm at 50 MHz. This work represents progress toward the development of quantitative imaging methods for intravascular applications.

Validation of Automated Border Detection in Intravascular Ultrasound Images

Intravascular ultrasound (IVUS) imaging provides cross-sectional views of the vessel lumen; however, lumen measurements still rely on operator-dependent border delineation and time-consuming lumen tracings. We tested a new system for automated lumen border detection in IVUS images based on acoustic quantification of blood and vessel wall. In 10 rabbits, 29 segments of the aorta were imaged in vivo using a 2.9-Fr IVUS catheter. IVUS images were obtained during motorized pullbacks of aortic segments of 18 mm length. Automated measurements of lumen dimensions were compared to automated measurements of a second pullback through the same segment, lumen measurements derived from visual border tracings in IVUS images, and to quantitative angiography. The automated system showed good reproducibility: Correlations for repeated measurements of lumen area, maximal and minimal lumen diameters were r = 0.97, r = 0.91, and r = 0.93, respectively. Automated measurements also correlated well to visual image analysis (lumen area, r = 0.97; maximal lumen diameter, r = 0.89; minimal lumen diameter, r = 0.89) and to angiographic measurements (lumen area, r = 0.93; lumen diameter, r = 0.95). In 12% of the images, the automated system overestimated lumen dimensions because of weak wall signals in the presence of echolucent structures next to the wall. Signal artifacts from the IVUS catheter itself or strong blood backscatter resulted in lumen underestimation in 6% of the images. Over- and underestimation of lumen by the border detection system were often associated with eccentric catheter position. Thus, lumen measurements in vivo IVUS images can be performed using an automated border detection system based on acoustic quantification of blood and vessel wall. The system allows reproducible and accurate measurements of lumen area and diameters. (ECHOCARDIOGRAPHY, Volume 13, November 1996)

Effects of ultraviolet light in vascular cells in vitro and in intact atherosclerotic explants: potential role of apoptosis in vascular biology

Complex cell-to-cell interactions are known to participate during vascular injury and remodeling, resulting in smooth muscle cell proliferation. Mechanical interventions have yielded little benefit in limiting this process and several site-specific genetic therapies are not yet clinically available. The aim of this study was to delineate the effect of very short wavelength ultraviolet (UVC) light therapy on the viability of macrophage and smooth muscle cells. Vascular cells were both treated in vitro and in intact explanted atherosclerotic aortic segments ex vivo with UVC light. Brief exposure to short wavelength UVC light in the absence of photosensitizers elicited a differential temporal and functional response among treated cells. However, dramatic reduction in both cellular viability and proliferative capacity with eventual cell demise was observed in all UVC-treated cells. Flow cytometry and immunohistochemical analyses revealed the presence of extensive DNA fragmentation, suggestive of apoptosis as a predominant pathway of cell death in these cells exposed to UVC light. We hypothesize that selective induction of apoptosis, in contrast to necrosis, with UVC light may represent a beneficial approach to interdict the complex biologic cascade of messengers that participate in the restenotic response to vascular injury.

Tissue characterization of arteries with 4 MHz ultrasound

Reflections of 4 MHz ultrasound pulses from 60 sites on 15 human aorta specimens were analysed using the plane-wave Born approximation (PWBA) inverse scattering method. The recovered high-resolution acoustic impedance profile for each site was correlated with an analysis (by microscope) of sections taken from that site. The shape of the impedance profile was found to be sensitive to the pathology of the tissue at each site. The severity of the various conditions could be gauged from the recovered impedance profiles. Some of the insonifications were done on aorta specimens that were shielded from the ultrasound transducer by a human tissue specimen about 1.25 cm thick in order to study the effects of intervening tissue. We found that it was possible to compensate for the distortion due to the frequency dependent attenuation of the intervening tissue. We conclude that ultrasound imaging, with the reflections analysed by means of PWBA inverse scattering provides a promising non-invasive tissue characterization modality.

Contrast-enhanced intravascular ultrasound: validation of a new technique for delineation of the vessel wall boundary

OBJECTIVES

We evaluated a new technique for delineation of the vessel wall surface during intravascular ultrasound imaging using echogenic contrast agents.

BACKGROUND

Intravascular ultrasound is used for detection of complex vessel wall structures after catheter-based interventions; however, differentiation between the lumen and these wall structures can be difficult.

METHODS

In 12 anesthetized dogs, intracoronary ultrasound was performed during intracoronary bolus injection (3 and 6 ml) of different contrast agents (hand-agitated saline solution, standard iohexol, sonicated iohexol, hand-agitated iohexol, SHU 454, SHU 508). Contrast intensity was quantified by videodensitometry, and contrast homogeneity was assessed qualitatively (grade 0 to 3).

RESULTS

Peak contrast intensities for SHU 454 and SHU 508 (mean [+/- SD] 48 +/- 9 and 36 +/- 6 U, respectively) were higher compared with standard, sonicated or agitated iohexol (16 +/- 3, 28 +/- 7 and 20 +/- 3 U, respectively) or with agitated saline solution (17 +/- 4 U); intensities were higher for 6 ml compared with that for 3 ml. Contrast homogeneity was higher for SHU 508 (mean [+/- SD] 3.0 +/- 0) and SHU 454 (2.7 +/- 0.5) compared with the other agents (standard iohexol 1.2 +/- 0.4, sonicated iohexol 2.0 +/- 0.5, agitated iohexol 1.8 +/- 0.6, agitated saline solution 1.0 +/- 0.4). Exact delineation of the vessel wall surface was possible in 100% of SHU 508 and in 88% of SHU 454 injections compared with 13% of agitated iohexol and 8% of sonicated iohexol injections. Accurate surface delineation was never achieved with standard iohexol or agitated saline solution. Shadowing of parts of the vessel wall by contrast material occurred at peak intensity of 75% of SHU 508 and 46% of SHU 454 injections but not with the other agents. No adverse physiologic reactions were noted, except for transient negative inotropic effects after 6 ml of SHU 508.

CONCLUSIONS

This preliminary study shows that delineation of the vessel wall boundary using echogenic contrast agents during intravascular ultrasound is safe and feasible. Because of higher contrast intensity and homogeneity, SHU 454 and SHU 508 are superior to other agents.