Contrast-enhanced B-mode US angiography in the assessment of experimental in vivo and in vitro atherosclerotic disease

Real-Time In Vivo Imaging of Human Liver Vasculature Using Coherent Flow Power Doppler: A Pilot Clinical Study

Power Doppler (PD) is a commonly used technique for flow detection and vessel visualization in radiology clinics. Despite its broad set of applications, PD suffers from multiple noise sources and artifacts, such as thermal noise, clutter, and flash artifacts. In addition, a tradeoff exists between acquisition time and Doppler image quality. These limit the ability of clinical PD imaging in deep-lying and small-vessel detection and visualization, particularly among patients with high body mass indices (BMIs). To improve the Doppler vessel detection, we have previously proposed coherent flow PD (CFPD) imaging and demonstrated its performance on porcine vasculature. In this article, we report on a pilot clinical study of CFPD imaging on healthy human volunteers and patients with high BMI to assess the clinical feasibility of the technique in liver imaging. In this study, we built a real-time CFPD imaging system using a graphical processing unit (GPU)-based software beamformer and a CFPD processing module. Using the real-time CFPD imaging system, the liver vasculature of 15 healthy volunteers with normal BMI below 25 and 15 patients with BMI greater than 25 was imaged. Both PD and CFPD image streams were produced simultaneously. The generalized contrast-to-noise ratio (gCNR) of the PD and CFPD images was measured to provide the quantitative evaluation of image quality and vessel detectability. Comparison of PD and CFPD image shows that gCNR is improved by 35% in healthy volunteers and 28% in high BMI patients with CFPD compared to PD. Example images are provided to show that the improvement in the Doppler image gCNR leads to greater detection of small vessels in the liver. In addition, we show that CFPD can suppress in vivo reverberation clutter in clinical imaging.

Does Quantification of Carotid Plaque Surface Irregularities Better Detect Symptomatic Plaques Compared to the Subjective Classification?

OBJECTIVES

To evaluate the interobserver agreement of color Doppler ultrasound (CDUS) and contrast-enhanced ultrasound (CEUS) for quantification of carotid plaque surface irregularities and to correlate objective and subjective measures with stroke occurrence.

METHODS

This work was an observational study involving 54 patients with 62 internal carotid artery or carotid bulb plaques (31 symptomatic) undergoing CDUS and CEUS between February 2016 and February 2018, with retrospective interpretation of prospectively acquired data. Plaques were included if causing moderate (50%-69%) or severe (70%-99%) stenosis based on velocity criteria, and their surface was classified as smooth, irregular, or ulcerated based on CEUS. The surface irregularities were quantified in the form of a surface irregularity index by 2 observers, based on CDUS and CEUS. The surface irregularity index was evaluated for interobserver agreement with CDUS and CEUS and correlated with the occurrence of stroke, as was the subjective characterization of the plaque surface.

RESULTS

Color Doppler ultrasound and CEUS showed good interobserver agreement (intraclass correlation coefficients, 0.979 and 0.952, respectively). Plaques were characterized as smooth in 30.6% of cases, irregular in 50%, and ulcerated in 19.4%. The subjective classification of the plaque surface did not correlate with stroke occurrence (P > .05, χ² ). Surface irregularity index values were significantly higher for symptomatic plaques with both CDUS and CEUS (P < .05).

CONCLUSIONS

Color Doppler ultrasound and CEUS can quantify carotid plaque surface irregularities with good interobserver agreement. The resulting quantitative measure was significantly higher in symptomatic plaques, whereas the subjective characterization of plaque surface failed to differ between symptomatic and asymptomatic plaques.

General principles and overview of vascular contrast-enhanced ultrasonography

Ultrasonography (US) is the first-line modality for the evaluation of vascular pathology. Although well-established for many diseases, US has inherent limitations that can occasionally hinder an accurate diagnosis. The value of US was improved by the introduction of microbubbles as ultrasonographic contrast agents (UCAs) and the emergence of contrast-enhanced ultrasonography (CEUS), following the introduction of second-generation UCAs and the emergence of modern contrast-specific techniques. CEUS offers valuable information about vascular disease, both on a macrovascular and a microvascular level, with well-established applications for carotid disease, post-interventional follow-up of abdominal aortic aneurysms, and the assessment of portal vein thrombosis. The purpose of this review is to discuss the principles of CEUS and to present an overview of its vascular applications.

A comparative study of color Doppler imaging and contrast-enhanced ultrasound for the detection of ulceration in patients with carotid atherosclerotic disease

OBJECTIVES

To evaluate the diagnostic accuracy of color Doppler imaging (CDI) and contrast-enhanced ultrasound (CEUS) for diagnosing carotid ulceration, having multi-detector computed tomography angiography (MDCTA) as the reference method.

METHODS

Patients with carotid disease referred for ultrasound (US), either due to the occurrence of neurovascular symptoms or for screening purposes, were included in this study if at least one plaque causing moderate (50-69%) or severe (70-99%) internal carotid artery stenosis was detected. Carotid US with CDI technique, CEUS, and MDCTA were performed in all patients, investigating the presence of ulceration. The agreement between modalities was evaluated using kappa statistics.

RESULTS

The study population included 54 patients (median age 62 years, inter-quartile range 16.2) and 66 carotid arteries. The mean degree of stenosis was 68.5% (SD 12.2%) while 47.1% of plaques were symptomatic. MDCTA characterized 28.8% of plaques as smooth, 45.5% irregular, and 24.3% ulcerated. Flow reversal was detected with CDI in 65.5% of ulcerations, while swirling of the microbubbles and neovessels adjacent to the ulcer were detected with CEUS in 17.64%. The agreement for ulceration diagnosis was moderate between CDI and CEUS (kappa 0.473) and between CDI and MDCTA (kappa 0.473) and very good between CEUS and MDCTA (kappa 0.921). The sensitivity, specificity, and positive and negative predictive values of CDI for the diagnosis of ulceration were 41.2%, 97.95%, 87.5%, 82.8% respectively, while CEUS respective measures were 94.1%, 97.95%, 94.1%, and 97.95%.

CONCLUSION

CEUS outperformed CDI in terms of agreement with MDCTA and diagnostic accuracy for the diagnosis of ulcerated carotid plaque.

KEY POINTS

• Superficial ulceration is a significant feature of carotid plaque vulnerability. • Color Doppler imaging has the potential to demonstrate carotid plaque ulceration but is characterized by limited sensitivity and moderate agreement with the reference method of multi-detector computed tomography angiography. • Contrast-enhanced ultrasound outperforms color Doppler imaging in terms of sensitivity for the detection of carotid plaque ulceration and in agreement with the reference method of multi-detector computed tomography angiography.

Contrast-enhanced ultrasound of the carotid system: a review of the current literature

Carotid disease is a major current health problem accounting for a significant part of stroke patients. Ultrasound with colour Doppler and spectral analysis is the primary imaging technique used for screening and diagnostic evaluation of the extracranial part of carotid arteries offering identification and grading of carotid disease. However, inherent limitations of this technique include flow-related artefacts like Doppler angle dependence and aliasing artefact which may sometimes hinder complete assessment of a stenotic part of the vessel, potentially failing to address clinically significant differential diagnosis issues. The intravenous use of microbubbles as an US contrast agent has been introduced for the supplementation of conventional technique. The value of contrast-enhanced ultrasound (CEUS) has been investigated in the evaluation of carotid disease leading to promising results. CEUS provides improved flow visualization free of artefacts and detailed plaque surface delineation, thus being able to accurately grade stenosis, identify carotid plaque ulcerations, differentiate occlusion from highly stenotic plaques and identify carotid dissection. Furthermore, microbubbles can be used to identify and grade intraplaque neovascularization, carotid wall inflammation in patients with arteritis, follow-up patients after carotid intervention and assist interventional procedures reducing the need for nephrotoxic contrast agents. The purpose of this review is to present and discuss the current literature regarding the various uses of CEUS in carotid arteries.

Imaging of the ulcerated carotid atherosclerotic plaque: a review of the literature

Abstract

Carotid atherosclerotic disease constitutes a major modern health problem whose diagnosis primarily relies on imaging. Grading of stenosis has been long used as the main factor for risk stratification and guiding of management. Nevertheless, increasing evidence has shown that additional plaque characteristics such as plaque composition and surface morphology play an important role in the occurrence of symptoms, justifying the term “vulnerable plaque”. Carotid plaque surface characteristics either in the form of surface irregularities or ulceration represent an important factor of vulnerability and are associated with the occurrence of neurologic symptoms. The delineation of the carotid plaque surface can be performed with virtually all imaging modalities including ultrasound, contrast-enhanced ultrasound, multi-detector computed tomography angiography, magnetic resonance angiography and the traditional reference method of angiography. These techniques have shown varying levels of diagnostic accuracy for the identification of ulcerated carotid plaques or plaque surface irregularities. As a consequence and given its high clinical significance, radiologists should be familiar with the various aspects of this entity, including its definition, classification, imaging findings on different imaging modalities and associations. The purpose of this review is to present the current literature regarding carotid plaque ulcerations and present illustrative images of ulcerated carotid plaques.

Teaching Points

• Plaque surface and ulceration represent risk factors for stroke in carotid disease.

• Characterisation of the plaque surface and ulcerations can be performed with every modality.

• US is the first-line modality for carotid disease and identification of ulcerations.

• The administration of microbubbles increases US accuracy for diagnosis of carotid ulceration.

• MDCTA and MRA are valuable for diagnosing ulceration and evaluating plaque composition.

Effect of contrast enhancement on measurement of carotid artery intimal medial thickness

Previous studies have used standard B-mode ultrasound to quantify the aggregate mean intimal medial thickness (IMT) of the near and far wall of the common carotid artery (CCA). Many investigators have had difficulty in accurately evaluating the near wall IMT secondary to difficulty in discerning the vessel lumen and intima. The purpose of this study is to determine the effect of contrast enhanced ultrasound on IMT measurement when compared with non-enhanced images. Twenty-six patients who had standard carotid ultrasounds completed over a 6-month period were evaluated, with 24 imaged by the same sonographer. Five to six measurements of the near and far walls were obtained over a 1 cm distance, beginning and ending 0.5 cm and 1.5 cm proximal to the carotid bifurcation. The measurements were made with and without the contrast agent OptisonTM (perflutren protein type-A microspheres), which was given as an IV bolus (0.5-0.7 cc). Of those imaged by the same sonographer, 40 carotid arteries were examined and a total of 867 measurements were obtained. A total of 10% of the carotid ultrasounds were restudied approximately 1 month after the initial interpretation to assess observer accuracy. The near wall CCA mean (SD) IMT was 0.075 (0.019)cm for left with contrast versus 0.067 (0.023)cm for left without contrast and 0.089 (0.024)cm for right with versus 0.071 (0.022)cm for right without, p 0.0001 both sides. For the far wall of the CCA, the mean (SD) IMT comparison was 0.075 (0.021)cm for left with versus 0.070 (0.016)cm for left without, p = 0.005, and 0.070 (0.023)cm for right with versus 0.070 (0.016) cm for right without, p = 0.68. In conclusion, contrast-enhanced IMT measurement showed a highly statistically significant difference in near carotid wall thickness determinations versus non-contrast values. The thicker measurement is in agreement with previously reported data showing that non-contrast images underestimated near wall common carotid IMT in histologic samples.

Assessment of the vulnerable carotid atherosclerotic plaque using contrast-enhanced ultrasonography

Carotid atherosclerosis represents a primary cause for cerebrovascular ischemic events and its contemporary management includes surgical revascularization for moderate to severe symptomatic stenoses. However, the role of invasive therapy seems to be questioned lately for asymptomatic cases. Numerous reports have suggested that the presence of neovessels within the atherosclerotic plaque remains a significant vulnerability factor and over the last decade imaging modalities have been used to identify intraplaque neovascularization in an attempt to risk-stratify patients and offer management guidance. Contrast-enhanced ultrasonography of the carotid artery is a relatively novel diagnostic tool that exploits resonated ultrasound waves from circulating microbubbles. This property permits vascular visualization by producing superior angiography-like images, and allows the identification of vasa vasorum and intraplaque microvessels. Moreover, plaque neovascularization has been associated with plaque vulnerability and ischemic symptoms lately as well. At the same time, attempts have been made to quantify contrast-enhanced ultrasonography signal using sophisticated software packages and algorithms, and to correlate it with intraplaque microvascular density. The aim of this review was to collect all recent data on the characteristics, performance, and prognostic role of contrast-enhanced ultrasonography regarding carotid stenosis management, and to produce useful conclusions for clinical practice.

Lumen segmentation and motion estimation in B-mode and contrast-enhanced ultrasound images of the carotid artery in patients with atherosclerotic plaque

In standard B-mode ultrasound (BMUS), segmentation of the lumen of atherosclerotic carotid arteries and studying the lumen geometry over time are difficult owing to irregular lumen shapes, noise, artifacts, and echolucent plaques. Contrast enhanced ultrasound (CEUS) improves lumen visualization, but lumen segmentation remains challenging owing to varying intensities, CEUS-specific artifacts and lack of tissue visualization. To overcome these challenges, we propose a novel method using simultaneously acquired BMUS&CEUS image sequences. Initially, the method estimates nonrigid motion (NME) from the image sequences, using intensity-based image registration. The motion-compensated image sequence is then averaged to obtain a single "epitome" image with improved signal-to-noise ratio. The lumen is segmented from the epitome image through an intensity joint-histogram classification and a graph-based segmentation. NME was validated by comparing displacements with manual annotations in 11 carotids. The average root mean square error (RMSE) was 112±73 μm . Segmentation results were validated against manual delineations in the epitome images of two different datasets, respectively containing 11 (RMSE 191±43 μm) and 10 (RMSE 351±176 μm ) carotids. From the deformation fields, we derived arterial distensibility with values comparable to the literature. The average errors in all experiments were in the inter-observer variability range. To the best of our knowledge, this is the first study exploiting combined BMUS&CEUS images for atherosclerotic carotid lumen segmentation.

Contrast-enhanced ultrasonography of the carotids

Contrast-enhanced ultrasonography of the carotids has recently emerged as a complementary examination to conventional carotid Doppler ultrasonography. It is an examination providing improved visualization of the vascular lumen, more accurate and detailed delineation of the vascular wall, and identification of atherosclerotic plaques. Moreover, contrast-enhanced ultrasonography has specific advantages over conventional ultrasonography and plays an important role in the diagnosis of the vulnerable carotid plaque, as it can identify intraplaque neovascularization and carotid plaque ulceration. Given the specific advantages and improved imaging of the carotids provided by this method, radiologists should be familiar with it. This pictorial essay illustrates the advantages of this technique and discusses its value in the imaging of carotid arteries.

Fully automated carotid plaque segmentation in combined contrast-enhanced and B-mode ultrasound

Carotid plaque segmentation in B-mode ultrasound (BMUS) and contrast-enhanced ultrasound (CEUS) is crucial to the assessment of plaque morphology and composition, which are linked to plaque vulnerability. Segmentation in BMUS is challenging because of noise, artifacts and echo-lucent plaques. CEUS allows better delineation of the lumen but contains artifacts and lacks tissue information. We describe a method that exploits the combined information from simultaneously acquired BMUS and CEUS images. Our method consists of non-rigid motion estimation, vessel detection, lumen-intima segmentation and media-adventitia segmentation. The evaluation was performed in training (n = 20 carotids) and test (n = 28) data sets by comparison with manually obtained ground truth. The average root-mean-square errors in the training and test data sets were comparable for media-adventitia (411 ± 224 and 393 ± 239 μm) and for lumen-intima (362 ± 192 and 388 ± 200 μm), and were comparable to inter-observer variability. To the best of our knowledge, this is the first method to perform fully automatic carotid plaque segmentation using combined BMUS and CEUS.

[A role of carotid endarterectomy in prevention of cerebral ischemic damage]

Authors present a literature review on the prevalence, clinical presentations, diagnosis and outcome of surgical treatment of atherosclerotic stenosis of the internal carotid artery.

Usefulness of contrast-enhanced ultrasound for detection of carotid plaque ulceration in patients with symptomatic carotid atherosclerosis

Previous data have indicated that carotid plaque ulceration is a strong predictor of cerebrovascular events. Standard ultrasound and color Doppler ultrasound (CDUS) scans have poor diagnostic accuracy for the detection of carotid plaque ulceration. The aim of the present prospective study was to assess the value of contrast-enhanced ultrasound (CEUS) scans for the detection of carotid plaque ulceration. The Institutional Ethics Committee approved the study protocol, and all patients provided informed consent. The patients had symptomatic stenosis of the internal carotid artery and underwent carotid computed tomographic angiography as part of their clinical evaluation. All patients underwent a CDUS examination in conjunction with CEUS. Carotid plaque ulceration was defined as the presence of ≥1 disruptions in the plaque-lumen border ≥1 × 1 mm. Carotid computed tomographic angiography was used as reference technique. The study population consisted of 20 patients (mean age 64 ± 9 years, 80% men), and 39 carotid arteries were included in the present analysis. Computed tomographic angiography demonstrated that the plaque surface was smooth in 15 (38%), irregular in 7 (18%) and ulcerated in 17 (44%) carotid arteries. The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of CDUS for the detection of ulceration was 29%, 73%, 54%, 46%, and 57%, respectively. The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of CEUS for the detection of ulceration was 88%, 59%, 72%, 63%, and 87%, respectively. CEUS had superior sensitivity and diagnostic accuracy for the assessment of carotid plaque ulceration compared with CDUS. CEUS improved the intrareader and inter-reader variability for the assessment of carotid plaque ulceration compared with CDUS. In conclusion, CEUS could be an additional method for the detection of carotid plaque ulceration. The role of CDUS for the assessment of carotid plaque ulceration seems limited.

Effect of carotid plaque screening using contrast-enhanced ultrasound on cardiovascular risk stratification

Cardiovascular risk stratification of asymptomatic patients is based on the assessment of risk factors. Noninvasive imaging of subclinical atherosclerosis may improve cardiovascular risk stratification, especially in patients with co-morbidities. The aim of this study was to investigate the effect of contrast-enhanced ultrasound (CEUS) of the carotid arteries on cardiovascular risk assessment. The study population consisted of 100 consecutive asymptomatic patients with ≥1 clinical risk factor for atherosclerosis. Cardiovascular risk was estimated by calculating the Prospective Cardiovascular Münster Heart Study (PROCAM) risk score. This score was divided into 3 subgroups: low (≤5%), intermediate (6% to 19%), and high (≥20%). Subclinical carotid atherosclerosis was assessed using standard ultrasound for intima-media thickness and plaque screening and CEUS for additional plaque screening. CEUS was performed using SonoVue contrast agent. Patients with subclinical atherosclerosis were considered to be at high cardiovascular risk. McNemar's test was used to compare PROCAM score to ultrasound findings. The mean PROCAM risk score was 9 ± 10; the PROCAM risk score was low in 72 patients (72%), intermediate in 17 patients (17%), and high in 11 patients (11%). A total of 21 patients (21%) had abnormal carotid intima-media thickness, 77% had plaques on conventional carotid ultrasound, and 88% had plaques on standard carotid ultrasound combined with CEUS. Detection of atherosclerosis led to the reclassification of 79 patients (79%) to high cardiovascular risk (p <0.001). In conclusion, CEUS changes the risk category as estimated by a traditional risk stratification model in most asymptomatic patients. CEUS may thus be an additional method for cardiovascular risk prediction in patient groups with co-morbidities.

Imaging atherosclerosis with hybrid [18F]fluorodeoxyglucose positron emission tomography/computed tomography imaging: what Leonardo da Vinci could not see

Prodigious efforts and landmark discoveries have led toward significant advances in our understanding of atherosclerosis. Despite significant efforts, atherosclerosis continues globally to be a leading cause of mortality and reduced quality of life. With surges in the prevalence of obesity and diabetes, atherosclerosis is expected to have an even more pronounced impact upon the global burden of disease. It is imperative to develop strategies for the early detection of disease. Positron emission tomography (PET) imaging utilizing [(18)F]fluorodeoxyglucose (FDG) may provide a non-invasive means of characterizing inflammatory activity within atherosclerotic plaque, thus serving as a surrogate biomarker for detecting vulnerable plaque. The aim of this review is to explore the rationale for performing FDG imaging, provide an overview into the mechanism of action, and summarize findings from the early application of FDG PET imaging in the clinical setting to evaluate vascular disease. Alternative imaging biomarkers and approaches are briefly discussed.

Current status and future developments of contrast-enhanced ultrasound of carotid atherosclerosis

B-mode and Doppler ultrasound are commonly used for the evaluation of atherosclerosis in the carotid arteries. Recently, contrast-enhanced ultrasound (CEUS) has been introduced as a technique to improve the detection of carotid atherosclerosis and evaluate the presence of intraplaque neovascularization, which is considered a marker of plaque vulnerability. The present review focuses on the role of CEUS for the assessment of atherosclerosis and plaque instability. Currently available literature and future developments with CEUS are discussed.

Assessment of subclinical atherosclerosis using contrast-enhanced ultrasound

AIMS

The sensitivity of standard carotid ultrasound and colour Doppler for the detection of subclinical atherosclerotic plaques is suboptimal. The aim of this study is to assess whether contrast-enhanced ultrasound (CEUS) added to standard carotid ultrasound improves the detection of subclinical atherosclerosis.

METHODS AND RESULTS

Carotid intima-media thickness (CIMT) measurement, standard carotid ultrasound including colour Doppler imaging, and CEUS were performed in 100 asymptomatic patients with one or more risk factors for atherosclerosis. CEUS was performed using intravenous administration of SonoVue™ contrast agent (Bracco S.p.A., Milan, Italy). CIMT, standard ultrasound, colour Doppler, and CEUS were reviewed by two independent observers. Standard ultrasound, colour Doppler, and CEUS were scored for the presence of atherosclerotic plaques. Subclinical atherosclerosis was diagnosed if patients had a CIMT above their age-corrected threshold value or if atherosclerotic plaques were present on standard carotid ultrasound clips or CEUS clips. McNemar's test was performed to compare between groups. Twenty-one patients (21%) had a thickened CIMT value and were considered to have subclinical atherosclerosis. Standard carotid ultrasound including colour Doppler demonstrated atherosclerotic plaques in 77 patients (77%). The addition of CEUS to the standard ultrasound protocol demonstrated atherosclerotic plaques in 88 patients (88%). The incorporation of CEUS into the standard carotid ultrasound protocol resulted in a significantly improved detection of patients with subclinical atherosclerosis (P < 0.01).

CONCLUSION

CEUS has an incremental value for the detection of subclinical atherosclerosis in the carotid arteries. Atherosclerotic plaques which were only detected with CEUS and not with standard carotid ultrasound and colour Doppler imaging were predominantly hypoechoic.

Imaging of atherosclerosis in WHHL rabbits using high-resolution ultrasound

Watanabe heritable hyperlipidemic (WHHL) rabbits provide an animal model of hypercholesterolemia and atherosclerotic progression. However, a large individual variation in plaque progression rate calls for serial investigations, as do treatment studies. In contrast to histopathology, transthoracic ultrasound imaging of the aortic arch is a noninvasive technique suitable for repeated investigations. We studied 34 WHHL rabbits by both techniques. Ultrasound correctly interpreted plaque morphology compared with histopathology of the same spot (location verified by needle puncture). Intima media thickness (IMT) measured by the two methods agreed well. Ultrasonic values were similar to the histopathologic average circumferential values when these were corrected for postmortem shrinkage. Finally, the transthoracic ultrasound technique demonstrated a significant increase in IMT over a 15-week period (p = 0.0002). We conclude that transthoracic ultrasound of aortic arch IMT in WHHL rabbits is a reliable and feasible technique for studies of plaque progression and the evaluation of interventions.

Diabetes mellitus and noninvasive imaging of atherosclerosis

With the aging of the baby-boom generation, non-insulin-requiring diabetes mellitus (type 2) has become a worldwide pandemic. The causes of the disease are complex and multifactorial and include genetic disposition, lifestyle choices, abnormal aging processes, and alterations in metabolic processes. The impetus for diagnosing the preclinical effects of diabetes generally has been the individual patient's risk for cardiovascular diseases, heart attack, stroke, and peripheral vascular diseases. This review highlights the newer noninvasive imaging methods designed to provide the early prevention of preclinical atherosclerosis.

Contrast Ultrasound Imaging of the Carotid Artery Vasa Vasorum and Atherosclerotic Plaque Neovascularization

Cardiovascular disease is associated with the aging of the population, obesity, metabolic syndrome, and diabetes. Therefore, it is important to develop non-invasive imaging systems to detect "at-risk" populations. New data suggest that contrast-enhanced ultrasound (CU) imaging of the carotid arteries enhances luminal irregularities (i.e., ulcers and plaques), improves near-wall, carotid intima-media thickness, and uniquely permits direct, real-time visualization of neovasculature of the atherosclerotic plaque and associated adventitial vasa vasorum. With continued clinical investigation, CU imaging of the carotid artery may afford an effective means to non-invasively identify atherosclerosis in "at-risk" populations while providing new standard for therapeutic monitoring.

Ultrasound of the liver

This article summarizes the recent advances in ultrasound imaging of the liver, especially contrast-enhanced imaging. Contrast-enhanced ultrasound has dramatically changed the role of ultrasound in liver tumor characterization, detection, and other applications. Available ultrasound contrast agents, agents under development, contrast-specific imaging techniques, and its applications in liver imaging are discussed.

Noninvasive estimation of the pressure gradient across stenoses using sonographic contrast: in vitro validation

OBJECTIVE

Because velocity measurements to estimate the degree of arterial stenosis are susceptible to local and systemic factors, we aimed to investigate the feasibility of estimating the pressure gradient across a stenosis noninvasively by using sonographic contrast.

METHODS

Using a gravity-fed flow system, a 1:4000 dilution of a contrast agent in water was circulated through silicone tubes that had either focal or long-segment stenoses of varying severity in a water bath. We measured the cross-sectional areas of the normal and stenotic regions with B-mode sonography and the flow velocity with spectral Doppler sonography and calculated the pressure gradients across the stenoses using the empirically derived Young mathematical model and the simplified Bernoulli equation. Estimated gradients were compared with those measured manometerically.

RESULTS

Both methods yielded estimates of pressure gradients that correlated with measured gradients (r > 0.988). In focal and long-segment stenoses, the Young model yielded gradients that agreed more closely with manometerically measured values than the Bernoulli equation (+/- 8% versus -24%-57%). Both methods were highly dependent on the ability to measure the luminal cross-sectional area. The presence of sonographic contrast in the vascular lumen highlighted the inner wall, allowing the accurate measurement of the luminal area to +/- 3.0%.

CONCLUSIONS

The pressure gradient can be estimated across stenoses noninvasively. The Young model was more accurate than the simplified Bernoulli equation in this model using steady flow. Estimated gradients are highly dependent on the definition of the vascular lumen, a process aided by the use of sonographic contrast.

Carotid arteries: contrast-enhanced US angiography--preliminary clinical experience

B-mode ultrasonographic (US) angiography enhanced with a microbubble-based US contrast agent (FS069) was evaluated in human subjects with carotid artery disease. Results at contrast material-enhanced US angiography and duplex US were compared with those at conventional angiography. Both US angiography and duplex US accurately depicted stenoses of 70% or more compared with those depicted at conventional angiography. The percentage diameter stenosis of the internal carotid artery measured at US angiography strongly correlated with that measured at conventional angiography (r = 0.988). The percentage area stenosis measured at US angiography strongly correlated with ex vivo measurements of the resected carotid plaque at magnetic resonance imaging (r = 0.979). US angiography depicted unsuspected wall irregularities, ulceration, and dissection.

Noninvasive surrogate markers of atherosclerosis

Noninvasive imaging techniques offer a unique opportunity to study the relation of surrogate markers to the development of atherosclerosis. These noninvasive imaging modalities include: (1) carotid artery, coronary, and aorta imaging; (2) left ventricular echocardiography imaging; (3) electron-beam computed tomography; (4) magnetic resonance imaging; and (5) ankle-brachial index. Because the incidence of coronary artery disease is a function of the development and progression of atherosclerosis, the use of noninvasive surrogate markers of atherosclerosis can aid in the diagnosis of cardiovascular disease through the identification of subclinical disease. Noninvasive imaging techniques provide an approach for identifying high-risk individuals who may benefit from active intervention to prevent clinical disease.