The powerful microbubble: from bench to bedside, from intravascular indicator to therapeutic delivery system, and beyond

Quantitative estimation of phospholipid molecules desorbed from a microbubble surface under ultrasound irradiation

Microbubbles have potential applications as drug and gene carriers, and drug release can be triggered by externally applied ultrasound irradiation while inside blood vessels. Desorption of molecules forming the microbubble shell can be observed under ultrasound irradiation of a single isolated microbubble, and the volume of desorbed molecules can be quantitatively estimated from the contact angle between the bubble and a glass plate. Microbubbles composed of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) shell and a poorly-soluble gas are created. When the microbubbles are exposed to a pulsed ultrasound, the contact angles increase dramatically; the percentage of DMPC molecules desorbed from the bubble surface reaches 70%. Vibration of a single bubble in the radial direction is measured using a laser Doppler vibrometer. The relationship between the vibrational characteristics and the amount of molecular desorption reveals that a larger vibrational amplitude of the bubble around the resonance size induces a larger amount of molecular desorption. These results support the possibility of controlling molecular desorption with pulsed ultrasound.

New Technologies in the Assessment of Carotid Stenosis: Beyond the Color-Doppler Ultrasound-High Frame Rate Vector-Flow and 3D Arterial Analysis Ultrasound

Atherosclerotic plaque in the carotid artery is the main cause of ischemic stroke, with a high incidence rate among people over 65 years. A timely and precise diagnosis can help to prevent the ischemic event and decide patient management, such as follow up, medical, or surgical treatment. Presently, diagnostic imaging techniques available include color-Doppler ultrasound, as a first evaluation technique, computed tomography angiography, which, however, uses ionizing radiation, magnetic resonance angiography, still not in widespread use, and cerebral angiography, which is an invasively procedure reserved for therapeutically purposes. Contrast-enhanced ultrasound is carving out an important and emerging role which can significantly improve the diagnostic accuracy of an ultrasound. Modern ultrasound technologies, still not universally utilized, are opening new horizons in the arterial pathologies research field. In this paper, the technical development of various carotid artery stenosis diagnostic imaging modalities and their impact on clinical efficacy is thoroughly reviewed.

3D-Arterial analysis software and CEUS in the assessment of severity and vulnerability of carotid atherosclerotic plaque: a comparison with CTA and histopathology

Purpose

Our purpose is to assess Multiparametric Ultrasound (MPUS) efficacy for evaluation of carotid plaque vulnerability and carotid stenosis degree in comparison with Computed Tomography angiography (CTA) and histology.

Material and methods

3D-Arterial Analysis is a 3D ultrasound software that automatically provides the degree of carotid stenosis and a colorimetric map of carotid plaque vulnerability.

We enrolled 106 patients who were candidates for carotid endarterectomy. Prior to undergoing surgery, all carotid artery plaques were evaluated with Color-Doppler-US (CDUS), Contrast-Enhanced Ultrasound (CEUS), and 3D Arterial analysis (3DAA) US along with Computerized Tomographic Angiography (CTA) to assess the carotid artery stenosis degree. Post-surgery, the carotid specimens were fixed with 10% neutral buffered formalin solution, embedded in paraffin and used for light microscopic examination to assess plaque vulnerability morphological features.

Results

The results of the CTA examinations revealed 91 patients with severe carotid stenoses with a resultant diagnostic accuracy of 82.3% for CDUS, 94.5% for CEUS, 98.4% for 3DAA, respectively. The histopathological examination showed 71 vulnerable plaques with diagnostic accuracy values of 85.8% for CDUS, 93.4% for CEUS, 90.3% for 3DAA, 92% for CTA, respectively.

Conclusions

The combination of CEUS and 3D Arterial Analysis may provide a powerful new clinical tool to identify and stratify “at-risk” patients with atherosclerotic carotid artery disease, identifying vulnerable plaques. These applications may also help in the postoperative assessment of treatment options to manage cardiovascular risks.

A Review of Clinical Applications for Super-resolution Ultrasound Localization Microscopy

Microvascular structure and hemodynamics are important indicators for the diagnosis and assessment of many diseases and pathologies. The structural and functional imaging of tissue microvasculature in vivo is a clinically significant objective for the development of many imaging modalities. Contrast-enhanced ultrasound (CEUS) is a popular clinical tool for characterizing tissue microvasculature, due to the moderate cost, wide accessibility, and absence of ionizing radiation of ultrasound. However, in practice, it remains challenging to demonstrate microvasculature using CEUS, due to the resolution limit of conventional ultrasound imaging. In addition, the quantification of tissue perfusion by CEUS remains hindered by high operator-dependency and poor reproducibility. Inspired by super-resolution optical microscopy, super-resolution ultrasound localization microscopy (ULM) was recently developed. ULM uses the same ultrasound contrast agent (i.e. microbubbles) in CEUS. However, different from CEUS, ULM uses the location of the microbubbles to construct images, instead of using the backscattering intensity of microbubbles. Hence, ULM overcomes the classic compromise between imaging resolution and penetration, allowing for the visualization of capillary-scale microvasculature deep within tissues. To date, many in vivo ULM results have been reported, including both animal (kidney, brain, spinal cord, xenografted tumor, and ear) and human studies (prostate, tibialis anterior muscle, and breast cancer tumors). Furthermore, a variety of useful biomarkers have been derived from using ULM for different preclinical and clinical applications. Due to the high spatial resolution and accurate blood flow speed estimation (approximately 1 mm/s to several cm/s), ULM presents as an enticing alternative to CEUS for characterizing tissue microvasculature in vivo. This review summarizes the principles and present applications of CEUS and ULM, and discusses areas where ULM can potentially provide a better alternative to CEUS in clinical practice and areas where ULM may not be a better alternative. The objective of the study is to provide clinicians with an up-to-date review of ULM technology, and a practical guide for implementing ULM in clinical research and practice.

Recent advances in vascular ultrasound imaging technology and their clinical implications

In this paper recent advances in vascular ultrasound imaging technology are discussed, including three-dimensional ultrasound (3DUS), contrast-enhanced ultrasound (CEUS) and strain- (SE) and shear-wave-elastography (SWE). 3DUS imaging allows visualisation of the actual 3D anatomy and more recently of flow, and assessment of geometrical, morphological and mechanical features in the carotid artery and the aorta. CEUS involves the use of microbubble contrast agents to estimate sensitive blood flow and neovascularisation (formation of new microvessels). Recent developments include the implementation of computerised tools for automated analysis and quantification of CEUS images, and the possibility to measure blood flow velocity in the aorta. SE, which yields anatomical maps of tissue strain, is increasingly being used to investigate the vulnerability of the carotid plaque, but is also promising for the coronary artery and the aorta. SWE relies on the generation of a shear wave by remote acoustic palpation and its acquisition by ultrafast imaging, and is useful for measuring arterial stiffness. Such advances in vascular ultrasound technology, with appropriate validation in clinical trials, could positively change current management of patients with vascular disease, and improve stratification of cardiovascular risk.

Acoustic characterization of tissue-mimicking materials for ultrasound perfusion imaging research

Materials with well-characterized acoustic properties are of great interest for the development of tissue-mimicking phantoms with designed (micro)vasculature networks. These represent a useful means for controlled in-vitro experiments to validate perfusion imaging methods such as Doppler and contrast-enhanced ultrasound (CEUS) imaging. In this work, acoustic properties of seven tissue-mimicking phantom materials at different concentrations of their compounds and five phantom case materials are characterized and compared at room temperature. The goal of this research is to determine the most suitable phantom and case material for ultrasound perfusion imaging experiments. The measurements show a wide range in speed of sound varying from 1057 to 1616 m/s, acoustic impedance varying from 1.09 to 1.71 × 10⁶ kg/m²s, and attenuation coefficients varying from 0.1 to 22.18 dB/cm at frequencies varying from 1 MHz to 6 MHz for different phantom materials. The nonlinearity parameter B/A varies from 6.1 to 12.3 for most phantom materials. This work also reports the speed of sound, acoustic impedance and attenuation coefficient for case materials. According to our results, polyacrylamide (PAA) and polymethylpentene (TPX) are the optimal materials for phantoms and their cases, respectively. To demonstrate the performance of the optimal materials, we performed power Doppler ultrasound imaging of a perfusable phantom, and CEUS imaging of that phantom and a perfusion system. The obtained results can assist researchers in the selection of the most suited materials for in-vitro studies with ultrasound imaging.

A Novel Statistical Optimization Algorithm for Estimating Perfusion Curves in Susceptibility Contrast-Enhanced MRI

Dynamic susceptibility contrast-enhanced magnetic resonance imaging is an important tool for evaluating intravascular indicator dynamics, which in turn is valuable for understanding brain physiology and pathophysiology. This procedure usually involves fitting a gamma-variate function to observed concentration-time curves in order to eliminate undesired effects of recirculation and the leakage of contrast agents. Several conventional curve-fitting approaches are routinely applied. The nonlinear optimization methods typically are computationally expensive and require reliable initial values to guarantee success, whereas a logarithmic linear least-squares (LL-LS) method is more stable and efficient, and does not suffer from the initial-value problem, but it can show degraded performance, especially when a few data or outliers are present. In this paper, we demonstrate, that the original perfusion curve-fitting problem can be transformed into a gamma-distribution-fitting problem by treating the concentration-time curves as a random sample from a gamma distribution with time as the random variable. A robust maximum-likelihood estimation (MLE) algorithm can then be readily adopted to solve this problem. The performance of the proposed method is compared with the nonlinear Levenberg-Marquardt (L-M) method and the LL-LS method using both synthetic and real data. The results show that the performance of the proposed approach is far superior to those of the other two methods, while keeping the advantages of the LL-LS method, such as easy implementation, low computational load, and dispensing with the need to guess the initial values. We argue that the proposed method represents an attractive alternative option for assessing intravascular indicator dynamics in clinical applications. Moreover, we also provide valuable suggestions on how to select valid data points and set the initial values in the two traditional approaches (LL-LS and nonlinear L-M methods) to achieve more reliable estimations.

Advances in vascular anatomy and pathophysiology using high resolution and multiparametric sonography

B-mode and Color Doppler are the first-line imaging modalities in cardiovascular diseases. However, conventional ultrasound (US) provides a lower spatial and temporal resolution (70-100 frames per second) compared to ultrafast technology which acquires several thousand frames per second. Consequently, the multiparametric ultrafast platforms manage new imaging algorithms as high-frequency ultrasound, contrast-enhanced ultrasound, shear wave elastography, vector flow, and local pulse wave imaging. These advances allow better ultrasound performances, more detailed blood flow visualization and vessel walls' characterization, and many future applications for vascular viscoelastic properties evaluation.In this paper, we provide an overview of each new technique's principles and concepts and the real or potential applications of these modalities on the study of the artery and venous anatomy and pathophysiology of the upper limb before and after creating a native or prosthetic arterio-venous fistula. In particular, we focus on high-frequency ultrasound that could predict cannulation readiness and its potential role in the venous valvular status evaluation before vascular access creation; on contrast-enhanced ultrasound that could improve the peri-operative imaging evaluation during US-guided angioplasty; on shear wave elastography and local pulse wave imaging that could evaluate preoperative vessels stiffness and their potential predictive role in vascular access failure; on vector flow imaging that could better characterize the different components of the vascular access complex flow.

Acoustic Nanodrops for Biomedical Applications

Acoustic nanodrops are designed to vaporize into ultrasound-responsive microbubbles, which presents certain challenges nonexistent for conventional nano-emulsions. The requirements of biocompatibility, vaporizability and colloidal stability has focused research on perfluorocarbons (PFCs). Shorter PFCs yield better vaporizability via their lower critical temperature, but they also dissolve more easily owing to their higher vapor pressure and solubility. Thus, acoustic nanodrops have required a tradeoff between vaporizability and colloidal stability in vivo. The recent advent of vaporizable endoskeletal droplets, which are both stable and vaporizable, may have solved this problem. The purpose of this review is to justify this premise by pointing out the beneficial properties of acoustic nanodrops, providing an analysis of vaporization and dissolution mechanisms, and reviewing current biomedical applications.

Review of ultrasound contrast agents in current clinical practice with special focus on DEFINITY® in cardiac imaging

Echocardiography is the most widely used noninvasive modality to evaluate the structure and function of the cardiac muscle in daily practice. However, up to 15-20% of echocardiograms are considered suboptimal. To enable accurate assessment of cardiac function and wall motion abnormality, the use of ultrasound microbubble contrast has shown substantial benefits in cases of salvaging nondiagnostic studies and enhancing the diagnostic accuracy in daily practice. DEFINITY^® is a perflutren based, lipid shelled microbubble contrast agent, which is US FDA approved for left ventricular opacification. The basis of ultrasound microbubbles, its development, and the clinical role of DEFINITY (characteristics, indications and case examples, side effect profile and existing evidence) is the subject of discussion in this review.

Exploiting Flow Dynamics for Superresolution in Contrast-Enhanced Ultrasound

Ultrasound (US) localization microscopy offers new radiation-free diagnostic tools for vascular imaging deep within the tissue. Sequential localization of echoes returned from inert microbubbles (MBs) with low concentration within the bloodstream reveals the vasculature with capillary resolution. Despite its high spatial resolution, low MB concentrations dictate the acquisition of tens of thousands of images, over the course of several seconds to tens of seconds, to produce a single superresolved image. Such long acquisition times and stringent constraints on MB concentration are undesirable in many clinical scenarios. To address these restrictions, sparsity-based approaches have recently been developed. These methods reduce the total acquisition time dramatically, while maintaining good spatial resolution in settings with considerable MB overlap. Here, we further improve the spatial resolution and visual vascular reconstruction quality of sparsity-based superresolution US imaging from low-frame rate acquisitions, by exploiting the inherent flow of MBs and utilize their motion kinematics. We also provide quantitative measurements of MB velocities and show that our approach achieves higher MB recall rate than the state-of-the-art techniques, while increasing contrast agents concentration. Our method relies on simultaneous tracking and sparsity-based detection of individual MBs in a frame-by-frame manner, and as such, may be suitable for real-time implementation. The effectiveness of the proposed approach is demonstrated on both simulations and an in vivo contrast-enhanced human prostate scan, acquired with a clinically approved scanner operating at a 10-Hz frame rate.

Contrast-enhanced ultrasonography for assessing neovascularization of carotid atherosclerotic plaque

Neovascularization of a carotid atherosclerotic plaque (AP) is associated with an increased risk of stroke. Contrast-enhanced ultrasonography (CEUS) is a widely used method for imaging intraplaque neovascularization in vivo. Unfortunately, there are no standardized guidelines for CEUS interpretation. The aim of this study was to identify the most reliable method for CEUS-based assessment of AP neovascularization. Seventy-eight AP were removed during carotid endarterectomy in 73 patients, of whom 5 had AP on both sides, and examined morphologically. All patients underwent preoperative duplex scanning and CEUS; Sonovue was used as a contrast agent. AP neovascularization was assessed on a 4-grade visual scale and with 3 different quantitative methods using QLAB software. On the visual scale (method 1), poorly (37%) and moderately (51%) vascularized plaques were the most common. Quantitative analysis (data were presented as Me (Q1; Q3)) revealed that the number of blood vessels per 1 cm2 of the plaque (method 2) was 16 (10; 26), the ratio of the total vessel area to the plaque area (method 3) was 6% (3; 9), and AP ROI (method 4) was 2.6 dB (1.8; 4.1). Significant correlations were demonstrated between the results produced by method 2 and method 3 (р < 0.0001), method 3 and method 2 (p = 0.0006), and between pathomorphological findings and the results produced by methods 1–3, especially method 2 (p < 0.004). AP ROI brightness did not correlate with other results. The presence of hyperechoic components (calcifications) in AP dramatically reduced the reliability of US-based intraplaque neovascularization assessment. The most accurate CEUS-based quantitative method for assessing intraplaque neovascularization is estimation of blood vessel number per 1 cm2 of the plaque.

Effect of Bubble Concentration on the in Vitro and in Vivo Performance of Highly Stable Lipid Shell-Stabilized Micro- and Nanoscale Ultrasound Contrast Agents

Ultrasound (US) is a widely used diagnostic imaging tool because it is inexpensive, safe, portable, and broadly accessible. Ultrasound contrast agents (UCAs) are employed to enhance backscatter echo and improve imaging contrast. The most frequently utilized UCAs are echogenic bubbles made with a phospholipid or protein-stabilized hydrophobic gas core. While clinically utilized, applications of UCAs are often limited by rapid signal decay (<5 min) in vivo under typical ultrasound imaging protocols. Here, we report on a formulation of lipid shell-stabilized perfluoropropane (C₃F₈) microbubbles and nanobubbles with a significantly prolonged in vivo stability. Microbubbles (875 ± 280 nm) of the target size were prepared by utilizing a multiple-step centrifugation cycle, while nanobubbles (299 ± 189 nm) were isolated from the activated vial using a single centrifugation step. To provide in-depth acoustic characterization of the new construct we evaluated the effect of size and concentration on their in vitro and in vivo performance. In vitro and in vivo characterization were carried out for a range of bubble concentrations normalized by total gas volume quantified via headspace gas chromatography/mass spectrometry (GC/MS). In vitro characterization revealed that nanobubbles at different concentrations are more consistently stable over time with the highest and lowest dilutions (50-fold decrease) only differing in US signal after 8 min exposure by 10.34%, while for microbubbles the difference was 86.46%. As expected, due to the difference in hydrodynamic diameter and scattering cross section difference, nanobubbles showed lower overall initial signal intensity. In vivo experiments showed that both microbubbles and nanobubbles with similar initial peak signal intensity are comparably stable over time with 66.8% and 60.6% remaining signal after 30 min, respectively. This study demonstrates that bubble concentration has significant effects on the persistence of both microbubbles and nanobubbles in vitro and in vivo, but the effects are more pronounced in larger bubbles. These effects should be taken into account when selecting the appropriate bubble parameters for future imaging applications.

Molecular Effects of Glycerol on Lipid Monolayers at the Gas-Liquid Interface: Impact on Microbubble Physical and Mechanical Properties

The production and stability of microbubbles (MBs) is enhanced by increasing the viscosity of both the formation and storage solution, respectively. Glycerol is a good candidate for biomedical applications of MBs, since it is biocompatible, although the exact molecular mechanisms of its action is not fully understood. Here, we investigate the influence glycerol has on lipid-shelled MB properties, using a range of techniques. Population lifetime and single bubble stability were studied using optical microscopy. Bubble stiffness measured by AFM compression is compared with lipid monolayer behavior in a Langmuir-Blodgett trough. We deduce that increasing glycerol concentrations enhances stability of MB populations through a 3-fold mechanism. First, binding of glycerol to lipid headgroups in the interfacial monolayer up to 10% glycerol increases MB stiffness but has limited impact on shell resistance to gas permeation and corresponding MB lifetime. Second, increased solution viscosity above 10% glycerol slows down the kinetics of gas transfer, markedly increasing MB stability. Third, above 10%, glycerol induces water structuring around the lipid monolayer, forming a glassy layer which also increases MB stiffness and resistance to gas loss. At 30% glycerol, the glassy layer is ablated, lowering the MB stiffness, but MB stability is further augmented. Although the molecular interactions of glycerol with the lipid monolayer modulate the MB lipid shell properties, MB lifetime continually increases from 0 to 30% glycerol, indicating that its viscosity is the dominant effect on MB solution stability. This three-fold action and biocompatibility makes glycerol ideal for therapeutic MB formation and storage and gives new insight into the action of glycerol on lipid monolayers at the gas-liquid interface.

Multiparametric ultrasound evaluation with CEUS and shear wave elastography for carotid plaque risk stratification

PURPOSE

To assess the diagnostic effectiveness of Multiparametric ultrasound (MPUS), which includes color Doppler ultrasound (CDUS), CEUS and Shear wave elastography (SWE), for evaluating carotid plaque as compared with CT-angiography (CTA) and histology.

MATERIALS AND METHODS

Forty-three consecutive patients scheduled to undergo carotid endarterectomy underwent MPUS. Then, after periods ranging from 2 days to 2 weeks, all underwent CTA. Each plaque was classified by means of dedicated scores for CEUS and SWE as compared with CTA features. At surgery, each plaque was removed in a single fragment to facilitate histological analysis, which evaluated 4 features: extension of the lipid core, thickness of the fibrous cap, inflammatory infiltrate (CD68 + and CD3 + markers) and the presence of intraplaque microvessels. For the CEUS, SWE and CTA, the following values for identifying plaque vulnerability were evaluated: sensitivity, specificity, accuracy, negative predictive value (NPV), positive predictive value (PPV) and Area under the curve (AUC). Cohen's kappa was used to evaluate the concordance between measurements in the different imaging methods. A p < 0.05 was considered statistically significant.

RESULTS

At histology, 31 out of 43 plaques were identified as vulnerable because of the presence of at least one of the following criteria: fibrous cap < 200 μm, lipid core, intraplaque hemorrhage, inflammatory infiltrate or intraplaque neovascularization. CTA showed a sensitivity of 87.1%, a specificity of 100%, a PPV of 100%, an NPV of 75% and an AUC of 93.5%. SWE showed a sensitivity of 87.1%, a specificity of 66.7%, a PPV of 87.1%, an NPV of 66.7% and an AUC of 76.9%. CEUS showed a sensitivity of 87.1%, a specificity of 58.3%, a PPV of 84.4%, an NPV of 63.6% and an AUC of 72.7%.

CONCLUSIONS

Multiparametric ultrasound is an effective modality to obtain comprehensive information on carotid plaques. Further studies are needed to determine whether it can be considered a diagnostic standard.

Impact of Cardiovascular Risk Factors and Pharmacologic Treatments on Carotid Intraplaque Neovascularization Detected by Contrast-Enhanced Ultrasound

BACKGROUND

Neovascularization is a marker of plaque vulnerability that can be assessed noninvasively using contrast-enhanced ultrasound (CEUS). The presence and extent of plaque neovascularization and their relation to cardiovascular risk factors and treatments were assessed in asymptomatic patients with carotid stenosis of intermediate severity and no indication for revascularization.

METHODS

Sixty-six patients aged 69 ± 8 years (59% men) were prospectively enrolled. Plaque neovascularization was assessed using CEUS with sulfur hexafluoride contrast in each of the four carotid segments bilaterally (a total of 528 segments). In each plaque, the presence or absence of contrast enhancement was assessed semiquantitatively as CEUS grade 1 (no signal or signal confined to the adventitia and/or shoulder of the plaque) or CEUS grade 2 (signal within the plaque).

RESULTS

Plaques were detectable in 289 of 528 carotid segments (54.7%). CEUS grade 2 was present in at least one plaque in 48 of 66 patients (72.7%) and was not influenced by stenosis severity or morphology. The highest CEUS grade 2 prevalence was observed in patients with diabetes and the lowest in those treated with angiotensin-converting enzyme inhibitors and statins, especially when low-density lipoprotein cholesterol was <100 mg/dL. Patients with multiple CEUS grade 2 plaques (20 of 66 [30%]) had both higher low-density lipoprotein and higher C-reactive protein.

CONCLUSION

Intraplaque neovascularization is frequent in asymptomatic patients with intermediate carotid stenosis and is more prevalent in those with diabetes. Low-density lipoprotein cholesterol < 100 mg/dL and treatment with angiotensin-converting enzyme inhibitors seem to confer protection from neovascularization, although larger interventional studies are necessary to confirm these data.

The inter-observer agreement in the assessment of carotid plaque neovascularization by contrast-enhanced ultrasonography: The impact of plaque thickness

BACKGROUND

The interobserver agreement in the assessment of the grade of carotid plaque neovascularization by contrast-enhanced ultrasonography is poorly established.

METHOD

We examined 140 carotid plaques in 66 patients (all patients had bilateral plaques, and 8 patients had 2 plaques on one side). We performed conventional and contrast-enhanced ultrasonography to analyze the presence of carotid plaque neovascularization, which was graded by two independent observers whose interobserver agreement (κ) was evaluated according to the thickness of carotid plaque.

RESULTS

For all carotid plaques, the mean κ was 0.689 (95% confidence interval 0.604-0.774). It was 0.689 (0.569-0.808), 0.637 (0.487-0.787), and 0.740 (0.585-0.896), respectively for carotid plaques with maximal thickness <2 mm, from 2 mm to 3 mm, and >3 mm.

CONCLUSION

The interobserver agreement for assessing carotid plaque neovascularization by using contrast-enhanced ultrasonography is substantial and acceptable for research purposes, regardless of the maximal thickness of the plaque.

Non-invasive imaging techniques and assessment of carotid vasa vasorum neovascularization: Promises and pitfalls

Carotid adventitia vasa vasorum neovascularization (VVn) is associated with the initial stages of arteriosclerosis and with the formation of unstable plaque. However, techniques to accurately quantify that neovascularization in a standard, fast, non-invasive, and efficient way are still lacking. The development of such techniques holds the promise of enabling wide, inexpensive, and safe screening programs that could stratify patients and help in personalized preventive cardiovascular medicine. In this paper, we review the recent scientific literature pertaining to imaging techniques that could set the stage for the development of standard methods for quantitative assessment of atherosclerotic plaque and carotid VVn. We present and discuss the alternative imaging techniques being used in clinical practice and we review the computational developments that are contributing to speed up image analysis and interpretation. We conclude that one of the greatest upcoming challenges will be the use of machine learning techniques to develop automated methods that assist in the interpretation of images to stratify patients according to their risk.

Improved plaque neovascularization following 2-year atorvastatin therapy based on contrast-enhanced ultrasonography: A pilot study

The present study assessed changes in carotid plaque neovascularization following long-term atorvastatin therapy (20 mg/day) using contrast-enhanced ultrasonography (CEUS). In this prospective case series, seven males (mean age, 68±9 years) and three females (mean age, 67±10 years) with a total of 13 carotid plaques underwent standard ultrasonography and CEUS at baseline, as well as after 1 and 2 years of atorvastatin treatment. The same plaques were then examined using real-time CEUS. The results of the enhanced intensity of plaque neovascularization at baseline were compared with results obtained during follow-up to examine the effects of long-term atorvastatin therapy. Standard ultrasonography revealed that 7 of the 13 carotid plaques were uniformly echolucent, whereas 6 carotid plaques were predominantly echolucent. CEUS revealed an enhanced intensity of 10.5±2.1 decibels (dB) prior to treatment, which decreased significantly to 7.3±2.6 dB following 2 years atorvastatin therapy (P<0.001). The ratio of enhanced intensity in the carotid artery lumen to that in the plaque was 3.10±1.10 at baseline and this value significantly increased to 4.96±2.98 following treatment for 2 years (P<0.001). The current pilot study therefore indicates that two-year atorvastatin therapy (20 mg/day) may reduce plaque neovascularization in the Chinese population.

Perivascular Perfusion on Contrast-Enhanced Ultrasound (CEUS) Is Associated with Inflammation in Patients with Acute Deep Vein Thrombosis

Background Inflammatory processes of the venous wall in acute deep vein thrombosis (DVT) play a role in thrombus formation and resolution. However, direct evaluation of the perivascular inflammation is currently not feasible. Objective To assess perivascular perfusion in acute proximal DVT using contrast-enhanced ultrasound (CEUS) reflecting perivenous inflammation and its association with systemic inflammatory markers in a single-centre, prospective observational study. Patients/Methods Twenty patients with proximal DVT underwent CEUS imaging in the thrombosed and contralateral popliteal vein at baseline and after 2 weeks and 3 months. Perfusion was quantified by measuring peak enhancement (PE) and wash-in rate (WiR) in a perivenous region after bolus injection of the contrast agent. High-sensitive C-reactive protein (hsCRP) and interleukin-6 (IL-6) were determined at the time of each CEUS imaging. Results PE and WiR were significantly higher in the thrombosed compared with the unaffected leg at baseline (1,007 vs. 34 au and 103 vs. 4 au/s) and 2-week follow-up (903 vs. 35 au and 70 vs. 4 au/s). Compared with baseline, PE and WiR in the thrombosed leg significantly decreased to 217 au and 18 au/s at 3-month follow-up.At baseline, hsCRP and IL-6 were elevated at 20.1 mg/mL and 8.2 pg/mL and decreased significantly to 2.8 mg/mL and 2.6 pg/mL at 2-week follow-up, remaining low after 3 months. There was a weak association between the level of inflammatory markers and the CEUS parameters at baseline on the thrombosed leg. Conclusion Elevated perivascular perfusion assessed by CEUS imaging is associated with the inflammatory response in acute DVT.

Hydrostatic Pressurization of Lung Surfactant Microbubbles: Observation of a Strain-Rate Dependent Elasticity

The microbubble offers a unique platform to study lung surfactant mechanics at physiologically relevant geometry and length scale. In this study, we compared the response of microbubbles (∼15 μm initial radius) coated with pure dipalmitoyl-phosphatidylcholine (DPPC) versus naturally derived lung surfactant (SURVANTA) when subjected to linearly increasing hydrostatic pressure at different rates (0.5-2.3 kPa/s) at room temperature. The microbubbles contained perfluorobutane gas and were submerged in buffered saline saturated with perfluorobutane at atmospheric pressure. Bright-field microscopy showed that DPPC microbubbles compressed spherically and smoothly, whereas SURVANTA microbubbles exhibited wrinkling and smoothing cycles associated with buckling and collapse. Seismograph analysis showed that the SURVANTA collapse amplitude was constant, but the collapse rate increased with the pressurization rate. An analysis of the pressure-volume curves indicated that the dilatational elasticity increased during compression for both shell types. The initial dilatational elasticity for SURVANTA was nearly twice that of DPPC at higher pressurization rates (>1.5 kPa/s), producing a pressure drop of up to 60 kPa across the film prior to condensation of the perfluorobutane core. The strain-rate dependent stiffening of SURVANTA shells likely arises from their composition and microstructure, which provide enhanced in-plane monolayer rigidity and lateral repulsion from surface-associated collapse structures. Overall, these results provide new insights into lung surfactant mechanics and collapse behavior during compression.

Differential Intensity Projection for Visualisation and Quantification of Plaque Neovascularisation in Contrast-Enhanced Ultrasound Images of Carotid Arteries

Studies have reported that intraplaque neovascularisation (IPN) is closely correlated with plaque vulnerability. In this study, a new image processing approach, differential intensity projection (DIP), was developed to visualise and quantify IPN in contrast-enhanced non-linear ultrasound image sequences of carotid arteries. DIP used the difference between the local temporal maximum and the local temporal average signals to identify bubbles against tissue non-linear artefact and noise. The total absolute and relative areas occupied by bubbles within each plaque were calculated to quantify IPN. In vitro measurements on a laboratory phantom were made, followed by in vivo measurements in which 24 contrast-enhanced non-linear ultrasound image sequences of carotid arteries from 48 patients were selected and motion corrected. The results using DIP were compared with those obtained by maximum intensity projection (MIP) and visual assessment. The results indicated that DIP can significantly reduce non-linear propagation tissue artefacts and is much more specific in detecting bubble signals than MIP, being able to reveal microbubble signals that are buried in tissue artefacts in the corresponding MIP image. A good correlation was found between microvascular area (MVA) (r = 0.83, p < 0.001)/microvascular density (r = 0.77, p < 0.001) obtained using DIP and the corresponding expert visual grades, comparing favourably to r = 0.26 and 0.23 obtained using MIP on the same data. In conclusion, the proposed method exhibits great potential in quantification of IPN in contrast-enhanced ultrasound images of carotid arteries.

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.

Targeting of Magnetic Nanoparticle-coated Microbubbles to the Vascular Wall Empowers Site-specific Lentiviral Gene Delivery in vivo

In the field of vascular gene therapy, targeting systems are promising advancements to improve site-specificity of gene delivery. Here, we studied whether incorporation of magnetic nanoparticles (MNP) with different magnetic properties into ultrasound sensitive microbubbles may represent an efficient way to enable gene targeting in the vascular system after systemic application. Thus, we associated novel silicon oxide-coated magnetic nanoparticle containing microbubbles (SO-Mag MMB) with lentiviral particles carrying therapeutic genes and determined their physico-chemical as well as biological properties compared to MMB coated with polyethylenimine-coated magnetic nanoparticles (PEI-Mag MMB). While there were no differences between both MMB types concerning size and lentivirus binding, SO-Mag MMB exhibited superior characteristics regarding magnetic moment, magnetizability as well as transduction efficiency under static and flow conditions in vitro. Focal disruption of lentiviral SO-Mag MMB by ultrasound within isolated vessels exposed to an external magnetic field decisively improved localized VEGF expression in aortic endothelium ex vivo and enhanced the angiogenic response. Using the same system in vivo, we achieved a highly effective, site-specific lentiviral transgene expression in microvessels of the mouse dorsal skin after arterial injection. Thus, we established a novel lentiviral MMB technique, which has great potential towards site-directed vascular gene therapy.

Atherosclerosis and Nanotechnology: Diagnostic and Therapeutic Applications

Over the past several decades, tremendous advances have been made in the understanding, diagnosis, and treatment of coronary artery disease (CAD). However, with shifting demographics and evolving risk factors we now face new challenges that must be met in order to further advance are management of patients with CAD. In parallel with advances in our mechanistic appreciation of CAD and atherosclerosis, nanotechnology approaches have greatly expanded, offering the potential for significant improvements in our diagnostic and therapeutic management of CAD. To realize this potential we must go beyond to recognize new frontiers including knowledge gaps between understanding atherosclerosis to the translation of targeted molecular tools. This review highlights nanotechnology applications for imaging and therapeutic advancements in CAD.

Microbubbles with a Self-Assembled Poloxamer Shell and a Fluorocarbon Inner Gas

The numerous applications of microbubbles in food science and medicine call for a better understanding and control of the effects of the properties of their shells on their stability and ability to resonate at chosen frequencies when submitted to an ultrasound field. We have investigated both millimetric and micrometric bubbles stabilized by an amphiphilic block copolymer, Poloxamer 188 (e.g., Pluronic F-68). Although Pluronic F-68 is routinely being used as a dispersing and foaming agent to facilitate phospholipid-based microbubble preparation, it has never been studied as a shell component per se. First, we investigated the adsorption kinetics of Pluronic F-68 at the interface between water and air, or air saturated with vapors of perfluorohexane (F-hexane), using bubble profile tensiometry analysis. F-Hexane was found to strongly accelerate the adsorption of Pluronic F-68 (at low concentrations) and decrease the interfacial tension values at equilibrium (at all concentrations). We also found that relatively stable microbubbles could unexpectedly be prepared from Pluronic F-68 in the absence of any other surfactant, but only when F-hexane was present. These bubbles showed an only limited volume increase over ∼3 h, while a 10-fold increase in size occurred within 200 s in the absence of a fluorocarbon. Remarkably, their deflation rate decreased when the Pluronic F-68 concentration decreased, suggesting that bubbles with semidilute copolymer coverage are more stable than those more densely covered by copolymer brushes. Single-bubble experiments using laser Doppler vibratometry showed that, by contrast with other surfactant-coated microbubbles, the resonance radius of the Pluronic F-68-coated microbubbles was lower than that of naked microbubbles, meaning that they are less elastic. It was also found that the bubble's vibrational displacement amplitude decreased substantially when the microbubbles were covered with Pluronic F-68, an effect that was further amplified by F-hexane.

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.

Improving performance of nanoscale ultrasound contrast agents using N,N-diethylacrylamide stabilization

The design of nanoscale yet highly echogenic agents for imaging outside of the vasculature and for ultrasound-mediated drug delivery remains a formidable challenge. We have previously reported on formulation of echogenic perfluoropropane gas nanobubbles stabilized by a lipid-pluronic surfactant shell. In the current work we describe the development of a new generation of these nanoparticles which consist of perfluoropropane gas stabilized by a surfactant and lipid membrane and a crosslinked network of N,N-diethylacrylamide. The resulting crosslinked nanobubbles (CL-PEG-NB) were 95.2±25.2nm in diameter and showed significant improvement in stability and retention of echogenic signal over 24h. In vivo analysis via ultrasound and fluorescence mediated tomography showed greater tumor extravasation and accumulation with CL-PEG-NB compared to microbubbles. Together these results demonstrate the capabilities and advantages of a new, more stable, nanometer-scale ultrasound contrast agent that can be utilized in future work for diagnostic scans and molecular imaging.

Contrast enhancement of carotid adventitial vasa vasorum as a biomarker of radiation-induced atherosclerosis

PURPOSE

Abnormal proliferation of adventitial vasa vasorum (vv) occurs early at sites of atherosclerosis and is thought to be an early biomarker of vascular damage. Contrast-enhanced ultrasound (CEUS) can detect this process. Its usefulness in irradiated arteries as a measure of accelerated atherosclerosis is unknown. This study investigates contrast intensity in carotid adventitia as an early marker of radiation-induced damage in head and neck cancer (HNC) patients.

MATERIALS/METHODS

Patients with HNC treated with a wedged-pair and matched neck technique or hemi-neck radiotherapy (RT) (unirradiated side as control) at least 2years previously were included. Patients had been prescribed a dose of at least 50Gy to the neck. CEUS was performed on both carotid arteries and a region of interest was selected in the adventitia of the far wall of both left and right distal common carotid arteries. Novel quantification software was used to compare the average intensity per pixel between irradiated and unirradiated arteries.

RESULTS

48 patients (34 males) with median age of 59.2years (interquartile range (IQR) 49.2-64.2) were included. The mean maximum point dose to the irradiated artery was 61.2Gy (IQR 52.6-61.8) and 1.1Gy (IQR 1.0-1.8Gy) to the unirradiated side. The median interval from RT was 59.4months (IQR 41-88.7). There was a significant difference in the mean (SD) contrast intensity per pixel on the irradiated side (1.1 (0.4)) versus 0.96 (0.34) on the unirradiated side (p=0.01). After attenuation correction, the difference in mean contrast intensity per pixel was still significant (1.4 (0.58) versus 1.2 (0.47) (p=0.02). Previous surgery or chemotherapy had no effect on the difference in contrast intensity between the 2 sides of the neck. Mean intensity per pixel did not correlate to traditional risk prediction models (carotid intima-medial thickness, QSTROKE score).

CONCLUSIONS

Proliferation of vv is demonstrated by increased contrast intensity in irradiated carotid arteries. This may be a useful, independent biomarker of radiation-induced carotid atherosclerosis when used as a tool to quantify neovascularization.

Contrast-enhanced ultrasound imaging of atherosclerotic carotid plaque neovascularization: a new surrogate marker of atherosclerosis?

An atherosclerotic plaque requires a nutrient blood supply, which is predominantly derived from arterial vasa vasorum. A variety of factors (environmental and genetic) contribute to the initiation and growth of atherosclerosis within vessel walls. Chemotactic factors, such as tissue ischemic and hypoxic factors, stimulate the release of vascular endothelial growth factor (VEGF) proteins, resulting in vessel wall angiogenesis. These developments often precede the formation of the luminal plaque. In this report, we describe the use of contrast-enhanced carotid ultrasound (CECU) imaging for the detection and quantification of intra-plaque neovascularization. The efficacy of CECU was measured against the neovascular density observed within the tissue specimens obtained at the time of carotid endarterectomy surgery. The objective of this study was to provide a histologic correlation between CECU and carotid artery atherosclerotic plaque neovascularization. Fifteen patients with significant atherosclerotic carotid artery disease received a CECU examination prior to undergoing a carotid endarterectomy (CEA). Two patients received bilateral endarterectomies, resulting in a total of 17 cases. At the time of surgery, carotid plaque samples were surgically removed and stained with specific vascular markers (CD31, CD34, von Willebrand factor, and hemosiderin) designed to identify the presence and degree of neovascularization. The intra-plaque neovascularization recorded on preoperative CECU was correlated with the degree of neovascularization noted in the tissue specimens. The CECU neovascularization was correlated to CD31-stained tissue specimens. This correlation value was 0.68 using Spearman's rank method. When CECU results were correlated with the other histologic markers (CD34, von Willebrand factor, and hemosiderin), a correlation of 0.50 was obtained. In conclusion, contrast-enhanced carotid ultrasound correlated to the presence and degree of intra-plaque neovascularization as determined from histology specimens.

Mathematical Models of Contrast Transport Kinetics for Cancer Diagnostic Imaging: A Review

Angiogenesis plays a fundamental role in cancer growth and the formation of metastasis. Novel cancer therapies aimed at inhibiting angiogenic processes and/or disrupting angiogenic tumor vasculature are currently being developed and clinically tested. The need for earlier and improved cancer diagnosis, and for early evaluation and monitoring of therapeutic response to angiogenic treatment, have led to the development of several imaging methods for in vivo noninvasive assessment of angiogenesis. The combination of dynamic contrast-enhanced imaging with mathematical modeling of the contrast agent kinetics enables quantitative assessment of the structural and functional changes in the microvasculature that are associated with tumor angiogenesis. In this paper, we review quantitative imaging of angiogenesis with dynamic contrast-enhanced magnetic resonance imaging, computed tomography, and ultrasound.

Intraplaque neovascularization as a novel therapeutic target in advanced atherosclerosis

INTRODUCTION

Atherosclerosis is a lipid-driven inflammatory process with a tremendously high mortality due to acute cardiac events. There is an emerging need for new therapies to stabilize atherosclerotic lesions. Growing evidence suggests that intraplaque (IP) neovascularisation and IP hemorrhages are important contributors to plaque instability.

AREAS COVERED

Neovascularization is a complex process that involves different growth factors and inflammatory mediators of which their individual significance in atherosclerosis remains poorly understood. This review discusses different aspects of IP neovascularization in atherosclerosis including the potential treatment opportunities to stabilize advanced plaques. Furthermore, we highlight the development of accurate and feasible in vivo imaging modalities for IP neovascularization to prevent acute events.

EXPERT OPINION

Although lack of a valuable animal model of IP neovascularization impeded the investigation of a causal and straightforward link between neovascularization and atherosclerosis, recent evidence shows that vein grafts in ApoE*3 Leiden mice as well as plaques in ApoE(-/-) Fbn1(C1039G+/-) mice are useful models for intraplaque neovessel research. Even though interference with vascular endothelial growth factor (VEGF) signalling has been widely investigated, new therapeutic opportunities have emerged. Cell metabolism, in particular glycolysis and fatty acid oxidation, appears to perform a crucial role in the development of IP neovessels and thereby serves as a promising target.

Contrast-Enhanced Ultrasound Angiogenesis Imaging by Mutual Information Analysis for Prostate Cancer Localization

OBJECTIVE

The role of angiogenesis in cancer growth has stimulated research aimed at noninvasive cancer detection by blood perfusion imaging. Recently, contrast ultrasound dispersion imaging was proposed as an alternative method for angiogenesis imaging. After the intravenous injection of an ultrasound-contrast-agent bolus, dispersion can be indirectly estimated from the local similarity between neighboring time-intensity curves (TICs) measured by ultrasound imaging. Up until now, only linear similarity measures have been investigated. Motivated by the promising results of this approach in prostate cancer (PCa), we developed a novel dispersion estimation method based on mutual information, thus including nonlinear similarity, to further improve its ability to localize PCa.

METHODS

First, a simulation study was performed to establish the theoretical link between dispersion and mutual information. Next, the method's ability to localize PCa was validated in vivo in 23 patients (58 datasets) referred for radical prostatectomy by comparison with histology.

RESULTS

A monotonic relationship between dispersion and mutual information was demonstrated. The in vivo study resulted in a receiver operating characteristic (ROC) curve area equal to 0.77, which was superior (p = 0.21-0.24) to that obtained by linear similarity measures (0.74-0.75) and (p <; 0.05) to that by conventional perfusion parameters (≤0.70).

CONCLUSION

Mutual information between neighboring time-intensity curves can be used to indirectly estimate contrast dispersion and can lead to more accurate PCa localization.

SIGNIFICANCE

An improved PCa localization method can possibly lead to better grading and staging of tumors, and support focal-treatment guidance. Moreover, future employment of the method in other types of angiogenic cancer can be considered.

A Review on Carotid Ultrasound Atherosclerotic Tissue Characterization and Stroke Risk Stratification in Machine Learning Framework

Cardiovascular diseases (including stroke and heart attack) are identified as the leading cause of death in today's world. However, very little is understood about the arterial mechanics of plaque buildup, arterial fibrous cap rupture, and the role of abnormalities of the vasa vasorum. Recently, ultrasonic echogenicity characteristics and morphological characterization of carotid plaque types have been shown to have clinical utility in classification of stroke risks. Furthermore, this characterization supports aggressive and intensive medical therapy as well as procedures, including endarterectomy and stenting. This is the first state-of-the-art review to provide a comprehensive understanding of the field of ultrasonic vascular morphology tissue characterization. This paper presents fundamental and advanced ultrasonic tissue characterization and feature extraction methods for analyzing plaque. Additionally, the paper shows how the risk stratification is achieved using machine learning paradigms. More advanced methods need to be developed which can segment the carotid artery walls into multiple regions such as the bulb region and areas both proximal and distal to the bulb. Furthermore, multimodality imaging is needed for validation of such advanced methods for stroke and cardiovascular risk stratification.

Imaging Modalities to Identity Inflammation in an Atherosclerotic Plaque

Atherosclerosis is a chronic, progressive, multifocal arterial wall disease caused by local and systemic inflammation responsible for major cardiovascular complications such as myocardial infarction and stroke. With the recent understanding that vulnerable plaque erosion and rupture, with subsequent thrombosis, rather than luminal stenosis, is the underlying cause of acute ischemic events, there has been a shift of focus to understand the mechanisms that make an atherosclerotic plaque unstable or vulnerable to rupture. The presence of inflammation in the atherosclerotic plaque has been considered as one of the initial events which convert a stable plaque into an unstable and vulnerable plaque. This paper systemically reviews the noninvasive and invasive imaging modalities that are currently available to detect this inflammatory process, at least in the intermediate stages, and discusses the ongoing studies that will help us to better understand and identify it at the molecular level.

The Use of Contrast-enhanced Ultrasonography for Imaging of Carotid Atherosclerotic Plaques: Current Evidence, Future Directions

Contrast-enhanced ultrasonography (CEUS) is a rapidly evolving modality for imaging carotid artery disease and systemic atherosclerosis. CEUS coupled with diagnostic ultrasonography predicts the degree of carotid artery stenosis and is comparable with computed tomography and magnetic resonance angiography. This article reviews the literature on the evolving role of CEUS for the identification and characterization of carotid plaques with an emphasis on detection of intra-plaque neovascularization and related high-risk morphologic features notably present in symptomatic patients. CEUS carotid imaging may play a prominent additive role in risk stratifying patients and serve as a powerful tool for monitoring therapeutic interventions.

Non-invasive molecular imaging of vulnerable atherosclerotic plaques

The growing discoveries coming from clinical and basic research during the past decades have revolutionized our knowledge regarding pathophysiologic mechanisms underlying the atherosclerotic process and its thrombotic complications. The traditional view focusing on the severity of stenosis of atherosclerotic plaque has given way to the evidence that the clinical complications of atherosclerotic vascular disease, particularly the propensity to develop thrombotic complications, are determined mainly by the biological composition of the plaque. This paradigm shift has reinforced the need to move from the sole anatomical assessment toward combined anatomic and functional imaging modalities enabling the molecular and cellular characterization of the disease on top of its structural properties. Together, the progress to identify molecular targets related to plaque vulnerability and the improvement of imaging techniques for the detection of such molecular targets have allowed us to obtain new important pathophysiological information. This might allow better patient stratification for the identification of subjects at high risk to develop premature atherosclerosis who might need an aggressive therapeutic approach. Nuclear techniques, magnetic resonance imaging, computed tomography angiography, and contrast-enhanced ultrasound represent the currently available non-invasive imaging modalities for molecular imaging which can provide different and complementary insights into the biological features of the atherosclerotic process. This clinical review will discuss the evidence and potential translational applications of the individual imaging techniques particularly concerning their ability to detect the main atherosclerotic features related to plaque vulnerability, such as plaque inflammation and intertwined neovascularization.

Ultrasound imaging beyond the vasculature with new generation contrast agents

Current commercially available ultrasound contrast agents are gas-filled, lipid- or protein-stabilized microbubbles larger than 1 µm in diameter. Because the signal generated by these agents is highly dependent on their size, small yet highly echogenic particles have been historically difficult to produce. This has limited the molecular imaging applications of ultrasound to the blood pool. In the area of cancer imaging, microbubble applications have been constrained to imaging molecular signatures of tumor vasculature and drug delivery enabled by ultrasound-modulated bubble destruction. Recently, with the rise of sophisticated advancements in nanomedicine, ultrasound contrast agents, which are an order of magnitude smaller (100-500 nm) than their currently utilized counterparts, have been undergoing rapid development. These agents are poised to greatly expand the capabilities of ultrasound in the field of targeted cancer detection and therapy by taking advantage of the enhanced permeability and retention phenomenon of many tumors and can extravasate beyond the leaky tumor vasculature. Agent extravasation facilitates highly sensitive detection of cell surface or microenvironment biomarkers, which could advance early cancer detection. Likewise, when combined with appropriate therapeutic agents and ultrasound-mediated deployment on demand, directly at the tumor site, these nanoparticles have been shown to contribute to improved therapeutic outcomes. Ultrasound's safety profile, broad accessibility and relatively low cost make it an ideal modality for the changing face of healthcare today. Aided by the multifaceted nano-sized contrast agents and targeted theranostic moieties described herein, ultrasound can considerably broaden its reach in future applications focused on the diagnosis and staging of cancer.

[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.

Cumulative phase delay between second harmonic and fundamental components--a marker for ultrasound contrast agents

Several imaging techniques aimed at detecting ultrasound contrast agents (UCAs) echo signals, while suppressing signals coming from the surrounding tissue, have been developed. These techniques are especially relevant for blood flow, perfusion, or contrast dispersion quantification. However, despite several approaches being presented, improving the understanding of the ultrasound/UCAs interaction may support further development of imaging techniques. In this paper, the physical phenomena behind the formation of harmonic components in tissue and UCAs, respectively, are addressed as a possible way to recognize the origin of the echo signals. Simulations based on a modified Rayleigh, Plesset, Noltingk, Neppiras, and Poritsky equation and transmission and backscattering measurements of ultrasound propagating through UCAs performed with a single element transducer and a submergible hydrophone, are presented. Both numerical and in vitro results show the occurrence of a cumulative time delay between the second harmonic and fundamental component which increases with UCA concentration and propagation path length through UCAs, and that was clearly observable at frequencies ( f0 = 2.5 MHz) and pressure regimes (mechanical index = 0.1) of interest for imaging. Most importantly, this delay is not observed in the absence of UCAs. In conclusion, the reported phenomenon represents a marker for UCAs with potential application for imaging.

Effects of noradrenaline and adenosine triphosphate on the degree on contrast enhancement in a rabbit model of atherosclerosis during contrast-enhanced ultrasonography

The aim of the study is to assess the effects of vasoactive agents on the degree of contrast enhancement in experimental atherosclerotic plaque during contrast-enhanced ultrasonography (CEUS). Abdominal aortic atherosclerosis was induced in 25 New Zealand white rabbits by a combination of cholesterol-rich diet and balloon endothelial denudation. Standard ultrasonography and CEUS were performed at baseline and during intravenous infusion of noradrenaline or adenosine triphosphate (ATP). The degree of contrast enhancement of the plaque after injection of contrast material was quantified by calculating the enhanced intensity in the plaque. The infusion of noradrenaline induced significant increase in systolic blood pressure (84 ± 13 mm Hg vs. 112 ± 20 mm Hg, p = 0.011) and significant decrease in the enhanced intensity in the plaque (7.52 ± 1.32 dB vs. 5.88 ± 1.33 dB, p < 0.001) during CEUS. The infusion of ATP resulted in the significant decrease in systolic blood pressure (80 ± 13 mm Hg vs. 65 ± 11 mm Hg, p = 0.005) and increase in the enhanced intensity in the plaque (7.52 ± 1.32 dB vs. 8.84 ± 1.55 dB, p < 0.001) during CEUS. The degree of contrast enhancement within an experimental atherosclerotic plaque during CEUS can be influenced by vasoactive agents and hemodynamic status.

Quantification of carotid plaque neovascularization using contrast-enhanced ultrasound with histopathologic validation

We sought to evaluate contrast-enhanced ultrasound (CEUS) imaging for the quantification of carotid plaque neovascularization. Seventeen patients underwent carotid endarterectomy after standard ultrasound and CEUS. Semiquantitative and quantitative analyses of contrast enhancement within the plaque were performed using a visual interpretation scale and quantitative analysis software, respectively. Enhancement intensity (dB) was measured at the plaque (EI(plaque)). Each specimen was stained with CD34 and CD68 to assess for microvessels and macrophages, respectively. Semiquantitative CEUS analyses were correlated with neovascularization at histology (r = 0.70, p = 0.002). Quantitative analysis was also correlated with neovascularization at histology (EI(plaque)r = 0.81, p < 0.001). EI(plaque) (r = 0.64, p = 0.01) was correlated with the degree of enhancement as assessed visually. Semiquantitative and quantitative analyses were not correlated with macrophage infiltration at the plaque. Contrast enhancement in the carotid plaque was correlated with neovascularity at the histopathologic exam. Furthermore, semiquantitative and quantitative measurements were highly correlated with each other, suggesting that either can be used to detect intraplaque neovascularization.