Detection and characterization of vascular lesions by intravascular ultrasound: an in vitro study correlated with histology

Hyperechogenic intimal lesions and wall thickness of the temporal and facial arteries in elderly patients with arterial occlusions of the eye

OBJECTIVE

To determine the association of arteriosclerosis, characterised by hyperechogenic intimal lesions (HIL), with wall thickness of the temporal and facial arteries in elderly patients with ocular arterial occlusions.

METHODS

Patients suffering from non-arteritic ocular perfusion disorders were included. High-resolution compression sonography (18 MHz) images of the temporal arteries (frontal and parietal branch at the upper margin of the auricle) and facial arteries (at the crossing point of the artery over the mandible) were analysed for the presence of HIL (grade 0: absent; grade 1: moderate; grade 2: severe). Characteristics of patients with and without evidence of HIL >grade 1 were compared.

RESULTS

In total, 330 cranial artery segments of 55 patients were analysed. HIL ≥grade 1 was present in 13.0% of all artery segments and in 38.1% of all patients. Patients with HIL ≥grade 1 in at least one arterial segment displayed significantly increased maximum wall thickness of the temporal arteries (0.62±0.23 mm vs 0.50±0.13 mm; p<0.01) and facial arteries (0.71±0.20 mm vs 0.54±0.19 mm; p=0.01). Patients with at least one temporal or facial artery segment with HIL were older, more often male and more frequently suffered from diabetes mellitus.

CONCLUSION

The presence of HIL goes along with a significantly increased wall thickness of the temporal and facial arteries. These findings should be considered when interpreting the results of sonography of the cranial arteries in the diagnostic workup of suspected giant cell arteritis.

Three-dimensional vascular and metabolic imaging using inverted autofluorescence

SIGNIFICANCE

Three-dimensional (3D) vascular and metabolic imaging (VMI) of whole organs in rodents provides critical and important (patho)physiological information in studying animal models of vascular network.

AIM

Autofluorescence metabolic imaging has been used to evaluate mitochondrial metabolites such as nicotinamide adenine dinucleotide (NADH) and flavine adenine dinucleotide (FAD). Leveraging these autofluorescence images of whole organs of rodents, we have developed a 3D vascular segmentation technique to delineate the anatomy of the vasculature as well as mitochondrial metabolic distribution.

APPROACH

By measuring fluorescence from naturally occurring mitochondrial metabolites combined with light-absorbing properties of hemoglobin, we detected the 3D structure of the vascular tree of rodent lungs, kidneys, hearts, and livers using VMI. For lung VMI, an exogenous fluorescent dye was injected into the trachea for inflation and to separate the airways, confirming no overlap between the segmented vessels and airways.

RESULTS

The kidney vasculature from genetically engineered rats expressing endothelial-specific red fluorescent protein TdTomato confirmed a significant overlap with VMI. This approach abided by the "minimum work" hypothesis of the vascular network fitting to Murray's law. Finally, the vascular segmentation approach confirmed the vascular regression in rats, induced by ionizing radiation.

CONCLUSIONS

Simultaneous vascular and metabolic information extracted from the VMI provides quantitative diagnostic markers without the confounding effects of vascular stains, fillers, or contrast agents.

Comprehensive analysis of clinico-pathological data reveals heterogeneous relations between atherosclerosis and cancer

AIMS

Atherosclerosis and cancer share common risk factors and involve similar molecular pathomechanisms. Most clinical and epidemiological studies show a positive correlation between atherosclerosis and smoking-related cancers and heterogeneous results for non-smoking-related cancers. However, up-to-date large-scale autopsy studies including a detailed analysis of cancer types are lacking. Therefore, we sought to investigate the relation between major cancer types and the grade of atherosclerosis in a recent well-powered autopsy cohort.

METHODS

In 2101 patients, both autopsy data and clinical data including demographics, disease groups, tumour type, cause of death and grade of atherosclerosis were reviewed and statistically analysed.

RESULTS

We found cancer in general is associated with less atherosclerosis (OR 0.60, p<0.0001). In particular, haematological neoplasm and sarcomas were associated with much less atherosclerosis (OR=0.45, p<0.0001 and OR=0.43, p=0.087), while carcinomas were associated with moderately less atherosclerosis (OR=0.72, p=0.002). Furthermore, non-smoking-related cancers were associated with much less atherosclerosis (OR=0.41, p<0.0001), while possibly smoking-related cancer and smoking-related cancer showed no significant association. In a comprehensive analysis of 21 cancer types, biliary tract cancer, lymphomas/lymphoid leukaemias and kidney cancer were associated with much less atherosclerosis (OR=0.19, p<0.0001; OR=0.41, p<0.0001; and OR=0.48, p=0.029). In an exploratory analysis of treatment strategies, we found that tumours with a recommendation of oxazaphosphorines and pyrimidine antagonist treatment were significantly associated with less atherosclerosis (OR=0.33, p=0.0068 and OR=0.58, p=0.012).

CONCLUSIONS

In conclusion, the study showed an inverse association between cancer and atherosclerosis postmortem that depends on the cancer type and suggests a possible impact of chemotherapy regimens.

Progress in atherosclerotic plaque imaging

Cardiovascular diseases are the primary cause of mortality in the industrialized world, and arterial obstruction, triggered by rupture-prone atherosclerotic plaques, lead to myocardial infarction and cerebral stroke. Vulnerable plaques do not necessarily occur with flow-limiting stenosis, thus conventional luminographic assessment of the pathology fails to identify unstable lesions. In this review we discuss the currently available imaging modalities used to investigate morphological features and biological characteristics of the atherosclerotic plaque. The different imaging modalities such as ultrasound, magnetic resonance imaging, computed tomography, nuclear imaging and their intravascular applications are illustrated, highlighting their specific diagnostic potential. Clinically available and upcoming methodologies are also reviewed along with the related challenges in their clinical translation, concerning the specific invasiveness, accuracy and cost-effectiveness of these methods.

Imaging the vulnerable plaque

Cardiovascular diseases are still the primary causes of mortality in the United States and in Western Europe. Arterial thrombosis is triggered by a ruptured atherosclerotic plaque and precipitates an acute vascular event, which is responsible for the high mortality rate. These rupture-prone plaques are called "vulnerable plaques." During the past decades, much effort has been put toward accurately detecting the presence of vulnerable plaques with different imaging techniques. In this review, we provide an overview of the currently available invasive and noninvasive imaging modalities used to detect vulnerable plaques. We will discuss the upcoming challenges in translating these techniques into clinical practice and in assigning them their exact place in the decision-making process.

Invasive imaging technologies: can we reconcile light and sound?

Since the introduction of intravascular, catheter-based invasive imaging and diagnostic tools in the catheterization laboratories two decades ago, the functional assessment of angiographically moderate or ambiguous lesions by fractional flow reserve measurements represents the established standard of care today. Likewise, intravascular ultrasound (IVUS) is widely accepted to guide treatment strategy in complex lesions, such as long or left main stem lesions. Developments are driven by the clinical interest to optimize treatment, prevent periprocedural complications, understand treatment failure and understand progression of atherosclerosis. As a result, a variety of devices are now clinically available that enable detection and monitoring of specific plaque features over time, such as the presence of necrotic core by IVUS-VH, a lipid-core plaque by near infrared (NIR) spectroscopy or a thin fibrous cap atheroma by optical coherence tomography (OCT). As the physical boundaries for both light and sound are different, these imaging technologies offer different advantages and limitations. Light-based technologies offer unparalleled high image resolution (OCT) or unparalleled high sensitivity and specificity for distinct plaque components (NIR spectroscopy), whereas conventional IVUS offers a much better tissue penetration. From a clinical perspective, both types of information are valuable. Ideally, this information should easily and in real time be available in the catheterization laboratory, consisting of co-registered datasets gained during a single catheter pullback. On this background, a combined NIR spectroscopy and IVUS catheter has recently been introduced for clinical use. The article discusses the potential and limitations of these different technologies. They may allow advanced coronary plaque diagnosis in a fast, accurate, reliable, user- and patient-friendly manner and, as such, can help to improve clinical practice today and therapeutic options in the future.

Ultrasound and light: friend or foe? On the role of intravascular ultrasound in the era of optical coherence tomography

More than 20 years after its introduction, intravascular ultrasound (IVUS) has outlived many other intracoronary techniques. IVUS was useful to solve many interventional problems and assisted us in understanding the dynamics of atherosclerosis. It serves as an established imaging endpoint in large progression-regression trial and as an important workhorse in many catheterization laboratories. Nowadays, increasingly complex lesions are treated with drug-eluting stents. The application of IVUS during such interventions can be very useful. Recently, optical coherence tomography (OCT), a light-based imaging technique, has entered the clinical arena. The “omnipresence” of OCT during scientific sessions and live courses with PCI may raise in many the question: Does IVUS have a future in the “era of OCT”? Three review articles, highlighted by this editorial, demonstrate the broad spectrum of current IVUS applications and underline the significant role of IVUS during the last two decades. OCT, the much younger technique, still has to prove its value. Yet OCT is likely to take over some of the current indications of IVUS as a research tool. In addition, OCT is currently gaining clinical significance for stent optimization during complex interventional procedures. Nevertheless, there is little doubt that IVUS still has a major role in studies on coronary atherosclerosis and for guidance of coronary stenting. Thus, ultrasound and light—are they friend or foe? In fact, both methods are good in their own rights. They are complementary rather than competitive. Moreover, in combination, at least for certain indications, they could be even better.

Intravascular ultrasound: principles and cerebrovascular applications

Intravascular sonography is a valuable tool for the morphologic assessment of coronary atherosclerosis and the effect of pharmacologic and nonpharmacologic interventions on the progression or stabilization of atherosclerosis. An analysis of the different modes, applications, and limitations is provided on the basis of review of existing data from multiple clinical case studies, trials, and mechanistic studies. Intravascular sonography has been used to assess the outcomes of different percutaneous interventions, including angioplasty and stent implantation, and to provide detailed characterization of atherosclerotic lesions, aneurysms, and dissections within the cerebrovascular circulation. Evolution of intravascular sonographic technology has led to the development of more sophisticated diagnostic tools such as color-flow, virtual histology, and integrated backscatter intravascular sonography. The technologic advancement in intravascular sonography has the potential of providing more accurate information prior, during, and after a medical or endovascular intervention. Continued assessment of this diagnostic technique in both the intracranial and extracranial circulation will lead to increased use in clinical practice with the intent to improve outcomes.

The performance of interventional loopless MRI antennae at higher magnetic field strengths

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8 +/- 0.2- and 9.7 +/- 0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of approximately 1 degrees C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.

Distribution of ultrasonic radiofrequency signal amplitude detects lipids in atherosclerotic plaque of coronary arteries: an ex-vivo study

BACKGROUND

Accumulation of lipids within coronary plaques is an important process in disease progression. However, gray-scale intravascular ultrasound images cannot detect plaque lipids effectively. Radiofrequency signal analysis could provide more accurate information on preclinical coronary plaques.

METHODS

We analyzed 29 zones of mild atheroma in human coronary arteries acquired at autopsy. Two histologic groups, i.e., plaques with a lipid core (group L) and plaques without a lipid core (group N), were analyzed by automatic calculation of integrated backscatter. One hundred regions of interest were set on the target zone. Radiofrequency signals from a 50 MHz transducer were digitized at 240 MHz with 12-bit resolution. The intensity of integrated backscatter and its distribution within each plaque were compared between the two groups.

RESULTS

Although the mean backscatter was similar between the groups, intraplaque variation of backscatter and backscatter in the axial direction were larger in group L than in group N (p = 0.02). Conventional intravascular ultrasound showed extremely low sensitivity for lipid detection, despite a high specificity. In contrast, a cut-off value>32 for the total variance of integrated backscatter identified lipid-containing plaque with a high sensitivity (85%) and specificity (75%).

CONCLUSION

Compared with conventional imaging, assessment of the intraplaque distribution of integrated backscatter is more effective for detecting lipid. As coronary atheroma progresses, its composition becomes heterogeneous and multi-layered. This radiofrequency technique can portray complex plaque histology and can detect the early stage of plaque progression.

Diagnosis and treatment of coronary vulnerable plaques

Thin-capped fibroatheroma is the morphology that most resembles plaque rupture. Detection of these vulnerable plaques in vivo is essential to being able to study their natural history and evaluate potential treatment modalities and, therefore, may ultimately have an important impact on the prevention of acute myocardial infarction and death. Currently, conventional grayscale intravascular ultrasound, virtual histology and palpography data are being collected with the same catheter during the same pullback. A combination of this catheter with either thermography capability or additional imaging, such as optical coherence tomography or spectroscopy, would be an exciting development. Intravascular magnetic resonance imaging also holds much promise. To date, none of the techniques described above have been sufficiently validated and, most importantly, their predictive value for adverse cardiac events remains elusive. Very rigorous and well-designed studies are compelling for defining the role of each diagnostic modality. Until we are able to detect in vivo vulnerable plaques accurately, no specific treatment is warranted.

Acute coronary syndromes: an emphasis shift from treatment to prevention; and the enduring challenge of vulnerable plaque detection in the cardiac catheterization laboratory

Rupture of vulnerable plaques is the main cause of acute coronary syndromes and myocardial infarctions. Identification of these vulnerable plaques is therefore essential to enable the development of treatment modalities to stabilize them. Several intravascular technologies, investigating coronary areas that will be responsible for future events, are highlighted in this review. The ideal technique would provide morphological, mechanical and biochemical information. Although several imaging techniques are currently under development, none of them alone provides such an all-embracing assessment. Optical coherence tomography has the advantage of high resolution, thermography has the potential to measure metabolism, and Raman spectroscopy obtains information on chemical components. Intravascular coronary ultrasound (IVUS) and IVUS-palpography are easy to perform and assess morphology and mechanical instability. Shear stress is an important mechanical parameter deeply influencing vascular biology. Nevertheless, all these techniques are still under investigation and, at present, none of them can unequivocally and comprehensively identify a vulnerable plaque and, most importantly, predict its further development. From a clinical point of view, most techniques currently assess only one feature of the vulnerable plaque. Thus, a combination of several modalities will be important in the future to ensure a high sensitivity and specificity in detecting vulnerable plaques.

Detection of vulnerable plaque in a murine model of atherosclerosis with optical coherence tomography

OBJECTIVES

The aim of this study was to evaluate the feasibility of optical coherence tomography (OCT) to identify the components of vulnerable plaques in a well-established murine model of human atherosclerosis.

BACKGROUND

Although the pathologic features that predict plaque rupture at autopsy are well known, the development of a technology to identify these high risk features in vivo is lacking. OCT uses reflected light to provide histology-like images of plaque with higher resolution than competing imaging modalities. Whether OCT can reliably identify the features of an atherosclerotic plaque that define it as vulnerable-thin fibrous cap, large lipid core, and high percent of lipid in the artery-requires further study.

METHODS

OCT images of the atherosclerotic innominate artery segments from the apolipoprotein E knockout (apoE(-/-)) mice were recorded and correlated with histology in both in vivo (n = 7) and well as in ex vivo experiments (n = 12).

RESULTS

Excellent correlation between the OCT and histology measurements for fibrous cap thickness, lipid core size, and percentage lipid content was found. The fibrous cap thicknesses examined span those of human fibrous caps known to rupture (< 65 microm). Regions of greatest light reflection in OCT images were observed when calcium hydroxy-apatite was scattered in lipid, less in fibrous tissue, and least in lipid.

CONCLUSIONS

These findings suggest that OCT holds promise for the identification of features defining vulnerable plaque including fibrous cap thickness, lipid core size, and the percentage of lipid content.

Diagnostic Accuracy of Optical Coherence Tomography and Integrated Backscatter Intravascular Ultrasound Images for Tissue Characterization of Human Coronary Plaques

OBJECTIVES

The purpose of the present study was to validate the diagnostic accuracy of optical coherence tomography (OCT), integrated backscatter intravascular ultrasound (IB-IVUS), and conventional intravascular ultrasound (C-IVUS) for tissue characterization of coronary plaques and to evaluate the advantages and limitations of each of these modalities.

BACKGROUND

The diagnostic accuracy of OCT for characterizing tissue types is well established. However, comparisons among OCT, C-IVUS, and IB-IVUS have not been done.

METHODS

We examined 128 coronary arterial sites (42 coronary arteries) from 17 cadavers; IVUS and OCT images were acquired on the same slice as histology. Ultrasound signals were obtained using an IVUS system with a 40-MHz catheter and digitized at 1 GHz with 8-bit resolution. The IB values of the ultrasound signals were calculated with a fast Fourier transform.

RESULTS

Using histological images as a gold standard, the sensitivity of OCT for characterizing calcification, fibrosis, and lipid pool was 100%, 98%, and 95%, respectively. The specificity of OCT was 100%, 94%, and 98%, respectively (Cohen's kappa = 0.92). The sensitivity of IB-IVUS was 100%, 94%, and 84%, respectively. The specificity of IB-IVUS was 99%, 84%, and 97%, respectively (Cohen's kappa = 0.80). The sensitivity of C-IVUS was 100%, 93%, and 67%, respectively. The specificity of C-IVUS was 99%, 61%, and 95%, respectively (Cohen's kappa = 0.59).

CONCLUSIONS

Within the penetration depth of OCT, OCT has a best potential for tissue characterization of coronary plaques. Integrated backscatter IVUS has a better potential for characterizing fibrous lesions and lipid pools than C-IVUS.

Toward rapid high resolution in vivo intravascular MRI: evaluation of vessel wall conspicuity in a porcine model using multiple imaging protocols

PURPOSE

To assess magnetic resonance (MR) pulse sequences for high resolution intravascular imaging.

MATERIALS AND METHODS

Intravascular imaging of the abdominal aorta and iliac arteries was performed in vivo in a porcine model at 1.5 T using catheter-mounted micro-receive coils. Ten protocols, including spin-echo (SE)-echo planar imaging (SE-EPI), segmented EPI, half-Fourier single-shot turbo spin-echo (HASTE), fast imaging with steady-state free precession (TrueFISP), turbo spin-echo (TSE), and SE acquisition schemes were employed in 13 trials. Images were analyzed by six expert raters with respect to wall-conspicuity, wall-to-lumen/tissue contrast, visible layers of the arterial wall, anticipated clinical usefulness, and overall image quality. Mean differences between sequence-types were evaluated using analysis of variance (ANOVA) between groups with planned comparisons.

RESULTS

The vessel wall was delineated in almost all protocols. Motion artifacts from physiological and device motion were reduced in fast techniques. The best contrast between the wall and surrounding tissue was provided by a HASTE protocol. Anatomic layers of the vessel wall were best depicted on dark blood T2-weighted TSE. Overall, TrueFISP was ranked highest on the remaining measures.

CONCLUSION

Dedicated catheter-coils combined with fast sequences have potential for in vivo characterization of vessel walls. TrueFISP offered the best overall image quality and acquisition speed, but suffered from the inability to delineate the multiple layers of the wall, which seems associated with dark blood- and T2-weighted contrast. We believe future intra-arterial trials should proceed from this study in normal artery imaging and initially focus on fast T2-weighted dark blood techniques in trials with pathology.

Clinical imaging of the vulnerable plaque in the coronary arteries: new intracoronary diagnostic methods

Rupture of a vulnerable plaque is the main cause of acute coronary syndromes and myocardial infarction. The features of rupture-prone atherosclerotic plaques have been previously described by pathologists. However, identification of vulnerable plaques in vivo is essential to study their natural history and to evaluate potential treatment modalities. Coronary angiography is the gold standard for the diagnosis of coronary artery disease, but it is unable to distinguish between stable and unstable plaques and to accurately predict future cardiac events. This current perspective describes the recently developed invasive imaging techniques to detect atherosclerotic vulnerable plaques in the coronary tree.

Detection of Lipid-Laden Atherosclerotic Plaque by Wavelet Analysis of Radiofrequency Intravascular Ultrasound Signals

OBJECTIVES

This study examined the feasibility of using a wavelet analysis of radiofrequency (RF) intravascular ultrasound (IVUS) signals in detecting lipid-laden plaque.

BACKGROUND

Wavelet analysis is a new mathematical model for assessing local changes in a geometrical profile of time-series signals.

METHODS

Radiofrequency IVUS signals of 85 arbitrarily selected vectors were acquired from 27 formalin-fixed noncalcified atherosclerotic plaques from human necropsy with a digitizer at 500 MHz with 8-bit resolution by use of a 40-MHz IVUS catheter. Wavelet analysis of these RF signals was performed using a Daubechies-2 wavelet to obtain a color-coded map of the correlation coefficient with the wavelet reconstructed over the x-y plane of the wavelet scale and the distance from the IVUS catheter. The plaque segment was then examined histologically after being stained with Masson's trichrome stain. This technique also was applied in vivo in 29 human coronary plaque segments. These segments were excised subsequently by directional coronary atherectomy and processed for histologic analysis.

RESULTS

In the in vitro study, histologic examination revealed lipid-laden segments in 29 vectors. When performing a wavelet analysis with the Daubechies-2 wavelet, the color-coded mapping revealed a different pattern in lipid-laden plaques compared with other types of plaque. Using this wavelet analysis, lipid-laden plaque could be detected with a sensitivity of 83% (24 of 29) and a specificity of 82% (46 of 56). In the in vivo study, fatty plaque could be detected with a sensitivity of 81% (13 of 16) and a specificity of 85% (11 of 13) with this method.

CONCLUSIONS

Wavelet analysis of RF IVUS signals enabled in vitro as well as in vivo detection of lipid-laden plaque. This method may be useful in assessing plaque vulnerability in patients with coronary artery disease.

Vascular remodeling and plaque composition between focal and diffuse coronary lesions assessed by intravascular ultrasound

Coronary remodeling and plaque composition were compared between focal and diffuse coronary lesions. Negative remodeling and fibrous and calcified plaque compositions contribute to stenosis development in diffuse lesions more frequently than in focal lesions.

Classification of arterial plaque by spectral analysis in remodelled human atherosclerotic coronary arteries

We aimed to characterise and to identify the predominant plaque type in vivo using unprocessed radiofrequency (RF) intravascular ultrasound (US) backscatter, in remodelled segments of human atherosclerotic coronary arteries. A total of 16 remodelled segments were identified using a 30-MHz intravascular ultrasound (IVUS) scanner in vivo. Of these, 9 segments were classified as positively remodelled (>1.05 of the total vessel area in comparison with the proximal and distal reference segments) and 7 as negatively remodelled (<0.95 of reference segment area). Spectral parameters (maximum power, mean power, minimum power and power at 30 MHz) were determined and plaque type was defined as mixed fibrous, calcified or lipid-rich. Positively remodelled segments had a larger total vessel area (16.5 +/- 1.1 mm2 vs. 8.7 +/- 0.9 mm2, p<0.01) and plaque area (7.3 +/- 1.1 mm2 vs. 4.4 +/- 0.8 mm2, p=0.05) than negatively remodelled segments. Both positively and negatively remodelled segments had a greater percentage of fibrous plaque (p<0.01) than calcified or lipid-rich plaque. Comparing positively and negatively remodelled segments, there was no significant difference between the proportion of fibrous, calcified or lipid-rich plaque. We have been able to characterise and to identify plaque composition in vivo in human atherosclerotic coronary arteries. Our data suggest that remodelled segments are predominantly composed of fibrous plaque, as identified by RF analysis, although plaque composition is similar, irrespective of the remodelling type.

Apolipoprotein A-I(Milano): current perspectives

PURPOSE OF REVIEW

Strategies to increase HDL are among the major targets of clinical research in atherosclerosis prevention. The mutant apolipoprotein A-I(Milano) has been associated with a reduced incidence of coronary disease in carriers. Furthermore, recombinant apolipoprotein A-I(Milano) has displayed remarkable atheroprotective activities and the possibility of directly reducing the burden of atherosclerosis in experimental models. This review is aimed at providing an update on the experimental studies in which apolipoprotein A-I(Milano), produced as a recombinant protein, has displayed important effects in the treatment of vascular diseases.

RECENT FINDINGS

In the past year, two reports have appeared, indicating that a single-dose administration of recombinant apolipoprotein A-I(Milano) dimers formulated into liposomes can reduce atheromas in models such as the apolipoprotein E-deficient mice and a rabbit model of carotid focal lesion, in which a direct 90 min infusion of the product reduced atheroma up to 30%. This finding was associated with an increase in HDL free cholesterol and the permanence of the recombinant product in the lesion for over 72 h.

SUMMARY

Recombinant apolipoprotein A-I(Milano), formulated as synthetic HDL with phospholipids, appears to exert a direct removing effect on arterial cholesterol. This is well evident in experimental animals and, more recently in clinical findings, as indicated by a dramatic increase in HDL free cholesterol after the infusion of different doses of the agent. As the product appears to be well tolerated and non-immunogenic, ongoing phase II studies in patients are being awaited with interest to obtain a 'proof of principle' for 'HDL therapy'.

Atherosclerotic plaque characterization by quantitative analysis using intravascular ultrasound: correlation with histological and immunohistochemical findings

The aim of this study was to clarify whether atherosclerotic plaque morphology, as defined by quantitative analysis with intravascular ultrasound (IVUS) images, was related to the immunohistochemical findings. Twenty-five coronary lesions in 25 patients who had ultrasound guidance during directional coronary atherectomy (DCA) were enrolled. The lesions retrieved by DCA were analyzed and divided into 3 groups (lesions infiltrated with both macrophages and lymphocytes: group IML; lesions infiltrated with macrophages but not lymphocytes: group IM; and non-infiltrated lesions: group NI). The mean plaque echo level divided by the mean adventitia echo level (MPEL/MAEL) and the heterogeneity of the distribution of plaque echo levels (HDPEL) were calculated. The proportion of patients with acute coronary syndromes was significantly different among the groups: IML (n=14), IM (n=5), and NI (71%, 0% and 17%, respectively; p<0.01). The pre-DCA HDPEL value was highest in group IML and lowest in group NI; however, no significant differences in MPEL/MAEL values were found. The results suggest that plaque morphology, as defined by IVUS images, was related to the immunohistochemical findings. The increase in HDPEL correlated with the presence of immune inflammation.

What has intravascular ultrasound taught us about plaque biology?

Intravascular ultrasound (IVUS) has a defined role in the cardiac catheterization laboratory to assess lesion severity and the procedural success of vascular interventions. However, IVUS has also contributed to our understanding of the biology of atherosclerosis and restenosis. In acute coronary syndromes, IVUS has revealed varying degrees of stenosis, thrombosis, and plaque derangement typical of the plaque disruption seen in many pathologic studies of patients who have died of this condition. IVUS has demonstrated that the culprit lesions of patients surviving acute coronary syndromes also tend to be softer, with less calcium, and tend to have more plaque with positive arterial remodeling (compensatory enlargement) than lesions causing stable coronary syndromes. Arterial remodeling is also an important component of restenosis after coronary interventions. IVUS has suggested that interventions that reduce restenosis tend to have a greater impact on preventing negative remodeling (constriction) rather than reducing neointimal proliferation. Oxidant stress may be an important contributor to negative remodeling, as IVUS has demonstrated this anatomy at sites of coronary artery spasm. Positive remodeling seen by IVUS is also associated with impaired endothelial vasomotor dysfunction, and IVUS studies have demonstrated the contribution of vasomotor tone to arterial elasticity. Future directions include integrating IVUS with other imaging modalities, such as angiography, to study the interaction of anatomic and physiologic factors in atherosclerosis progression, and using the raw ultrasound signal to distinguish plaque components and differences in wall strain that may identify vulnerable plaques.

Impact of coronary plaque morphology as assessed by IVUS computer-aided analysis on mechanisms of balloon angioplasty and stenting

This study was performed in order to quantitate structural coronary plaque modifications after balloon angioplasty and stenting and to evaluate the impact of plaque morphology on the mechanisms of lumen enlargement during angioplasty. Plaque morphology was studied by computer-aided analysis of 60 cross-sectional intravascular ultrasound (IVUS) images of the target lesion in 20 patients undergoing percutaneous coronary angioplasty. Based on a computer-aided video densitometry classification of plaque morphology, three groups of plaques were defined based on the slope value of a fifth polynomial regression of the plaque gray-level distribution. In groups A and B, balloon angioplasty provided significant increases in lumen area (P < 0.0001) and vessel area (P < 0.05) without a reduction in plaque area; neither parameter increased in group C. In group A, stenting was associated with an additional lumen enlargement (P < 0.0001) due to plaque reduction (P < 0.05). In groups B and C, stenting further increased lumen area (P < 0.0001) by improving vessel area (P < 0.001) but without plaque reduction. Balloon angioplasty and stenting provided a significant decrease in plaque area in group A as compared to groups B (P < 0.05) and C (P < 0.01). Finally, vessel area improvement was greater in group B than in groups A (P < 0.01) and C (P < 0.05). The mechanisms underlying lumen enlargement after coronary angioplasty are highly dependent on plaque morphology as defined by an IVUS computer-aided analysis and may differ between balloon angioplasty and stenting.

Detection of fibrous cap in atherosclerotic plaque by intravascular ultrasound by use of color mapping of angle-dependent echo-intensity variation

BACKGROUND

The thickness of the fibrous cap is a major determinant in the vulnerability of atherosclerotic plaque to rupture. It has been demonstrated that intravascular ultrasound (IVUS) backscatter from fibrous tissue is strongly dependent on the ultrasound beam angle of incidence. This study investigated the feasibility of using a new IVUS color mapping technique representing the angle-dependent echo-intensity variation to determine the thickness of the fibrous cap in atherosclerotic plaque.

METHODS AND RESULTS

Nineteen formalin-fixed noncalcified human atherosclerotic plaques from necropsy were imaged in vitro with a 30-MHz IVUS catheter. The IVUS catheter was moved coaxially relative to the plaque. The images showing maximum and minimum echo intensity of the plaque surface were selected to calculate the angle-dependent echo-intensity variation. A colorized representation of the echo-intensity variation in the plaque was obtained from the 2 IVUS images. A clearly bordered area with large variation in echo intensity was revealed for each plaque surface in the colorized IVUS image. The thickness (x, mm) of this area correlated significantly with that of fibrous cap (y, mm) measured from histologically prepared sections as y=1.05x-0.01 (r=0.81, P:<0.0001). Bland-Altman analysis also supported the reliability of this method (mean difference, 0.00+/-0.10 mm).

CONCLUSIONS

This novel technique for color mapping the echo-intensity variation in IVUS provided an accurate representation of the thickness of the fibrous cap in atherosclerotic plaque. This method may be useful in assessing plaque vulnerability to rupture in atherosclerosis.

Residual plaque burden, delivered dose, and tissue composition predict 6-month outcome after balloon angioplasty and beta-radiation therapy

BACKGROUND

Inhomogeneity of dose distribution and anatomic aspects of the atherosclerotic plaque may influence the outcome of irradiated lesions after balloon angioplasty (BA). We evaluated the influence of delivered dose and morphological characteristics of coronary stenoses treated with beta-radiation after BA.

METHODS AND RESULTS

Eighteen consecutive patients treated according to the Beta Energy Restenosis Trial 1.5 were included in the study. The site of angioplasty was irradiated with the use of a beta-emitting (90)Sr/(90)Y source. With the side branches used as anatomic landmarks, the irradiated area was identified and volumetric assessment was performed by 3D intracoronary ultrasound imaging after treatment and at 6 months. The type of tissue, the presence of dissection, and the vessel volumes were assessed every 2 mm within the irradiated area. The minimal dose absorbed by 90% of the adventitial volume (D(v90)Adv) was calculated in each 2-mm segment. Diffuse calcified subsegments and those containing side branches were excluded. Two hundred six coronary subsegments were studied. Of those, 55 were defined as soft, 129 as hard, and 22 as normal/intimal thickening. Plaque volume showed less increase in hard segments as compared with soft and normal/intimal thickening segments (P<0.0001). D(v90)Adv was associated with plaque volume at follow-up after a polynomial equation with linear and nonlinear components (r = 0.71; P = 0.0001). The multivariate regression analysis identified the independent predictors of the plaque volume at follow-up: plaque volume after treatment, D(v90)Adv, and type of plaque.

CONCLUSIONS

Residual plaque burden, delivered dose, and tiss composition play a fundamental role in the volumetric outcome at 6-month follow-up after beta-radiation therapy and BA.

Three-dimensional reconstruction of the coronary artery wall by image fusion of intravascular ultrasound and bi-plane angiography

BACKGROUND

Intravascular ultrasound (IVUS) is becoming increasingly accepted for assessing coronary anatomy. However, its utility in visualizing and quantifying coronary morphology has been limited by its 2D tomographic nature. This study presents a 3D reconstruction technique that accurately preserves 3D geometric information.

METHODS AND RESULTS

Images obtained from manual IVUS pullbacks and continuous bi-plane angiography were fused, using angiography to reconstruct the transducer trajectory and aid in solving for the correct rotational orientation. A novel 3D active surface method automatically identified the luminal and medial-adventitial borders which, when superimposed on the transducer trajectory, could be surface-rendered for visualization and morphometry. Segmentation agreed well with manual assessment, and 3D luminal shape matched that of angiography when projected to 2D.

CONCLUSIONS

We conclude that this method provides an accurate reconstruction of the vessel's anatomy, which accounts for the true curvature of the vessel.

Flow estimation using an intravascular imaging catheter

Coronary flow assessment can be useful for determining the hemodynamic severity of a stenosis and to evaluate the outcome of interventional therapy. We developed a method for measuring the transverse flow through the imaging plane of an intravascular ultrasound (IVUS) catheter. This possibility has raised great clinical interest since it permits simultaneous assessment of vessel geometry and function with the same device. Furthermore, it should give more accurate information than combination devices because lumen diameter and velocity are determined at the same location. Flow velocity is estimated based on decorrelation estimation from sequences of radiofrequency (RF) traces acquired at nearly the same position. Signal gating yields a local estimate of the velocity. Integrating the local velocity over the lumen gives the quantitative flow. This principle has been calibrated and tested through computer modeling, in vitro experiments using a flow phantom and in vivo experiments in a porcine animal model, and validated against a Doppler element containing guide wire (Flowire) in humans. Originally the method was developed and tested for a rotating single element device. Currently the method is being developed for an array system. The great advantage of an array over the single element approach would be that the transducer has no intrinsic motion. This intrinsic motion sets a minimal threshold in the detectable velocity components. Although the principle is the same, the method needs some adaptation through the inherent different beamforming of the transducer. In this paper various aspects of the development of IVUS flow are reviewed.

Intraluminal ultrasonic palpation: assessment of local and cross-sectional tissue stiffness

Many intravascular therapeutic techniques for the treatment of significant atherosclerotic lesions are mechanical in nature: examples are angioplasty, stenting and atherectomy. The selection of the most adequate treatment would be advantageously aided by knowledge of the mechanical properties of the lesion and surrounding tissues. Based on the success of intravascular ultrasound (IVUS) in accurately depicting the morphology of atheromatous lesions, ultrasonic tissue characterisation has been proposed as a tool to determine the composition of atheroma. We describe the addition of local compliance information to the IVUS image in the form of a colour-coded line congruent with the lumen perimeter. The technique involves analysis of echo signals obtained at two or more states of incremental intravascular pressure. Using vessel phantoms and specimens, we demonstrate the utility of intravascular compliance imaging. The palpograms are able to identify lesions of different elasticity independently of the echogenicity contrast, because the information provided by the elastograms is generally independent of that obtained from the IVUS image. Thus, the palpogram can complement the characterisation of lesion from the IVUS image. We also describe cross-sectional measures of elasticity that are based on the elastogram. Finally, natural extensions of intravascular palpation to other endoluminal ultrasound applications are proposed.

Serial automated three-dimensional intravascular ultrasound analysis of the self-expanding Radius stent

Automated 3-dimensional intravascular ultrasound (IVUS) analysis was used to assess status of the treated coronary artery immediately and 6 months after placement of a self-expanding Radius stent in 15 patients. Serial 3-dimensional IVUS analysis demonstrated gradual stent expansion that countered neointimal proliferation and preserved the lumen.

Classification of arterial plaque by spectral analysis of in vitro radio frequency intravascular ultrasound data

To test whether radio-frequency analysis of coronary plaques predicts the histological classification, r.f. data were collected using a 30 MHz intravascular ultrasound scanner. Two hundred ninety-nine regions-of-interest from eight postmortem coronary arteries were selected and identified by histology as falling into one of seven different tissue types. These are loose fibrous tissue (n = 78), moderate fibrous tissue (n = 27), dense fibrous tissue (n = 33), microcalcification (n = 14), calcified plaque (n = 55), lipid/fibrous mixture (n = 51) and homogeneous areas of lipid pool (n = 29). On the basis of a previous study, four spectral parameters were calculated for each of the regions-of-interest: maximum power (dB), mean power (dB), spectral slope (dB/MHz) over the bandwidth 18-35 MHz and the intercept of the spectral slope with the 0 Hz axis (dB). A minimum-distance classifier using the Mahalanobis (1948) distance was applied to the data. Following resubstitution of the training data into the classifier, the total correctly classified was 54%. The data were reclassified using three broader tissue groups: (1) calcified plaque, (2) lipid pool and (3) a mixed fibrous category, incorporating loose fibrous tissue, moderate fibrous tissue, dense fibrous tissue, lipid/fibrous mixture and microcalcification. The total correctly classified was 86%. Using "leave-one-out" cross-validation, the classification rates were 48% for seven tissue subgroups and 83% for three broader categories of tissue type.

Advances in ultrasound biomicroscopy

The visualisation of living tissues at microscopic resolution is attracting attention in several fields. In medicine, the goals are to image healthy and diseased tissue with the aim of providing information previously only available from biopsy samples. In basic biology, the goal may be to image biological models of human disease or to conduct longitudinal studies of small-animal development. High-frequency ultrasonic imaging (ultrasound biomicroscopy) offers unique advantages for these applications. In this paper, the development of ultrasound biomicroscopy is reviewed. Aspects of transducer development, systems design and tissue properties are presented to provide a foundation for medical and biological applications. The majority of applications appear to be developing in the 40-60-MHz frequency range, where resolution on the order of 50 microm can be achieved. Doppler processing in this frequency range is beginning to emerge and some examples of current achievements will be highlighted. The current state of the art is reviewed for medical applications in ophthalmology, intravascular ultrasound, dermatology, and cartilage imaging. Ultrasound biomicroscopic studies of mouse embryonic development and tumour biology are presented. Speculation on the continuing evolution of ultrasound biomicroscopy will be discussed.

Remodeling of atherosclerotic coronary arteries varies in relation to location and composition of plaque

The aim of this study was to determine the contribution of morphologic characteristics and location of plaque in remodeling of atherosclerotic coronary arteries. Consecutive intravascular ultrasound studies performed in native coronary arteries before an intervention were included in the study. Total vessel, lumen and plaque + media areas were measured at target lesion, and distal and proximal references. Remodeling index was calculated as target total vessel area/proximal reference total vessel area, and categorized into 3 groups based on relative total vessel-area ratio: (1) > 1.1 (group A, adequate remodeling); (2) 0.9 to 1.1 (group B, failure of compensatory enlargement); and (3) <0.9 (group C, coronary shrinkage). Eighty-nine narrowings were assessed in 80 intravascular ultrasound studies. Thirty-eight lesions (43%) were defined as soft and 51 (57%) as hard. Soft plaques were more prevalent in group A than in groups B and C (p = 0.001). Conversely, the arc of calcium was larger in group C lesions (p = 0.005). At distal segments, group A lesions were more prevalent than those in groups B and C, whereas at proximal segments group C lesions were more prevalent (p = 0.007). Multivariate analysis identified the arc of calcium and the location of plaque at distal segments as independent predictors of compensatory enlargement (odds ratio 0.94, 95% confidence interval 0.90 to 0.99; odds ratio 4.6; 95% confidence interval 1.4 to 15.7, respectively), whereas hard plaques were an independent predictor of coronary shrinkage (odds ratio 4.6; 95% confidence interval 1.7 to 12.5). In conclusion, composition and location of plaque appeared to be major determinants of vessel remodeling during the process of atherosclerosis.

Comparison of normal and diseased pulmonary artery morphology by intravascular ultrasound and histological examination

The study was performed to determine the morphological characteristics of normal and diseased pulmonary arteries by ultrasound (intravascular ultrasound, IVUS) and histology. Forty-nine cadaver segments of pulmonary arteries from 16 postmortem patients were imaged in vitro by IVUS and compared to matched histological sections. The pulmonary vasculature of 11 patients with pulmonary hypertension was investigated in vivo by IVUS. In the in vitro study, 34 of a total of 143 histological sections of the segmental pulmonary arteries showed fibrotic wall components; the remaining 109 sections had regular components. Imaged by IVUS, the wall of the regular and fibrotic arteries revealed a single layer histologically, representing the intima, media, adventitia and connective tissue. Adjacent lung tissue could be detected by IVUS and was confirmed by the histological section. In three patients with a history of chronic embolic pulmonary hypertension, IVUS revealed thrombi or a double vessel wall layer. Histologically, the material extracted after pulmonary thromboendarterectomy in two patients showed wall-adherent thrombus features of early organization. In all other patients only a single wall layer was seen. The vessel walls (intima, media, adventitia and connective tissue) of regular and slightly fibrosed pulmonary arteries show only a single layer. Wall-adherent organized thrombi in chronic embolic pulmonary hypertension can be detected by IVUS as a second inner vessel layer. Thus IVUS may represent an additional tool for detecting chronic embolic pulmonary hypertension when the results of pulmonary angiography or computed tomography are not definite.

Screening of ruptured plaques in patients with coronary artery disease by intravascular ultrasound

AIM

To visualise the characteristics of ruptured plaques by intravascular ultrasound (IVUS) and to correlate plaque characteristics with clinical symptoms to establish a quantitative index of plaque vulnerability.

METHODS

144 consecutive patients with angina were examined using IVUS. Ruptured plaques, characterised by a plaque cavity and a tear on the thin fibrous cap, were identified in 31 patients (group A), of whom 23 (74%) presented with unstable angina. Plaque rupture was confirmed by injecting contrast medium filling the plaque cavity during IVUS examination. Of the patients without plaque rupture (group B, n = 108), only 19 (18%) had unstable angina.

RESULTS

No significant differences were found between groups A and B in relation to plaque and vessel area (p > 0.05). Mean (SD) per cent stenosis in group A was less than in group B, at 56.2 (16.5)% v 67.9 (13.4)%; p < 0.001. Area of the emptied plaque cavity in group A (4.1 (3.2) mm2) was larger than the echolucent zone in group B (1.32 (0.79) mm2) (p < 0.001). The plaque cavity to plaque ratio in group A (38.5 (17.1)%) was larger than the echolucent area to plaque ratio in group B (11.2 (8.9)%) (p < 0.001). The thickness of the fibrous cap in group A was less than in group B, at 0.47 (0.20) mm v 0.96 (0.94) mm; p < 0.001.

CONCLUSIONS

Plaques seem to be prone to rupture when the echolucent area is larger than 4.1 (3.2) mm2, when the echolucent area to plaque ratio is greater than 38.5 (17.1)%, and when the fibrous cap is thinner than 0.7 mm. IVUS can identify plaque rupture and vulnerable plaques. This may influence patient management and treatment.

High-resolution intravascular magnetic resonance imaging: monitoring of plaque formation in heritable hyperlipidemic rabbits

BACKGROUND

The individual makeup of atherosclerotic plaque has been identified as a dominant prognostic factor. With the use of an intravascular magnetic resonance (MR) catheter coil, we evaluated the effectiveness of high-resolution MR in the study of the development of atherosclerotic lesions in heritable hyperlipidemic rabbits.

METHODS AND RESULTS

Sixteen hyperlipidemic rabbits were investigated at the ages of 6, 12, 24, and 36 months. The aorta was studied with digital subtraction angiography and high-resolution MR with the use of a surface coil and an intravascular coil that consisted of a single-loop copper wire integrated in a 5F balloon catheter. Images were correlated with histological sections regarding wall thickness, plaque area, and plaque components. Digital subtraction angiography revealed no abnormalities in the 6- and 12-month-old rabbits and only mild stenoses in the 24- and 36-month-old rabbits. High-resolution imaging with surface coils resulted in an in-plane resolution of 234x468 microm. Delineation of the vessel wall was not possible in younger rabbits and correlated only poorly with microscopic measurements in the 36-month-old rabbits. Intravascular images achieved an in-plane resolution of 117x156 microm. Increasing thickness of the aortic wall and plaque area was observed with increasing age. In the 24- and 36-month-old animals, calcification could be differentiated from fibrous and fatty tissue on the basis of the T2-fast spin echo images, as confirmed by histological correlation.

CONCLUSIONS

Atherosclerotic evolution of hyperlipidemic rabbits can be monitored with high-resolution intravascular MR imaging. Image quality is sufficient to determine wall thickness and plaque area and to differentiate plaque components.

In-vitro validation, with histology, of intravascular ultrasound in renal arteries

OBJECTIVE

To investigate the feasibility of using intravascular ultrasound to characterize normal and diseased renal arteries.

MATERIALS AND METHODS

Forty-four renal artery specimens from 21 humans, removed at autopsy, were studied with intravascular ultrasound in vitro. From each vascular specimen, two to four sets of corresponding intravascular ultrasound images and histologic sections were subjected to qualitative analysis. The renal arterial wall was considered normal by intravascular ultrasound when the wall thickness (intima and media) was 0.5 mm or less. On intravascular ultrasound imaging, a distinction was made between bright lesions with or without peripheral shadowing (i.e. calcification). Histological sections were examined and fibromuscular lesions were scored with or without calcifications. Quantitative analysis of a multitude of intravascular ultrasound cross-sections (interval 5 mm) included assessment of the lumen area, vessel area, plaque area and percentage area obstructed. The target site (smallest lumen area) was compared with a reference site (largest lumen area before the first major side branch).

RESULTS

Of the 130 corresponding intravascular ultrasound images and histologic sections analysed, 55 were normal and 75 presented a bright lesion on ultrasound; in 31 lesions, peripheral shadowing was involved. The sensitivity of the intravascular ultrasound in detecting calcifications was 87%, and the specificity was 89%. Lumen area reduction at the target site was associated with vessel and plaque area enlargement in eight specimens, with plaque area enlargement in 12 specimens and with a vessel area reduction in 21 specimens.

CONCLUSIONS

Intravascular ultrasound is a reliable technique for distinguishing renal arteries with or without a lesion. Both plaque development and local vessel narrowing may result in renal artery stenosis.

Role of intravascular ultrasound imaging in identifying vulnerable plaques

A plaque that has a large lipid core and a thin fibrous cap may undergo rupture. Once it ruptures, it may lead to thrombus formation and subsequent vessel occlusion. To identify unstable plaques before they rupture is essential for clinical management and patient's prognosis. Intravascular ultrasound (IVUS) opens a new window for the assessment of plaque morphology to identify vulnerable plaques and plaque rupture. We examined 144 patients with angina and ischemic ECG changes using IVUS. Ruptured plaques, characterized by a plaque cavity and a tear on the thin fibrous cap, were identified in 31 patients (group A) of which 23/31 (74%) clinically presented as unstable angina. Plaque rupture was confirmed by injecting contrast medium filling the plaque cavity during IVUS examination. Of the patients without plaque rupture (group B, n = 108), only 19 (18%) had unstable angina. No significant differences between the 2 groups were found concerning the vessel and plaque areas (p > 0.05). The percent stenosis in group A (56.2 +/- 16.5%) was significantly lower than in group B (67.9 +/- 13.4%) (p < 0.001). Area of the plaque cavity in group A (4.1 +/- 3.2 mm2) was significantly larger than the echolucent zone in group B (1.32 +/- 0.79 mm2) (p < 0.001). The plaque cavity/plaque ratio in group A (38.5 +/- 17.1%) was larger than the echolucent area/plaque ratio in group B (11.2 +/- 8.9%) (p < 0.001). The thickness of the fibrous cap in group A (0.47 +/- 0.20 mm) was significantly thinner than that (0.96 +/- 0.94 mm) in group B (p < 0.001).

CONCLUSIONS

Plaques seem to be prone to rupture when the echolucent area is larger than 1 mm2, the echolucent area/plaque ratio greater than 20% and the fibrous cap thinner than 0.7 mm. IVUS has the capacity of identifying plaque rupture and vulnerable plaques. This may have potential influence on patients management and therapy.

Tissue characterization in intravascular ultrasound images

Intravascular ultrasound (IVUS) imaging permits direct visualization of vascular pathology. It has been used to evaluate lumen and plaque in coronary arteries and its clinical significance for guidance of coronary interventions is increasingly recognized. Conventional manual evaluation is tedious and time-consuming. This paper describes a highly automated approach to segmentation of coronary wall and plaque, and determination of plaque composition in individual IVUS images and pullback image sequences. The determined regions of plaque were classified in one of three classes: soft plaque, hard plaque, or hard plaque shadow. The method's performance was assessed in vitro and in vivo in comparison with observer-defined independent standards. In the analyzed images and image sequences, the mean border positioning error of the wall and plaque borders ranged from 0.13-0.17 mm. Plaque classification correctness was 90%.