Characterisation of coronary atherosclerotic morphology by spectral analysis of radiofrequency signal: in vitro intravascular ultrasound study with histological and radiological validation

Assessment and characterization of time-related differences in plaque composition by intravascular ultrasound-derived radiofrequency analysis in heart transplant recipients

BACKGROUND

The survival of heart transplant patients is limited by cardiac allograft vasculopathy (CAV). Intravascular ultrasound (IVUS) and IVUS-derived radiofrequency plaque composition analysis (IVUS-RF) provide further information about the process of coronary atherosclerosis.

METHODS

In this study we aimed to assess the time-dependent differences in disease progression in patients with CAV. Fifty-six patients were divided into three groups according to time interval after transplantation (Group I: 1 to 3 months, 18 patients; Group II, 1 to 5 years, 20 patients; Group III: 5 to 15 years, 18 patients).

RESULTS

IVUS-RF revealed time-dependent increases in all plaque components. The largest increase was shown for fibrotic, fibrofatty and necrotic tissue between Groups I and II. Dense calcium area increased uniformly in all groups. IVUS-RF-derived plaque type analysis revealed predominantly fibrotic plaques in all groups with a decrease of frequency over time. Fibrolipidic and fibrotic-calcific plaques increased uniformly. High-risk lesions, such as thick-cap fibroatheromas (FAs), increased in Groups I and II and decreased in Group III. Thin-cap FAs were detected only in Group III.

CONCLUSIONS

IVUS-RF, as compared with gray-scale IVUS, provides better detailed information about the development of CAV by plaque morphology and composition analysis in different stages after heart transplantation. Serial IVUS-RF analysis in these patients may improve the stratification of heart transplant recipients.

Impact of intramural thrombus in coronary arteries on the accuracy of tissue characterization by in vivo intravascular ultrasound radiofrequency data analysis

Virtual Histology (VH) intravascular ultrasound (IVUS) allows differentiation between 4 different tissue phenotypes. However, the current classification tree for analysis cannot differentiate the presence of intramural thrombus. The aim of this study was to evaluate the impact of intramural thrombus for correlative accuracy between in vitro histopathology of coronary atherosclerotic plaque obtained by directional coronary atherectomy and corresponding in vivo tissue characterization obtained by VH IVUS. Coronary IVUS imaging of 30 coronary artery lesions was obtained using a 20-MHz phased-array IVUS catheter with a motorized pull-back system at set 0.5 mm/s. The debulking region of the in vivo histologic image was predicted from comparison between pre- and post-first debulking VH IVUS images. Cross-sectional histologic slices were cut every 0.5 mm starting from the most proximal part of the formalin-fixed debulking tissue. Histologic slices were divided into 2 groups by the presence or absence of pathologic thrombus. A total of 259 in vitro histologic slices were obtained, and pathologic thrombus was detected in 81 slices. Correlation was favorable, with high sensitivity for all plaque components, but specificities for fibrous (thrombus slices vs nonthrombus slices 36% vs 94%) and fibrofatty (9% vs 60%) tissue were lower in thrombus slices. Therefore, predictive accuracies for the 2 plaque components were lower in thrombus slices (fibrous tissue 78% vs 99%, fibrofatty tissue 68% vs 83%, respectively). In conclusion, intramural thrombus was colored as fibrous or fibrofatty by VH IVUS, reducing VH accuracy in these kinds of lesions.

Changes in unstable coronary atherosclerotic plaque composition after balloon angioplasty as determined by analysis of intravascular ultrasound radiofrequency

The effects of balloon angioplasty (BA) on plaque distribution remain incompletely documented. In 20 patients with unstable angina pectoris, intravascular ultrasound gray scale and radiofrequency analyses were performed before and after BA. Composition of the plaque was 61% fibrotic tissue, 15% fibrofatty tissue, 15% necrotic tissue, and 7% dense calcium tissue. After BA, 35% of lumen enlargement was due to an increase in total vessel area and 65% to a significant decrease in plaque area. This resulted from a longitudinal redistribution of the tissue toward the reference segments. Radiofrequency analysis showed that the fibrous and fibrofatty tissues were able to redistribute longitudinally, whereas calcium remained at the same level. A third of necrotic tissue was lost after BA. In conclusion, in unstable plaques, BA resulted in a longitudinal redistribution of fibrotic and fibrofatty tissues and disappearance of 1/3 of necrotic tissue.

Virtual histology

As a luminogram, coronary angiography provides a good overview of the coronary artery tree. Using quantitative coronary measurements, the degree of coronary obstruction can be determined. The limitation of coronary angiography is that it does not provide information on the arterial wall structure and therefore cannot assess the extent of atherosclerosis. Knowledge about adaptive coronary remodelling processes as compensatory enlargement of the coronary artery has focused diagnostic interest on the non-stenotic lesions of the coronary tree. Intravascular ultrasound (IVUS) can reveal discrepancies between the extent of coronary atherosclerosis and angiography imaging by in vivo plaque imaging. Spectrum analysis of IVUS-derived radiofrequency (RF) data enables a more detailed analysis of plaque composition and morphology. Preliminary in vitro studies correlated four histological plaque components with a specific spectrum analysis of the RF data. The different components (fibrous, fibrofatty, necrotic core and dense calcium) are colour coded. Coronary tissue maps were reconstructed from RF data using IVUS-Virtual Histology (VH IVUS) software (Real-Time VH, Volcano Corporation, Rancho Cordova, California, USA). VH IVUS has the potential to detect high-risk lesions and can provide new insights into the pathophysiology of coronary artery disease. VH IVUS allows the differentiation of different lesion types based on information derived from histopathology. The in vivo specific histological analysis of coronary atherosclerosis may allow better stratification of treatment of patients with coronary artery disease.

In vivo plaque characterization using intravascular ultrasound-virtual histology in a porcine model of complex coronary lesions

OBJECTIVE

To determine the accuracy of detection of different tissue types of intravascular ultrasound-virtual histology (IVUS-VH) in a porcine model of complex coronary lesions.

METHODS AND RESULTS

Coronary lesions were induced by injecting liposomes containing human oxidized low-density lipoprotein into the adventitia of the arteries. IVUS-VH imaging was performed in vivo at 8.2+/-1.6 weeks after injection. A total of 60 vascular lesions were analyzed and compared with their correspondent IVUS-VH images. Correlation analysis was performed using linear regression models. Compared with histology, IVUS-VH correctly identified the presence of fibrous, fibro-fatty, and necrotic tissue in 58.33%, 38.33%, and 38.33% of lesions, respectively. The sensitivity of IVUS-VH for the detection of fibrous, fibro-fatty, and necrotic core tissue was 76.1%, 46%, and 41.1% respectively. A linear regression analysis performed for each individual plaque component did not show strong correlation that would allow significant prediction of individual values.

CONCLUSIONS

In a porcine model of complex coronary lesions, IVUS-VH was not accurate in detecting the relative amount of specific plaque components within each individual corresponding histological specimen.

Improved visualization of high-intensity focused ultrasound lesions

Spectral parameter imaging in both the fundamental and harmonic of backscattered radio-frequency (RF) data were used for immediate visualization of high-intensity focused ultrasound (HIFU) lesion sites. A focused 5-MHz HIFU transducer with a coaxial 9-MHz focused single-element diagnostic transducer was used to create and scan lesions in chicken breast and freshly excised rabbit liver. B-mode images derived from the backscattered RF signal envelope were compared with midband fit (MBF) spectral parameter images in the fundamental (9-MHz) and harmonic (18-MHz) bands of the diagnostic probe. Images of HIFU-induced lesions derived from the MBF to the calibrated spectrum showed improved contrast (approximately 3 dB) of tumor margins versus surround compared with images produced from the conventional signal envelope. MBF parameter images produced from the harmonic band showed higher contrast in attenuated structures (core, shadow) compared with either the conventional envelope (3.3 dB core; 11.6 dB shadow) or MBF images of the fundamental band (4.4 dB core; 7.4 dB shadow). The gradient between the lesion and surround was 3.4 dB/mm, 6.9 dB/mm and 17.2 dB/mm for B-mode, MBF-fundamental mode and MBF-harmonic mode, respectively. Images of threshold and "popcorn" lesions produced in freshly excised rabbit liver were most easily visualized and boundaries best-defined using MBF-harmonic mode.

A hypothesis for vulnerable plaque rupture due to stress-induced debonding around cellular microcalcifications in thin fibrous caps

In this article, we advance a hypothesis for the rupture of thin fibrous cap atheroma, namely that minute (10-mum-diameter) cellular-level microcalcifications in the cap, which heretofore have gone undetected because they lie below the visibility of current in vivo imaging techniques, cause local stress concentrations that lead to interfacial debonding. New theoretical solutions are presented for the local stress concentration around these minute spherical inclusions that predict a nearly 2-fold increase in interfacial stress that is relatively insensitive to the location of the hypothesized microinclusions in the cap. To experimentally confirm the existence of the hypothesized cellular-level microcalcifications, we examined autopsy specimens of coronary atheromatous lesions using in vitro imaging techniques whose resolution far exceeds conventional magnetic resonance imaging, intravascular ultrasound, and optical coherence tomography approaches. These high-resolution imaging modalities, which include confocal microscopy with calcium-specific staining and micro-computed tomography imaging, provide images of cellular-level calcifications within the cap proper. As anticipated, the minute inclusions in the cap are very rare compared with the numerous calcified macrophages observed in the necrotic core. Our mathematical model predicts that inclusions located in an area of high circumferential stress (>300 kPa) in the cap can intensify this stress to nearly 600 kPa when the cap thickness is <65 microm. The most likely candidates for the inclusions are either calcified macrophages or smooth muscle cells that have undergone apoptosis.

Accuracy of In Vivo Coronary Plaque Morphology Assessment

OBJECTIVES

The goal of the present study was to compare the accuracy of in vivo tissue characterization obtained by intravascular ultrasound (IVUS) radiofrequency (RF) data analysis, known as Virtual Histology (VH), to the in vitro histopathology of coronary atherosclerotic plaques obtained by directional coronary atherectomy.

BACKGROUND

Vulnerable plaque leading to acute coronary syndrome (ACS) has been associated with specific plaque composition, and its characterization is an important clinical focus.

METHODS

Virtual histology IVUS images were performed before and after a single debulking cut using directional coronary atherectomy. Debulking region of in vivo histology image was predicted by comparing pre- and post-debulking VH images. Analysis of VH images with the corresponding tissue cross section was performed.

RESULTS

Fifteen stable angina pectoris (AP) and 15 ACS patients were enrolled. The results of IVUS RF data analysis correlated well with histopathologic examination (predictive accuracy from all patients data: 87.1% for fibrous, 87.1% for fibro-fatty, 88.3% for necrotic core, and 96.5% for dense calcium regions, respectively). In addition, the frequency of necrotic core was significantly higher in the ACS group than in the stable AP group (in vitro histopathology: 22.6% vs. 12.6%, p = 0.02; in vivo virtual histology: 24.5% vs. 10.4%, p = 0.002).

CONCLUSIONS

Correlation of in vivo IVUS RF data analysis with histopathology shows a high accuracy. In vivo IVUS RF data analysis is a useful modality for the classification of different types of coronary components, and may play an important role in the detection of vulnerable plaque.

Coronary plaque composition of nonculprit lesions, assessed by in vivo intracoronary ultrasound radio frequency data analysis, is related to clinical presentation

BACKGROUND

Identification of subclinical high-risk plaques is potentially important because they may have greater likelihood of rupture and subsequent thrombosis. The purpose of this study was to assess the relationship between plaque composition determined by intravascular ultrasound (IVUS) radio frequency (RF) data analysis and clinical presentation.

METHODS

In 55 patients, a nonculprit vessel with < 50% diameter stenosis was studied with IVUS. Tissue maps were reconstructed from RF data using IVUS-Virtual Histology software.

RESULTS

Mean percentage of the different plaque components were 0.99% +/- 0.9%, calcium; 68.04% +/- 9.8%, fibrous; 19.31% +/- 7.3%, fibrolipidic; and 9.43% +/- 6.6%, lipid core. Mean lipid core percentage was significantly larger in patients with acute coronary syndrome (ACS) when compared with stable patients (12.26% +/- 7.0% vs 7.40% +/- 5.5%, P = .006). In addition, stable patients showed more fibrotic vessels (70.97% +/- 9.3% vs 63.96% +/- 9.1%, P = .007). There was no significant difference for either mean calcium (1.20% +/- 1.1% vs 0.83% +/- 0.7%, P = .124) or fibrolipidic (20.57% +/- 6.9% vs 18.40% +/- 7.6%, P = .281) percentages in ACS and stable patients, respectively. Vessel area obstruction did not differ between groups (46.49% +/- 10.9% vs 42.83% +/- 11.8%, P = .221). There was a significant, albeit weak, positive correlation between lipid core percentage and stenosis severity as determined by vessel area obstruction (r = 0.34, P = .015).

CONCLUSIONS

In this study, plaque characterization of nonculprit vessels using spectral analysis of IVUS RF data analysis was significantly related to clinical presentation. Percentage of lipid core, a feature related to acute coronary events and worse prognosis, was significantly larger in patients with ACS. Conversely, stable patients showed more fibrotic content.

A general solution for catheter position effects for strain estimation in intravascular elastography

Intravascular ultrasound (US) elastography reveals the elastic properties of vascular tissue and plaque. However, misalignment of the US catheter in the vessel lumen can cause incorrect strain estimation in intravascular US elastography caused by strain projection artifacts. In this paper, we present a general theoretical solution where the impact of catheter eccentricity, tilt and noncoplanar errors on the strain estimates are derived. Appropriate corrections to strain estimates can then be applied with prior knowledge of the catheter position information to reduce the strain projection artifacts. Simulations using a frequency-domain-based algorithm that models intravascular US imaging before and after a specified deformation are presented. The simulations are used to verify the theoretical derivations for two displacement situations (linear and nonlinear) under intraluminal pressure, with and without stress decay. The linear displacement case demonstrates that the correction factor is dependent only on the angle between the US beam and the cross-sectional plane of the vessel. For the nonlinear displacement case, where a l/r stress decay in the displacement is modeled, the correction factor becomes a more complicated function of the azimuthal angle.

Methodological considerations and approach to cross-technique comparisons using in vivo coronary plaque characterization based on intravascular ultrasound radiofrequency data analysis: insights from the Integrated Biomarker and Imaging Study (IBIS)

Grey scale intravascular ultrasound (IVUS) is a valuable clinical tool to assess the extent and severity of coronary atheroma. However, it cannot reliably identify plaques with a high-risk of future clinical events. Serial IVUS studies to assess the progression and/or regression of atherosclerotic plaques demonstrated only modest effects, of pharmacological intervention on plaque burden, even when clinical efficacy is documented. Spectral analysis of radiofrequency ultrasound data (IVUS-virtual histology (IVUS-VH), Volcano Therapeutics, Rancho Cordova, CA) has the potential to characterize accurately plaque composition. The Integrated Biomarker and Imaging Study (IBIS) evaluated both invasive and non-invasive imaging techniques along with the assessment of novel biomarkers to characterize sub-clinical atherosclerosis. IVUS-VH was not included at the start of the IBIS protocol. The purpose of this paper is to describe the methodology we used to obtain and analyse IVUS-VH images and the approach to cross-correlations with the other techniques.

Rationale and methods of the integrated biomarker and imaging study (IBIS): combining invasive and non-invasive imaging with biomarkers to detect subclinical atherosclerosis and assess coronary lesion biology

Death or myocardial infarction, the most serious clinical consequences of atherosclerosis, often result from plaque rupture at non-flow limiting lesions. Current diagnostic imaging with coronary angiography only detects large plaques that already impinge on the lumen and cannot accurately identify those that have a propensity to cause unheralded events. Accurate evaluation of the composition or of the biomechanical characteristics of plaques with invasive or non-invasive methods, alone or in conjunction with assessment of circulating biomarkers, could help identify high-risk patients, thus providing the rationale for aggressive treatments in order to reduce future clinical events. The IBIS (Integrated Biomarker and Imaging Study) study is a prospective, single-center, non-randomized, observational study conducted in Rotterdam. The aim of the IBIS study is to evaluate both invasive (quantitative coronary angiography, intravascular ultrasound (IVUS) and palpography) and non-invasive (multislice spiral computed tomography) imaging techniques to characterize non-flow limiting coronary lesions. In addition, multiple classical and novel biomarkers will be measured and their levels correlated with the results of the different imaging techniques. A minimum of 85 patients up to a maximum of 120 patients will be included. This paper describes the study protocol and methodological solutions that have been devised for the purpose of comparisons among several imaging modalities. It outlines the analyses that will be performed to compare invasive and non-invasive imaging techniques in conjunction with multiple biomarkers to characterize non-flow limiting subclinical coronary lesions.

Geometrical validation of intravascular ultrasound radiofrequency data analysis (Virtual Histology) acquired with a 30 MHz boston scientific corporation imaging catheter

Recently, the plaque characterization field was explored with the use of the substrate (frequency domain analysis) rather than the envelope (amplitude or gray-scale imaging) of the intravascular ultrasound (IVUS) radiofrequency data. However, there is no data about the agreement of quantitative outcome between the two methods. The aim of this study was to assess the correlation and agreement between quantitative coronary ultrasound and the geometrical measurements provided by the spectral analysis of ultrasound radiofrequency data [IVUS-Virtual Histology (IVUS-VH), Volcano Therapeutics). Twenty-five patients were included in this study. The IVUS catheter used was a commercially available mechanical sector scanner (Ultracross 2.9 Fr 30 MHz catheter, Boston Scientific) covered with an outer sheath. IVUS-VH significantly underestimated lumen [relative difference (RD)=14.8+/-5.6; P<0.001], vessel (RD=14.1+/-4.8; P<0.001), and plaque (RD=11.5+/-10.8; P<0.001) cross-sectional areas (CSAs). Nevertheless, when adjusted for the ultrasound propagation delay caused by the sheath, relative differences of measurements were remarkably low (0.49%+/-6.3%, P=0.64 for lumen; 2.33%+/-4.6%, P=0.007 for vessel; and 4.2%+/-10.4%, P=0.005 for plaque CSA). These data suggest that the volumetric output of the IVUS-VH software underestimates measurements when acquired with a 30 MHz catheter. However, after applying a mathematical adjustment method for the ultrasound propagation delay caused by the outer sheath of the 30 MHz catheter, relative differences of direct measurements were negligible. These results suggest that ultrasound radiofrequency data analysis could provide, aside from precise compositional data, an accurate geometrical output.

Peripheral application of intravascular ultrasound virtual histology

Intravascular ultrasound (IVUS) provides direct depiction of coronary artery anatomy. Traditional use of this tomographic imaging modality has been in determination of geometric measurements of an artery, such as lumen or plaque size. However, by analyzing the backscattered or radiofrequency (RF) data it is possible to glean information on the composition of plaques. This chapter describes the theory of spectral analysis and its application clinical practice.

Carotid plaque spaces relate to symptoms and ultrasound scattering

We have previously found that spectral analysis of ultrasound (US) can discriminate in vitro plaques from asymptomatic and symptomatic (transient ischemic attack within previous 4 weeks) patients. That study found no differences in percentages of lipid or thrombus between the two groups by optical microscopy/planimetry. The present study was to find out if another feature from the microscopy could show a difference. The number and size of spaces resulting from cell death or new blood vessels were measured to see if they related to symptoms or could help explain US differentiation. Twelve plaques from each group were examined by optical microscopy. The sizes and concentrations of two kinds of spaces, endothelial lined (vessel spaces) and unlined (tissue spaces), were correlated both with symptoms and also with the US tissue characterization scores from the previous study. Symptomatic vs. asymptomatic plaques showed a higher concentration and a larger size: 0.87 vs. 0.21 per mm(2) (p < 0.005) and 154 vs. 110 microm (p < 0.02). A discriminant function of spaces with symptoms as dependent variables correctly identified 91.7% of the plaques (p < 0.001). The concentration in plaques previously designated by US as true positive plaques or true negative was 1.21 vs. 0.22 per mm(2) (p < 0.005). spaces were increased in plaques of symptomatic patients and were related to UTC scores. Both the lined and unlined spaces were useful as predictors.

Regularized autoregressive analysis of intravascular ultrasound backscatter: improvement in spatial accuracy of tissue maps

Autoregressive (AR) models are qualified for analysis of stochastic, short-time data, such as intravascular ultrasound (IVUS) backscatter. Regularization is required for AR analysis of short data lengths with an aim to increase spatial accuracy of predicted plaque composition and was achieved by determining suitable AR orders for short data records. Conventional methods of determining order were compared to the use of trend in the mean square error for determining order. Radio-frequency data from 101 fibrous, 56 fibro-lipidic, 50 calcified, and 70 lipid-core regions of interest (ROIs) were collected ex vivo from 51 human coronary arteries with 30 MHz unfocused IVUS transducers. Spectra were computed for AR model orders between 3-20 for data representing ROIs of two sizes (32 and 16 samples at 100 MHz sampling frequency) and were analyzed in the 17-42 MHz bandwidth. These spectra were characterized based on eight previously identified parameters. Statistical classification schemes were computed from 75% of the data and cross-validated with the remaining 25% using matched histology. The results determined the suitable AR order numbers for the two ROI sizes. Conventional methods of determining order did not perform well. Trend in the mean square error was identified as the most suitable factor for regularization of short record lengths.

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.

Spectral parameter imaging for detection of prognostically significant histologic features in uveal melanoma

Specific extracellular matrix patterns in uveal melanoma are associated with metastatic risk. The laminin-rich composition and dimensions (on the order of a wavelength or less) of these structures suggest that acoustic backscatter might be affected by their presence. In this study, 10-MHz radiofrequency (RF) ultrasound (US) data were acquired before surgical removal of 117 eyes with uveal malignant melanoma. Histologic sections were evaluated for the presence of matrix patterns and acoustic backscatter was characterized using calibrated spectrum analysis. Statistical correlations between acoustic and histologic patterns were determined and linear discriminant analysis (LDA) and radial basis networks (RBN) were used to develop classification models for histologically based risk groups. Statistically significant correlations were found between acoustic parameters and the presence of histologic matrix-rich patterns. Retrospective classification accuracies of 74.4% and 78.6% were obtained with LDA and RBN, respectively. Leave-one-out analyses indicated estimated predictive accuracies of 71.8% and 75.0% for LDA and RBN, respectively.

Thrombus characterization with intravascular ultrasound: potential to predict successful thrombolysis

PURPOSE

To define analysis methods using radiofrequency (RF) data from intravascular ultrasound (IVUS) to characterize thrombus so that the success or failure of thrombolysis can be predicted.

METHODS

Experimentation was done in 2 phases: first, 40 clots created from platelet-rich (n=20) and platelet-poor (n=20) plasma were imaged with 20 and 30-MHz IVUS probes. The digitized RF signals were analyzed to determine the attenuation and slope of attenuation characteristics as a reference standard for the second part of the study. In this phase, 20 perfusion trials were performed with 10 platelet-rich and 10 platelet-poor samples in an in vitro arterial perfusion model; the composition of the samples was blinded during interrogation with IVUS. Alteplase (0.1 mg) was then infused directly into the thrombus for 1 hour; restoration of flow and amount of lysis were quantified.

RESULTS

Significant differences were detected for the integrated attenuation value (p<0.001) between platelet-poor and platelet-rich clots interrogated with a 30-MHz probe, but no differences were noted using the 20-MHz catheter. However, both transducers were capable of differentiating platelet-rich and platelet-poor clots using a slope of attenuation analysis, which was successful in identifying all 20 (100%) of the blinded clots in the in vitro model. The speed and completeness of lysis were significantly higher (p<0.05) for platelet-rich (78%) than platelet-poor (21%) clots.

CONCLUSIONS

IVUS preprocessed imaging is capable of differentiating various types of thrombus. This information can be used to predict the success or failure of pharmacological lysis techniques.

Intracoronary ultrasound in acute coronary syndromes: from characterization of vulnerable plaques to guidance of percutaneous treatment of complex stenoses

Our current knowledge on the substrate and genesis of acute coronary syndromes (ACS) results from the integration of pathological, angiographic, and intracoronary imaging techniques. To summarize briefly the current paradigm, eight differentiated stages of development of atherosclerotic lesions are currently accepted, defined not only by the cellular elements involved, but also by the appearance of sudden alterations of plaque structure and coronary thrombosis. The latter constitutes not only the dominant substrate for the most devastating manifestations of coronary artery disease, but also accelerates plaque size at a faster pace than in earlier stages. The composition of atherosclerotic plaque varies significantly along the different evolutive stages, and thus includes cellular (macrophage, smooth muscle cells) and noncellular elements (glicosaminglycan or collagen-rich cellular matrix, extracellular lipid deposits, calcification, fresh, or organized thrombus) in a varying proportion. Furthermore, a dynamic process of vessel remodeling occurs along the atherosclerotic process, resulting, in most cases, in a protective mechanism against myocardial ischemia by preserving luminal dimensions during plaque enlargement. Intravascular ultrasound (IVUS) is one of the intracoronary imaging techniques that has contributed to the understanding of these changes in man. In addition, IVUS has the potential of being a useful clinical tool for predicting the chances of future acute coronary events by identifying vulnerable plaques, of characterizing which is the culprit lesion in ACS, and in guiding revascularization procedures in the treacherous field of thrombotic coronary syndromes. In this article, we review the current evidence on the potential of IVUS imaging for fulfilling these purposes.

Coronary plaque classification with intravascular ultrasound radiofrequency data analysis

BACKGROUND

Atherosclerotic plaque stability is related to histological composition. However, current diagnostic tools do not allow adequate in vivo identification and characterization of plaques. Spectral analysis of backscattered intravascular ultrasound (IVUS) data has potential for real-time in vivo plaque classification.

METHODS AND RESULTS

Eighty-eight plaques from 51 left anterior descending coronary arteries were imaged ex vivo at physiological pressure with the use of 30-MHz IVUS transducers. After IVUS imaging, the arteries were pressure-fixed and corresponding histology was collected in matched images. Regions of interest, selected from histology, were 101 fibrous, 56 fibrolipidic, 50 calcified, and 70 calcified-necrotic regions. Classification schemes for model building were computed for autoregressive and classic Fourier spectra by using 75% of the data. The remaining data were used for validation. Autoregressive classification schemes performed better than those from classic Fourier spectra with accuracies of 90.4% for fibrous, 92.8% for fibrolipidic, 90.9% for calcified, and 89.5% for calcified-necrotic regions in the training data set and 79.7%, 81.2%, 92.8%, and 85.5% in the test data, respectively. Tissue maps were reconstructed with the use of accurate predictions of plaque composition from the autoregressive classification scheme.

CONCLUSIONS

Coronary plaque composition can be predicted through the use of IVUS radiofrequency data analysis. Autoregressive classification schemes performed better than classic Fourier methods. These techniques allow real-time analysis of IVUS data, enabling in vivo plaque characterization.

Discrimination of early/intermediate and advanced/complicated coronary plaque types by radiofrequency intravascular ultrasound analysis

Radiofrequency intravascular ultrasound (IVUS-RF) analysis, as an extension of conventional IVUS imaging, may provide more accurate plaque discrimination. Thirty-two autopsy atherosclerotic coronary arteries were investigated. Corresponding sectors in different plaques were matched by histologic and RF analysis. Histologic analysis utilized the American Heart Association plaque classification. The backscattered ultrasound RF signal was analyzed by fast-Fourier transform, providing the underlying frequency components of its power spectrum. The normalized backscattered signal power (in decibels [dB]) for frequencies between 15.3 and 40.3 MHz was then measured for plaque discrimination. Advanced/complicated plaque types showed a higher signal power at all frequencies than early/intermediate lesion types (p <0.001 to p = 0.005). Discrimination of advanced/complicated lesion types was best at 15.3 MHz, with a cut-off point of 2.5 dB (sensitivity 93%, specificity 79%), and second best at 17.6 MHz (sensitivity 87%, specificity 71%, cut-off point 1.9 dB). With conventional IVUS, plaque discrimination was weaker; the best sensitivity for diagnosing early/intermediate lesion types was reached for "soft plaque" (sensitivity 63%, specificity 73%). Compared with conventional IVUS, IVUS-RF can discriminate between advanced/complicated and early/intermediate coronary atherosclerotic lesions with relatively high sensitivity and specificity in vitro.

Effects of transducer position on backscattered intensity in coronary arteries

Acute myocardial infarction is a frequent cause of sudden death, and is typically initiated by the rupture of coronary artery plaques. The likelihood and severity of rupture are influenced by the plaque structures and components. Radiofrequency (RF) intravascular ultrasound (US) (IVUS-RF) measurements extend current IVUS imaging techniques and may eventually enable the in vivo identification of these features. However, IVUS-RF measurements are affected by the transducer's instantaneous position in the vessel. Specifically, backscattered intensity (BI) decreases as either the distance between the tissue and the transducer increases, or as the beam's angle of incidence on the tissue increases. IVUS-RF data were acquired from seven disease-free coronary arteries in vitro. The 0-dB level for BI was defined as the peak intensity of the reflection from a stainless-steel flat reflector at each distance. The baseline BI measured in adventitial tissue was -32.5 dB (at 0 degrees, 0 mm) with angle and distance dependencies of -0.172 dB/ degrees and -3.37 dB/mm. In contrast, the BI from combined intima and media was -38.2 dB with dependencies of -0.111 dB/ degrees and -4.46 dB/mm (p < 0.05 for all three parameters). Acknowledging and compensating for these effects may allow IVUS-RF to develop into a rapidly deployable tool for the clinical detection of vulnerable plaques and to monitor coronary artery disease progression and regression.

Assessing spectral algorithms to predict atherosclerotic plaque composition with normalized and raw intravascular ultrasound data

Spectral analysis of backscattered intravascular ultrasound (IVUS) data has demonstrated the ability to characterize plaque. We compared the ability of spectral parameters (e.g., slope, midband fit and y-intercept), computed via classic Fourier transform (CPSD), Welch power spectrum (WPSD) and autoregressive (MPSD) models, to classify plaque composition. Data were collected ex vivo from 32 human left anterior descending coronary arteries. Regions-of-interest (ROIs), selected from histology, comprised 64 collagen-rich, 24 fibrolipidic, 23 calcified and 37 calcified-necrotic regions. A novel quantitative method was used to correlate IVUS data with corresponding histologic sections. Periodograms of IVUS samples, identified for each ROI, were used to calculate spectral parameters. Statistical classification trees (CT) were computed with 75% of the data for plaque characterization. The remaining data were used to assess the accuracy of the CTs. The overall accuracies for normalized spectra with CPSD, WPSD and MPSD were, respectively, 84.7%, 85.6% and 81.1% (training data) and 54.1%, 64.9% and 37.8% (test data). These numbers were improved to 89.2%, 91.9% and 89.2% (training) and 62.2%, 73% and 59.5% (test) when the calcified and calcified-necrotic regions were combined for analysis. Most CTs misclassified a few fibrolipidic regions as collagen, which is histologically acceptable, and the unnormalized and normalized spectra results were similar.

Morphology of vulnerable coronary plaque: insights from follow-up of patients examined by intravascular ultrasound before an acute coronary syndrome

OBJECTIVES

To determine the morphologic features of coronary plaques associated with acute coronary syndrome, we prospectively followed patients with atherosclerotic disease identified by intravascular ultrasound (IVUS).

BACKGROUND

Although clinical evaluation of the vulnerable atherosclerotic plaque is important, few data exist regarding the morphology of the vulnerable plaque in clinical settings.

METHODS

We examined 114 coronary sites without significant stenosis by angiography (<50% diameter stenosis) in 106 patients. All the sites exhibited atherosclerotic lesions by IVUS. These lesions consisted of 22 concentric and 92 eccentric plaques with a percent plaque area averaging 59 +/- 12%.

RESULTS

During the follow-up period of 21.8 +/- 6.4 months (range 1 to 24), 12 patients had an acute coronary event at a previously examined coronary site at an average of 4.0 +/- 3.4 months after the initial IVUS study. All the preexisting plaques related to the acute events exhibited an eccentric pattern and the mean percent plaque area was 67 +/- 9%, which was greater than plaque area in the other 90 patients without acute events (57 +/- 12%, p < 0.05). There was no statistically significant difference in lumen area between two patient groups (6.7 +/- 3.0 vs. 7.5 +/- 3.7 mm2). Among 12 coronary sites with an acute occlusion, 10 sites contained the echolucent zones, eight of these shallow and two deep, likely representing a lipid-rich core. In 90 sites without acute events, an echolucent zone in the shallow portion was seen at only four sites (p < 0.05).

CONCLUSIONS

Large eccentric plaque containing an echolucent zone by IVUS can be at increased risk for instability even though the lumen area is preserved at the time of initial study. Compensatory enlargement of vessel wall due to remodeling may contribute to the relatively small degree of stenosis by angiography.

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.