Intravascular ultrasound. Looking below the surface of vascular disease

Stent visualization methods to guide percutaneous coronary interventions and assess long-term patency

Evaluation of acute percutaneous coronary intervention (PCI) results and long-term follow-up remains challenging with ongoing stent designs. Several imaging tools have been developed to assess native vessel atherosclerosis and stent expansion, improving overall PCI results and reducing adverse cardiac events. Quantitative coronary analysis has played a crucial role in quantifying the extent of coronary artery disease and stent results. Digital stent enhancement methods have been well validated and improved stent strut visualization. Intravascular imaging remains the gold standard in PCI guidance but adds costs and time to the procedure. With a recent shift towards non-invasive imaging assessment and coronary computed tomography angiography imaging have shown promising results. We hereby review novel stent visualization techniques used to guide PCI and assess stent patency in the modern PCI era.

Coronary plaque redistribution after stent implantation is determined by lipid composition: A NIRS-IVUS analysis

BACKGROUND

The composition of plaque impacts the results of stenting. The following study evaluated plaque redistribution related to stent implantation using combined near-infrared spectroscopy and intravascular ultrasound (NIRS-IVUS) imaging.

METHODS

The present study included 49 patients (mean age 66 ± 11 years, 75% males) presenting with non-ST elevation myocardial infarction (8%), unstable angina (49%) and stable coronary artery disease (43%). The following parameters were analyzed: mean plaque volume (MPV, mm3), plaque burden (PB, %), remodeling index (RI), and maximal lipid core burden index in a 4 mm segment (maxLCBI4mm). High-lipid burden lesions (HLB) were defined as by maxLCBI4mm > 265 with positive RI. Otherwise plaques were defined as low-lipid burden lesions (LLB). Measurements were done in the target lesion and in 4 mm edges of the stent before and after stent implantation.

RESULTS

MPV and maxLCBI4mm decreased in both HLB (MPV 144.70 [80.47, 274.25] vs. 97.60 [56.82, 223.45]; maxLCBI4mm: 564.11 ± 166.82 vs. 258.11 ± 234.24, p = 0.004) and LLB (MPV: 124.50 [68.00, 186.20] vs. 101.10 [67.87, 165.95]; maxLCBI4mm: 339.07 ± 268.22 vs. 124.60 ± 160.96, p < 0.001), but MPV decrease was greater in HLB (28.00 [22.60, 57.10] vs. 13.50 [1.50, 28.84], p = 0.019). Only at the proximal stent edge of LLB, maxLCBI4mm decreased (34 [0, 207] vs. 0 [0, 45], p = 0.049) and plaque burden increased (45.48 [40.34, 51.55] vs. 51.75 [47.48, 55.76], p = 0.030).

CONCLUSIONS

NIRS-IVUS defined HLB characterized more significant decreases in plaque volume by stenting. Plaque redistribution to the proximal edge of the implanted stent occurred only in LLB.

Combined NIRS and IVUS imaging detects vulnerable plaque using a single catheter system: a head-to-head comparison with OCT

AIMS

The presence of thin-cap fibroatheromas (TCFA) is associated with high risk of acute coronary syndrome, hence their early detection may identify high-risk patients. In the present study we investigated the ability of a combined imaging catheter with near-infrared spectroscopy (NIRS) plus intravascular ultrasound (IVUS) to detect TCFA in patients with stable coronary artery disease.

METHODS AND RESULTS

Optical coherence tomography (OCT) and combined NIRS-IVUS assessment were performed on identical coronary segments. IVUS analysis provided per-segment minimal cross-sectional area (CSA), plaque length (PL), plaque burden (PB), plaque volume (PV), and remodelling index (RI). OCT was used as the gold-standard reference to define TCFA (fibrous cap thickness <65 μm). Plaque lipid content was estimated by NIRS (lipid core burden index [LCBI]). OCT-defined TCFA was present in 18 of 76 segments. IVUS revealed that OCT-defined TCFA were positively remodelled lesions with greater PB and PV, smaller CSA, and longer PL, while NIRS revealed greater LCBI per 2 mm segment (LCBI2mm) (all p<0.001). Greatest accuracy for OCT-defined TCFA detection was achieved using LCBI2mm >315 with RI >1.046 as a combined criterion value.

CONCLUSIONS

OCT-defined TCFA are characterised by positive vessel remodelling, high plaque burden and greater lipid core burden as assessed by dual NIRS-IVUS imaging.

Optimizing circular ring arrays for forward-looking IVUS imaging

Forward-looking (FL) catheter-based imaging systems are highly desirable for guiding interventions in intra- vascular ultrasound (IVUS) applications. One of the main challenges of array-based FL-IVUS systems is the large channel count, which results in increased system complexity. Synthetic phased-array processing with a reduced firing count simplifies the front-end and, hence, can enable 3-D real-time imaging. Recently, we have investigated dual-ring arrays suitable for IVUS imaging, in which the two concentric circular arrays are used separately as transmit (Tx) and receive (Rx) arrays. In this study, we present different optimized array designs based on dual and single circular rings which are suitable for synthetic phased-array processing with a reduced number of firings. To obtain an optimal firing set that produces low side lobes in the wideband response, we use a simulated annealing algorithm. In the simulations, we use 1.2-mm-diameter array configurations with 64 Tx and 58 Rx elements, a center frequency of 20 MHz and fractional bandwidths of 50% and 80%. The results show that optimized dual-ring arrays provide 8 dB improvements in peak near side-lobe level with no widening in the main lobe width when compared with full and other sparse co-arrays.

Intravascular ultrasound assessment of the association between spatial orientation of ruptured coronary plaques and remodeling morphology of culprit plaques in ST-elevation acute myocardial infarction

The aim of this study was to assess the association between the spatial location of plaque rupture and remodeling pattern of culprit lesions in acute anterior myocardial infarction (MI). Positive remodeling suggests a potential surrogate marker of plaque vulnerability, whereas plaque rupture causes thrombus formation followed by coronary occlusion and MI. Intravascular ultrasound (IVUS) can determine the precise spatial orientation of coronary plaque formation. We studied 52 consecutive patients with acute anterior MI caused by plaque rupture of the culprit lesion as assessed by preintervention IVUS. The plaques were divided into those with and without positive remodeling. We divided the plaques into three categories according to the spatial orientation of plaque rupture site: myocardial (inner curve), epicardial (outer curve), and lateral quadrants (2 intermediate quadrants). Among 52 plaque ruptures in 52 lesions, 27 ruptures were oriented toward the epicardial side (52%), 18 toward the myocardial side (35%), and 7 in the 2 lateral quadrants (13%). Among 35 plaques with positive remodeling, plaque rupture was observed in 21 (52%) on the epicardial side, 12 (34%) on the myocardial side, and 2 (6%) on the lateral side. However, among 17 plaques without positive remodeling, plaque rupture was observed in 6 (35%), 6 (35%), and 5 (30%), respectively (p = 0.047). Atherosclerotic plaques with positive remodeling showed more frequent plaque rupture on the epicardial side of the coronary vessel wall in anterior MI than those without positive remodeling.

Relationship between tissue characterization with 40 MHz intravascular ultrasound imaging and 64-slice computed tomography

BACKGROUND

Identification of coronary plaque composition is important for selecting the treatment strategy, and 64-slice computed tomography (CT) is a noninvasive method of characterizing atherosclerotic plaques. However, the correlation between plaque characteristics detected by CT and intravascular ultrasound (IVUS) is not clear. A 40 MHz IVUS imaging system (iMap-IVUS) has recently been developed to evaluate plaque composition. The aim of this study was to compare iMap-IVUS with 64-slice CT angiography for the characterization of non-calcified coronary plaques.

METHODS AND RESULTS

Both 64-slice CT angiography and iMap-IVUS were performed in 19 patients (38 plaques). CT values were measured as Hounsfield units (HU) in circular regions of interest (ROI) drawn on the plaques. The iMap-IVUS system analyzed coronary plaques as fibrotic, lipidic, necrotic, or calcified tissue based on the radiofrequency spectrum. A positive correlation was found between CT values and the percentage of fibrotic plaque (r=0.34, p=0.036) or calcified plaque (r=0.40, p=0.011). Conversely, a negative correlation was found between CT values and the percentage of lipidic plaque (r=-0.41, p=0.01), or necrotic plaque (r=-0.41, p=0.01).

CONCLUSIONS

Good correlations were observed between the characteristics of non-calcified plaque determined by iMap-IVUS and the CT values of plaque detected by 64-slice CT scanning.

Coronary plaque quantification by voxel analysis: dual-source MDCT angiography versus intravascular sonography

OBJECTIVE

The purpose of this study was to evaluate a voxel-based analytic technique for quantification of noncalcified coronary artery plaque with intravascular sonography as a standard of reference.

SUBJECTS AND METHODS

Intravascular sonography and dual-source MDCT angiography prospectively performed on 12 patients resulted in identification of 20 segments containing noncalcified plaque. Four of these segments were used to establish reference measurements of 0.6-mm proximal wall thickness with a 0-HU cutoff between the epicardial fat and outer wall and an individually adjusted threshold for the interface between the wall and lumen. With these data, consecutive circular layers of the outer wall were subtracted from a 3D volume to determine the plaque plus medial layer and the actual plaque volume in the other 16 segments. Accuracy of the voxel technique was assessed by comparing the results with intravascular sonographic findings.

RESULTS

Both the total plaque burden (plaque plus medial layer) and the actual plaque volume had good concordance with intravascular sonographic findings (49.6 +/- 20 mm (3) vs 56.7 +/- 23.6 mm (3), p = 0.076; 26.5 +/- 14.8 mm (3) vs 30.9 +/- 15.3 mm (3), p = 0.09). Corresponding correlation coefficients were r = 0.76 and r = 0.79. The method had good reproducibility, the an intraclass correlation coefficients being 0.93 for total plaque burden and 0.90 for actual plaque volume.

CONCLUSION

Voxel analysis can be used for accurate and reproducible quantification not only of plaque burden but also of actual plaque volume.

Accuracy of dual-source CT in the characterisation of non-calcified plaque: use of a colour-coded analysis compared with virtual histology intravascular ultrasound

Non-invasive assessment of plaque volume and composition is important for risk stratification and long-term studies of plaque stabilisation. Our aim was to evaluate dual-source computed tomography (DSCT) and colour-coded analysis in the quantification and classification of coronary atheroma. DSCT and virtual histology intravascular ultrasound (IVUS-VH) were prospectively performed in 14 patients. 22 lesions were compared in terms of plaque volume, maximal per cent vessel stenosis and percentages of fatty, fibrous or calcified components. Plaque characterisation was performed with software that automatically segments luminal or outer vessel boundaries and uses CT attenuation for a colour-coded plaque analysis. Good correlation was found for per cent vessel stenosis in DSCT (53+/-13%) and IVUS (51+/-14%; r(2) = 0.70). Mean volumes for entire plaque and non-calcified atheroma were 68.5+/-33 mm(3) and 56.7+/-30 mm(3), respectively, in DSCT and 60.8+/-29 mm(3) and 55.8+/-26 mm(3), respectively, in IVUS. Mean percentages of fatty, fibrous or calcified components were 28.2+/-6%, 53.2+/-9% and 18.7+/-13%, respectively, in DSCT and 29.9+/-5%, 55.3+/-12% and 14.4+/-9%, respectively, in IVUS-VH. Significant overestimation was present for the entire plaque and the volume of calcified plaque (p = 0.03; p = 0.0004). Although good correlation with IVUS was obtained for the entire plaque (r(2) = 0.76) and non-calcified plaque volume (r(2) = 0.84), correlation proved very poor and insignificant for percentage plaque composition. Interclass correlation coefficients for non-calcified plaque volume and percentages of fatty, fibrous or calcified components were 0.99, 0.99, 0.95 and 0.98, respectively, and intraclass coefficients were 0.98, 0.93, 0.98 and 0.99, respectively. We found that using Hounsfield unit-based analysis, DSCT allows for accurate quantification of non-calcified plaque. Although percentage plaque composition proves highly reproducible, it is not correlated with IVUS-VH.

Pioglitazone reduces the necrotic-core component in coronary plaque in association with enhanced plasma adiponectin in patients with type 2 diabetes mellitus

BACKGROUND

Pioglitazone has a preventive effect on cardiovascular disease, but its ability to stabilize coronary plaque is unknown.

METHODS AND RESULTS

A prospective, randomized trial was conducted in 54 patients with type 2 diabetes and stable angina pectoris, randomly assigned to either a pioglitazone group or control group. Non-culprit, angiographically mild-to-moderate obstructive lesions were examined with virtual histology intravascular ultrasound (VH-IVUS) at coronary intervention and 6 months later. In total, 42 lesions of 22 patients in the pioglitazone group and 44 lesions of 24 patients in the control group were analyzed. After 6 months, patients in the pioglitazone group had significantly improved blood sugar, high-sensitivity C-reactive protein, and plasma adiponectin levels. VH-IVUS analysis revealed that, although the total plaque-to-vessel volume was not changed in either group, the necrotic-core area had significantly decreased in the pioglitazone group (-4.6+/-5.9% vs 1.1+/-9.3%, P=0.001). There was a significant inverse correlation between the change in plasma adiponectin levels and the change in necrotic-core area (r=-0.46, P<0.0001).

CONCLUSIONS

Pioglitazone may stabilize coronary plaque by reducing the necrotic-core component, in association with enhanced plasma adiponectin levels.

Characterization of coronary atherosclerosis by dual-source computed tomography and HU-based color mapping: a pilot study

To assess HU-based color mapping for characterization of coronary plaque, using intravascular ultrasound virtual histology (IVUS-VH) as a standard of reference. Dual-source computed tomography and IVUS-VH were prospectively performed in 13 patients. In five lesions, HU thresholds of the color-coding software were calibrated to IVUS-VH. In a 15-lesion verification cohort, volumes of vessel, lumen and plaque or percentages of lipid, fibrous and calcified components were obtained through use of pre-set HU cut-offs as well as through purely visual adjustment of color maps. Calibrated HU ranges for fatty or fibrous plaque, lumen and calcification were -10-69, 70-158, 159-436 and 437+. Using these cut-offs, HU-based analysis achieved good agreement of plaque volume with IVUS (47.0 vs. 51.0 mm(3)). Visual segmentation led to significant overestimation of atheroma (61.6 vs. 51.0 mm(3); P = 0.04) Correlation coefficients for volumes of vessel, lumen and plaque were 0.92, 0.87 and 0.83 with HU-based analysis or 0.92, 0.85 and 0.71 with visual evaluation. With both methods, correlation of percentage plaque composition was poor or insignificant. HU-based plaque analysis showed good reproducibility with intra-class correlation coefficients being 0.90 for plaque volume and 0.81, 0.94 or 0.98 for percentages of fatty, fibrous or calcified components. With use of optimized HU thresholds, color mapping allows for accurate and reproducible quantification of coronary plaque.

Association of coronary plaque composition and arterial remodelling: a virtual histology analysis by intravascular ultrasound

BACKGROUND

Conflicting data have been reported about the correlation between plaque composition assessed by virtual histology (VH) and remodelling index (RI).

AIM

To evaluate, in a larger patient population, the relationship between plaque morphology obtained by VH and arterial remodelling.

METHODS AND RESULTS

VH intravascular ultrasound was performed on 95 non-bifurcation native significant lesions (>75% stenosis) in 85 patients. Positive remodelling (defined as RI > or =1.05) was present in 28 lesions, whereas intermediate/negative remodelling (RI <1.05) was present in 67 lesions. Compared with intermediate/negative remodelling, positive remodelling was associated with an increased frequency of patients with acute coronary syndrome (n = 13 (52%) vs n = 15 (25%); p = 0.017), and with a greater plaque burden (mean (SD) 78.3 (6.3)% vs 73.2 (6.8)%, p = 0.001). At the minimal lumen site, necrotic core was significantly smaller in lesions with positive remodelling (median (interquartile range) 5.0% (2.2-11.0%)) than in lesions with intermediate/negative remodelling (median (interquartile range) 9.0% (4.0-16.0%); p = 0.048). No differences were observed in the rate of thin-cap fibroatheroma or in the presence of multiple necrotic core layers, and there were no statistical differences for fibrous, fibro fatty and dense calcium percent plaque area at the minimum lumen diameter (MLD), or for the entire lesion length between both groups.

CONCLUSIONS

In vivo VH analysis shows that lesions with positive remodelling have statistically less necrotic core percent area at the MLD site compared with intermediate/negative remodelling lesions.

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.

Cardiac hemodynamics, coronary circulation and interventional cardiology

Microcirculation is the functional end of the coronary circulation and it plays a key role in the regulation of coronary blood flow, both on the local and global scales. A good understanding of its function under physiological and pathophysiological conditions is crucial but, because of its micro-scale, access to this part of the coronary circulation is extremely difficult and requires a considerable amount of innovation and new technologies. Dynamics of the coronary circulation provide the true vehicle by which blood supply reaches the myocardium- coronary vasculature is only the conducting component of that vehicle. It is highly unlikely that the pulsatile nature of the flow, the capacitance of the conducting vessels and the constant pounding of coronary vasculature by surrounding tissue are not part of the design, regulation, and function of the coronary circulation. Interventions, whether to assess or to correct coronary stenosis, continue to be the main clinical avenue to dealing with coronary heart disease. Clinical decisions rely heavily on the ability to determine the true morphology of an occlusive lesion, to predict the future course of that lesion and to assess the functional toll on coronary blood supply which it will inflict at each stage.

Quantitative real-time blood flow estimation with intravascular ultrasound in the presence of in-plane flow

Previously, we showed a source of error in blood flow estimation introduced by in-plane flow using a slow-time finite-impulse response (FIR) filter-bank method measuring blood flow through the image plane of an intravascular ultrasound (IVUS) catheter array. There is a monotonic relationship between flow velocity and the normalized second moment of the slow-time spectrum when flow is orthogonal to the image plane of a side-looking catheter array. However, this relationship changes in the presence of in-plane flow, as slow-time spectra shift and spread with varying in-plane and out-of-plane components. These two effects increase the normalized spectral second moment, resulting in flow overestimates. However, by resampling the received signal with variable time delay from pulse to pulse (i.e., tilting the slow-time signals), the slow-time spectrum shifts back to direct current (DC), and the orthogonal estimation method can be used. We present a method to correct this overestimation and accurately estimate blood flow through the image plane in real time. Initially, the tilt delay needed to shift the slow-time spectrum back to DC at each point within the flow field is calculated. Knowing this tilt delay, a tilted slow-time signal is obtained for the velocity component normal to the image plane, and its spectrum is estimated using a filter-bank. That spectrum then is used to estimate the flow speed using a mapping function closely related to the monotonic relationship between the slow-time spectrum and flow speed observed for orthogonal flow. To accurately estimate flow angles, we modified the filter-bank algorithm, applying slow-time filter coefficients in a tilted arrangement and studying the slow-time spectral energy as a function of tilt. The slow-time spectral estimate is constructed with the tilted output of eight narrow, band-pass filters from a filter-bank. Independent simulations show that, for blood slowing at angles between +/-6 degrees and +/-15 degrees at a speed of 300 mm/s, flow velocity would be overestimated by as much as 38.79% and 249%, respectively, using the direct filter-bank approach. However, this error can be corrected using the modified method presented here, reducing the maximum overestimation error by a factor of 2.69 and 10.88 for those angles, respectively. Although the remaining error is not negligible, the volume flow rate, calculated by integrating the flow velocity over the entire vessel lumen, differs by only 3% or less from the true value over the angular range considered here. This represents an improvement of a factor of 40 over uncompensated estimates at maximum flow angles. Consequently, the modified real-time method can quantitatively measure flow in most IVUS applications in which the catheter's image plane is not precisely orthogonal to the flow direction.

Blood flow estimation error with intravascular ultrasound due to in-plane component of flow

Previously, we presented a real-time method to measure blood flow perpendicular to the image plane of an intravascular ultrasound (IVUS) imaging system using a slow-time FIR (finite impulse response) filter bank. Any in-plane flow introduces error in the flow measurement using the filter bank algorithm. Simulations show that for a flow angle of +/- 10 degrees and velocities between 200 mm/s and 300 mm/s, the energy within the lowest frequency band filter is 6.92 to 7.80 times higher than for perpendicular flow in the worst case. We present a variation of the FIR filter bank algorithm, applying filter coefficients in a tilted fashion to slow-time signals (i.e., combining slow-time and fast-time). An appropriate tilt, which depends on the flow angle and velocity, corrects for the increased energy under the frequency bands. In this case, the energy under the lowest frequency band filter for an angle of +/- 10 degrees and velocities ranging from 200 mm/s to 300 mm/s is 2.09 to 2.94 times higher than for perpendicular flow, yielding greater than a factor of three improvement in the worst case over the original slow-time method. Moreover, the average energy over the vessel determined with the appropriate tilt is within 2-3% of the true value.

Retrospective image-based gating of intracoronary ultrasound images for improved quantitative analysis: The intelligate method

Quantitative analysis of intracoronary ultrasound (ICUS) studies is performed on a series of tomographic cross-sectional ICUS images acquired during a motorized 0.5 mm/sec catheter pullback. Catheter displacement in the vascular lumen during the cardiac cycle causes an anatomically shuffled ICUS study, which results in a sawtooth-shaped appearance of the coronary segment in longitudinal reconstructed views in quantitative coronary ultrasound software packages. This hampers contour detection and leads to a laborious time-consuming semiquantitative analysis process that may produce inaccurate results. To solve these problems, in the past, online ECG-gated acquisition hardware has been applied. This article describes a novel image-based gating method called Intelligate, which features automatic retrospective selection of end-diastolic frames from videotaped or digitally stored ICUS studies. Our evaluation shows that there are no quantitative differences between analysis results of hardware ECG-gated and Intelligated ICUS studies.

Coronary intravascular ultrasound: implications for understanding the development and potential regression of atherosclerosis

The incremental value of intravascular ultrasound (IVUS), compared with angiographic analysis of coronary atherosclerosis, originates principally from 2 key features-its tomographic perspective and the ability to image coronary atheroma directly. Whereas angiography depicts the cross-sectional coronary anatomy as a planar silhouette of the lumen, ultrasound directly images the atheroma within the vessel wall, allowing measurement of atheroma size, distribution, and to some extent, composition. Although angiography remains the principal method to assess the extent of coronary atherosclerosis and to guide percutaneous coronary interventions, IVUS is rapidly altering conventional paradigms in the diagnosis and therapy of coronary artery disease. Thus, IVUS has become a vital adjunctive imaging modality for the aggressive coronary interventional cardiologist. As such, ultrasound has earned a role as a viable complementary technique relative to angiography, rather than an alternative to conventional angiographic methods. This article reviews the rationale, technical advantages and limitations, and interpretation of intravascular ultrasonography from the perspective of the general and invasive cardiologist. We emphasize the impact that IVUS studies have had on our understanding of the atherosclerotic coronary artery disease process, because these findings have important implications for all cardiologists. We then review several trials that are currently using intravascular ultrasonography for the study of coronary artery disease regression.

Spatial orientation of atherosclerotic plaque in non-branching coronary artery segments

It has been postulated that atherosclerotic plaque deposition is spatially related to regions of low shear in non-branching vessel segments. Intravascular ultrasound (IVUS) allows precise spatial orientation of coronary artery plaque formation in humans. The objective of this study was to test the hypothesis that coronary plaques have a higher prevalence on the myocardial side in regions that encounter low surface shear stress. IVUS allows the determination of the inner versus the outer curve of the vessel based on vascular and perivascular landmarks. We studied 30 consecutive patients pre-intervention using IVUS and measured vessel area, lumen area and plaque area (vessel-lumen area) during a motorized pullback at 1 mm intervals. Vessel segments near a side branch (within two times the diameter of the vessel) were excluded from analysis because of flow disturbances. All plaques were classified as concentric or eccentric and all eccentric plaques were further divided with respect to their spatial orientation in the vessel into quadrants: myocardial (inner curve, lower shear stress), epicardial (outer curve, higher shear stress) and lateral (two quadrants intermediate). A total of 613 cross-sections were analyzed in 14 left anterior descending, six left circumflex, and ten right coronary arteries. Plaque distribution was found to be concentric in 321 (52.4%) and eccentric in 292 (47.6%) cross sections. Of all eccentric plaques, 184 cross sections were oriented toward the myocardial side (62.6%) compared to only 54 toward the epicardial side (17.3%) and 54 in the 2 lateral quadrants (19.5%, P<0.001). No difference in plaque area (6.75+/-2.70 vs. 6.76+/-2.60 mm(2)), vessel area (15.28+/-4.73 vs. 15.35+/-4.40 mm(2)), or plaque thickness (1.26+/-0.37 vs. 1.25+/-0.43 mm) was noted between myocardial or epicardial plaques. These results suggest that atherosclerotic plaques develop more frequently on the myocardial side of the vessel wall, which may relate to lower shear stress. However, plaque size is similar on the epicardial and myocardial side.

The role of intravascular ultrasound imaging in vascular brachytherapy

Intracoronary brachytherapy has recently emerged as a new therapy to prevent restenosis. Initial experimental work was achieved in animal models and the results were assessed by histomorphometry. Initial clinical trials used angiography to guide dosimetry and to assess efficacy. Intravascular ultrasound (IVUS) permits tomographic examination of the vessel wall, elucidating the true morphology of the lumen and transmural components, which cannot be investigated on the lumenogram obtained by angiography. This paper reviews the use of IVUS in the clinical studies of brachytherapy conducted to date. IVUS allows clinicians to make a thorough assessment of the remodeling of the vessel and appears to have a major role to play in facilitating understanding of the underlying mechanisms of action in this emerging field. The authors propose that state-of-the-art IVUS techniques should be employed to further knowledge of the mechanisms of action of brachytherapy in atherosclerotic human coronary arteries.

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.

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.

Impact of intravascular ultrasound in understanding transplant coronary artery disease

Intravascular ultrasound imaging has been recognized as a sensitive tool to study early transplant vasculopathy lesions. An early examination performed soon after transplantation allows one to study donor atherosclerosis. Further, serial follow-up imaging with meticulous site matching provides important information regarding the progression of donor atherosclerosis and the development of transplant vasculopathy lesions. This review highlights the contribution of intravascular imaging in understanding the transplant coronary artery disease. This review also outlines various methodologies employed by different investigators, and stresses the strengths and weaknesses of these methodologies for correct interpretation as well as comparison of data from different studies.

Stepwise intravascular ultrasound (IVUS) guidance of high-pressure coronary stenting does not result in an improved acute or long-term outcome: a randomized comparison to "final-look" IVUS assessment

The objective of this study was to evaluate the potential benefit of stepwise intravascular ultrasound (IVUS)-guided coronary stent deployment compared to angiographic stent implantation with final IVUS assessment only. Acute procedural success and 6-month angiographic follow-up were compared in both groups. Intravascular ultrasound was performed using a 20- or 30-MHz mechanically rotated catheter in 85 patients who were prospectively randomized to group A (n=42; IVUS-guided) and group B (n=43; angiography +/- final IVUS assessment). There was no difference in the number of stents implanted (1.5+/-0.9 stents/lesion in group A and 1.3+/-0.6 stents/lesion in group B), the duration of the procedure, or the amount of contrast medium used. Defined criteria of optimal stent deployment (stent apposition, stent symmetry, complete coverage of dissections, >90% in-stent lumen area/reference lumen area) were achieved in 54.2% in group A and 56.6% in group B (NS). Angiographic follow-up was 87.1% at 6+/-2 months, and clinical follow-up was 100% at 8+/-1 months. There was no significant difference in restenosis rate (33.3% vs. 34.9%) applying a binary >50% diameter stenosis criterion for both groups. There was no significant difference in minimal in-stent lumen area at both baseline (7.91+/-2.64 mm2 vs. 7.76+/-2.21 mm2) and follow-up (5.84+/-2 mm2 vs. 5.52+/-1.87 mm2). With regard to immediate procedural lumen gain and rate of restenosis, multiple IVUS examinations during the procedure showed no advantage compared to final IVUS assessment only.

Intravascular ultrasound: basic interpretation

IVUS provides a new gold standard for visualization and measurement of coronary artery disease. Morphologic and morphometric observations by IVUS are in general considerably more detailed and accurate than those obtained by angiography. IVUS has led to new insights into the pathophysiology of coronary plaque accumulation with respect to adaptive vessel responses (remodeling) and their exhaustion (de-remodeling, shrinkage). Further technologic refinements need to focus on issues such as improvement in resolution and miniaturization of IVUS catheters to enhance the applicability of this imaging technique.

Intravascular elasticity imaging using ultrasound: feasibility studies in phantoms

A technique is described for measuring the local hardness of the vessel wall and atheroma using intravascular ultrasound. Strain images were constructed using the relative local displacements, which are estimated from the time shifts between gated echo signals acquired at two levels of intravascular pressure. Time shifts were estimated using one-dimensional correlation with bandlimited interpolation around the peak. Tissue-mimicking phantoms with typical morphology and hardness topology of some atherosclerotic vessels were constructed. Hard and soft regions could be distinguished on the strain image, independently of their contrast in echogenicity. Thus, the potential of ultrasonic hardness imaging to provide information that may be unavailable from the echogram alone was demonstrated. The strain images of the homogeneous and layered phantoms showed some artifacts that need to be corrected for, to obtain images of the modulus of elasticity. For in vitro and in vivo experiments, the spatial resolution of the technique needs to be improved. Furthermore, two-dimensional correlation techniques may be necessary in case of nonradial expansion and an off-centre catheter position.

Morphometric analysis in three-dimensional intracoronary ultrasound: an in vitro and in vivo study performed with a novel system for the contour detection of lumen and plaque

Currently, automated systems for quantitative analysis by intracoronary ultrasound (ICUS) are restricted to the detection of the lumen. The aim of this study was to determine the accuracy and reproducibility of a new semiautomated contour detection method, providing off-line identification of the intimal leading edge and external contour of the vessel in three-dimensional ICUS. The system allows cross-sectional and volumetric quantification of lumen and of plaque. It applies a minimum-cost algorithm and the concept that edge points derived from previously detected longitudinal contours guide and facilitate the contour detection in the cross-sectional images. A tubular phantom with segments of various luminal dimensions was examined in vitro during five catheter pull-backs (1 mm/sec), and subsequently 20 diseased human coronary arteries were studied in vivo with 2.9F 30 MHz mechanical ultrasound catheters (200 images per 20 mm segment). The ICUS measurements of phantom lumen area and volume revealed a high correlation with the true phantom areas and volumes (r = 0.99); relative mean differences were -0.65% to 3.86% for the areas and 0.25% to 1.72% for the volumes of the various segments. Intraob-server and interobserver comparisons showed high correlations (r = 0.95 to 0.98 for area and r = 0.99 for volume) and small mean relative differences (-0.87% to 1.08%), with SD of lumen, plaque, and total vessel measurements not exceeding 7.28%, 10.81%, and 4.44% (area) and 2.66%, 2.81%, and 0.67% (volume), respectively. Thus the proposed analysis system provided accurate measurements of phantom dimensions and can be used to perform highly reproducible area and volume measurements in three-dimensional ICUS in vivo.

Clinical and invasive 7-month follow-up of a patient with a synthetic coronary graft

The PossisR polytetrafluoroethylene Permaflow is a synthetic coronary graft which incorporates a Venturi resistor and which is anastomosed to the superior vena cava. Multiple anastomoses to the coronary vessels can be established in sequence until the graft is finally connected to the aorta. This aortocaval fistula enables permanent flow within the synthetic graft. We present the post-operative clinical and invasive findings of a 69-year-old female patient with a PossisR coronary graft. Follow-up angiography 3 and 7 months post-operatively showed patency of both coronary anastomoses and sufficient run-off to the native vessel segments. The left-right shunt induced by the graft was hemodynamically insignificant. Left ventricular function had improved significantly. 30 MHz intravascular ultrasound of the proximal part of the PossisR graft did not show an extra inner layer at the luminal side of the synthetic graft wall, so that relevant intimal thickening could be excluded after an interval of 7 months post implantation. Using an intravascular imaging technique, focal atherosclerotic lesions or thrombotic graft wall alterations could also be ruled out.

In vivo ultrasonic parametric imaging of carotid atherosclerotic plaque by videodensitometric technique

Extensive experimental and clinical data show that the ultrasonic image conveys information on the biochemical composition of the atherosclerotic plaque, ie, the relative content of lipids (hypoechoic), fibrous tissue (hyperechoic), and calcific deposits (very echogenic with shadowing). A more dishomogeneous echo structure of the plaque is also more often associated with clinically complicated carotid plaques. To date, however, the assessment of plaque density and homogeneity by transcutaneous B-mode imaging remains subjective and qualitative. The aim of this study was to assess whether plaque echodensity and homogeneity might be established on a more objective and quantitative basis by description of the spatial distribution of echo amplitude (referred to as tissue texture) applied to digitized images, obtained with commercially available B-mode transcutaneous imaging systems. A total of 47 B-mode images derived from echotomographic studies in 10 patients were digitized off line. For each region of interest, a set of first-order (mean gray level, standard deviation, skewness, kurtosis: mathematical descriptors of the shape of the frequency distribution of gray-level histogram) and of second-order (entropy, angular moment: mathematical descriptors of the spatial distribution of gray levels within the region of interest) textural parameters were evaluated. The visual, concordant reading by two independent, experienced observers assigned the plaques on the basis of qualitatively assessed echodensity to three groups: "soft" (n = 18), "fibrotic" (n = 20), "calcific" (n = 9). Regarding spatial gray-level distribution, 46 plaques would be separated into "homogenous" (n = 17) and "dishomogeneous" (n = 29). On digitized images, the normalized mean gray level was the most effective first-order textural parameter for distinguishing soft (24.2 +/- 12.4 arbitrary units in a zero to 255 scale) from fibrotic (64.5 +/- 16.4) and calcific plaques (125.3 +/- 24.5), P < 0.01 for all intergroup differences. "Homogeneous" plaques were separated from "heterogeneous" ones on the basis of entropy (5 +/- 1 vs 7.9 +/- 9.7; P < 0.01), whereas the values of angular second moment overlapped (1.542E-3 + 1.334E-3 vs 5.181E-4 +/- 2.5615E-4, P = ns). In conclusion, quantitative texture analysis of ultrasonic images derived from transcutaneous, high-resolution, commercially available B-scan systems is feasible in man and provides a quantitative operator-independent assessment of plaque echodensity and homogeneity.

Experimental studies on an efficient catheter array imaging system

A new synthetic aperture system for intraluminal imaging has been tested using a 32 element, 20 MHz circular array wrapped around the surface of a catheter appropriate for coronary artery applications. This system is based on an optimal reconstruction method that has been extended to reduce grating lobes using a slight modification to classic synthetic aperture data acquisition. Optimal reconstruction filters have been derived for two different operating modes based on this new data acquisition strategy. Imaging results on a wire target phantom show that spatial resolution is a simple linear function of depth, reaching a minimum 6 dB beam width of approximately 2.2 wavelengths. Sidelobe levels are inherently high for this system because of the small number of firings used to synthesize an aperture. Optimal reconstruction filters, however, can reduce these sidelobes to at least -20 dB in all cases. Finally, images of an excised segment of porcine femoral artery demonstrate the overall performance of the system as an intraluminal imager.

Angiographic, intravascular ultrasound and functional findings early after orthotopic heart transplantation

Accelerated graft atherosclerosis is responsible for increased mortality and morbidity among heart transplant recipients. The aim of this in-vivo study was to evaluate coronary atherosclerotic vessel alterations and endothelial function. Seventeen patients (14 males; mean age 49.3 years; range 24 to 69) were studied an average of 11 weeks (range 5 to 21) after heart transplantation because of coronary artery disease (n = 8), dilative cardiomyopathy (n = 7), mitral valve replacement (n = 1) and left atrial metastases of a leiomyosarcoma (n = 1). Mean age of the donor hearts (9 males) was 29 years (range 12 to 55). All recipients underwent biplane ventriculography and coronary angiography. In this study population, a total of 120 coronary segments (main stem, 21; left anterior descending artery, 85; circumflex artery, 14) were analyzed by intravascular ultrasound (20 MHz, 3.5F). In 13 patients, acetylcholine was infused into the proximal left anterior descending artery (10(-8) to 10(-5) M) to evaluate vasomotion within this segment. Regional contraction abnormalities were documented in 2 patients. Nine segments angiographically showed non-critical stenoses (5 patients). Intravascular ultrasound detected 52 cross-sectional areas with a three-layer pattern indicating intimal thickening. Mean circumferential extension of intimal proliferation was 192 degrees, mean intimal thickness 0.35 mm. Only 5 segments of the sonographically pathological cross-sectional areas showed angiographical evidence of atherosclerotic lesions. Intracoronary administration of acetylcholine at doses of 10(-8) and 10(-7) M resulted in vasoconstriction of the examined coronary segment in only 2 patients; the intracoronary application of acetylcholine at doses of 10(-6) and 10(-5) M revealed coronary vasoconstriction in 10 of the total of 13 patients. Using intravascular ultrasound, coronary artery lesions in heart transplant recipients can already be depicted at a very early stage. The abnormal response to acetylcholine in most of the heart recipients is independent of the extent of atherosclerotic vessel abnormalities documented by ultrasound or angiography.

Usefulness of a prototype intravascular ultrasound imaging in evaluation of aortic dissection and comparison with angiographic study, transesophageal echocardiography, computed tomography, and magnetic resonance imaging

The aim of this study was to evaluate the accuracy of information obtained with a prototype intravascular ultrasound (IVUS) system in chronic aortic dissection by comparing results with angiography, transesophageal echocardiography (TEE), computed tomography, or magnetic resonance imaging. We assigned 15 patients to IVUS imaging after they underwent angiography. The detection rate of the intimal flap was 100% in all segments of the aorta, and the detection rate of the intimal tear was 0%, 50%, 50%, and 77.8% in the ascending, arch, descending, and abdominal aorta, respectively. IVUS demonstrated 100% of the celiac and renal arteries, and 80% of the superior mesenteric arteries as well as their relation to dissection. It clarified the origin of 12 of 60 main abdominal branches (20%) which were not clear on the angiogram. It also determined the distal extent of the dissection in all cases. With regard to the size of the vessel, there was a good correlation between IVUS and computed tomographic values (r = 0.98, p < 0.01). No complications occurred in any patient. IVUS accurately demonstrated thrombus or spontaneous echo contrast in the false lumen that was confirmed with computed tomography or TEE, or both. It was especially useful in evaluating the abdominal aorta with regard to determining the size of the vessel, the extent of dissection, the relation of the branches to the false lumen, and the detection of intimal tears--important information for follow-up of patients and for planning surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

Composition of coronary atherosclerotic plaque in the intima and media affects intravascular ultrasound measurements of intimal thickness

OBJECTIVES

The purpose of this study was to evaluate how intravascular ultrasound-determined thickness and reflectivity of the inner echogenic layer of coronary artery plaque is affected by changes in collagen, elastin, proteoglycan, calcium and lipid content in the intima and media.

BACKGROUND

Coronary artery plaque often results in disruption of the internal elastic lamina and in increased collagen in the media as well as increased intimal lipid, calcium and proteoglycan content. How these factors affect intravascular ultrasound-derived measurements of intimal thickness are unknown.

METHODS

Twenty-one coronary artery segments from 10 patients with varying degrees of coronary artery disease were obtained fresh and unfixed at autopsy. Intravascular ultrasound (30-MHz, 3.5F catheter) was performed at regions in the vessel where a three-layered region was evident. Quantitative measurements of inner echogenic and deeper echolucent layer thickness were determined as well as estimates of ultrasound reflectivity and compared with histologic measurements of intimal and medial thickness and collagen, proteoglycan, lipid and calcium content.

RESULTS

In plaque > 300 microns thick and equivalent histologic levels of collagen, inner echogenic layer reflectivity was greater in regions with a greater amount of calcium and proteoglycan and lesser amounts of lipid. Inner echogenic layer thickness correlated well with intimal thickness but had the best correlation with intimal and medial thickness when medial collagen content was increased (r = 0.89). The median observed difference between intimal and inner echogenic thickness was 160 (p < 0.05 compared with zero), whereas the median observed difference between inner echogenic thickness and intimal plus medial thickness when medial collagen content was increased was 60 (not significantly different from zero).

CONCLUSIONS

The inner echogenic layer of human coronary artery plaque may represent both intima and media. The echolucent layer by ultrasound may represent only the basal section of the media that does not contain high collagen content.

Visualisation of arterial structure in vivo with intravascular ultrasound

BACKGROUND

Contrast angiography provides a silhouette of the arterial lumen, but does not give information about arterial wall structure. Catheter-tip ultrasound transducers can now provide a cross-sectional image of the arterial wall. This study examined the pathological correlation of intravascular ultrasound images and the accuracy of ultrasound measurements of vascular geometry.

METHODS

Intravascular ultrasound images were obtained with a mechanically rotated catheter-tip transducer and recorded on videotape. Initial validation studies were performed in fresh, post-mortem arterial specimens, which were filled with saline at physiological pressures. Ultrasound images at specific sites were compared with the pathological findings at that site and measurements of luminal diameter were compared with corresponding angiographic measurements. Subsequently, intravascular ultrasound was employed to examine the aorta, ilio-femoral and coronary arteries in patients undergoing balloon angioplasty.

RESULTS

The pathological correlations showed that intravascular ultrasound can detect early initial thickening and mild atherosclerotic lesions that do not result in luminal deformation. Ultrasound images provided definition of calcified, fibrotic and lipid-filled lesions. Ultrasound measurements of luminal diameter correlated well with pathology measurements (r = 0.93), as did ultrasound measurements of plaque area (r = 0.89). The in vivo studies demonstrated that intravascular ultrasound can define atheroma lesions not evident on contrast angiography and permits detailed evaluation of the results of interventions such as balloon angioplasty.

CONCLUSIONS

Intravascular ultrasound provides a unique window upon arterial structure and pathology in humans. Ultrasound images allow accurate measurements of vascular geometry and define early atheromatous lesions that are not evident with angiography.

Use of intracoronary ultrasonography in assessing pharmacotherapy for myocardial ischemia

Intracoronary ultrasonography can provide morphologic and physiologic information on coronary vasomotor responses to pharmacotherapy. Preliminary studies indicate a high correlation between dimensions determined by 2-dimensional echocardiography, angiography, and pathology. Similarly, the emerging data on intracoronary Doppler flow velocity responses beyond atherosclerotic obstructions before, during, and after coronary balloon occlusion will provide further insights into myocardial oxygen supply and its responses to pharmacotherapy during controlled myocardial ischemia.

Intravascular ultrasound in coronary atherosclerosis: a new approach to clinical assessment

Intravascular ultrasound evaluation of the coronary arteries by means of a selective coronary catheter attached to an ultrasound unit has afforded precise depiction of coronary lumen diameter and area at the level of the catheter tip. The arterial wall at this level can be evaluated for lipid, fibrous tissue, calcification, wall dissections, and intraluminal thrombi. The technique has the advantage over coronary angioscopy and angiography in that it does not require infusions or injections to allow visualization, and it has the ability to depict the inside of the arterial wall. The current disadvantages include the inability to visualize the vessel segments distal to the catheter tip. Three-dimensional reconstruction techniques allow depiction of the segment of the artery traversed by the catheter tip. The use of Doppler ultrasound imaging provides information on coronary flow velocities through coronary obstructions. Intravascular ultrasound images may provide information that complements the coronary arteriogram and may have an impact on patient care and clinical investigation strategies.