Reproducibility of volumetric quantification in intravascular ultrasound images

Imaging atherosclerotic plaque inflammation by fluorodeoxyglucose with positron emission tomography: ready for prime time?

Inflammation is a determinant of atherosclerotic plaque rupture, the event leading to most myocardial infarctions and strokes. Although conventional imaging techniques identify the site and severity of luminal stenosis, the inflammatory status of the plaque is not addressed. Positron emission tomography imaging of atherosclerosis using the metabolic marker fluorodeoxyglucose allows quantification of arterial inflammation across multiple vessels. This review sets out the background and current and potential future applications of this emerging biomarker of cardiovascular risk, along with its limitations.

Cross-sectional, prospective study of MRI reproducibility in the assessment of plaque burden of the carotid arteries and aorta

Introduction

This cross sectional study was conducted to test reproducibility of analysis of MRI parameters in carotids and thoracic descending aorta (TOA), evaluate the correlation of plaque burden and associations with subject age and gender.

Methods

Three hundred subjects, with cardiovascular risk factors, underwent a black blood MRI of both carotids and TOA. Mean wall area, wall thickness, lumen area, total vessel area and wall area/total vessel area (WA/TVA) ratio were manually measured. Inter-reader and intra-reader-reproducibility was tested on 187 and 20 randomly chosen subjects respectively.

Results

The intra-observer-reproducibility for the analysis was high (Intraclass-Correlation-Coefficients (ICC’s >0.8), except mean WA/TVA ratio of TOA. Similarly, the inter-observer reproducibility was acceptable (ICC’s >0.7 for mean wall area, lumen area and total vessel area). MRI parameters in aorta and carotids increased with age for both sexes (p<0.001). Except for mean wall thickness of TOA and WA/TVA ratio, MRI parameters were significantly higher in males than in females. All MRI measurements except the mean wall thickness and WA/TVA ratio were highly reproducible. There was good correlation for mean wall area between carotids and aorta compatible with the systemic nature of atherosclerosis. Similar to clinical presentation of cardiovascular diseases we found greater values in most MRI parameters (except for WA/TVA ratio) in males than in females and with increasing age.

Conclusions

These data suggest that analysis of most MRI measurements of plaque burden is reproducible and that there is correlation between plaque burden between carotids and aorta validating the systemic distribution of the disease.

Comparison of Methods for Magnetic Resonance-Guided [18-F]Fluorodeoxyglucose Positron Emission Tomography in Human Carotid Arteries

Background and Purpose— Inflammation is a major risk factor for atherosclerotic plaque rupture and clinical events. Previous studies have shown that plaque [ 18 F]fluorodeoxyglucose (FDG) uptake correlates with macrophage content. In this study we examined the reproducibility of 3 methods of quantifying plaque FDG uptake in the carotid arteries using positron emission tomography (PET). The correlation between 2 simplified uptake parameters (standardized uptake value [SUV], vessel wall-to-blood ratio [VBR]) and a gold standard technique (influx rate [K i ]) was also determined. We used MRI to correct carotid plaque FDG uptake for partial volume error.

Methods— Seven patients with a recent carotid territory transient ischemic attack underwent imaging twice within 8 days using MR and FDG-PET. MR coregistered to PET was used to delineate regions of interest, and to facilitate partial volume correction (PVC).

Results— SUV was the most reproducible parameter irrespective of whether it was normalized by body surface area (BSA), lean body mass, or weight (intraclass correlation coefficient=0.85, 0.88, and 0.90, respectively). VBR correlated better to K i than SUV ( r =0.58 VBR, r =0.46 SUV BSA ). PVC improved these correlations to r =0.81 VBR and r =0.76 SUV BSA , and only slightly degraded the reproducibility of SUV (intraclass correlation coefficient=0.83–0.85).

Conclusions— MR-guided FDG-PET is a highly reproducible technique in the carotid artery and the excellent anatomic detail provided by MR facilitates PVC. Of the methods examined, SUV BSA PVC appears to represent the best compromise between reproducible and accurate determination of FDG metabolism in carotid artery vessel wall.

(18)Fluorodeoxyglucose positron emission tomography imaging of atherosclerotic plaque inflammation is highly reproducible: implications for atherosclerosis therapy trials

OBJECTIVES

This study tested the near-term reproducibility of (18)fluorodeoxyglucose positron emission tomography (FDG-PET) imaging of atherosclerosis.

BACKGROUND

It is known that FDG-PET can measure inflammation within the aorta, carotid, and vertebral arteries with histologic validation in humans and animal models of disease. By tracking changes in inflammation over time, PET could be used as a surrogate marker of antiatheroma drug efficacy. However, the short-term variability and reproducibility of the technique are unknown.

METHODS

We imaged the carotid arteries and aorta in 11 subjects with FDG-PET/computed tomography twice, 14 days apart. We assessed interobserver and intraobserver agreement and interscan variability.

RESULTS

Interscan plaque FDG variability over 2 weeks was very low; intraclass correlation coefficients (ICC) ranged between 0.79 and 0.92. Interobserver agreement was high across all territories imaged except aortic arch (ICC values from 0.90 to 0.97, arch 0.71). Intraobserver agreement was high, with ICC values between 0.93 and 0.98.

CONCLUSIONS

Spontaneous change in plaque FDG uptake is low over 2 weeks, with favorable inter- and intraobserver agreement. Power calculations suggest that drug studies using FDG-PET imaging would require few subjects compared with other imaging modalities. This study strengthens the case for FDG-PET as a noninvasive plaque imaging technique.

Meta-analysis of the studies assessing temporal changes in coronary plaque volume using intravascular ultrasound

To assess the temporal effect of statin therapy on coronary atherosclerotic plaque volume measured by intravascular ultrasound (IVUS), we searched PubMed for eligible studies published between 1990 and January 2006. Inclusion criteria for retrieved studies were (1) IVUS volume analysis at baseline and follow-up and (2) statin therapy in > or =1 group of patients. All data of interest were abstracted in prespecified structured collection forms. Statistical analysis was performed with Review Manager 4.2. Random-effect weighted mean difference (WMD) was used as summary statistics for comparison of continuous variables. Nine studies of 985 patients (with 11 statin treatment arms) were selected. After a mean follow-up of 9.8 +/- 4.9 months, we found a significant decrease in coronary plaque volume (WMD -5.77 mm(3), 95% confidence interval -10.36 to -1.17, p = 0.01), with no significant heterogeneity across studies (p = 0.47). Prespecified subgroup analyses showed similar trends. Studies in which the achieved low-density lipoprotein (LDL) cholesterol level was <100 mg/dl showed a trend for plaque regression (WMD -7.88 mm(3), 95% confidence interval -16.31 to 0.55, p = 0.07), whereas studies in which the achieved level of LDL cholesterol was > or =100 mg/dl, the trend was less evident (WMD -4.22 mm(3), 95% confidence interval -10.27 to 1.82, p = 0.17). Plaque volume remained essentially unchanged in patients not treated with statins (WMD 0.13 mm(3), 95% confidence interval -4.42 to 4.68, p = 0.96). In conclusion, statin therapy, particularly when achieving the target LDL level, appears to promote a significant regression of coronary plaque volume as measured by IVUS.

Applications of Intravascular Ultrasound in the Treatment of Peripheral Occlusive Disease

Intravascular ultrasound (IVUS) has emerged as a useful and often necessary adjunct in a rising number of catheter-based peripheral interventions. IVUS catheters enable luminal and transmural cross-sectional imaging of peripheral vessels with high dimensional accuracy and provide detailed information about lesion morphology. IVUS is able to guide the optimal choice of appropriate angioplasty technique, guide the delivery of endovascular devices, and assess the immediate outcome of an intervention. In this review we discuss the role of IVUS for peripheral occlusive diseases, specifically the application of IVUS technology during percutaneous transluminal angioplasty (PTA), intravascular stent placement, crossing total occlusions, and venous obstructive disease.

Sample size calculation for clinical trials using magnetic resonance imaging for the quantitative assessment of carotid atherosclerosis

PURPOSE

To provide sample size calculation for the quantitative assessment of carotid atherosclerotic plaque using non-invasive magnetic resonance imaging in multi-center clinical trials. METHODS. As part of a broader double-blind randomized trial of an experimental pharmaceutical agent, 20 asymptomatic placebo-control subjects were recruited from 5 clinical sites for a multi-center study. Subjects had 4 scans in 13 weeks on GE 1.5 T scanners, using TOF, T1-/PD-/T2- and contrast-enhanced Tl-weighted images. Measurement variability was assessed by comparing quantitative data from the index carotid artery over the four time points. The wall/outer wall (W/OW) ratio was calculated as wall volume divided by outer wall volume. The percent lipid-rich/necrotic core (%LR/NC) and calcification (%Ca) were measured as a proportion of the vessel wall. For %LR/NC and %Ca, only those subjects that exhibited LR/NC or Ca components were used in the analysis.

RESULTS

Measurement error was 5.8% for wall volume, 3.2% for W/OW ratio, 11.1% for %LR/NC volume and 18.6% for %Ca volume. Power analysis based on these values shows that a study with 14 participants in each group could detect a 5% change in W/OW ratio, 10% change in wall volume, and 20% change in %LR/NC volume (power = 80%, p < .05). The calculated measurement errors presume any true biological changes were negligible over the 3 months that subjects received placebo.

CONCLUSION

In vivo MRI is capable of quantifying plaque volume and plaque composition, such as %lipid-rich/necrotic core and %calcification, in the clinical setting of a multi-center trial with low inter-scan variability. This study provides the basis for sample size calculation of future MRI trials.

Volumetric Quantitative Analysis of Tissue Characteristics of Coronary Plaques After Statin Therapy Using Three-Dimensional Integrated Backscatter Intravascular Ultrasound

OBJECTIVES

The purpose of the present study was twofold: 1) to evaluate the usefulness of three-dimensional (3D) integrated backscatter (IB) intravascular ultrasound (IVUS) for quantitative tissue characterization of coronary plaques; and 2) to use this imaging technique to determine if six months of statin therapy alters the tissue characteristics of coronary plaques.

BACKGROUND

Three-dimensional IVUS techniques for quantitative tissue characterization of plaque composition have not been developed.

METHODS

Radiofrequency (RF) signals were obtained using an IVUS system with a 40-MHz catheter. The IB values of the RF signal were calculated and color-coded. The 3D reconstruction of the color-coded map was performed by computer software. A total of 18 IB IVUS images were captured at an interval of 1 mm in each plaque. A total of 52 patients with hyperlipidemia were randomized to treatment with pravastatin (20 mg/day, n = 17), atorvastatin (20 mg/day, n = 18), or diet (n = 17) for six months. The tissue characteristics of arterial plaque in each patient (one arterial segment per patient) were analyzed with 3D IB IVUS before and after treatment.

RESULTS

Significant increases of fibrous volume (pravastatin: 25.4 +/- 6.5% to 28.1 +/- 6.1%; atorvastatin: 26.2 +/- 5.7% to 30.1 +/- 5.5%) and mixed lesion volume (atorvastatin: 25.5 +/- 6.6% to 28.7 +/- 5.1%) and a reduction of lipid volume (pravastatin: 25.5 +/- 5.7% to 21.9 +/- 5.3%; atorvastatin: 26.5 +/- 5.2% to 19.9 +/- 5.5%) were observed after statin therapy.

CONCLUSIONS

Statin therapy reduced the lipid component in patients with stable angina without reducing the degree of stenosis. Three-dimensional IB IVUS offers the potential for quantitative volumetric tissue characterization of coronary atherosclerosis.

Non-invasive measures of pulse wave velocity correlate with coronary arterial plaque load in humans

OBJECTIVE

Arterial stiffness is an emerging major risk factor for cardiovascular morbidity and mortality. The aim of the present study was to assess if coronary artery plaque load correlates with non-invasive measures of arterial stiffness.

DESIGN

Prospective investigational study.

SETTING

Tertiary university hospital centre.

PATIENTS

Patients undergoing elective diagnostic coronary angiography.

INTERVENTIONS AND MAIN OUTCOME MEASURES

Coronary artery plaque burden was assessed using a 30 MHz intravascular ultrasound catheter during an automated pullback. Proximal coronary artery plaque volume was determined using a validated edge-detection algorithm following three-dimensional computerized reconstruction. Central arterial stiffness was assessed in each patient using applanation tonometry to radial, carotid and femoral pulses, with derivation of aortic pressure augmentation and pulse wave velocity using pulse wave analysis.

RESULTS

In 35 patients (61 +/- 2 years), proximal coronary arterial plaque volume was 5.9 +/- 0.6 mm3/mm of vessel. Plaque volume correlated positively with carotid-radial pulse wave velocity (r = 0.47, P = 0.008) and appeared to correlate with carotid-femoral pulse wave velocity (r = 0.34, P = 0.07). Aortic augmentation (r = 0.24, P = 0.16), augmentation index (r = 0.3, P = 0.08), and pulse pressure (r = 0.22, P = 0.2) did not correlate significantly with proximal coronary artery plaque volume.

CONCLUSIONS

Non-invasive measures of carotid-radial pulse wave velocity correlate with the extent of coronary artery plaque volume and may be a useful non-invasive surrogate marker for the extent of coronary atherosclerosis. Our findings are consistent with the suggestion that central aortic stiffness may promote the development of coronary atherosclerosis and ischaemic heart disease.

An Automated Method for Lumen and Media–Adventitia Border Detection in a Sequence of IVUS Frames

In this paper, we present a method for the automated detection of lumen and media-adventitia border in sequential intravascular ultrasound (IVUS) frames. The method is based on the use of deformable models. The energy function is appropriately modified and minimized using a Hopfield neural network. Proper modifications in the definition of the bias of the neurons have been introduced to incorporate image characteristics. A simulated annealing scheme is included to ensure convergence at a global minimum. The method overcomes distortions in the expected image pattern, due to the presence of calcium, employing a specialized structure of the neural network and boundary correction schemas which are based on a priori knowledge about the vessel geometry. The proposed method is evaluated using sequences of IVUS frames from 18 arterial segments, some of them indicating calcified regions. The obtained results demonstrate that our method is statistically accurate, reproducible, and capable to identify the regions of interest in sequences of IVUS frames.

Automated three-dimensional assessment of coronary artery anatomy with intravascular ultrasound scanning

BACKGROUND

Angiography allows the definition of advanced, severe stages of coronary artery disease, but early atherosclerotic lesions, which do not lead to luminal stenosis, are not identified reliably. In contrast, intravascular ultrasound scanning allows the precise characterization and quantification of a wide range of atherosclerotic lesions, independent of the severity of luminal stenosis.

METHODS

Three-dimensional (3-D) reconstruction of entire coronary segments is possible with the integration of sequential 2-dimensional tomographic images and allows volumetric analysis of coronary arteries.

RESULTS

Automated systems able to recognize lumen and vessel borders and to display 3-D images are becoming available.

CONCLUSION

These systems have the potential for on-line 3-D image reconstruction for clinical decision-making and fast routine volumetric analysis in research studies. This review describes 3-D intravascular ultrasound scanning acquisition, analysis, and processing, and the associated technical challenges.

Impact of vessel curvature on the accuracy of three-dimensional intravascular ultrasound: validation by phantoms and coronary segments

BACKGROUND

Three-dimensional intravascular ultrasound (IVUS) is used for volumetric assessment of arteriosclerotic plaque burden and restenotic tissue at follow-up after coronary interventions. However, the accuracy of these measurements, especially in tortuous vessels, is unclear.

METHODS

A commercially available electrocardiogram (ECG)-gated 3-dimensional-IVUS system was tested in volume-validated straight and curved hydrocolloid phantoms and in volume-validated coronary specimens. Catheter withdrawal (30 MHz, 3.2F) was triggered using standardized ECG source with 0.2-mm step intervals per cardiac cycle simulation.

RESULTS

On the basis of automated phantom volume measurements, IVUS overestimated true phantom volume (relative error = [measured V - true V]/true V x 100) by a median of 0.9%, 0.25%, and 1.96% for straight, mildly curved, and severely curved segments, respectively. The true volume of the coronary specimens was overestimated by a median of 5.79%.

CONCLUSION

A median percentage deviation of 3-dimensional-IVUS-measured volumes from the true volumes of less than 10% in phantoms and coronary artery segments can be achieved.

Three-dimensional intravascular ultrasound assessment of abdominal aortic aneurysm necks

PURPOSE

To document the accuracy of an automated analysis system for measuring lumen diameter and neck lengths of abdominal aortic aneurysms (AAAs) from intravascular ultrasound (IVUS) images and to describe additional features associated with 3-dimensional (3D) IVUS imaging.

METHODS

Twenty-two aortic aneurysms were studied with IVUS. Lumen diameters obtained using the automated analysis system were compared with manual measurements from axial IVUS scans, as were neck lengths obtained using automated analysis versus those measured with the aid of a displacement sensing device. Automated analyses were repeated by a second observer. Agreement was expressed as the coefficient of variation (CV).

RESULTS

Twenty proximal aortic, 6 distal aortic, and 3 iliac necks were available for analysis. Comparison between automated analysis and manual measurements for lumen diameter revealed a difference of 0.45 +/- 0.42 mm (mean +/- SD, Pearson's r = 0.99, p < 0.001, CV = 2.1%) and a difference of 0.05 +/- 0.12 cm (r = 0.99, p = 0.04, CV = 4.1%) for neck length. Interobserver difference for lumen diameter was 0.13 +/- 0.66 mm (r = 0.99, p < 0.001, CV = 3.4%) and 0.05 +/- 0.11 cm for length measurements (r = 0.99, p = 0.02, CV = 3.5%). The 3D IVUS imaging facilitated the identification of neck configuration.

CONCLUSIONS

Automated analysis of IVUS images allows accurate measurement of the lumen diameter of proximal and distal AAA necks and gives length measurements comparable to those of manual analysis. Longitudinal display of IVUS images aids in the elucidation of neck anatomy.