Manual planimetric measurement of carotid plaque volume using three-dimensional ultrasound imaging

Characteristics and evaluation of atherosclerotic plaques: an overview of state-of-the-art techniques

Atherosclerosis is an important cause of cerebrovascular and cardiovascular disease (CVD). Lipid infiltration, inflammation, and altered vascular stress are the critical mechanisms that cause atherosclerotic plaque formation. The hallmarks of the progression of atherosclerosis include plaque ulceration, rupture, neovascularization, and intraplaque hemorrhage, all of which are closely associated with the occurrence of CVD. Assessing the severity of atherosclerosis and plaque vulnerability is crucial for the prevention and treatment of CVD. Integrating imaging techniques for evaluating the characteristics of atherosclerotic plaques with computer simulations yields insights into plaque inflammation levels, spatial morphology, and intravascular stress distribution, resulting in a more realistic and accurate estimation of plaque state. Here, we review the characteristics and advancing techniques used to analyze intracranial and extracranial atherosclerotic plaques to provide a comprehensive understanding of atheroma.

Uses of ultrasound in stroke prevention

Ultrasound methods are useful in stroke prevention in several ways. Measurement of carotid plaque burden, as either total plaque area (TPA) or total plaque volume (TPV) are strong predictors of cardiovascular risk: much stronger than intima-media thickness, which does not represent true atherosclerosis, but a biologically and genetically distinct phenotype. Measurement of plaque burden is also useful for the study of genetics, and of new risk factors such as toxic products of the intestinal microbiome. Carotid plaque burden is highly correlated with and as predictive of risk as coronary calcium scores, but is less costly and does not require radiation. Furthermore, because carotid plaques change in time over a period of months, they can be used for a new approach to vascular prevention: "Treating arteries instead of treating risk factors". In high-risk patients with asymptomatic carotid stenosis (ACS), this approach, implemented in 2003 in our clinics, was associated with a >80% reduction of stroke and myocardial infarction over 2 years. "Treating arteries without measuring plaque would be like treating hypertension without measuring blood pressure". Ultrasound methods can also be used to assess plaque vulnerability, by detecting echolucency, ulceration and plaque inhomogeneity on assessment of plaque texture. Transcranial Doppler (TCD) embolus detection is useful for risk stratification in patients with ACS; patients with two or more microemboli in an hour of monitoring have a 1-year risk of 15.6%, vs. 1% without microemboli, so this very clearly distinguishes which patients with ACS could benefit from intervention. TCD saline studies are more sensitive than trans-esophageal echocardiography for detection of patent foramen ovale, and more predictive of recurrent stroke. These methods should be more widely used, to reduce the increasing burden of stroke in our aging populations.

Area-Preserving Mapping of 3D Carotid Ultrasound Images Using Density-Equalizing Reference Map

Atherosclerotic plaques are focal and tend to occur at arterial bends and bifurcations. To quantitatively monitor the local changes in the carotid vessel-wall-plus-plaque thickness (VWT) and compare the VWT distributions for different patients or for the same patients at different ultrasound scanning sessions, a mapping technique is required to adjust for the geometric variability of different carotid artery models. In this work, we propose a novel method called density-equalizing reference map (DERM) for mapping 3D carotid surfaces to a standardized 2D carotid template, with an emphasis on preserving the local geometry of the carotid surface by minimizing the local area distortion. The initial map was generated by a previously described arc-length scaling (ALS) mapping method, which projects a 3D carotid surface onto a 2D non-convex L-shaped domain. A smooth and area-preserving flattened map was subsequently constructed by deforming the ALS map using the proposed algorithm that combines the density-equalizing map and the reference map techniques. This combination allows, for the first time, one-to-one mapping from a 3D surface to a standardized non-convex planar domain in an area-preserving manner. Evaluations using 20 carotid surface models show that the proposed method reduced the area distortion of the flattening maps by over 80% as compared to the ALS mapping method. The proposed method is capable of improving the accuracy of area estimation for plaque regions without compromising inter-scan reproducibility.

Three-Dimensional Echographic Evaluation of Carotid Artery Disease

The introduction of three-dimensional echography (3D echo) in vascular field is not recent, but it still remains a seldom-used technique because of the costs of ultrasound probe and the need of dedicated laboratories. Therefore, despite significant prognostic implications, the high diagnostic accuracy in plaque definition, and the relative ease of use, 3D echo in vascular field is a niche technique. The purpose of this review is mainly clinical and intends to demonstrate the potential strength of a 3D approach, including technical aspects, in order to present to clinicians and imagers the appealing aspects of a noninvasive and radiation-free methodology with relevant diagnostic and prognostic correlates in the assessment of carotid atherosclerosis. A comprehensive literature search (since 1990s to date) using the PubMed, MEDLINE, and Cochrane libraries databases has been conducted. Articles written in English have been assessed, including reviews, clinical trials, meta-analyses, and interventional/observational studies. Manual cross-referencing was also performed, and relevant references from selected articles were reviewed. The search was limited to studies conducted in humans. Search terms, retrieved also with PubMed Advanced search and AND/OR Boolean operators (mainly in title and abstract), included three-dimensional, echo, stroke/transient ischemic attack, predictors, carotid, imaging, and biomarkers.

Three-Dimensional Ultrasound of Carotid Plaque

Measurement of plaque burden is different from measurement of carotid intima-media thickness (IMT). Carotid total plaque area is a stronger predictor of cardiovascular risk than IMT, and in contrast to progression of IMT, which does not predict cardiovascular events, progression of total plaque area and total plaque volume strongly predict cardiovascular events. Measurement of plaque burden is useful in genetic research, and in evaluation of new therapies for atherosclerosis. Perhaps more importantly, it can be used for management of patients. A strategy called "treating arteries instead of treating risk factors" markedly reduces risk among patients with asymptomatic carotid stenosis.

Fully automatic plaque segmentation in 3-D carotid ultrasound images

Automatic segmentation of the carotid plaques from ultrasound images has been shown to be an important task for monitoring progression and regression of carotid atherosclerosis. Considering the complex structure and heterogeneity of plaques, a fully automatic segmentation method based on media-adventitia and lumen-intima boundary priors is proposed. This method combines image intensity with structure information in both initialization and a level-set evolution process. Algorithm accuracy was examined on the common carotid artery part of 26 3-D carotid ultrasound images (34 plaques ranging in volume from 2.5 to 456 mm(3)) by comparing the results of our algorithm with manual segmentations of two experts. Evaluation results indicated that the algorithm yielded total plaque volume (TPV) differences of -5.3 ± 12.7 and -8.5 ± 13.8 mm(3) and absolute TPV differences of 9.9 ± 9.5 and 11.8 ± 11.1 mm(3). Moreover, high correlation coefficients in generating TPV (0.993 and 0.992) between algorithm results and both sets of manual results were obtained. The automatic method provides a reliable way to segment carotid plaque in 3-D ultrasound images and can be used in clinical practice to estimate plaque measurements for management of carotid atherosclerosis.

Vascular wall imaging of vulnerable atherosclerotic carotid plaques: current state of the art and potential future of endovascular optical coherence tomography

As stroke is one of the leading causes of death and long-term morbidity worldwide, the research community has studied cardiac embolic sources, as well as vessel wall pathologies. For the latter, attention has been focused on defining morphologic tissue features associated with catastrophic events stemming from the carotid artery. Multiple noninvasive imaging modalities are currently being used to image and classify carotid atherosclerotic plaques, such as MR imaging, CT, and sonography, in an effort to provide clinically relevant predictive metrics for use in patient risk stratification and to define appropriate treatment options. This article compares and contrasts these existing clinical imaging modalities along with discussion of a new endovascular technique originally developed for cardiology, OCT, with which 3D comprehensive high-resolution images of the arterial wall can be acquired.

Advantages and pitfalls of three-dimensional ultrasound imaging of carotid bifurcation

OBJECTIVES

Several specialists use three-dimensional (3D) ultrasound as adjuvant imaging technique in their clinical practice. It has been applied to study carotid plaque morphology, surface and volume during atherosclerosis progression. Nonetheless, no papers have so far described the use of this technique in conditions different than carotid stenosis, such as bifurcation anatomy changes of the caliber and vessel course modifications.

METHODS

Patients admitted to our ultrasound laboratory for vascular screening were submitted to standard carotid duplex and to 3D ultrasound reconstruction of the carotid bifurcation.

RESULTS

Forty normal subjects, 7 patients with caliber alterations (4 carotid bulb ectasia and 3 internal carotid lumen narrowing), 45 patients with course variations (tortuosities and kinking) and 35 patients with internal carotid artery stenosis of various degrees have been investigated.

CONCLUSIONS

3D ultrasound is a feasible technique. It can improve carotid axis imaging through a better presentation of caliber variations and vessel course 'at a glance'. 3D ultrasound from the inward flow can provide imaging of the stenosis, but stenosis quantification should always take into account the assessment of plaque morphology and vessel wall.

Magnetic resonance angiography, digital subtraction angiography and Doppler ultrasonography in detection of carotid artery stenosis: a comparison with findings from histological specimens

BACKGROUND

Patients' life expectancy, clinical symptomatology and the extent of carotid stenosis are the most important factors when deciding whether to perform carotid endarterectomy (CEA) in patients with carotid stenosis. Therefore, the accuracy of measuring carotid stenosis is of utmost importance.

METHODS

Patients with internal carotid artery (ICA) stenosis were investigated by digital subtraction angiography (DSA), Doppler ultrasonography (DUS) and magnetic resonance angiography (MRA). Atherosclerotic plaque specimens were transversally cut into smaller segments and histologically processed. The slides were scanned and specimens showing maximal stenosis were determined; the minimal diameter and the diameter of the whole plaque were measured. DSA, DUS and MRA measurements were obtained in 103 patients. A comparison between preoperative and histological findings was performed. In addition, correlation coefficients were computed and tested.

RESULTS

Results show a significant correlation for each of the diagnostic procedures. Mean differences in the whole cohort between preoperative measurements and the histological measurements are as follows: angiographic measurement of carotid stenosis underestimated histological measurement by 14.5% and MRA by 0.7%, but DUS overestimated by 6.6%. The results in severe stenosis (> or =70%) are as follows: angiographic measurement underestimated the histological measurements by 2.3%, but MRA overestimated by 12.1% and DUS by 11.3%. The results in moderate stenosis (50-69%): angiographic measurement underestimated the histological measurements by 12.3%, but MRA overestimated by 0.2% and DUS by 7.2%. The results in mild stenosis (30-49%): angiographic measurement underestimated the histological measurements by 24.7% and MRA by 7.6%, but DUS overestimated by 3.3%.

CONCLUSIONS

Our study confirms that DSA underestimates moderate and mild ICA stenosis. DUS slightly overestimated moderate ICA stenosis and highly overestimated high-grade ICA stenosis. MRA proved to be accurate in detecting moderate ICA stenosis, but slightly underestimated mild stenosis and overestimated high-grade stenosis. The surgeon should be aware of these discrepancies when deciding whether to perform CEA in patients with ICA stenosis.

Nonrigid registration of three-dimensional ultrasound and magnetic resonance images of the carotid arteries

Atherosclerosis at the carotid bifurcation can result in cerebral emboli, which in turn can block the blood supply to the brain causing ischemic strokes. Noninvasive imaging tools that better characterize arterial wall, and atherosclerotic plaque structure and composition may help to determine the factors which lead to the development of unstable lesions, and identify patients at risk of plaque disruption and stroke. Carotid magnetic resonance (MR) imaging allows for the characterization of carotid vessel wall and plaque composition, the characterization of normal and pathological arterial wall, the quantification of plaque size, and the detection of plaque integrity. On the other hand, various ultrasound (US) measurements have also been used to quantify atherosclerosis, carotid stenosis, intima-media thickness, total plaque volume, total plaque area, and vessel wall volume. Combining the complementary information provided by 3D MR and US carotid images may lead to a better understanding of the underlying compositional and textural factors that define plaque and wall vulnerability, which may lead to better and more effective stroke prevention strategies and patient management. Combining these images requires nonrigid registration to correct the nonlinear misalignments caused by relative twisting and bending in the neck due to different head positions during the two image acquisition sessions. The high degree of freedom and large number of parameters associated with existing nonrigid image registration methods causes several problems including unnatural plaque morphology alteration, high computational complexity, and low reliability. Thus, a "twisting and bending" model was used with only six parameters to model the normal movement of the neck for nonrigid registration. The registration technique was evaluated using 3D US and MR carotid images at two field strengths, 1.5 and 3.0 T, of the same subject acquired on the same day. The mean registration error between the segmented carotid artery wall boundaries in the target US image and the registered MR images was calculated using a distance-based error metric after applying a "twisting and bending" model based nonrigid registration algorithm. An average registration error of 1.4 +/- 0.3 mm was obtained for 1.5 T MR and 1.5 +/- 0.4 mm for 3.0 T MR, when registered with 3D US images using the nonrigid registration technique presented in this paper. Visual inspection of segmented vessel surfaces also showed a substantial improvement of alignment with this nonrigid registration technique compared to rigid registration.

A "twisting and bending" model-based nonrigid image registration technique for 3-D ultrasound carotid images

Atherosclerosis at the carotid bifurcation resulting in cerebral emboli is a major cause of ischemic stroke. Most strokes associated with carotid atherosclerosis can be prevented by lifestyle/dietary changes and pharmacological treatments if identified early by monitoring carotid plaque changes. Registration of 3-D ultrasound (US) images of carotid plaque obtained at different time points is essential for sensitive monitoring of plaque changes in volume and surface morphology. This registration technique should be nonrigid, since different head positions during image acquisition sessions cause relative bending and torsion in the neck, producing nonlinear deformations between the images. We modeled the movement of the neck using a "twisting and bending" model with only six parameters for nonrigid registration. We evaluated the algorithm using 3-D US carotid images acquired at two different head positions to simulate images acquired at different times. We calculated the mean registration error (MRE) between the segmented vessel surfaces in the target image and the registered image using a distance-based error metric after applying our "twisting and bending" model-based nonrigid registration algorithm. We achieved an average registration error of 0.80 +/-0.26 mm using our nonrigid registration technique, which was a significant improvement in registration accuracy over rigid registration, even with reduced degrees-of-freedom compared to the other nonrigid registration algorithms.