Closed contour edge detection of blood vessel lumen and outer wall boundaries in black-blood MR images

Quantitative measurements of the blood vessel wall area may provide useful information of atherosclerotic plaque burden, progression and/or regression. Magnetic resonance imaging is a promising technique for identifying both luminal and outer wall boundaries of the human blood vessels. Currently these boundaries are primarily defined manually, a process viewed as labor intensive and subject to significant operator bias. Fully automated post-processing techniques used for identifying the lumen and wall boundaries, on the other hand, are also problematic due to the complexity of signal features in the vicinity of the blood vessels. The goals of this study were to develop a robust, automated closed contour edge detection algorithm, apply this algorithm to high resolution human carotid artery images, and assess its accuracy, and reproducibility. Our algorithm has proven to be sensitive to various contrast situations and is reasonably accurate and highly reproducible.     

3D ultrasound analysis of carotid plaque volume and surface morphology

Morphological characterization of carotid plaques has been used for risk stratification and evaluation of response to therapy, evaluation of new risk factors, genetic research, and for quantifying effects of new anti-atherosclerotic therapies. We developed a 3D US system that allows detailed studies of carotid plaques in 3D. Our software includes 3D reconstruction, viewing, manual and semi-automated segmentation of carotid plaques, and surface morphology analysis to be used for quantitative tracking of plaque changes. We evaluated our plaque quantification software by examining plaque volume measurement accuracy, variability, and plaque surface morphology. We used vascular test phantoms to study segmentation accuracy, and used 48 3D US carotid plaques of patients ranging in size from 13.2 mm(3) to 544.0 mm(3) to study plaque volume measurement variability. We compared results from the semi-automated plaque measurements to the results obtained from manual measurements, which were used as the "gold" standard. We developed a surface plaque morphology quantification technique based on the segmented plaque surface curvature and used it to analyze plaques. Accuracy of plaque volume measurements for the simulated plaques ranged from 4.2% to 1.5% for volumes ranging from 68.5 mm(3) to 286 mm(3). The variability study showed that coefficients of variation in the measurement of plaque volume decreased with increasing plaque size for both inter- (90.8-3.9%) and intra-observer (70.2-3.1%) measurements over the plaque sizes measured. Surface morphology analysis showed that 1 mm ulceration could be quantified and monitored for changes over time. The automated plaque quantification approach showed a little higher intra-observer variability than the manual technique, and its performance was better for segmenting the wall than the lumen. Our results indicate that our approach is sensitive tool and can be used in studies of plaque progression and regression as it relates to atherosclerosis treatment effects and can be used effectively in longitudinal studies for direct measurement carotid plaque volume.     

Discrimination of components in atherosclerotic plaques from human carotid endarterectomy specimens by magnetic resonance imaging ex vivo

Specific MRI techniques have been used to determine the dimensional and compositional properties of atherosclerotic lesions in carotid endarterectomy tissues. A quantitative comparison of areas of specific features in typical tissue segments was performed using MR images and histologic images. The mean difference for the measurements by the two methods was 4.5% for the total vessel, 5.3% for the internal carotid artery lumen, and 5.0% for the external carotid lumen. For other less abundant components, the mean difference was 14.2%. For direct characterization, individual tissue components were isolated by microdissection and their T1 and T2 relaxation times measured. Highly calcified areas typically had rather short T1 (452-837 ms) and short T2 (10.4-18.4 ms). In contrast, regions enriched in lipid had much longer T1 (1,380-1,480 ms) and longer T2 (35.3-49.0 ms). Other components such as thrombus had intermediate T1 (1,180 ms) and short T2 (15.4 ms). T2 parametric imaging was used as a complementary approach for segmentation and quantitation of tissue components. In fresh tissue, several different components exhibited different T2 ranges: calcified/solid lipid (13-18 ms). cellular/ECM (9-30 ms), fluid lipid (35-40 ms): fibrous (50-60 ms). These results demonstrate the utility of MRI for identifying and quantifying specific components of atherosclerotic plaque ex vivo, and suggest its value for these measurements in vivo as well.     

Comparison of carotid vessel wall area measurements using three different contrast-weighted black blood MR imaging techniques

Measuring carotid artery plaque burden from MRI is a reliable method for monitoring regression and progression of atherosclerosis. However, to measure all available images would be very time consuming, and in practice the image quality (IQ) of these images may be inconsistent, which can directly impact the quality of measurement. It is hypothesized that if IQ is comparable among different contrast weighted images, then carotid artery area measurements obtained from different contrast images of the same location will produce identical results. To test this, T1, proton density and T2 weighted images were acquired from ten patients (51 +/- 7 years old). Carotid lumen and vessel wall area was measured using a custom designed software program. The results showed strong agreement evidenced with only small differences on both lumen (mean: 40.5 mm(2)) and wall (mean: 52.6 mm(2)) area measurement among different weighted images. The maximum absolute mean differences are less than 2.7 mm(2) and 4.4 mm(2), and 90(th) percentile of the absolute differences are 5.6 mm(2) and 8.2 mm(2) respectively. In conclusion, different contrast weighted images with high and comparable IQ will yield similar results in lumen and vessel wall area measurement. At each matched location, it is recommended that the image with the highest IQ be used for area measurement.     

Tracking regression and progression of atherosclerosis in human carotid arteries using high-resolution magnetic resonance imaging

BACKGROUND AND PURPOSE: Magnetic resonance imaging (MRI) can accurately and reproducibly measure the volume of atherosclerotic plaque in human carotid arteries. Atherosclerotic plaques may either progress or regress over time, depending on individual risk factors and treatment regimens. This study was designed to determine if regression or progression of human carotid atherosclerosis in patients receiving statin therapy over 24 months can be detected by high-resolution MRI. METHODS: In 11 subjects who had undergone unilateral carotid endarterectomy and were on statin therapy, volumes for total carotid artery, concentric wall (normal wall), eccentric wall (plaque), and lumen were quantified at 0, 16 and 24 months using a 1.5-T human imager equipped with 6-cm phased array coils. RESULTS: The interobserver mean coefficient of variation (CV) was lowest for the lumen volume (3.1%) and highest for the plaque volume (9.8%). The interscan mean CV was lowest for the total artery volume (3.2%) and highest for the plaque volume (9.9%). As much as 26% regression and 35% progression were observed in individual subject's carotid artery eccentric wall (plaque) volumes over time. Mean eccentric wall volume increased 5% by 16 months and 8% by 24 months. Mean total wall volume increased slightly at both 16 and 24 months (+1.2% and +1.8%). CONCLUSIONS: High-resolution MRI provides a noninvasive reproducible method of tracking changes in carotid atherosclerosis. This pilot study detected changes in individual subjects at both 16 and 24 months. MRI tracking of changes in atherosclerotic plaques should prove useful in assessing vascular disease risk and monitoring the efficacy of interventions designed to induce regression or retard progression.     

Computerized quantification of carotid artery stenosis using MRA axial images

Currently, the North American symptomatic carotid endarterectomy trial, European carotid surgery trial, and common carotid method are used to measure the carotid stenosis for determining candidates for carotid endarterectomy using the projection angiography from different modalities such as digital subtraction angiography, rotational angiography, computed tomography angiography and magnetic resonance angiography. A new computerized carotid stenosis measuring system was developed using MR angiography axial image to overcome the drawbacks of conventional carotid stenosis measuring methods, to reduce the variability of inter-observer and intra-observer. The gray-level thresholding is one of the most popular and efficient methods for image segmentation. We segmented the carotid artery and lumen from three-dimensional time-of-flight MRA axial images using gray-level thresholding technique. Using the measured intima-media thickness value of common carotid artery for each case, we separated carotid artery wall from the segmented carotid artery region. After that, the regions of segmented carotid without artery wall were divided into region of blood flow and plaque. The calculation of carotid stenosis degree was performed as follows: carotid stenosis grading = (area measure of plaque/area measure of blood flow region and plaque) * 100%. No previous study has developed the carotid stenosis measuring method using MRA axial image. The new computerized stenosis measuring system has advantage over conventional caliper measuring methods; it will not only greatly increase the speed of stenosis measuring but also reduce the variability between readers. It should also reduce the variability between different institutions.     

A fast screening protocol for carotid plaques imaging using 3D multi-contrast MRI without contrast agent

PurposeTo implement a fast (~ 15 min) MRI protocol for carotid plaque screening using 3D multi-contrast MRI sequences without contrast agent on a 3 Tesla MRI scanner.Materials and methods7 healthy volunteers and 25 patients with clinically confirmed transient ischemic attack or suspected cerebrovascular ischemia were included in this study. The proposed protocol, including 3D T1-weighted and T2-weighted SPACE (variable-flip-angle 3D turbo spin echo), and T1-weighted magnetization prepared rapid acquisition gradient echo (MPRAGE) was performed first and was followed by 2D T1-weighted and T2-weighted turbo spin echo, and post-contrast T1-weighted SPACE sequences. Image quality, number of plaques, and vessel wall thicknesses measured at the intersection of the plaques were evaluated and compared between sequences.ResultsAverage examination time of the proposed protocol was 14.6 min. The average image quality scores of 3D T1-weighted, T2-weighted SPACE, and T1-weighted magnetization prepared rapid acquisition gradient echo were 3.69, 3.75, and 3.48, respectively. There was no significant difference in detecting the number of plaques and vulnerable plaques using pre-contrast 3D images with or without post-contrast T1-weighted SPACE. The 3D SPACE and 2D turbo spin echo sequences had excellent agreement (R = 0.96 for T1-weighted and 0.98 for T2-weighted, p < 0.001) regarding vessel wall thickness measurements.ConclusionThe proposed protocol demonstrated the feasibility of attaining carotid plaque screening within a 15-minute scan, which provided sufficient anatomical coverage and critical diagnostic information. This protocol offers the potential for rapid and reliable screening for carotid plaques without contrast agent.     

A purpose-built neck coil for black-blood DANTE-prepared carotid artery imaging at 7 T

Atherosclerotic plaques in the bifurcation of the carotid arteries can pose a significant health risk due to possible plaque rupture and subsequent stroke. The assessment of plaques, and evaluation of the risk they pose, can be performed with Black-Blood (BB) vessel wall magnetic resonance imaging. However, resolution at standard clinical field strengths (up to 3 T) is limited, hampering reliable assessment and diagnosis. The aim of this study was to investigate the benefits of 7 T MRI using a BB application that has been successful at clinical field strengths. Therefore, for BB imaging, each sequence was preceded with ‘Delay Alternating with Nutation for Tailored Excitation’ (DANTE) preparation pulses for blood signal suppression. A coil comprising a 4-channel Tx array was designed and built to provide the required excitation coverage for the DANTE train; and a 4-channel Rx array was constructed to target the carotid bifurcation. Human and phantom results showed satisfactory blood suppression and comparable SNR and CNR to 3 T, therefore demonstrating the feasibility of the application at 7 T. However, the imposed SAR restrictions led to long scan times and subsequent motion artifacts. Thus, more accurate local SAR supervision schemes are required which could lead to a further improvement of BB DANTE vessel wall imaging at 7 T.     

Asymmetric radial expansion and contraction of rat carotid artery observed using a high-resolution ultrasound imaging system

The geometry of carotid artery bifurcation is of high clinical interest because it determines the characteristics of blood flow that is closely related to the formation and development of atherosclerotic plaque. However, information on the dynamic changes in the vessel wall of carotid artery bifurcation during a pulsatile cycle is limited. This pilot study investigated the cyclic changes in carotid artery geometry caused by blood flow pulsation in rats. A high-resolution ultrasound imaging system with a broadband scanhead centered at 40 MHz was used to obtain longitudinal images of the rat carotid artery. A high frame rate retrospective B-scan imaging technique based on the use of electrocardiogram to trigger signal acquisition was used to examine precisely the fast arterial wall motion. Two-dimensional geometry data obtained from nine rats showed that the rat carotid artery asymmetrically contracts and dilates during each cardiac cycle. Systolic/diastolic vessel diameters near the upstream and downstream regions from the bifurcation were 0.976 ± 0.011/0.825 ± 0.015 mm and 0.766 ± 0.015/0.650 ± 0.016 mm, respectively. Their posterior/anterior wall displacement ratios in the radial direction were 41.0 ± 14.9% and 2.9 ± 1.6%, respectively. These results indicate that in the vicinity of bifurcation, the carotid artery favorably expands to the anterior side during the systolic phase. This phenomenon was observed to be more prominent in the downstream region near the bifurcation. The cyclic variation pattern in wall movement varies depending on the measurement site, which shows different patterns at far upstream and downstream of the bifurcation. The asymmetric radial expansion and contraction of the rat carotid artery observed in this study may be useful in studying the hemodynamic etiology of cardiovascular diseases because the pulsatile changes in vessel geometry may affect the local hemodynamics that determines the spatial distribution of wall shear stress, one of important cardiovascular risk factors. Further systematic study is needed to clarify the effects of wall elasticity, branch angle and vessel diameter ratio on the asymmetric wall motion of carotid artery bifurcation.     

Assessment of the reliability of the determination of carotid artery lumen sizes by quantitative image processing of magnetic resonance angiograms and images

In order to use MR imaging to assess progression or regression of atherosclerosis, one must have an idea of the reproducibility of the imaging and image processing techniques. The ability of dark-blood MRI and semiautomated image processing to reproducibility measure the inner boundary of the carotid arteries was evaluated and compared with results obtained using bright-blood MRA. MRI and MRA images were obtained for two normal and two diseased volunteers six times each over a short period of time (6 months). The carotid bifurcation was used to align slices from different imaging sessions. The area for each vessel (right and left common, internal and external carotid artery) was determined for the six imaging sessions. The standard deviations of each lumen area normalized to the average area were computed for each vessel segment for each volunteer. For the common, internal, and external carotids, the averaged normalized standard deviations for MRI were 8, 12, and 17% and for MRA were 6, 8, and 13%. Lumen sizes obtained by MRI and MRA were found to be not statistically different. Eccentric plaques not seen on MRA were visualized by MRI. In conclusion, dark-blood MRI with semiautomated image processing yields reliable lumen areas that are in agreement with those obtained by MRA.     

Magnetic resonance imaging in a model of atherosclerosis: use of a collar around the rabbit carotid artery

Transverse cardiac-cycle gated high resolution magnetic resonance images have been obtained from the neck of the New Zealand white rabbit both in normal animals and from those in which a collar had been earlier positioned around one carotid artery. The study included animals fed on normal and on high cholesterol diets with the surgical modification having been demonstrated previously to cause a rapid and reproducible lesion resembling early atherosclerosis. The aim of the work was to investigate the attainable spatial resolution and sensitivity at a field strength of 2 T using a large radiofrequency transmitter system and a surface coil receiver with which spin-echo images have been obtained. Visualization was enhanced using a three-dimensional interpolation technique. An image resolution of 200 microns was readily obtained but was shown to be insufficient for delineating pathological features within the artery wall such as intimal layer thickening. The results have been compared with histopathological findings which confirmed that any morphological changes were within the pixel resolution of the image. Extensions to the methodology are proposed which should be able to detect atherosclerotic changes with a resolution of 50 microns within a feasible imaging time. In addition, the MRI study of how the surgical intervention alters the artery shape and curvature was carried out and the MRI demonstrated that collar implantation in general does not occlude the artery and causes only a slight and gradual degree of curvature to the vessel.     

High resolution simultaneous imaging of intracranial and extracranial arterial wall with improved cerebrospinal fluid suppression

PurposeTo develop a technique for three dimensional (3D) high resolution joint imaging of intracranial and extracranial arterial walls with improved cerebrospinal fluid (CSF) suppression and good blood suppression based on T1 weighted sampling perfection with application optimized contrast using different angle evolutions (T1w-SPACE) and to compare this technique (hereafter, iSPACE) with alternating with nutation for tailored excitation (DANTE) prepared SPACE sequence (DANTE-SPACE) for their CSF suppression performance around the mid cerebral arteries (MCA) and blood suppression at carotid arteries.Materials and methodsEight volunteers and twelve patients were prospectively recruited in this institutional review board approved study. A custom designed 32-channel coil set covering the intracranial and extracranial arteries was used for signal reception. Imaging was performed in each subject using DANTE-SPACE and iSPACE. Signal-to-noise ratios (SNR) of the vessel walls at the MCA and carotid arteries, and contrast-to-noise ratios (CNR) between vessel wall and CSF at the MCA and between vessel wall and lumen at carotid arteries from the two sequences were compared.ResultsIn volunteers, contrast between CSF and white matter (surrogate for vessel wall signal) at the M2 segments in iSPACE was 67.9% higher than in DANTE-SPACE. At the carotid region, the SNR of vessel wall in iSPACE was 11.6% higher than DANTE-SPACE while the CNR in iSPACE was 13% higher than DANTE-SPACE. In patients, images with 0.6 mm isotropic resolution were obtained in 7.5 min. iSPACE showed 70.9% improvement in CNR between plaque and CSF at the M2 segments compared to DANTE-SPACE.ConclusionSimultaneous extracranial and intracranial arterial wall imaging using iSPACE improved CSF suppression significantly at the M2 segment of MCA while blood suppression was comparable to DANTE-SPACE. The technique achieved 3D images with 0.6 mm isotropic spatial resolution and took 7.5 min using a custom made coil set. Using this technique, intracranial plaque visualization was improved with no observable image SNR degradation.     

Optimized 3D bright blood MRI of aortic plaque at 3 T

PURPOSE: To evaluate the feasibility of an optimized bright blood MRI protocol at 3 T in combination with contrast agent administration for the detection and characterization of aortic high-risk plaques for the improved workup of acute stroke patients. MATERIALS AND METHODS: ECG synchronized T1-weighted 3D gradient echo MRI was performed in 45 acute stroke patients. Data were acquired with high near isotropic spatial resolution (approximately 1 mm(3)) covering the entire thoracic aorta. To compensate for breathing and vessel motion artifacts, images were collected using respiratory navigator gating in combination with short diastolic data acquisition windows adjusted on a patient-by-patient basis. In patients with aortic plaques > or =3 mm in thickness, gadolinium contrast agent was administered and both pre- and post-contrast T1-weighted 3D measurements with identical vessel coverage were performed. RESULTS: Bright blood 3D MRI detected 33 high-risk plaques with an average maximum plaque thickness of 4.2+/-1.0 mm in 23 of 45 acute stroke patients. The availability of pre- and post-contrast images acquired within the same session enhanced the identification of calcified plaque components in 77% of all analyzed plaques: post-contrast MRI clearly improved the delineation of hypointense plaque cores in 23 of 30 cases and assisted in the classification of core shape and of core fraction. CONCLUSION: 3D bright blood MRI at 3 T was feasible for the detection of aortic high-risk sources and may help to improve the detection of causes of cerebral embolism in acute stroke patients.     

Compressed sensing based simultaneous black- and gray-blood carotid vessel wall MR imaging

ObjectiveIn this study, we sought to demonstrate the blood suppression performance, image quality and morphological measurements for compressed sensing (CS) based simultaneous 3D black- and gray-blood imaging sequence (CS-siBLAG) in carotid vessel wall MR imaging.Materials and methodsSeven healthy volunteers and five patients were recruited. Healthy subjects underwent five CS-siBLAG scans with 1, 2, 3, 4 and 5-fold accelerations. Signal-to-tissue ratio (STR) and contrast-to-tissue ratio (CTR) were computed as the measures of flowing signal suppression performance and the image quality for black-blood imaging of the technique. Vessel lumen area (LA) and wall area (WA) were compared between fully sampled acquisition and each accelerated acquisition. Patients underwent three CS-siBLAG scans with 1, 3 and 5-fold accelerations as well as a 3D time of flight (3D TOF) scan. Two radiologists reviewed the under-sampled black- and gray-blood image quality.ResultsSTR and CTR values obtained with 2 to 5-fold accelerations were not significantly different from those with full acquisition. LA and WA measured at 2 ×, 3 ×, 4 × and 5 × were all highly correlated to the corresponding values at 1 ×. For patients imaging, two radiologists both found that the dual-contrast images at 3 × acceleration exhibited comparable image quality to that of the fully sampled acquisition, and that the images at 5 × exhibited slightly blurred vessel wall and outer vessel wall boundaries.ConclusionBy combining the CS under-sampling pattern and reconstruction, pseudo-centric phase encoding order and dual blood contrast sequences, this technique provides spatially registered black- and gray-blood images and excellent visualization for vessel wall imaging and gray-blood imaging in a short scan time.     

三层膜管壁结构的颈动脉超生仿真

颈动脉超声仿真 有助于血管壁超声检测及信号处理方法的性能评价。提出三层膜管壁结构的颈动脉超声仿真方法。根据组织散射点分布的形状和密度与超声回声斑点分布的变化规律,对临床图像统计分析,确定对应组织散射点分布、密度及强度等特征参数,利用FIELD II产生超声射频回波信号并得到B超图像。30 例仿真试验结果表明,血管壁内、中、外膜及血流回声的统计分布与临床B超图像的一致;与中心频率为8 MHz的结果相比,12 MHz仿真的内、中、外膜厚度及管腔直径测量均值与预设值相等,最大相对误差分别为4.01%, 1.25%, 0.04%及0.15%,仿真效果更为逼真。      

3D high-resolution contrast enhanced MRI of carotid atheroma — a technical update

Objective

Development of a fast 3D high-resolution magnetic resonance imaging (MRI) protocol for improved carotid artery plaque imaging.

Methods

Two patients with carotid atherosclerosis disease underwent 3D high-resolution MRI which included time-of-flight and T₁-weighted variable flip angle, fast-spin-echo (FSE) imaging, pre- and post-intravenous gadolinium-based contrast agent administration.

Results

Good quality images with intrinsic blood suppression were obtained pre- and post-contrast administration using a 3D FSE sequence. The plaque burden, lipid core volume, hemorrhage volume and fibrous cap thickness were well determined.

Conclusions

3D high-resolution MR imaging of carotid plaque using TOF and 3D FSE can achieve high isotropic resolution, large coverage, and excellent image quality within a short acquisition time.     

An ultrasound simulation method for carotid arteries with a wall structure of three membranes

Ultrasound simulation for carotid arteries is helpful to the performance assessments of vessel wall detection and signal processing methods by using ultrasound techniques.An ultrasound simulation method of carotid artery wall with a three-membrane structure is proposed in present study.According to the ultrasound speckle distributions varying with the shapes and densities of scatterer distributions,as well as the statistic results of the clinical images,the parameters of distributions,densities and intensities of scatterers for different kinds of tissues in the carotid artery phantoms are determined.Each region is acoustically characterized using FIELD II software to produce the radio frequency echo signals,from which ultrasound images are derived.The results based on 30 simulations show that the echo distributions of the intimae,mediae,adventitias and blood are consistent with the clinical ones.Moreover,compared with the results from the central frequency of 8 MHz,the mean measurements for thicknesses of the intima,media and adventitia membranes,as well as the lumen diameter from the simulation images based on 12 MHz are the same as the preset ones,and the maximum relative errors are the 4.01%,1.25%,0.04%and 0.15%,respectively.The simulation under this condition is more realistic.     

An entropy-based approach to automatic image segmentation of satellite images

An entropy-based image segmentation approach is introduced and applied to color images obtained from Google Earth. Segmentation refers to the process of partitioning a digital image in order to locate different objects and regions of interest. The application to satellite images paves the way to automated monitoring of ecological catastrophes, urban growth, agricultural activity, maritime pollution, climate changing and general surveillance. Regions representing aquatic, rural and urban areas are identified and the accuracy of the proposed segmentation methodology is evaluated. The comparison with gray level images revealed that the color information is fundamental to obtain an accurate segmentation.     

Automated vessel segmentation using cross-correlation and pooled covariance matrix analysis

Time-resolved contrast-enhanced magnetic resonance angiography (CE-MRA) provides contrast dynamics in the vasculature and allows vessel segmentation based on temporal correlation analysis. Here we present an automated vessel segmentation algorithm including automated generation of regions of interest (ROIs), cross-correlation and pooled sample covariance matrix analysis. The dynamic images are divided into multiple equal-sized regions. In each region, ROIs for artery, vein and background are generated using an iterative thresholding algorithm based on the contrast arrival time map and contrast enhancement map. Region-specific multi-feature cross-correlation analysis and pooled covariance matrix analysis are performed to calculate the Mahalanobis distances (MDs), which are used to automatically separate arteries from veins. This segmentation algorithm is applied to a dual-phase dynamic imaging acquisition scheme where low-resolution time-resolved images are acquired during the dynamic phase followed by high-frequency data acquisition at the steady-state phase. The segmented low-resolution arterial and venous images are then combined with the high-frequency data in k-space and inverse Fourier transformed to form the final segmented arterial and venous images. Results from volunteer and patient studies demonstrate the advantages of this automated vessel segmentation and dual phase data acquisition technique.     

An assessment of the sharpness of carotid artery tissue boundaries with acquisition voxel size and field strength

A measure of the sharpness of vessel wall interfaces in carotid artery MRI may be useful for assessing the conspicuity of the wall's features. An edge detection technique was used to measure the signal intensity gradients in 2D time-of-flight (2D-TOF) and double-inversion recovery black-blood (DIR-BB) carotid artery images of normal subjects that were acquired at 1.5 T with 0.55 x 0.55 x 2.0-mm (0.6 mm3) acquisition voxels and zero filled to reduce the in-plane reconstructed voxel size by one half in each dimension as well as with 0.27 x 0.27 x 2.0-mm (0.15 mm3) acquisition voxels and at 3.0 T with 0.27 x 0.27 x 2.0-mm (0.15 mm3) acquisition voxels using surface coils. The gradient intensities of the lumen-to-background interface varied closely with the contrast-to-noise ratio of the 2D-TOF imaging. For the DIR-BB imaging, in which higher spatial frequency artery structures are visible, the gradient intensities at the interfaces were higher than theoretically predicted at both field strengths with smaller acquisition voxels. The use of acquisition voxels smaller than those previously used at 1.5 T can improve the visualization of carotid artery structures at 1.5 and 3.0 T with surface coil reception.