Quantification of vessel wall motion and cyclic strain using cine phase contrast MRI: in vivo validation in the porcine aorta

In Vivo Visualization and Quantification of Rat Laryngeal Blood Supply After Hydration Challenge

OBJECTIVES

Systemic dehydration decreases total body blood volume; however, hemodynamic alterations at the level of local organs, such as the larynx, remain unclear. Here we sought to quantify superior thyroid artery (STA) blood flow after dehydration and rehydration using in vivo magnetic resonance angiography (MRA) and ultrasound imaging in a rat model.

METHODS

Male Sprague-Dawley rats (N = 17) were included in this prospective, repeated measures design. Rats first underwent MRA to determine baseline STA cross-sectional area, followed by high-frequency in vivo ultrasound imaging to measure STA blood velocity at baseline. Next, rats were systemically dehydrated (water withholding), followed by rehydration (water ad-lib). Ultrasound imaging was repeated immediately after dehydration and following rehydration. The STA blood velocity and STA cross-sectional area were used to compute STA blood flow. Three rats served as temporal controls for ultrasound imaging. To determine if the challenges to hydration status affected the STA cross-sectional area, four rats underwent only MRA at baseline, dehydration, and rehydration.

RESULTS

Systemic dehydration resulted in 10.5% average body weight loss. Rehydration resulted in average body weight gain of 10.9%. Statistically significant reductions were observed in STA mean blood flow rate after dehydration. Rehydration reversed these changes to pre-dehydration levels. No significant differences were observed in STA cross-sectional area with dehydration or rehydration.

CONCLUSION

Systemic dehydration decreased blood flow in the superior thyroid artery. Rehydration restored blood flow in the STA. Change in hydration status did not alter the STA cross-sectional area. These preliminary findings demonstrate the feasibility of using ultrasound and MRA to quantify hemodynamic changes and visualize laryngeal blood vessels.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:779-785, 2024.

The Potential for Quantifying Regional Distributions of Radial and Shear Strain in the Thoracic and Abdominal Aortic Wall Using Spiral Cine DENSE Magnetic Resonance Imaging

Aortic displacement encoding with stimulated echoes (DENSE) magnetic resonance imaging (MRI) was recently developed to assess heterogeneities in aortic wall circumferential strain (CS). However, previous studies neglected potential radial and shear strain (RSS) distributions. Herein, we present an improved aortic DENSE MRI postprocessing method to assess the feasibility of quantifying all components of the two-dimensional (2D) strain tensor. 32 previously acquired 2D DENSE scans from three distinct aortic locations were re-analyzed. Contrasting previous studies, displacements of the inner and outer aortic wall layers were processed separately to preserve RSS. Differences in regional strain between the new and old postprocessing methods were evaluated, along with interobserver, intraobserver, and interscan repeatability for all strain components. The new postprocessing method revealed an overall mean absolute difference in regional CS of 0.01 ± 0.01 compared to the prior method, with minimal impact on CS repeatability. Mean absolute magnitudes of regional RSS increased significantly compared to changes in CS (radial 0.04 ± 0.05, p < 0.001; shear 0.04 ± 0.04, p = 0.02). Most repeatability metrics for RSS were significantly worse than for CS. The unique distributions of RSS for each axial location associated well with local periaortic structures and mean aortic displacement. The new postprocessing method captures heterogeneous distributions of nonzero RSS which may provide new information for improving clinical diagnostics and computational modeling of heterogeneous aortic wall mechanics. However, future studies are required to improve the repeatability of RSS and assess the influence of partial volume effects.

Demonstration of circumferential heterogeneity in displacement and strain in the abdominal aortic wall by spiral cine DENSE MRI

BACKGROUND

Knowledge of tissue properties of the abdominal aorta can improve understanding of vascular disease and guide interventional approaches. Existing MRI methods to quantify aortic wall displacement and strain are unable to discern circumferential heterogeneity.

PURPOSE

To assess regional variation in abdominal aortic wall displacement and strain as a function of circumferential position using spiral cine displacement encoding with stimulated echoes (DENSE).

STUDY TYPE

Prospective.

POPULATION

Cardiovascular disease-free men (n = 8) and women (n = 9) ages 30-42.

SEQUENCES

Prospective electrocardiogram (ECG)-gated and navigator echo-gated spiral, cine 2D DENSE and retrospective ECG-gated phase contrast MR (PCMR) sequences at 3T.

ASSESSMENT

In-plane displacement values of the aortic wall acquired with DENSE were used to determine radial and circumferential aortic wall motion. A quadrilateral-based 2D strain calculation method was implemented to determine strain from the displacement field. Peak displacement and its radial and circumferential contributions as well as peak circumferential strain were compared among eight circumferential wall segments. Distensibility was calculated using PCMR and compared with homogenized circumferential strain.

STATISTICAL TESTS

To account for repeated measurements in volunteers, linear mixed models for mean sector values were created for displacement magnitude, circumferential displacement, radial displacement, and circumferential strain. Comparisons were made between sectors. Calculated distensibility and homogenized circumferential strain were compared using Bland-Altman analysis. Statistical significance was defined as P < 0.05.

RESULTS

Displacement was highest in the anterior wall (1.5 ± 0.7 mm) and was primarily in the radial as compared with circumferential direction (1.04 ± 0.05 mm vs. 0.81 ± 0.42 mm). Circumferential strain was highest in the lateral walls (left 0.16 ± 0.05 and right 0.21 ± 0.12) with homogenized circumferential strain of 0.14 ± 0.05.

DATA CONCLUSION

DENSE imaging in the abdominal aortic wall demonstrated that the anterior aortic wall exhibits the greatest displacement, while the lateral wall experiences the largest circumferential strain.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:731-743.

Imaging and finite element analysis: a methodology for non-invasive characterization of aortic tissue

Characterization of the mechanical properties of arterial tissues usually involves an invasive procedure requiring tissue removal. In this work we propose a non-invasive method to perform a biomechanical analysis of cardiovascular aortic tissue. This method is based on combining medical imaging and finite element analysis (FEA). Magnetic resonance imaging (MRI) was chosen since it presents relatively low risks for human health. A finite element model was created from the MRI images and loaded with systolic physiological pressures. By means of an optimization routine, the structural material properties were changed until average strains matched those measured by MRI. The method outlined in this work produced an estimate of the in situ properties of cardiovascular tissue based on non-invasive image datasets and finite element analysis.

MRI strain imaging of the carotid artery: present limitations and future challenges

Rupture of atherosclerotic plaques in the carotid artery is a main cause of stroke. Current diagnostics are not sufficient to identify all rupture-prone plaques, and studies have shown that biomechanical factors improve current plaque risk assessment. Strain imaging may be a valuable contribution to this risk assessment. MRI is a versatile imaging technique that offers various methods that are capable of measuring tissue strain. In this review, MR imaging techniques with displacement (DENSE), velocity (PC MRI), or strain (SENC) encoding protocols are discussed, together with post-processing techniques based on time-resolved MRI data. Although several MRI techniques are being developed to improve time-resolved MR imaging, current technical limitations related to spatial and temporal resolutions render MRI strain imaging currently unfit for carotid plaque strain evaluation. A novel approach using non-rigid image registration of MR images to determine strain in carotid arteries based on black blood cine MRI is proposed in this review. This and other post-processing techniques based on time-resolved MRI data may provide a good estimate of plaque strain, but are also dependent on the spatial and temporal resolution of the MR images. However, they seem to be the most promising approach for MRI based plaque strain analysis in the near future.

Longitudinal differences in the mechanical properties of the thoracic aorta depend on circumferential regions

Understanding the mechanical behavior of the arterial wall and its spatial variations is essential for the study of vascular physiopathology and the design of biomedical devices that interact with the arterial wall. Although it is generally accepted that the aortic wall gets stiffer along its length, the spatial variations in the mechanical behavior of the thoracic aorta are not well understood. In this study, therefore, we investigate both longitudinal and circumferential variations in the mechanical properties of the porcine descending thoracic aorta. Using a previously developed experimental method and stress-strain analysis, the stress, stretch, tangent modulus (TM), and pressure-strain elastic modulus (PSEM) are estimated in the range of in vivo pressure. The results show that the longitudinal differences of both TM and PSEM are statistically significant in the posterior region but not in the anterior region. Both moduli are greater in the posterior distal region when compared with the other test regions. The findings of this study meet a need for clarifying the region investigated, especially in circumferential region, to study the regional variations in biomechanics of blood vessels.

Methods for robust in vivo strain estimation in the carotid artery

A hierarchical block-matching motion tracking algorithm for strain imaging is presented. Displacements are estimated with improved robustness and precision by utilizing a Bayesian regularization algorithm and an unbiased subsample interpolation technique. A modified least-squares strain estimator is proposed to estimate strain images from a noisy displacement input while addressing the motion discontinuity at the wall-lumen boundary. Methods to track deformation over the cardiac cycle incorporate a dynamic frame skip criterion to process data frames with sufficient deformation to produce high signal-to-noise displacement and strain images. Algorithms to accumulate displacement and/or strain on particles in a region of interest over the cardiac cycle are described. New methods to visualize and characterize the deformation measured with the full 2D strain tensor are presented. Initial results from patients imaged prior to carotid endarterectomy suggest that strain imaging detects conditions that are traditionally considered high risk including soft plaque composition, unstable morphology, abnormal hemodynamics and shear of plaque against tethering tissue can be exacerbated by neoangiogenesis. For example, a maximum absolute principal strain exceeding 0.2 is observed near calcified regions adjacent to turbulent flow, protrusion of the plaque into the arterial lumen and regions of low echogenicity associated with soft plaques. Non-invasive carotid strain imaging is therefore a potentially useful tool for detecting unstable carotid plaque.

Preliminary findings in quantification of changes in septal motion during follow-up of type B aortic dissections

OBJECTIVE

To quantify longitudinal changes in intra-arterial septum (IS) motion with two-dimensional (2D) phase-contrast magnetic resonance imaging (2D pcMRI) in type B aortic dissections (AD) to improve the understanding of AD and its midterm development.

METHODS

From a database of 42 patients who underwent a dynamic magnetic resonance imaging (MRI) examination at the Acute Aortic Treatment Center of The Methodist DeBakey Heart & Vascular Center, 2D pcMRI image data was available from 10 patients with type B AD for both short-term (mean, 6.6 days; range, 1-10 days; n = 7) and midterm follow-up (mean, 155 days; range, 60-324; n = 5). IS motion was quantified as motion of IS boundary points averaged over the cardiac cycle. Relative change in IS motion was expressed as percent change compared with initial presentation. Maximum IS extension (true lumen [TL] expansion) and contraction (TL compression), IS fraction in phase with aortic flow and correlation of IS motion with aortic flow (IS compliance) were quantified.

RESULTS

IS motion at initial presentation was 0.68 ± 0.2 mm and was reduced at short-term (0.48 ± 0.3 mm; P = .07) and midterm (0.5 ± 0.2 mm; P = .1) follow-up. Trend in relative change of IS motion was variable during short-term follow-up: reduced in three subjects (-75% ± 6%) and elevated in four subjects (48% ± 23%). During midterm follow-up, relative change in IS motion was reduced in four subjects (28% ± 19%) and slightly elevated in one (6.2%). IS contraction decreased with follow-up while IS extension slightly increased. Fraction of IS moving in phase with aortic flow increased but IS compliance decreased, suggesting increasing IS stiffness.

CONCLUSIONS

Reduction of IS motion in AD is seen with short-term and midterm follow-up. Intersubject variability of this trend is high at short-term follow-up but low at midterm follow-up. Detailed analysis of IS motion parameters indicate reduction of IS contraction and IS compliance with time. This has potential implications for endovascular management of type B aortic dissections, as expansion of aortic stent grafts can be limited by a stiff IS.

Analysis of peripheral artery velocity tracing in a porcine model

BACKGROUND

The aim of the study was to trace the peripheral artery velocity with ultrasound in pigs and provide inference on diagnosis of the type, location and severity of vascular diseases.

MATERIALS AND METHODS

Limb tightening, adrenaline administration and arterial wall pinching were performed independently in six pigs, and then the evolution of the external iliac artery or femoral artery velocity tracing were monitored.

RESULTS

With the increase of the extents of hindlimb tightening, peak systolic velocity (PSV) of ipsilateral external iliac artery turned from 36.33±1.77 cm/s to 59.72±2.67 cm/s, minimum post-principal wave velocity (MPV from 13.68±1.11 cm/s to -7.48±0.82 cm/s, peak diastolic velocity (PDV) from 19.31±0.86 cm/s to 8.98±0.45 cm/s, and, end diastolic velocity (EDV) from 13.2±0.45 cm/s to 0. With the increase of the dose of the epinephrine injection, PSV increased from 36.33±1.77 cm/s to 43.97±2.15 cm/s but then decreased to 35.43±3.01 cm/s, and MPV negatively increased to -23.53±0.82 cm/s after decreasing from 13.68±1.11 cm/s to 0. PDV and EDV gradually decreased to zero. With the increase of the stenosis severity in the abdominal aortic wall pinching, PSV was reduced and had a linearly negative correlation with the stenosis severity (R=0.983, R2=0.967). MPV gradually increased, and its direction reversed when the stenosis severity increased, then diminished when the blood flow was occluded by more than 2/3.

CONCLUSIONS

The formation of peripheral artery velocity is the result of concurrent effects of cardiac ejection, vascular resistance, effective circulating blood volume and elastic recoil. Vascular resistance exerts pronounced effects on the diastolic waveform, and the occurrence of backward wave indicates that the downstream circulation resistance significantly increases.

In vivo quantification of murine aortic cyclic strain, motion, and curvature: implications for abdominal aortic aneurysm growth

PURPOSE

To develop methods to quantify cyclic strain, motion, and curvature of the murine abdominal aorta in vivo.

MATERIALS AND METHODS

C57BL/6J and apoE(-/-) mice underwent three-dimensional (3D) time-of-flight MR angiography to position cardiac-gated 2D slices at four locations along the abdominal aorta where circumferential cyclic strain and lumen centroid motion were calculated. From the 3D data, a centerline through the aorta was created to quantify geometric curvature at 0.1-mm intervals. Medial elastin content was quantified with histology postmortem. The location and shape of abdominal aortic aneurysms (AAAs), created from angiotensin II infusion, were evaluated qualitatively.

RESULTS

Strain waveforms were similar at all locations and between groups. Centroid motion was significantly larger and more leftward above the renal vessels than below (P < 0.05). Maximum geometric curvature occurred slightly proximal to the right renal artery. Elastin content was similar around the circumference of the vessel. AAAs developed in the same location as the maximum curvature and grew in the same direction as vessel curvature and motion.

CONCLUSION

The methods presented provide temporally and spatially resolved data quantifying murine aortic motion and curvature in vivo. This noninvasive methodology will allow serial quantification of how these parameters influence the location and direction of AAA growth.

Automated analysis of four-dimensional magnetic resonance images of the human aorta

The purpose of the study was to demonstrate the accuracy and clinical utility of an automated method of image analysis of 4D (3D + time) magnetic resonance (MR) imaging of the human aorta. Serial MR images of the entire thoracic aorta were acquired on 32 healthy individuals. Graph theory based segmentation was applied to the images and cross sectional area (CSA) was determined for the entire length of thoracic aorta. Mean CSA was compared between the 3 years. CSA values at the level of sinuses of Valsalva and sino-tubular junction were used to calculate average diameters for comparison to Roman-Devereux norms. A robust automated segmentation method was developed that accurately reproduced CSA measurements for the entire length of thoracic aorta in serially acquired scans with a 1% error compared to expert tracing. Calculated aortic root diameters based on CSA correlated with Roman-Devereux norms. Mean CSA for the aortic root agreed well with previously published manually derived values. Automated analysis of 4D MR images of the thoracic aorta provides accurate and reproducible results for CSA in healthy human subjects. The ability to simultaneously analyze the entire length of thoracic aorta throughout the cardiac cycle opens the door to the calculation of novel indices of aortic biophysical properties. These novel indices may lead to earlier detection of patients at risk for adverse events.

An image processing algorithm for the in-vivo quantification and visualization of septum motion in type III B - aortic dissections with cine magnetic resonance imaging

Currently, there is no method to predict outcome of endovascular treatment (EVAR) of type III B aortic dissections (TB-AD). A new image processing algorithm is presented for quantifying IS displacement from cine 2D phase contrast magnetic resonance images (2D pcMRI) towards a new classification of TB-AD based on IS mobility. Bulk motion of the true aortic lumen (tAB) center (ALC), maximum, minimum and average displacement of the boundary points composing the IS and tAB excluding the IS were quantified at two locations in one patient. Correlations of the ALC motion and the averaged temporal displacement AD(t) of IS and tAB excluding IS with the aortic flow waveform were calculated. Range of ALC motion was similar in both locations (average 0.56 mm, max 1.37 mm) and correlated with the aortic flow waveform in the abdominal aorta but not the thoracic aorta. Range of displacement of the IS was from 1.27 mm to -1.64 mm (average 0.09 + or - 0.07 mm) in the thoracic aorta, and from 0.38 mm to -3.38 mm (average 0.42 + or - 0.23 mm) in the abdominal aorta. tAB motion excluding the IS was 1.21 mm to 0.84 mm (thoracic, average 0.13 + or - 0.07 mm) and 0.52 mm to -1.88 mm (abdominal, average 0.37 + or - 0.11 mm). AD(t) for IS and tAB excluding the IS both correlated with aortic flow in the abdominal aorta only.

Phase-contrast MRI-based elastography technique detects early hypertensive changes in ex vivo porcine aortic wall

PURPOSE

To measure the elastic properties of ex vivo porcine aortas in control and hypertensive groups using a phase contrast magnetic resonance imaging (MRI)-based elastography technique.

MATERIALS AND METHODS

Female domestic pigs were randomized to a normal control group (N; n=5) or a renovascular hypertension group (HT; n=5) for the duration of 3 months. Mean arterial pressure was significantly higher in the hypertension group than in the control group (173+/-12 vs. 115+/-11 mmHg, P

RESULTS

The Young's modulus-wall thickness product, a reflection of vascular stiffness, was significantly higher in the hypertension group than in the control group (0.571+/-0.080 vs. 0.419+/-0.026, P<0.05). Histological analysis and staining confirmed increased intima-media thickness and collagen content in the hypertensive aorta, while elastin staining showed no difference.

CONCLUSION

The current study shows that MR elastography offers a method to study the physiologic changes in the arterial wall secondary to early hypertension.

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Key Words

• mri
• elastography
• aorta

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MESH Headings

• Animals
• Aorta, Abdominal/pathology
• Aorta, Abdominal/ultrastructure
• Blood Pressure
• Body Weight
• Cholesterol/blood
• Disease Models, Animal
• Elasticity Imaging Techniques/methods
• Female
• Hypertension/diagnosis
• Magnetic Resonance Imaging/methods
• Microscopy, Phase-Contrast
• Swine

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Grants

• R37 EB001981 NIBIB NIH HHS
• DK73608 NIDDK NIH HHS
• R01 EB001981-08 NIBIB NIH HHS
• R01 DK073608 NIDDK NIH HHS
• R01 EB001981 NIBIB NIH HHS
• EB001981 NIBIB NIH HHS

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Affiliation(s)

David A Woodrum Radiology Department, Mayo Clinic, Rochester, Minnesota 55905, USA.
Joerg Herrmann
Amir Lerman
Anthony J Romano
Lilach O Lerman
Richard L Ehman

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Evaluation of wall motion and dynamic geometry of the inferior vena cava using intravascular ultrasound: implications for future device design

PURPOSE

To use intravascular ultrasound (IVUS) to define the wall motion of the inferior vena cava (IVC) during normal respiratory cycles and evaluate its dynamic geometry during Valsalva maneuvers.

METHODS

Between September 2005 and October 2006, 10 patients who were having IVC filters placed underwent IVUS prior to filter implantation. With the anesthetized patient in a supine position, a 10-second IVUS recording of IVC motion below the renal veins was made during both normal respiratory cycles and Valsalva maneuvers. Diameters (n = 100 measurements) were measured from the epicenter of the lumen in both a long and short axis. Changes in diameters were evaluated using a Student t test for paired data; variations in IVC wall motion circumference of the vessel were compared using an analysis of variance for repeated measurements. Intra-/interobserver variability was analyzed with Bland-Altman plots.

RESULTS

The mean IVC diameter was 14.3+/-4.1 mm in the short axis and 23.2+/-3.5 mm in the long axis. There was significant variation in infrarenal IVC wall movement about the circumference, with 1.4+/-0.2 mm (range 0.6-1.8) displacement in the short axis and 1.0+/-0.2 mm (range 0.2-1.4) displacement in the long axis during the normal respiratory cycle (p = 0.04). In the short axis, the IVC diameter significantly increased with Valsalva from 14.3+/-4.1 to 19.6+/-1.2 mm (p = 0.0001); in the long axis, the diameter increased from 23.2+/-3.5 to 24+/-1.2 mm (p = 0.02). With Valsalva, there was a significantly greater change in the short axis (30.9%+/-4.8%) compared to the long axis (3.4%+/-2.2%; p = 0.0001). There were no significant differences in the interobserver and intraobserver measurements.

CONCLUSION

In the supine position, the IVC is elliptical and deforms anisotropically during the normal respiratory cycle. The greatest displacement (36%) is in the short axis during a Valsalva maneuver. These profound changes within the venous system will require intracaval devices to have active fixation to prevent migration. Devices should be designed to accommodate these changes to prevent fatigue failure.

Increased anterior abdominal aortic wall motion: possible role in aneurysm pathogenesis and design of endovascular devices

PURPOSE

To determine whether variations in aortic wall motion exist in mammalian species other than humans and to consider the potential implications of such variations.

METHODS

M-mode ultrasound was used to measure abdominal aortic wall motion in 4 animal species [mice (n=10), rats (n=8), rabbits (n=7), and pigs (n=5)], and humans (n=6). Anterior wall displacement, posterior wall displacement, and diastolic diameter were measured. The ratio of displacement to diameter and cyclic strain were calculated.

RESULTS

Body mass varied from 24.1+/-2.4 g (mouse) to 61.8+/-13.4 kg (human); aortic diameter varied from 0.53+/-0.07 mm (mouse) to 1.2+/-1 mm (human). Anterior wall displacement was 2.5 to 4.0 times greater than posterior among the species studied. The ratios of wall displacement to diastolic diameter were similar for the anterior (range 9.40%-11.80%) and posterior (range 2.49%-3.91%) walls among species. The ratio of anterior to posterior displacement (range 2.47-4.03) and aortic wall circumferential cyclic strain (range 12.1%-15.7%) were also similar. An allometric scaling exponent was experimentally derived relating anterior wall (0.377+/-0.032, R2=0.94) and posterior wall (0.378+/-0.037, R2=0.93) displacement to body mass.

CONCLUSION

Abdominal aortic wall dynamics are similar in animals and humans regardless of aortic size, wih more anterior than posterior wall motion. Wall displacement increases linearly with diameter, but allometrically with body mass. These data suggest increased dynamic strain of the anterior wall. Increased strain, corresponding to increased elastin fatigue, may help explain why human abdominal aortic aneurysms initially develop anteriorly. Aortic wall motion should be considered when developing endovascular devices, since asymmetric motion may affect device migration, fixation, and sealing.

Valvular endothelial cells and the mechanoregulation of valvular pathology

Endothelial cells are critical mediators of haemodynamic forces and as such are important foci for initiation of vascular pathology. Valvular leaflets are also lined with endothelial cells, though a similar role in mechanosensing has not been demonstrated. Recent evidence has shown that valvular endothelial cells respond morphologically to shear stress, and several studies have implicated valvular endothelial dysfunction in the pathogenesis of disease. This review seeks to combine what is known about vascular and valvular haemodynamics, endothelial response to mechanical stimuli and the pathogenesis of valvular diseases to form a hypothesis as to how mechanical stimuli can initiate valvular endothelial dysfunction and disease progression. From this analysis, it appears that inflow surface-related bacterial/thrombotic vegetative endocarditis is a high shear-driven endothelial denudation phenomenon, while the outflow surface with its related calcific/atherosclerotic degeneration is a low/oscillatory shear-driven endothelial activation phenomenon. Further understanding of these mechanisms may help lead to earlier diagnostic tools and therapeutic strategies.

Endovascular Repair of Thoracic and Abdominal Aortic Aneurysms: Pre- and Postprocedural Imaging

Endovascular repair of thoracic and abdominal aortic aneurysms is a safe alternative to conventional open surgical repair. Clinical success, however, is highly dependent on patient selection. Diagnostic vascular imaging has an essential role for this selection process. Following endovascular aneurysm repair (EVAR), patients require long-term surveillance and again vascular imaging serves an integral function. This article reviews EVAR selection criteria and post-EVAR assessment and then discusses the imaging modalities used to evaluate these patients, namely multi-detector-row computed tomographic angiography, magnetic resonance imaging/angiography, duplex ultrasonography, and catheter angiography.