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Obstructive sleep apnea syndrome is one of the most common sleep disorders. To treat patients with this health problem, it is important to detect the severity of this syndrome and occlusion sites in each patient. The goal of this study is to test the hypothesis that the cure of obstructive sleep apnea syndrome by maxillomandibular advancement surgery can be predicted by analyzing the effect of anatomical airway changes on the pressure effort required for normal breathing using a high-fidelity, 3-D numerical model. The employed numerical model consists of: (1) 3-D upper airway geometry construction from patient-specific computed tomographic scans using an image segmentation technique, (2) mixed-element mesh generation of the numerically constructed airway geometry for discretizing the domain of interest, and (3) computational fluid dynamics simulations for predicting the flow field within the airway and the degree of severity of breathing obstruction. In the present study, both laminar and turbulent flow simulations were performed to predict the flow field in the upper airway of the selected patients before and after maxillomandibular advancement surgery. Patients of different body mass indices were also studied to assess their effects. The numerical results were analyzed to evaluate the pressure gradient along the upper airway. The magnitude of the pressure gradient is regarded as the pressure effort required for breathing, and the extent of reduction of the pressure effort is taken to measure the success of the surgery. The description of the employed numerical model, numerical results from simulations of various patients, and suggestion for future work are detailed in this paper.

Medical device user interfaces are increasingly complex, resulting in a need for evaluation in clinicallyaccurate settings. Simulation of these interfaces can allow for evaluation, training, and use for research without the risk of harming patients and with a significant cost reduction over using the actual medical devices. This pilot project was phase 1 of a study to define and evaluate a methodology for development of simulated medical device interface technology to be used for education, device development, and research. Digital video and audio recordings of interface interactions were analyzed to develop a model of a smart intravenous medication infusion pump user interface. This model was used to program a high-fidelity simulated smart intravenous medication infusion pump user interface on an inexpensive netbook platform.

The objective of this study was to examine the role of body mass and subcutaneous fat in injury severity and pattern sustained by overweight drivers. Finite element models were created to represent the geometry and properties of subcutaneous adipose tissue in the torso with data obtained from reconstructed magnetic resonance imaging data-sets. The torso adipose tissue models were then integrated into the standard multibody dummy models together with increased inertial parameters and sizes of the limbs to represent overweight occupants. Frontal crash simulations were carried out considering a variety of occupant restraint systems and regional body injuries were measured. The results revealed that differences in body mass and fat distribution have an impact on injury severity and pattern. Even though the torso adipose tissue of overweight subjects contributed to reduce abdominal injury, the momentum effect of a greater body mass of overweight subjects was more dominant over the cushion effect of the adipose tissue, increasing risk of other regional body injuries except abdomen. Through statistical analysis of the results, strong correlations (p 0.01) were found between body mass index and regional body injuries except neck injury. The analysis also revealed that a greater momentum of overweight males leads to greater forward torso and pelvic excursions that account for higher risks (p 0.001) of head, thorax and lower extremity injury than observed in non-overweight males. The findings have important implications for improving the vehicle and occupant safety systems designed for the increasing global obese population.

This study was designed to determine if three-dimensional (3D) laser scanning techniques could be used to collect accurate anthropometric measurements, compared with traditional methods. The use of an alternative 3D method would allow for quick collection of data that could be used to change the parameters used for facepiece design, improving fit and protection for a wider variety of faces. In our study, 10 facial dimensions were collected using both the traditional calipers and tape method and a Konica-Minolta Vivid9i laser scanner. Scans were combined using RapidForm XOR software to create a single complete facial geometry of the subject as a triangulated surface with an associated texture image from which to obtain measurements. A paired t-test was performed on subject means in each measurement by method. Nine subjects were used in this study: five males (one African-American and four Caucasian females) and four females displaying a range of facial dimensions. Five measurements showed significant differences (p0.05), with most accounted for by subject movements or amended by scanning technique modifications. Laser scanning measurements showed high precision and accuracy when compared with traditional methods. Significant differences found can be very small changes in measurements and are unlikely to present a practical difference. The laser scanning technique demonstrated reliable and quick anthropometric data collection for use in future projects in redesigning respirators.

The objective of this paper is the reconstruction of upper airway geometric models as hybrid meshes from clinically used Computed Tomography (CT) data sets in order to understand the dynamics and behaviors of the pre- and postoperative upper airway systems of Obstructive Sleep Apnea Syndrome (OSAS) patients by viscous Computational Fluid Dynamics (CFD) simulations. The selection criteria for OSAS cases studied are discussed because two reasonable pre- and postoperative upper airway models for CFD simulations may not be created for every case without a special protocol for CT scanning. The geometry extraction and manipulation methods are presented with technical barriers that must be overcome so that they can be used along with computational simulation software as a daily clinical evaluation tool. Eight cases are presented in this paper, and each case consists of pre- and postoperative configurations. The results of computational simulations of two cases are included in this paper as demonstration.

In this paper, we present a technique for generating solution adaptive meshes that uses feature-aligned surfaces embedded in the computational domain. A feature-detection algorithm is first employed to identify features of interest. Surfaces associated with these features are then meshed, and a volume mesh that respects these embedded surface meshes is then generated. This approach is promising because it provides a mechanism for controlling local grid quality as well as automatically aligning cell faces with the feature. For flows that include topologically and/or geometrically complex features, we describe a mesh refinement technique that can be used in tandem with the embedded surface meshes to provide a mechanism for refining throughout the computational domain. We include results from two cases to illustrate how the method works.

The amelioration of obstructive sleep apnea syndrome (OSAS) by maxillomandibular advancement (MMA) surgery can be predicted by analyzing anatomical airway changes with 3-dimensional (3D) geometrical reconstruction and computational fluid dynamics. Computer Enabling Technology Lab (ETLab) and Computational Simulation Lab (CSLab) can be used to analyze anatomic airway change for previously operated patients with a clinical cure of OSAS. MMA surgery reduces airway resistance and pressure effort (gradient) of OSAS by increasing the dimension of the airway. ETLab has been used to reconstruct the upper airway as a 3D computer model (bone and soft tissue surrounding the pharyngeal airway) from existing helical computed tomography scan format of OSAS patients. ETLab can compare and construct the geometry with numerical meshes of the airway between pre- and postoperative MMA by the use of bioengineering software. This technology uses high-fidelity computation fluid dynamic simulations, developed at the CSLab, for prediction and analysis of the flow field in the airway for pre- and postoperative MMA. It is possible to use the simulation to predict the likely success of future treatment and develop a prognostic factor. The soft- and hard-tissue mesh is used to determine the pre- and postoperative differences in the facial and pharyngeal tongue base for soft-tissue change associated with hard-tissue movement. This correlation predicts the amount of surgical movement necessary to create an adequate airflow. These results help define the surgical techniques in OSAS for more precise identification of upper airway anatomical features. This process correlates the area and pressure change at the velopharynx, oropharynx, and retroglossal space of the upper airway by the ETLab. Results can compare with polysomnogram and cure rates. 3D computer analysis can be used to test flow dynamics in the human airway for surgical treatment of OSAS.

This paper describes a new efficient automatic remeshing method for three-dimensional hybrid meshes for viscous flow simulations to accommodate the meshes with changes of small devices quickly and easily. This remeshing method has two notable advantages. First, hybrid meshes can be generated automatically from a baseline mesh when the small devices are moved and/or deformed. This enables shape optimization techniques to find better models quickly, because tens or hundreds of meshes are usually required during the process. Second, the updated hybrid meshes are the same as the baseline mesh except for elements around the small devices. Their effect can be evaluated more accurately. Solution data from the baseline mesh can be shared with new hybrid meshes (e.g., as an initial condition to expedite the convergence of computational simulations). The remeshing method is applied to the Japan Aerospace Exploration Agency's high-lift-configuration standard model with a nacelle chine in different locations to demonstrate its capability. Computational simulations are also performed to further discuss the effectiveness of the remeshing method.

An octree-based mesh generation method is proposed to create reasonable-quality, geometry-adapted unstructured hexahedral meshes automatically from triangulated surface models without any sharp geometrical features. A new, easy-to-implement, easy-to-understand set of refinement templates is developed to perform local mesh refinement efficiently even for concave refinement domains without creating hanging nodes. A buffer layer is inserted on an octree core mesh to improve the mesh quality significantly. Laplacian-like smoothing, angle-based smoothing and local optimization-based untangling methods are used with certain restrictions to further improve the mesh quality. Several examples are shown to demonstrate the capability of our hexahedral mesh generation method for complex geometries. Copyright © 2008 John Wiley & Sons, Ltd.

This paper proposes a method for the creation of hybrid meshes with embedded surfaces for viscous flow simulations as an extension of the multiple marching direction approach (AIAA J. 2007; 45(1):162–167). The multiple marching direction approach enables to place semi-structured elements around singular points, where valid semi-structured elements cannot be placed using conventional hybrid mesh generation methods. This feature is discussed first with a couple of examples. Elements sometimes need to be clustered inside a computational domain to obtain more accurate results. For example, solution features, such as shocks, vortex cores and wake regions, can be extracted during the process of adaptive mesh generation. These features can be represented as surface meshes embedded in a computational domain. Semi-structured elements can be placed around the embedded surface meshes using the multiple marching direction approach with a pretreatment method. Tetrahedral elements can be placed easily instead. A couple of results are presented to demonstrate the capability of the mesh generation method. Copyright © 2008 John Wiley & Sons, Ltd.

Mesh generation is a critical step in high fidelity computational simulations. High-quality and high-density meshes are required to accurately capture the complex physical phenomena. A robust approach for a parallel framework has been developed to generate large-scale meshes in a short period of time. A coarse tetrahedral mesh is generated first to provide the basis of block interfaces and then is partitioned into a number of sub-domains using METIS partitioning algorithms. A volume mesh is generated on each sub-domain in parallel using an advancing front method. Dynamic load balancing is achieved by evenly distributing work among the processors. All the sub-domains are combined to create a single volume mesh. The combined volume mesh can be smoothed to remove the artifacts in the interfaces between sub-domains. A void region is defined inside each sub-domain to reduce the data points during the smoothing operation. The scalability of the parallel mesh generation is evaluated to quantify the improvement on shared- and distributed-memory computer systems.

This paper describes the method to generate hybrid meshes composed of triangular prisms, pyramids, hexahedra, and tetrahedra for viscous computational fluid dynamics simulations. Surface triangulation is performed using a direct advancing front method or a modified mesh-decimation method. From a surface mesh, a near-field mesh is generated using an advancing layer approach. To generate high-quality meshes and to mesh around singular points, multiple marching directions are prepared from nodes on sharp convex corners. Special placement of elements around the sharp convex corners avoids using generalized elements. Tetrahedral meshing is then performed to fill the rest of the domain using an advancing front method. The hybrid mesh generation method is applied to several models to demonstrate its capability.

In this paper, we propose efficient and robust unstructured mesh generation methods based on computed tomography (CT) and magnetic resonance imaging (MRI) data, in order to obtain a patient-specific geometry for high-fidelity numerical simulations. Surface extraction from medical images is carried out mainly using open source libraries, including the Insight Segmentation and Registration Toolkit and the Visualization Toolkit, into the form of facet surface representation. To create high-quality surface meshes, we propose two approaches. One is a direct advancing front method, and the other is a modified decimation method. The former emphasizes the controllability of local mesh density, and the latter enables semi-automated mesh generation from low-quality discrete surfaces. An advancing-front-based volume meshing method is employed. Our approaches are demonstrated with high-fidelity tetrahedral meshes around medical geometries extracted from CT/MRI data. Copyright © 2005 John Wiley & Sons, Ltd.

High fidelity simulations using mesh-based computational technologies such as computational fluid dynamics and computational structural mechanics are very important in providing valuable performance information of a design. These simulations, however, require accurate geometry representation as well as high quality meshes about the design in order to obtain accurate data. Unfortunately, while three-dimensional complex geometry is involved, it is a challenge to maintain geometry fidelity while trying to obtain high quality meshes. This paper discusses detailed information associated with non-uniform rational B-splines (NURBS) and its utilizations for geometry and mesh generation. It will go beyond regular curve and surface formulations to include NURBS volume and dynamic NURBS. Detailed NURBS formulations are described through mathematical expressions and applications to demonstrate the versatility and reliability of NURBS in geometry and mesh generation that enable high performance computational simulations.

Arteriovenous (AV) grafts and fistulas used for hemodialysis frequently develop intimal hyperplasia (IH) at the venous anastomosis of the graft, leading to flow-limiting stenosis, and ultimately to graft failure due to thrombosis. Although the high AV access blood flow has been implicated in the pathogenesis of graft stenosis, the potential role of needle turbulence during hemodialysis is relatively unexplored. High turbulent stresses from the needle jet that reach the venous anastomosis may contribute to endothelial denudation and vessel wall injury. This may trigger the molecular and cellular cascade involving platelet activation and IH, leading to eventual graft failure. In an in-vitro graft/needle model dye injection flow visualization was used for qualitative study of flow patterns, whereas laser Doppler velocimetry was used to compare the levels of turbulence at the venous anastomosis in the presence and absence of a venous needle jet. Considerably higher turbulence was observed downstream of the venous needle, in comparison to graft flow alone without the needle. While turbulent RMS remained around 0.1m∕s for the graft flow alone, turbulent RMS fluctuations downstream of the needle soared to 0.4–0.7m∕s at 2 cm from the tip of the needle and maintained values higher than 0.1m∕s up to 7–8 cm downstream. Turbulent intensities were 5–6 times greater in the presence of the needle, in comparison with graft flow alone. Since hemodialysis patients are exposed to needle turbulence for four hours three times a week, the role of post-venous needle turbulence may be important in the pathogenesis of AV graft complications. A better understanding of the role of needle turbulence in the mechanisms of AV graft failure may lead to improved design of AV grafts and venous needles associated with reduced turbulence, and to pharmacological interventions that attenuate IH and graft failure resulting from turbulence.

Background Surgery is a highly technical field that combines continuous decision-making with the coordination of spatiovisual tasks. Objective We designed a virtual interactive presence and augmented reality (VIPAR) platform that allows a remote surgeon to deliver real-time virtual assistance to a local surgeon, over a standard Internet connection. Methods The VIPAR system consisted of a "local" and a "remote" station, each situated over a surgical field and a blue screen, respectively. Each station was equipped with a digital viewpiece, composed of 2 cameras for stereoscopic capture, and a high-definition viewer displaying a virtual field. The virtual field was created by digitally compositing selected elements within the remote field into the local field. The viewpieces were controlled by workstations mutually connected by the Internet, allowing virtual remote interaction in real time. Digital renderings derived from volumetric MRI were added to the virtual field to augment the surgeon's reality. For demonstration, a fixed-formalin cadaver head and neck were obtained, and a carotid endarterectomy (CEA) and pterional craniotomy were performed under the VIPAR system. Results The VIPAR system allowed for real-time, virtual interaction between a local (resident) and remote (attending) surgeon. In both carotid and pterional dissections, major anatomic structures were visualized and identified. Virtual interaction permitted remote instruction for the local surgeon, and MRI augmentation provided spatial guidance to both surgeons. Camera resolution, color contrast, time lag, and depth perception were identified as technical issues requiring further optimization. Conclusion Virtual interactive presence and augmented reality provide a novel platform for remote surgical assistance, with multiple applications in surgical training and remote expert assistance.

Background Obesity and motor vehicle crash (MVC) injuries are two parallel epidemics in the United States. An important unanswered question is if there are sex differences in the associations between the presence of obesity and non-fatal MVC injuries. Objectives To further understand the association between obesity and non-fatal motor vehicle crash injuries, particularly the sex differences in these relations. Methods We examined this question by analyzing data from the 2003 to 2007 National Automotive Sampling System Crashworthiness Data System (NASS CDS). A total of 10, 962 drivers who were aged 18 years or older and who survived frontal collision crashes were eligible for study. Results Male drivers experienced a lower rate of overall non-fatal MVC injuries than did female drivers (38.1% vs. 52.2%) but a higher rate of severe injuries (0.7% vs. 0.2%). After adjusting for change in velocity (ΔV) during the crashes, obese male drivers showed a much higher risk [logistic coefficients of BMI for moderate, serious, and severe injury are 0.0766, 0.1470, and 0.1792, respectively; all p

The grid generation (or mesh generation) process is a critical yet challenging step in the simulations that solve numerically nonlinear partial differential equations (PDEs) representative of fluid dynamics. It is often a tedious and labor-intensive process when complex geometries are involved. There are many techniques in the grid generation process. In this chapter, concepts, strategies, algorithms, and software tools employed in the grid generation process are described. A brief outline of the grid generation process and adaptive grid generation is presented. Structured grid systems with Cartesian and curvilinear strategies and unstructured grid systems with quadtree/octree, Delaunay triangulation, advancing front, and advancing layers methods are summarized to provide readers with a clear overview of these important approaches. Software tools and literature sources for further information on grid generation techniques and applications are stated. In the final section, future trends in grid generation and technology vision are provided. Computational examples are presented throughout this chapter to demonstrate applications of grid strategies. Keywords: structured, unstructured, and hybrid grid generation; algebraic, elliptic, and hyperbolic systems; curvilinear grids; Cartesian grids; Delaunay triangulation; advancing front methods; adaptive grid refinement; redistribution; Computer Aided Geometry Design (CAGD)

Fluid flow simulations through a valve often require evaluation of the valve in multiple opening positions. A mesh has to be generated for the valve for each position and compounding. The problem is the fact that the valve is typically part of a larger feedline system. In this paper, we propose to develop a system to create meshes for feedline systems with parametrically controlled valve openings. Herein we outline two approaches to generate the meshes for a valve in a feedline system at multiple positions. There are two issues that must be addressed. The first is the creation of the mesh on the valve for multiple positions. The second is the generation of the mesh for the total feedline system including the valve. For generation of the mesh on the valve, we will describe the use of topology matching and mesh generation parameter transfer. For generation of the total feedline system, we will describe two solutions that we have implemented. In both cases the valve is treated as a component in the feedline system. In the first method the geometry of the valve in the feedline system is replaced with a valve at a different opening position. Geometry is created to connect the valve to the feedline system. Then topology for the valve is created and the portion of the topology for the valve is topology matched to the standard valve in a different position. The mesh generation parameters are transferred and then the volume mesh for the whole feedline system is generated. The second method enables the user to generate the volume mesh on the valve in multiple open positions external to the feedline system, to insert it into the volume mesh of the feedline system, and to reduce the amount of computer time required for mesh generation because only two small volume meshes connecting the valve to the feedline mesh need to be updated.

We proposed a new, easy-to-implement, easy-to-understand set of refinement templates in our previous paper to create geometry-adapted all-hexahedral meshes easily [Ito et al., “Octree-Based Reasonable-Quality Hexahedral Mesh Generation Using a New Set of Refinement Templates,” International Journal for Numerical Methods in Engineering, Vol. 77, Issue 13, March 2009, pp. 1809-1833, DOI: 10.1002/nme.2470]. This paper offers an extension of the template set to refine concave domains more efficiently. In addition, two new options, temporal rotation and temporal local inflation, are introduced to create hexahedral meshes with fewer elements for long objects or objects with thin regions. Several examples are shown to discuss the improvements of the mesh generation method.

The pediatric pelvis is vulnerable to injuries in motor vehicle collisions, sport activities, and fall accidents. Pelvic fractures and injury mechanisms in children differ substantially from those found in adults [1]. While the injury mechanisms and tolerances of the adult pelvis have been fairly well characterized through cadaveric experiments and computational models, efforts related to the pediatric pelvis have been limited due to difficulties in acquiring and testing pediatric cadavers. The objective of this study was to develop a finite element (FE) model of a 10-year-old (10YO) human pelvis to provide more comprehensive understanding of injury mechanisms experienced by children.Copyright © 2008 by ASME

The influence of wall motion on the hemodynamic characteristics of the human femoral bifurcation and its effects on the development of peripheral artery disease has not been previously investigated. This study aimed in investigating the hemodynamics of a compliant patient-specific femoral artery bifurcation model by a fluid structure interaction (FSI) scheme. The complex physiological geometry of the femoral artery bifurcation was reproduced from sequentially obtained transverse CT scan images. Velocity waveforms derived from phase contrast MR images were extracted and mapped to define boundary conditions. Equations governing blood flow and wall motion were solved using an FSI framework that utilizes commercial codes: FLUENT for computational fluid dynamics and ANSYS for computational structural dynamics. The results showed that wall compliance decreased flow velocities at the relatively high curvature geometries including common and superficial femoral artery (SFA), and it created strong recirculation in the profunda femoris artery close to the bifurcation. In the SFA region near the apex, time averaged wall shear stress (TAWSS) differences up to 25% between compliant and rigid models were observed. The compliant model also exhibited lower TAWSS and oscillatory shear at the superior section of the common femoral artery close to the bifurcation. The presence of wall motion, however, created minor differences in the general flow-field characteristics. We conclude that wall motion does not have significant influence on the global fluid dynamic characteristics of the femoral artery bifurcation. Longer arterial segments need to be simulated to see the effect of wall motion on tortuousity which was previously cited as an important factor in the development of atherosclerosis at the femoral artery.

A three-dimensional (3D) geometric model obtained from a 3D device or other approaches is not necessarily watertight due to the presence of geometric deficiencies. These inadequacies must be repaired to create a valid surface mesh on the model as a pre-process of computational engineering analyses. This procedure has been a tedious and labor-intensive step, as there are many kinds of deficiencies that can make the geometry to be nonwatertight, such as gaps and holes. It is still challenging to repair discrete surface models based on available geometric information. The focus of this paper is to develop a new automated method for patching holes on the surface models in order to achieve watertightness. It describes a numerical algorithm utilizing Non-Uniform Rational B-Splines (NURBS) surfaces to generate smooth triangulated surface patches for topologically simple holes on discrete surface models. The Delaunay criterion for point insertion and edge swapping is used in this algorithm to improve the outcome. Surface patches are generated based on existing points surrounding the holes without altering them. The watertight geometry produced can be used in a wide range of engineering applications in the field of computational engineering simulation studies.

Computational Fluid Dynamics (CFD) has become a crucial tool to predict and analyze flow field within a domain. When engineers understand the complex flow patterns, reliable results can be provided. However, accuracy improvement, confidence development, throughput increase, and cost reduction are the four key challenges to CFD. Solution throughput in the generation of aerodynamics, propulsion, and fluid dynamics simulations that involve parametric and sensitivity studies of design and analysis must be significantly improved. Such parametric and sensitivity studies require a large number of CFD simulation runs. However, preparing and managing such a large number of CFD simulation jobs is a challenging problem even for experienced users; compounded by the complexity of running these simulations with different cluster environments on the Department of Defense (DoD) High Performance Computing (HPC) systems. Moreover, different groups of users may prefer a different CFD solver that requires a different input file format and outputs solutions in various flavors.

This paper deals with structured grid generation using Floater’s parametrization algorithm for surface triangulation. It gives an outline of the algorithm in the context of structured grid generation. Then it explains how the algorithm can be used to generate a structured grid over a singular NURBS surface patch. This is an alternate method to the known carpeting method of reparametrization for structured grid generation over a NURBS surface patch. The paper also explains how to generate a structured grid over a four sided trimmed patch of a facetted surface using the parametrization algorithm. All the procedures are explained using examples.

‡This paper describes the implementation of the Unstructured Grid Consortium (UGC) Application Programming Interface (API) for unstructured mesh generation into a preexisting grid generation library. Modifications to bring the edge and structured surface meshing capabilities of the library into conformance with the UGC specification are described. The flexibility afforded by the API for rapidly moving technology between applications is demonstrated by implementing the UGC compliant library into two separate mesh-generation applications. The lessons learned from this effort are presented and suggestions for future improvement to the UGC API are provided.

This paper describes the method to generate hybrid meshes comprised of triangular prisms, pyramids, hexahedra and tetrahedra for viscous computational fluid dynamics (CFD) simulations. Surface triangulation is performed using a direct advancing front method or a modified mesh-decimation method. From a surface mesh, a near-field mesh is generated using an advancing layer approach. To generate high quality meshes and to mesh around singular points, multiple marching directions are prepared from nodes on sharp convex corners. Tetrahedral meshing is then performed to fill the rest of the domain using an advancing front method. The hybrid mesh generation method is applied to several models to demonstrate its capability.

Purpose To evaluate different grades of in-stent stenosis in a nickel-titanium stent with MRI. Materials and Methods Magnetic resonance phase velocity mapping (MR-PVM) was used to measure flow velocity through a 9-mm NiTi stent with three different degrees of stenosis in a phantom study. The tested stenotic geometries were 1) axisymmetric 75%, 2) axisymmetric 90%, and 3) asymmetric 50%. The MR-PVM data were subsequently compared with the velocities from computational fluid dynamic (CFD) simulations of identical conditions. Results Good quantitative agreement in velocity distribution for the 50% and 75% stenoses was observed. The agreement was poor for the 90% stenosis, most likely due to turbulence and the high-velocity gradients found in the small luminal area relative to the pixel resolution in our imaging settings. Conclusion The accuracy of the MRI velocities inside the stented area renders MRI a modality that may be used to assess moderate to severe in-stent restenosis (ISR) in medium-sized vascular stents in peripheral vessels, such as the iliac, carotid, and femoral arteries. Advances in MR instrumentation may provide sufficient resolution to obtain adequate velocity information from smaller vessels, such as the coronary arteries, and allow MRI to substitute for invasive and expensive catheterization procedures currently in clinical use. J. Magn. Reson. Imaging 2005;22:248–257. © 2005 Wiley-Liss, Inc.