In vivo monitoring of the inflammatory response in a stented mouse aorta model.

The popularity of vascular stents continues to increase for a variety of applications, including coronary, lower limb, renal, carotid, and neurovascular disorders. However, their clinical effectiveness is hindered by numerous postdeployment complications, which may stimulate inflammatory and fibrotic reactions. The purpose of this study was to evaluate the vessel inflammatory response via in vivo imaging in a mouse stent implantation model. Corroded and noncorroded self-expanding miniature nitinol stents were implanted in mice abdominal aortas, and novel in vivo imaging techniques were used to assess trafficking and accumulation of fluorescent donor monocytes as well as cellular proliferation at the implantation site. Monocytes were quantitatively tracked in vivo and found to rapidly clear from circulation within hours after injection. Differences were found between the test groups with respect to the numbers of recruited monocytes and the intensity of the resulting fluorescent signal. Image analysis also revealed a subtle increase in matrix metalloproteinase activity in corroded compared with the normal stented aortas. In conclusion, this study has been successful in developing a murine stent inflammation model and applying novel in vivo imaging tools and methods to monitor the complex biological processes of the host vascular wall response.

The Computational Fluid Dynamics Rupture Challenge 2013--Phase II: Variability of Hemodynamic Simulations in Two Intracranial Aneurysms.

With the increased availability of computational resources, the past decade has seen a rise in the use of computational fluid dynamics (CFD) for medical applications. There has been an increase in the application of CFD to attempt to predict the rupture of intracranial aneurysms, however, while many hemodynamic parameters can be obtained from these computations, to date, no consistent methodology for the prediction of the rupture has been identified. One particular challenge to CFD is that many factors contribute to its accuracy; the mesh resolution and spatial/temporal discretization can alone contribute to a variation in accuracy. This failure to identify the importance of these factors and identify a methodology for the prediction of ruptures has limited the acceptance of CFD among physicians for rupture prediction. The International CFD Rupture Challenge 2013 seeks to comment on the sensitivity of these various CFD assumptions to predict the rupture by undertaking a comparison of the rupture and blood-flow predictions from a wide range of independent participants utilizing a range of CFD approaches. Twenty-six groups from 15 countries took part in the challenge. Participants were provided with surface models of two intracranial aneurysms and asked to carry out the corresponding hemodynamics simulations, free to choose their own mesh, solver, and temporal discretization. They were requested to submit velocity and pressure predictions along the centerline and on specified planes. The first phase of the challenge, described in a separate paper, was aimed at predicting which of the two aneurysms had previously ruptured and where the rupture site was located. The second phase, described in this paper, aims to assess the variability of the solutions and the sensitivity to the modeling assumptions. Participants were free to choose boundary conditions in the first phase, whereas they were prescribed in the second phase but all other CFD modeling parameters were not prescribed. In order to compare the computational results of one representative group with experimental results, steady-flow measurements using particle image velocimetry (PIV) were carried out in a silicone model of one of the provided aneurysms. Approximately 80% of the participating groups generated similar results. Both velocity and pressure computations were in good agreement with each other for cycle-averaged and peak-systolic predictions. Most apparent "outliers" (results that stand out of the collective) were observed to have underestimated velocity levels compared to the majority of solutions, but nevertheless identified comparable flow structures. In only two cases, the results deviate by over 35% from the mean solution of all the participants. Results of steady CFD simulations of the representative group and PIV experiments were in good agreement. The study demonstrated that while a range of numerical schemes, mesh resolution, and solvers was used, similar flow predictions were observed in the majority of cases. To further validate the computational results, it is suggested that time-dependent measurements should be conducted in the future. However, it is recognized that this study does not include the biological aspects of the aneurysm, which needs to be considered to be able to more precisely identify the specific rupture risk of an intracranial aneurysm.

The effect of head rotation on the geometry and hemodynamics of healthy vertebral arteries.

The geometric and hemodynamic characteristics of the left and right vertebral arteries (LVA, RVA) of six healthy volunteers were investigated for the supine (S) and the prone position (P) a common sleeping posture with head rotation. MRI images were used to reconstruct the subject specific three-dimensional solid models of the LVA and RVA from the level of the carotid bifurcation to the vertebrobasilar junction (VJ). Geometric parameters such as cross sectional area ratio, curvature, tortuosity and branch angle were estimated. MR-PCA was used to obtain the blood flow waveforms for the two positions and computational fluid dynamics (CFD) were used to assess the flow field in terms of wall shear stress (WSS) relative residence times (RRT) and localized normalized helicity (LNH). Significant geometric changes but moderate flow changes were observed for both vertebral arteries with head rotation. The CFD results at the VJ show that head rotation causes changes in the WSS distribution, RRT and LNH. Further studies are warranted to assess the clinical significance of the results in terms of atherosclerosis development at the VJ and how the observed geometric changes may affect blood flow to the brain in healthy subjects and vertebral artery stenosis patients, and in terms of increased rapture susceptibility in vertebrobasilar aneurysm patients.

Impact of head rotation on the individualized common carotid flow and carotid bifurcation hemodynamics.

This paper aims at evaluating the changes that head rotation poses on morphological and flow characteristics of the carotid bifurcation (CB) and on the distribution of parameters that are regarded as important in atherosclerosis development, such as relative particle residence time (RRT), normalized oscillatory shear index (nOSI), and helicity (HL). Using a subject-specific approach, six healthy volunteers were MR-scanned in two head postures: supine neutral and prone with rightward head rotation. Cross-sectional flow velocity distribution was obtained using phase-contrast MRI at the common carotid artery (CCA). Our results indicate that peak systolic flow rate is reduced at the prone position in most cases for both CCAs. Morphological MR images are used to segment and construct the CB models. Numerical simulations are performed and areas exposed to high helicity or unfavorable hemodynamics are calculated. Head rotation affects the instantaneous spatial extent of high helicity regions. Posture-related observed differences in the distribution of nOSI and RRT suggest that inlet flow waveform tends to moderate geometry-induced changes in the qualitative and quantitative distribution of atherosclerosis-susceptible wall regions. Overall, presented results indicate that an individualized approach is required to fully assess the postural role in atherosclerosis development and in complications arising in stenotic and stented vessels.

Variability of computational fluid dynamics solutions for pressure and flow in a giant aneurysm: the ASME 2012 Summer Bioengineering Conference CFD Challenge.

Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry reconstructed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest.

Stent overlapping and geometric curvature influence the structural integrity and surface characteristics of coronary nitinol stents.

Preliminary studies have revealed that some stents undergo corrosion and fatigue-induced fracture in vivo, with significant release of metallic ions into surrounding tissues. A direct link between corrosion and in-stent restenosis has not been clearly established; nonetheless in vitro studies have shown that relatively high concentrations of heavy metal ions can stimulate both inflammatory and fibrotic reactions, which are the main steps in the process of restenosis. To isolate the mechanical effects from the local biochemical effects, accelerated biomechanical testing was performed on single and overlapping Nickel-Titanium (NiTi) stents subjected to various degrees of curvature. Post testing, stents were evaluated using Scanning Electron Microscopy (SEM) to identify the type of surface alterations. Fretting wear was observed in overlapping cases, in both straight and curved configurations. Stent strut fractures occurred in the presence of geometric curvature. Fretting wear and fatigue fractures observed on stents following mechanical simulation were similar to those from previously reported human stent explants. It has been shown that biomechanical factors such as arterial curvature combined with stent overlapping enhance the incidence and degree of wear and fatigue fracture when compared to single stents in a straight tube configuration.

Effect of head posture on the healthy human carotid bifurcation hemodynamics.

Head and neck postures may cause morphology changes to the geometry of the carotid bifurcation (CB) that alter the low and oscillating wall shear stress (WSS) regions previously reported as important in the development of atherosclerosis. Here the right and left CB were imaged by MRI in two healthy subjects in the neutral head posture with the subject in the supine position and in two other head postures with the subject in the prone position: (1) rightward rotation up to 80°, and (2) leftward rotation up to 80°. Image-based computational models were constructed to investigate the effect of posture on arterial geometry and local hemodynamics. The area exposure to unfavorable hemodynamics, based on thresholds set for oscillatory shear index (OSI), WSS and relative residence time, was used to quantify the hemodynamic impact on the wall. Torsion of the head was found to: (1) cause notable changes in the bifurcation and internal carotid artery angles and, in most cases, on cross-sectional area ratios for common, internal and external carotid artery, (2) change the spatial distribution of wall regions exposed to unfavorable hemodynamics, and (3) cause a marked change in the hemodynamic burden on the wall when the OSI was considered. These findings suggest that head posture may be associated with the genesis and development of atherosclerotic disease as well as complications in stenotic and stented vessels.

The role of vascular calcification in inducing fatigue and fracture of coronary stents.

Traditional approaches for in-vitro pulsatile and fatigue testing of endovascular stents do not take into consideration the pathologies of the stented vessel and their associated biomechanical effects. One important pathology is calcification, which may be capable of inducing changes in the vessel wall leading to inhomogeneous distribution of stresses combined with wall motion during the cardiac cycle. These local property changes in the region adjacent to stents could directly influence in-vivo stent performance. Seven cases containing a total of 18 stents were obtained from autopsy. Radiographs were evaluated and vessels were sectioned for histology and stent topographical analysis. Stents were retrieved by chemical removal of surrounding tissue and surfaces were evaluated using 3D digital optical and scanning electron microscopy for biomechanical abrasion and fracture features. Pathologic complications such as restenosis and thrombus formation were assessed from histological sections. Direct evidence of fracture was found in 6 of the 7 cases (in 12 out of 18 stents; 9 drug eluting and 3 bare metal). The degree of stent alterations was variable, where separation of segments due to fracture occurred mostly in drug-eluting stents. All fracture surfaces were representative of a high cycle fatigue mechanism. These fractures occurred in complex lesions involving the presence of diffuse calcification alone, or in combination with vessel angulations and multiple overlapping stents. Morphologic analysis of tissue at or near some fracture sites showed evidence of thrombus formation and/or neointimal tissue growth.

Effect of posture change on the geometric features of the healthy carotid bifurcation.

Segmented cross-sectional MRI images were used to construct 3-D virtual models of the carotid bifurcation in ten healthy volunteers. Geometric features, such as bifurcation angle, internal carotid artery (ICA) angle, planarity angle, asymmetry angle, tortuosity, curvature, bifurcation area ratio, ICA/common carotid artery (CCA), external carotid artery (ECA)/CCA, and ECA/ICA diameter ratios, were calculated for both carotids in two head postures: 1) the supine neutral position; and 2) the prone sleeping position with head rotation to the right ( ∼ 80°). The results obtained have shown that head rotation causes 1) significant variations in bifurcation angle [32% mean increase for the right carotid (RC) and 21% mean decrease for the left carotid (LC)] and internal carotid artery angle (97% mean increase for the RC, 43% mean decrease for the LC); 2) a slight increase in planarity and asymmetry angles for both RC and LC; 3) minor and variable curvature changes for the CCA and for the branches; 4) slight tortuosity changes for the braches but not for the CCA; and 5) unsubstantial alterations in area and diameter ratios (percentage changes %). The significant geometric changes observed in most subjects with head posture may also cause significant changes in bifurcation hemodynamics and warrant future investigation of the hemodynamic parameters related to the development of atherosclerotic disease such as low oscillating wall shear stress and particle residence times.

In-vivo corrosion and local release of metallic ions from vascular stents into surrounding tissue.

OBJECTIVES: To evaluate retrieved bare metal vascular stents and surrounding tissue. BACKGROUND: Limited information is available regarding the condition of stent surfaces and their interaction with vascular tissue following implantation. Corrosion of stents presents two main risks: release of metallic ions into tissue and deterioration of the mechanical properties of stents which may contribute to fracture. Release of heavy metal ions could alter the local tissue environment leading to up-regulation of inflammatory mediators and promote in-stent restenosis. METHODS: Nineteen cases were collected from autopsy, heart explants for transplant, and vascular surgery (23 vessels containing 33 bare metal stents). A method was developed for optimal tissue dissolution and separation of the stent/tissue components without inducing stent corrosion. When available, chemical analysis was performed to assess metallic content in both the control and dissolved tissue solutions. Electron microscopy and digital optical microscopy imaging were used to evaluate stents. RESULTS: Twelve of the 33 stents showed varying degrees of corrosion. Metallic levels in the tissue surrounding the corroded stents were significantly higher (0.5-3.0 mcg/cm² stent) than in control solutions (0-0.30 mcg/cm² stent) and in tissue surrounding stents that did not undergo corrosion (0- 0.20 mcg/cm² stent). CONCLUSIONS: Corrosion of some retrieved stents is described which leads to transfer of heavy metal ions into surrounding tissue. The contribution of this metallic ion release to the mechanisms of in-stent restenosis as well as its effect on the mechanical properties of stents is unknown and requires further investigation.

Clinical device-related article surface characterization of explanted endovascular stents: evidence of in vivo corrosion.

Limited information exists regarding the in vivo stability of endovascular stents. Nine excised human vascular segments with implanted stents (n = 16) manufactured from stainless steel, nickel-titanium, tantalum, and cobalt-based alloys were analyzed. The stent/tissue components were separated using an established tissue dissolution protocol and control and explanted stents were evaluated by digital optical and electron microscopy. Metallic content in surrounding tissues was measured by mass spectroscopy. Surface alterations, consistent with corrosion mediated by electrochemical and mechanical factors, were observed in 9 of the 16 explanted stents and were absent from control stents. Tissue dissolved from around corroded stents corresponded with a higher metallic content. The effect of these changes in the microtopography of stents on their mechanical properties (fatigue strength and fracture limit) in addition to the potential for released metallic debris contributing to the biological mechanisms of in-stent restenosis supports the need for further investigations.

A spheroidal control volume for the quantitative measurement of regurgitant flow by cardiac MRI.

PURPOSE: We sought to show that a spheroidally shaped control volume (CV), formed from a minimal MRI data set, can be used to measure regurgitant flow through a defective cardiac valve consistently and accurately under a variety of flow conditions. MATERIALS AND METHODS: Using a pulsatile flow pump and phantoms simulating severe valvular regurgitation, we acquired 31 scans of two or three radially oriented slices, using a variety of flow waveforms and regurgitant volumes of 12 to 55 ml. Data sets included high- and low-resolution scans, and variable-rate sparse sampling was also applied to reduce the scan time. An oblate spheroid was placed in the pump chamber opposite the jet and fit as tightly as possible to isomagnitude velocity contours at 25% of the velocity encoding limit. RESULTS: Normalized regurgitant volumes (NRVs) expressed as a percentage of the pump setting were obtained from the product of the spheroid surface area with the velocities normal to it. Mean +/- SD NRV values were 96.8 +/- 6.6% for all scans. Imaging times in the breath-hold range were obtained using reduced resolution and variable-rate sparse sampling approaches without significant degradation in accuracy. CONCLUSION: In our preliminary findings, the spheroidal CV method showed clear potential for the development of a robust, clinically feasible technique for the measurement of regurgitant volume.

Correction of temporal misregistration artifacts in jet flow by conventional phase-contrast MRI.

PURPOSE: To show that accuracy of jet flow representation by magnetic resonance (MR) phase-contrast (PC) velocity-encoded (VE) cine imaging is dominated by error terms resulting from the temporal distribution of data, and to present a generally applicable data interpolation-based approach to correct for this phenomenon. MATERIALS AND METHODS: Phase-contrast data were acquired in a stenotic orifice flow phantom using a physiologic pulsatile flow waveform. A temporally registered scan, acquired without data segmentation or interleaving was obtained (17 minutes) and taken as the reference (REF). Conventional PC data sets were acquired using segmentation and data interleaving. An enhanced temporal registration (ETR) algorithm was applied to the acquired data to temporally interpolate component sets and output data at matching time points, thereby reducing temporal dispersion. RESULTS: Compared to the REF data, conventionally processed PC data consistently overestimated peak velocities in laminar jet flow regions (127% +/- 28%) and exhibited relatively weak correlations (r = 0.67 +/- 0.23). The ETR-processed data better represented peak velocities (101% +/- 13%, P < 0.001) and correlated more closely with the REF data (r = 0.94 +/- 0.05, P < 0.001). CONCLUSION: The temporal distribution of PC data impacts the accuracy of velocity representation in pulsatile jet flow. A temporal registration postprocessing algorithm can minimize loss of accuracy.

Effects of venous needle turbulence during ex vivo hemodialysis on endothelial morphology and nitric oxide formation.

Arteriovenous grafts used for hemodialysis frequently develop intimal hyperplasia (IH), which ultimately leads to graft failure. Although the turbulent jet from the dialysis needle may contribute to vessel wall injury, its role in the pathogenesis of IH is relatively unexplored. In the current study, using bovine aortic endothelial cells (BAEC) cultured on the inner surface of a compliant tube, we evaluated the effects of simulated hemodialysis conditions on morphology and nitric oxide (NO) production. The flows via the graft and needle were 500 ml/min (Reynolds number=819) and 100ml/min (Reynolds number=954), respectively. In the presence of the needle jet for 6h, 19.3% (+/-1.53%) of BAEC were sheared off, whereas no loss of BAEC was observed in the presence of graft flow alone (P<0.05). In the presence of graft flow alone, assessment of cell orientation by the Saltykov method revealed that BAEC were oriented along the flow direction. This alignment, however, was lost in the presence of needle flow. Finally, NO production was also significantly decreased in the presence of the needle flow compared to the presence of graft flow alone (16+/-3.1 vs 34.7+/-1.9 nmol/10(6)cells/h, P<0.05). NO is a key player in vascular homeostasis mechanisms modulating vasomotor tone, inhibiting inflammation and smooth muscle cell proliferation. Thus, the loss of NO signaling and the loss of endothelial integrity caused by needle jet turbulence may contribute to the cascade of events leading to IH formation during hemodialysis.

Severe, diffuse coronary artery spasm after drug-eluting stent placement.

OBJECTIVES: Three cases of severe, diffuse coronary artery spasm after drug-eluting stent placement at our institution prompted this review. BACKGROUND: Drug-eluting stents have gained widespread use due to extraordinarily low rates of restenosis. Despite these generally superior clinical outcomes, the specter of rare idiosyncratic reactions remains a concern. METHODS: We performed searches of Medline and the U.S. FDA Manufacturer And User facility Device Experience database (MAUDE) to identify and describe spasm after coronary stent placement. Searches included drug-eluting and bare-metal stents. Institutional cases are reviewed. Location, time course and outcome of cases are described. RESULTS: Thirteen cases of spasm were identified after stent placement. Seven cases occurred after Cypher drug-eluting stent placement, 2 after Taxus drug-eluting stent placement, 1 after BiodivYsio and 3 after bare-metal stents. Five patients experienced diffuse, multivessel spasm--2 after Cypher, 2 after Taxus, and 1 after a Velocity stent. Of these 5 patients, 2 died. An additional 2 required intra-aortic balloon pump placement for cardiogenic shock. Another had persistent symptomatic, diffuse coronary spasm documented by angiography at 1 year. CONCLUSIONS: We describe coronary spasm after stent placement, particularly after drug-eluting stents. Outcomes associated with diffuse severe spasm after stenting are poor, and the pathophysiology remains poorly understood.

Evaluation of in-stent stenosis by magnetic resonance phase-velocity mapping in nickel-titanium stents.

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.

Turbulent flow evaluation of the venous needle during hemodialysis.

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 0.1 m/s for the graft flow alone, turbulent RMS fluctuations downstream of the needle soared to 0.4-0.7 m/s at 2 cm from the tip of the needle and maintained values higher than 0.1 m/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.

Magnetic resonance phase velocity mapping through NiTi stents in a flow phantom model.

PURPOSE: To assess constant and pulsatile flow velocity within the lumen of a peripheral NiTi stent using phase velocity mapping for comparison with independent assessments of flow velocity in a phantom model. MATERIALS AND METHODS: A 9 x 20-mm stent installed in flexible tubing was placed in a phantom filled with stationary fluid. Constant and pulsatile flow (produced by a pump programmed to produce a simulation of the carotid artery flow) was assessed using phase velocity mapping at 4.1 T (for constant flow) and at 1.5 T (for pulsatile flow). In all cases 256 x 256 gradient echo phase velocity maps were acquired. For the pulsatile flow condition, cine images with acquisition gated to the pump cycle were acquired with 40 msec temporal resolution across the simulated cardiac cycle. Computed flow volume rates were compared with fluid volume collection for the constant flow model, and with ultrasonic Doppler flow meter measurements for the pulsatile model. RESULTS: The data showed that volume flow rate assessments by phase velocity mapping agreed with independent measurements within 10% to 15%. CONCLUSION: Phase velocity mapping of the lumen of peripheral size NiTi stents is possible in an in vitro model.

Alternative control volume geometries for measuring regurgitant flow through a valve.

Control-volume (CV) methods applied to magnetic resonance velocity-encoded cine images of the convergent proximal flow field of a regurgitant valve have been shown to measure reverse blood flow volume accurately. Spatial and temporal averaging are known to affect accuracy, but the effects of slice thickness and orientation relative to the flow field have not been systematically studied, nor have CV configurations requiring fewer scans been explored. Further, surface area calculations at the intersection of CV walls are a previously unrecognized source of error. Using a computational fluid dynamics model of steady flow through an orifice, we evaluated five different CV configurations in terms of accuracy, time costs, and clinical potential. CVs incorporating a basal wall were affected by blurring of axial velocity gradients near the orifice, and voxel grid alignment relative to the orifice was the most significant source of inaccuracy. Errors in surface area calculations at plane intersections produced deviations of 7-20%, depending on configuration. A CV formed by slices parallel to the orifice plane was deemed clinically unusable, while a cylindrical CV yielded good accuracy in simulated tests and showed potential for practical implementation based on scan time, ease of view selection, and visualization of the flow field.

A flow visualization study of an anatomic coronary artery anastomosis model with an implant.

Flow Streamlining Devices is a new tool in Coronary Artery Bypass Grafting (CABG). They aim in: a) Performing a sutureless anastomosis to reduce thrombosis at the veno-arterial junction, and b) Providing a hemodynamically efficient scaffolding to reduce secondary flow disturbances. Thrombosis and flow disturbances are factors that have been reported as contributing factors to the development of intimal hyperplasia (IH) and failure of the graft. By reducing thrombosis and flow disturbances, it is expected that IH will be inhibited and the lifetime of the graft extended. To evaluate the hemodynamic benefits of such an implant, two models were designed and fabricated. One simulated the geometry of the conventional anastomosis without an implant, and the other simulated an anastomosis with a flow streamlining implant. Identical flow conditions relevant to a coronary anastomosis were imposed on both models and flow visualization was performed with dye injection and a digital camera. Results showed reduction of disturbances in the presence of the implant. This reduction seems to be favorable to hemodynamic streamlining which may create conditions that may inhibit the initialization of IH. However, the compliance and geometric mismatch between the anastomosis and the implant created a disturbance at the rigid compliant wall interface, which should be eliminated prior to clinical applications.