Convective mass transfer at the carotid bifurcation

The convective conditions in regions of hemodynamic separation may produce uneven local mass transfer at the arterial wall which may lead to an atherogenic response. This study estimates the potential variation in local mass transfer of oxygen at the human carotid bifurcation under steady flow conditions. The three-dimensional separated flow at the bifurcation was studied using a computational analysis of the basic conservation equations of mass, momentum, and species. Mass transfer between the blood and the wall was estimated throughout the sinus region for a condition where the concentration at the wall was constant. Flow separation at the carotid bifurcation created a complex concentration field. The mass transfer was five times lower along the outer wall of the carotid sinus than the artery wall immediately upstream or downstream of the sinus. The region of low mass transfer was similar to the region of low shear stress but not identical. This distribution of low mass transfer correlated strongly with intimal thickening as measured previously from human specimens. Quantitative differences in mass transfer at the arterial wall should not be discarded as an important mechanism by which hemodynamics influences atherogenesis at this site of clinical disease.

Reduced blood flow accelerates intimal hyperplasia in endarterectomized canine arteries

The purpose of this study was to evaluate a technique that accelerates intimal hyperplasia by reduction of blood flow. Bilateral endarterectomies were performed in both femoral and carotid arteries in six dogs. One week later, all animals underwent banding of an artery distal to the injured region to reduce the blood flow by 50%. The contralateral injured arteries served as controls. At 11 weeks, the specimens were harvested and analyzed. Five of 12 (42%) of the flow-restricted arteries and nine of 12 (75%) of the non-flow-restricted arteries were patent at 11 weeks (P<0.05). Marked stenotic intimal hyperplastic lesions developed in the flow-restricted arteries (69% stenosis) as compared with the non-flow-restricted arteries (37% stenosis). Mean(s.d.) intimal thickness, intimal areas, and intimal/medial area ratio were 0.52(0.19) mm, 3.17(1.11) mm2, and 1.12(0.33)%, respectively, in the flow-restricted arteries. Their counterparts in the non-flow-restricted arteries were 0.21(0.09) mm, 1.70(1.09) mm2, and 0.58(0.14)%, respectively (P<0.05). Extracellular matrix comprised 48% of total intimal volumes in the flow-restricted arteries. Cell proliferation and occluded arteries were also characterized. These data demonstrate that reduction of blood flow significantly accelerated intimal hyperplasia and occlusion rates in endarterectomized arteries. Advanced intimal hyperplastic lesions (>50% stenosis) possess a high extracellular matrix content. This new animal model is a reliable generator of advanced stenotic lesions in a relatively short time period and can be used to study biologic mechanisms of stenosis and evaluate therapeutic interventions.

Computational simulation of turbulent signal loss in 2D time-of-flight magnetic resonance angiograms

Time-of-flight magnetic resonance (MR) angiography is currently limited in the evaluation of arterial stenoses by flow-induced signal loss. This signal loss has been attributed to phase dispersion and to phase misregistration. We have developed a fluid mechanics model of 2D time-of-flight MR angiograms to study the amount of signal loss caused by random turbulence. The simulations were created by stochastic analysis of particle pathlines determined by computational fluid dynamics for turbulent flow. The images obtained by the model compare well to actual MR images of flow in stenoses. By selectively removing the random turbulent motion in the simulation, it can be seen that random phase dispersion is the dominant mechanism of signal loss. Phase misregistration and mean flow phase dispersion act as secondary effects. The MR simulation model recreates accurately the variation of signal loss over a range of echo times. The model can be used further to explore and design new pulse sequences. For example, the current study showed that high slew rate gradient waveforms can significantly reduce poststenotic signal loss. In conclusion, computational modeling of MR angiography can be a useful approach for the analysis of MRA signal loss and the design of improved pulse sequences.

Boundary layer infusion of nitric oxide reduces early smooth muscle cell proliferation in the endarterectomized canine artery

To evaluate the direct effect of nitric oxide (NO) on vascular smooth muscle cell (SMC) proliferation in vivo, we used an expanded polytetrafluoroethylene (ePTFE)-based local infusion device to deliver an NO donor, proline/NO (PROLI/NO), to the luminal boundary layer of endarterectomized artery and the distal anastomosis of the graft in a canine model. Once delivered to the blood, PROLI/NO releases NO by a mechanism involving pH-dependent decomposition. Six dogs underwent bilateral femoral artery endarterectomies. ePTFE infusion devices, blindly primed with PROLI/NO to one artery or proline to the contralateral vessel, were anastomosed proximal to the injured segments so that each animal served as its own control. PROLI/NO or proline was continuously delivered for 7 days from an osmotic reservoir, through the wall of the graft infusion device. Euthanasia was carried out at 7 days, and the processed specimens were blindly analyzed for SMC proliferation at both graft anastomoses and endarterectomized segments by a bromodeoxyuridine index assay. All dogs survived with no clinical side effects. In comparing the treated and control vessels, NO released from PROLI/NO significantly reduced SMC proliferation by 43% (13.24 +/- 1.24% versus 23.24 +/- 1.01%, P = 0.004) at the distal anastomoses and by 68% (10.58 +/- 1.63% versus 25.17 +/- 3.39%, P = 0.007) at endarterectomized segments. However, there was no significant difference in blood flow measurements between treated and control arteries (56.25 +/- 6.50 ml/min versus 46.50 +/- 3.20 ml/min, P = 0.094). These data demonstrate that local boundary layer infusion of NO released from PROLI/NO significantly reduces SMC proliferation in injured arteries with no effect on regional blood flow. This study suggests a new strategy to inhibit early SMC proliferation in injured arteries and probably to control intimal hyperplastic lesion formation in the manipulated vessels.

Time-course study of intimal hyperplasia in the endarterectomized canine artery

In an effort to characterize intimal hyperplastic lesions, we have undertaken a time-course study in a canine endarterectomy model of intimal hyperplasia. Twenty dogs underwent surgical endarterectomies of the carotid arteries. A total of 23 of 27 (85%) injured arteries were patent, which consisted of 6, 8, 5, and 4 arteries found to be patent at 1, 2, 5, and 11 weeks, respectively. Measurable intimal thickening developed at 1 week (0.08 +/- 0.01 mm) and at 2 weeks (0.13 +/- 0.02 mm), maximized at 5 weeks (0.29 +/- 0.03 mm), and subsided at 11 weeks (0.21 +/- 0.01 mm) after injury. Endothelial cells covering intimal hyperplastic tissues were seen only at 11 weeks. The intimal cell proliferation rate reached a maximum of 24% at 1 week, decreased dramatically at 2 weeks, and remained at low levels but higher than baseline levels at 5 and 11 weeks. Extracellular matrix (ECM) content accounted for 29% of total intimal volume at 1 week after endarterectomy and increased to 37, 40, and 47% at 2, 5, and 11 weeks, respectively. These data demonstrate that maximum intimal cell proliferation occurs at 1 week and maximum intimal hyperplasia at 5 weeks after arterial injury. Intimal ECM content increased with time after injury throughout the duration of this study. The uniform and consistent intimal lesion that was established in this large animal model is clinically relevant and can be used to study cellular and molecular mechanisms of restenosis and to evaluate therapeutic interventions.

Improved measurement of pressure gradients in aortic coarctation by magnetic resonance imaging

OBJECTIVES

This study evaluated whether magnetic resonance imaging (MRI) and magnetic resonance (MR) phase velocity mapping could provide accurate estimates of stenosis severity and pressure gradients in aortic coarctation.

BACKGROUND

Clinical management of aortic coarctation requires determination of lesion location and severity and quantification of the pressure gradient across the constricted area.

METHODS

Using a series of anatomically accurate models of aortic coarctation, the laboratory portion of this study found that the loss coefficient (K), commonly taken to be 4.0 in the simplified Bernoulli equation delta P = KV2, was a function of stenosis severity. The values of the loss coefficient ranged from 2.8 for a 50% stenosis to 4.9 for a 90% stenosis. Magnetic resonance imaging and MR phase velocity mapping were then used to determine coarctation severity and pressure gradient in 32 patients.

RESULTS

Application of the new severity-dependent loss coefficients found that pressure gradients deviated from 1 to 17 mm Hg compared with calculations made with the commonly used value of 4.0. Comparison of MR estimates of pressure gradient with Doppler ultrasound estimates (in 22 of 32 patients) and with catheter pressure measurements (in 6 of 32 patients) supports the conclusion that the severity-based loss coefficient provides improved estimates of pressure gradients.

CONCLUSIONS

This study suggests that MRI could be used as a complete diagnostic tool for accurate evaluation of aortic coarctation, by determining stenosis location and severity and by accurately estimating pressure gradients.

The accuracy of magnetic resonance phase velocity measurements in stenotic flow

Nuclear magnetic resonance (MR) can be used to measure velocities in fluid flow using the technique of phase velocity mapping. Advantages of MR velocimetry include the simultaneous mapping of the entire flow field through a non-contacting, magnetic window. The phase velocity mapping technique assumes that velocity is constant over the measurement time (typically around 10 ms). For many fluid flows, this assumption is not valid. The current study showed that MR phase velocity measurements of velocity through stenotic flow can be in error by over 100% immediately upstream and downstream of the stenosis throat and by 20% far downstream of the throat in comparison with laser Doppler anemometer measurements taken at the same location. Highly turbulent flow also led to significant errors in velocity measurement. These errors can be attributed to several sources including low signal-to-noise ratio, additional phase shifts due to non-constant velocities, and non-stationary transit-time effects. Velocity measurement errors could be reduced to under 30% at all measurement locations through the use of MR sequences with high signal-to-noise ratios, low echo times, and thick slices.

Recombinant mitotoxin basic fibroblast growth factor-saporin reduces venous anastomotic intimal hyperplasia in the arteriovenous graft

BACKGROUND

The plant cytotoxin saporin (SAP) is a potent ribosome-inactivating protein. When conjugated to basic fibroblast growth factor (FGF2), it selectively kills proliferating cells that have upregulated FGF receptors. In this study, we evaluated the effect of the recombinant chimeric mitotoxin rFGF2-SAP on venous anastomotic intimal hyperplasia, a major cause of failure of arteriovenous (AV) grafts.

METHODS AND RESULTS

Recently designed expanded polytet-rafluoroethylene-based local infusion devices were implanted bilaterally as femoral AV conduits in six dogs. The venous anastomoses were the sites of continuous delivery of rFGF2-SAP (2.7 micrograms.kg-1.d-1) to one side and vehicle (4.6 microL.kg-1.d-1) as control to the contralateral side for 14 days. All animals survived, and all grafts were patent. Liver enzyme levels and histological analyses of liver, kidneys, and brain were normal, indicating the absence of systemic toxicity. Morphometric measurements and measurements of cell proliferation by bromodeoxyuridine index analysis were performed at both arterial and venous anastomoses. There were no significant differences between the treated grafts and the control grafts in intimal hyperplasia and intimal cell proliferation at the arterial anastomoses. In contrast, rFGF2-SAP reduced intimal thickness by 32%, intimal area by 40%, and cell proliferation index by 33% at the treated venous anastomoses compared with the control venous anastomoses (P < .05).

CONCLUSIONS

These data demonstrate that local infusion of rFGF2-SAP significantly reduces venous anastomotic intimal hyperplasia and cell proliferation without systemic toxicity. This study suggests a new strategy for reducing intimal hyperplasia by the selective killing of proliferating smooth muscle cells with a potent chimeric mitotoxin through a novel local infusion device.

Tenascin: a potential role in human arteriovenous PTFE graft failure

To investigate the involvement of tenascin (TN) in human neointimal hyperplastic lesion formation, we studied 12 human arteriovenous (AV) polytetrafluoroethylene (PTFE) loop grafts removed at the time of graft revision. Immunoperoxidase technique was used to determine TN and proliferating cell nuclear antigen (PCNA) as an index of cell proliferation. The venous anastomotic neointimal hyperplastic lesion was analyzed as three layers, each one-third of the thickness. TN was distributed in the neointima as follows: (1) in the luminal layer, all lesions were intensely stained; (2) in the middle layer, 9 of 12 (75%) lesions had moderate reactivity; (3) in the deep layer near the PTFE grafts, 3 of 12 (25%) lesions were strongly stained and 5 of 12 (42%) had moderate reactivity; and (4) 90 +/- 6% of microvessels within the neointima showed intense periendothelial staining. PCNA indices were as follows: luminal layer, 29 +/- 11%; middle layer, 9 +/- 5%; deep layer near the graft, 9 +/- 7%; and microvessel containing intimal fields, 67 +/- 4%. The cell proliferation rate was significantly higher in the luminal layer than in the middle or deep layers of neointima (P < 0.05). The PCNA index in microvessel-containing intimal fields was three to eight times that of avascular fields (P < 0.001). These data demonstrate that TN is distributed in a pattern similar to that of cell proliferation in human neointimal hyperplastic lesions. The results suggest that TN expression may have a potential role in neovascularization and neointimal growth in human AV PTFE graft failure.

Pulsatile flow in the human left coronary artery bifurcation: average conditions

The localization of atherosclerosis in the coronary arteries may be governed by local hemodynamic features. In this study, the pulsatile hemodynamics of the left coronary artery bifurcation was numerically simulated using the spectral element method for realistic in vivo anatomic and physiologic conditions. The velocity profiles were found to be skewed in both the left anterior descending and the circumflex coronary arteries. Velocity skewing arose from the bifurcation as well as from the curvature of the artery over the myocardial surface. Arterial wall shear stress was significantly lower in the bifurcation region, including the side walls. The greatest oscillatory behavior was localized to the outer wall of the circumflex artery. The time-averaged mean wall shear stress varied from about 3 to 98 dynes/cm2 in the left coronary artery system. The highly localized distribution of low and oscillatory shear stress along the walls strongly correlates with the focal locations of atheroma in the human left coronary artery.

Comparison of phantom and computer-simulated MR images of flow in a convergent geometry: implications for improved two-dimensional MR angiography

The signal loss that occurs in regions of disturbed flow significantly decreases the clinical usefulness of MR angiography in the imaging of diseased arteries. This signal loss is most often attributed to turbulent flow; but on a typical MR angiogram, the signal is lost in the nonturbulent upstream region of the stenosis as well as in the turbulent downstream region. In the current study we used a flow phantom with a forward-facing step geometry to model the upstream region. The flow upstream of the step was convergent, which created high levels of convective acceleration. This region of the flow field contributes to signal loss at the constriction, leading to overestimation of the area of stenosis reduction. A computer program was designed to simulate the image artifacts that would be caused by this geometry in two-dimensional time-of-flight MR angiography. Simulated images were compared with actual phantom images and the flow artifacts were highly correlated. The computer simulation was then used to test the effects of different orders of motion compensation and of fewer pixels per diameter, as would be present in MR angiograms of small arteries. The results indicated that the computational simulation of flow artifacts upstream of the stenosis provides an important tool in the design of optimal imaging sequences for the reduction of signal loss.

Determination of wall shear stress in the aorta with the use of MR phase velocity mapping

MR phase velocity mapping was used to calculate wall shear stress (WSS) in the suprarenal and infrarenal abdominal aorta, two sites with very different proclivities for development of a atherosclerosis. For the eight subjects studied, the average value of the mean (time averaged over the cardiac cycle) WSS in the suprarenal aorta was 10.4 dynes/cm2 at the posterior wall and 8.6 at the anterior wall. In the infrarenal aorta, WSS values were 4.7 at the posterior wall and 6.1 at the anterior wall. Peak WSS over the cardiac cycle was 48 and 54 at the anterior and posterior walls of the suprarenal aorta, respectively, and 33 and 30 at the anterior and posterior walls of the infrarenal aorta, respectively. Wide variation was found in both mean and peak WSS values among subjects. However, for 28 of 32 locations examined, mean and peak WSS were higher in the suprarenal aorta than in the infrarenal aorta. Because atherosclerosis is more likely to form in the infrarenal aorta than in the suprarenal aorta, this study supports the hypothesis that low WSS is a localizing factor for atherosclerosis, and high WSS may act as a deterrent against formation of atherosclerosis.

Flow in T-bifurcations: effect of the sharpness of the flow divider

The local geometry of a bifurcation has been hypothesized to be a potential geometrical risk factor for the development of atherosclerosis. While flow division and branch area ratios clearly affect the flow field, the importance of the flow divider shape is not as clear. A fast spectral element computational fluid mechanics (CFD) solver was used to simulate flow through 90 degrees T-bifurcations with three different flow divider shapes. Other factors, such as flow partition, area ratio, and bifurcation angle, were kept constant. A Reynolds number range of 15 to 350 was studied to bracket experimental results in the literature. The variation in the sharpness of the corners had a dramatic effect on both the flow field and wall shear stress distribution in the side branch, but little effect on the flow in the main tube. The magnitude of reverse velocities and wall shear stress in the side branch increased linearly over a physiological range of Reynolds number and corner shape. This paper verifies the accuracy and usefulness of spectral element CFD in studying three-dimensional hemodynamics.

Turbulent fluctuation velocity: the most significant determinant of signal loss in stenotic vessels

Studies of flow in a 90%-stenosis phantom were conducted to elucidate the parameters and mechanisms responsible for signal loss in MR angiographic images. The studies independently evaluated the effect of velocity, Reynolds number, turbulent fluctuation velocity, and turbulence intensity on the amount of post-stenotic signal loss. Results suggested that the magnitude of the turbulent fluctuation velocity, not merely the presence of turbulence or the intensity of turbulence, was the parameter that determined the extent of the signal loss. The study suggests that future flow phantom studies should be conducted with fluids having physiologic velocities and viscosities to obtain accurate levels of turbulent fluctuation velocities and hence reproduce results of in-vivo signal-loss patterns. The mechanism for signal loss is that the temporal and spatial variations of the turbulent fluctuation velocity cause a range of phases to be present within a voxel. Examination of the theoretical aspects of fluid turbulence suggest that shortening gradient durations and imaging during diastole may help reduce signal loss.

A scaling law for wall shear rate through an arterial stenosis

Atherosclerosis of the human arterial system produces major clinical symptoms when the plaque advances to create a high-grade stenosis. The hemodynamic shear rates produced in high-grade stenoses are important in the understanding of atheromatous plaque rupture and thrombosis. This study was designed to quantify the physiologic stress levels experienced by endothelial cells and platelets in the region of vascular stenoses. The steady hemodynamic flow field was solved for stenoses with percent area reductions of 50, 75, and 90 percent over a range of physiologic Reynolds numbers (100-400). The maximum wall shear rate in the throat region can be shown to vary by the square root of the Reynolds number. The shear rate results can be generalized to apply to a range of stenosis lengths and flow rates. Using dimensions typical for a human carotid or coronary artery, wall shear rates were found to vary from a maximum of 20,000 s-1 upstream of the throat to a minimum of -630 s-1 in the recirculation zone for a 90 percent stenosis. An example is given which illustrates how these values can be used to understand the relationship between hemodynamic shear and platelet deposition.

Fluid wall shear stress measurements in a model of the human abdominal aorta: oscillatory behavior and relationship to atherosclerosis

Clinically significant atherosclerosis in the human aorta is most common in the infrarenal segment. This study was initiated to test the hypothesis that flowfield properties are closely related to the localization of plaques in this segment of the arterial system. Wall shear stress was calculated from magnetic resonance velocity measurements of pulsatile flow in an anatomically accurate model of the human abdominal aorta. The wall shear stress values were compared with intimal thickening from 15 post-mortem aortas measured by quantitative morphometry of histological cross sections obtained at standard locations. Wall shear stress oscillated in direction throughout most of the infrarenal aorta, most prominently in the distal region. The time-averaged mean wall shear stress (-1.7 to 1.4 dyn/cm2) was lowest near the posterior wall in this region. These hemodynamic parameters coincided with the locations of maximal intimal thickening. Statistical correlation between oscillatory shear and intimal thickness yielded r = 0.79, P < 0.00001. Low mean shear stresses correlated nearly as well (r = -0.75, P < 0.00005). Comparison of our data with surface maps of Sudan Red staining and early lesions as reported by others revealed similar conclusions. In contrast, pulse and maximum shear stresses did not correlate with plaque localization as has been shown for other sites of selective involvement by atherosclerosis (r < 0.345). Simulated exercise conditions markedly changed the magnitude and pattern of wall shear stress in the distal abdominal aorta. These results demonstrate that in the infrarenal aorta, regions of low mean and oscillating wall shear stresses are predisposed to the development of plaque while regions of relatively high wall shear stress tend to be spared.

Pulsatile velocity measurements in a model of the human abdominal aorta under resting conditions

Oscillations in near-wall velocity direction have been found to correlate with atherosclerotic plaque localization in the carotid sinus bifurcation. However, it remains unproven whether these conditions could account for the localization of the disease at other sites where atherosclerosis forms. The abdominal aorta is an important site of clinical disease in a relatively straight segment of artery. This study was initiated to quantify the velocity field in the abdominal aorta in order to determine if local differences in hemodynamic velocity directions could account for the localization of disease in this segment. Magnetic Resonance Imaging velocimetry was used to measure the pulsatile velocity profiles in an anatomically accurate in vitro model of the abdominal aorta. Velocities measured in the suprarenal aorta were laminar and reversed minimally, comparing well with theoretical solutions of Womersley flow (r = 0.96). The time-averaged velocity was +3.0 cm/s near-wall at a distance of 1 mm away from the wall. In the infrarenal aorta, the maximal velocities were skewed toward the anterior wall. At the posterior wall, velocity oscillated in direction and was retrograde for 82 percent of the cardiac cycle. The time-averaged velocity near the posterior wall was -12.5 cm/s as compared to +3.00 cm/s near the anterior wall. At the aortic bifurcation, the locations of maximal and minimal velocities in this slice were concentrated near the lateral posterior walls. This study quantifies the magnitude of low and oscillatory velocity that may exist in the abdominal aorta and suggests that there is a strong relationship between the velocities in the retrograde direction under resting conditions and the distribution of atherosclerotic plaque.

Unsteady entrance flow development in a straight tube

The entrance conditions for pulsatile flow are important in the understanding blood flow out of the heart and in developing regions at branches. The pulsatile entrance flow was solved using a spectral element simulation of the full unsteady Navier-Stokes equations. A mean Reynolds number of 200 and a range of Womersley parameters from 1.8 to 12.5 was used for a sinusoidal inlet flow waveform 1 + sin (omega t). Variations in the entrance length were observed during the pulsatile cycle. The amplitude of the entrance length variation decreased with an increase in the Womersley parameter. The phase lag between the entrance length and the inlet flow waveform increased for Womersley parameter alpha up to 5.0 and decreased for alpha larger than 5.0. For low alpha, the maximum entrance length during pulsatile flow was approximately the same as the steady entrance length for the peak flow. For high varies; is directly proportional to, the pulsatile entrance length was more uniform during the cycle and tended to the entrance length for the mean flow. The wall shear rate reached its far downstream value after only about half of the entrance length and also exhibited a dependence on alpha. The results quantify the entrance conditions typically encountered in studies of the arterial system.

Hemodynamics in the abdominal aorta: a comparison of in vitro and in vivo measurements

In vivo measurements of blood velocity profiles are difficult to obtain and interpret, since the parameters that govern the normally highly complex flow situation may not be fully quantified or understood at the time of measurement. In vitro flow models have been used often to better understand vascular hemodynamics. The assumptions made in the design of these models limit the applicability of the results. In this study, in vitro flow measurements made in a carefully designed model of the abdominal aorta were compared with in vivo measurements obtained with magnetic resonance imaging. In the suprarenal aorta, the velocity profiles were mostly forward and axisymmetric in both the in vitro and in vivo cases. In the infrarenal aorta, there was extensive flow reversal noted near the posterior wall in both cases. In the aortic bifurcation, two peaks of flow reversal were noted near the lateral posterior walls, and M-shaped velocity profiles were observed in late diastole. The in vitro and in vivo measurements exhibited good qualitative agreement. The in vitro model was accurate in modeling the in vivo hemodynamics of the abdominal aorta. The complex phenomena observed in vivo were explained on the basis of knowledge gained from the in vitro study.

Pulsatile velocity measurements in a model of the human abdominal aorta under simulated exercise and postprandial conditions

This study examines the hemodynamics of the abdominal aorta during physiological changes in flow rates and pulse rate that occur under exercise and postprandial conditions. Hemodynamic measurements were performed using an in vitro model which took into account seven major branches, the curvature, and the pulsatile nature of blood flow of the abdominal aorta. Magnetic Resonance Imaging velocimetry employing phase-velocity encoding was used to measure the pulsatile axial velocity profiles for the entire cross-section at three axial locations. Under simulated exercise conditions, the forward velocities were approximately double those seen during rest, and the flow reversal seen for resting conditions was greatly reduced. Near the posterior wall of the infrarenal aorta, the velocities were negative for only 21 percent of the cardiac cycle as compared with 82 percent for resting conditions. Postprandial conditions produced a 25 percent reduction in peak velocity and a 33 percent reduction in mean velocity near the left anterior wall of the aorta just distal to the superior mesenteric artery (in comparison with resting conditions). The changes that can occur in abdominal aorta hemodynamics under different physiologic conditions may affect the rate of progression of atherosclerosis at this site.

Computer modeling of the abdominal aorta using magnetic resonance images

An approach is described for creating a 3-D computer model of the abdominal aorta from just two projective images. The aorta is modeled by conical segments connecting circular cross sections. Accuracy of this technique is within 1 mm. From the 3-D computer model, quantitative measurements of vessel diameter, length, and position are available for any subset of the arterial structure. Visualization is enhanced by displaying the computer model rather than a direct set of images obtained from different perspectives. Ambiguities from overlapping branches can be resolved by rotating the model or by eliminating the interfering structures. This approach has been applied in both phantom studies, in which quantitative comparisons were made, and in vivo studies, in which qualitative evaluations were made.

Collapse of diseased arteries with eccentric cross section

The mechanism of atherosclerotic plaque rupture is not fully understood. Mechanical stress may be one of the factors contributing to the instability of the fibrous plaque cap. The existence of a severe stenosis may lower the transmural pressure enough to cause the collapse of arteries leading to high concentrated compressive and tensile stresses. This study presents quantitative estimates of the stresses and deformations in the collapsed thick-walled artery. The results of large deformation finite element calculations identify the locations of the high stress concentrations and their magnitudes which cannot be precisely predicted under a thin-wall assumption. The maximum compressive stress calculated reached 80% of the Young's modulus for fairly small negative transmural pressures. Results are useful to predict likely locations of the plaque cap rupture due to compressive stresses. The tube law of area as a function of transmural pressure showed a large discrepancy from a thin-wall calculation. The buckling pressure calculated for the outer-to-inner wall surface radius ratio of 1.60 was nearly 30% lower than that of the elastic thin-wall buckling theory. An increase in eccentricity further reduced this buckling pressure. The results indicate that a thick plaque which is eccentric increases the likelihood of collapse of stenotic arteries.