Pulsatile flow past St. Jude Medical bileaflet valve. An in vitro study.

An in vitro hemodynamic study of the St. Jude Medical bileaflet aortic prosthesis was performed in a mock circulatory system simulating physiological pulsatile flow. The study included measurements of pressure drop across the valves, percent regurgitation, velocity, and turbulence in a model human aorta. The measurements indicated that pressure drop (mean systolic pressure drop of 6.2 mm Hg), percent regurgitation (10.15%), and turbulent normal stresses immediately downstream from the valve (825 dynes/cm2) were better than those with other prosthetic valves and bioprostheses. The flow development in the aorta was not significantly affected by the orientation of the bileaflet valve in the root of the aorta. However, velocity measurements immediately downstream from the valves showed flow reversal and separation in the vicinity of the hinge points of the leaflets where thrombus formation has been previously reported.

Effect of valve orientation on flow development past aortic valve prostheses in a model human aorta.

The effect of valve orientation on flow development in a model human aorta was studied by means of a qualitative flow visualization technique. The model replicated the geometry of the human aorta and the experiment simulated a physiologically realistic pulsatile flow. The following valves were studied: Starr-Edwards Stellite, Starr-Edwards silicone, Björk-Shiley spherical disc, Björk-Shiley convexo-concave disc, and Hall-Kaster tilting disc. All the valves had a tissue anulus diameter of 27 mm. With the ball-in-cage valves, the flow in the ascending aorta was predominantly axial and uniform throughout systole, while vortex formation was observed downstream from the ball. With the tilting disc valves, the flow development in the aorta was a function of the orientation of the valves. With the major flow orifice directed toward the commissure between the right and noncoronary cusps, the fluid motion was predominantly in the axial direction through early systole. A vortex developed along the wall of lesser curvature of the aorta with the progression of systole. In early diastole, a well-defined flow reversal was observed along the lesser curvature of the aorta. With the major flow orifice directed toward the left coronary cusp, the fluid motion, although predominantly axial, was not uniform in the ascending aorta. Regions of relative stasis present near the wall of greater curvature subsequently developed into a trapped vortex throughout the cardiac cycle. With the major flow orifice directed more posteriorly, an improved fluid dynamic characteristic was observed, and there was no trapped vortex present near the wall of greater curvature. The flow visualization study in the model human aorta suggests that, from a fluid dynamic point of view, orientation of the major flow orifice of the tilting disc valve toward the wall of lesser curvature is not advisable.

A method for automatic edge detection and volume computation of the left ventricle from ultrafast computed tomographic images.

RATIONALE AND OBJECTIVES: Detection of endocardial and epicardial borders of the left ventricle (LV) using various imaging modalities is time-consuming and prone to interpretive error. An automatic border detection algorithm is presented that is used with ultrafast computed tomographic images of the heart to compute cavity volumes. METHODS: The basal-level slice is identified, and the algorithm automatically detects the endocardial and epicardial borders of images from the basal to the apical levels. From these, the ventricular areas and chamber volumes are computed. The algorithm uses the Fuzzy Hough Transform, region-growing schemes, and optimal border-detection techniques. The cross-sectional areas and the chamber volumes computed with this technique were compared with those from manually traced images using canine hearts in vitro (n = 8) and studies in clinical patients (n = 27). RESULTS: Though the correlation was good (r = .88), the algorithm overestimated the LV epicardial area by 4.8 +/- 6.4 cm2, though this error was not statistically different from zero (P > .05). There was no difference in endocardial areas (r = .95, P > .05). The algorithm tended to underestimate the end-diastolic volume (r = .94) and the end-systolic volume (r = .94), although these errors were not statistically different from zero (P > .05). The algorithm tended to underestimate the ejection fraction (r = .80), although this error was not statistically different from zero (P > .05). CONCLUSIONS: Automatic detection of myocardial borders provides the clinician with a useful tool for calculating chamber volumes and ejection fractions. The algorithm, with the corrections suggested, provides an accurate estimation of areas and volumes. This algorithm may be useful for contour border identification with ultrasound, positron-emission tomography, magnetic resonance imaging, and other imaging modalities in the heart, as well as other structures.

Arterial enhancement in acute cerebral ischemia: clinical and angiographic correlation.

PURPOSE: To investigate the cause and clinical significance of arterial enhancement (AE) in contrast-enhanced T1-weighted MR examinations after acute cerebral ischemia. METHODS: Contrast MR examinations and conventional angiograms of 17 patients studied following an acute ischemic event or an internal carotid occlusion were retrospectively reviewed. MR and angiographic studies were performed within 1 day of each other. The presence of AE was correlated with both angiographic findings and patient clinical status. RESULTS: AE was not confined to patients with angiographic evidence of complete arterial occlusion. Only 64% of patients demonstrating AE had complete occlusion angiographically. Complete arterial occlusion did not always correlate with AE. In two of nine patients with complete occlusion, no AE was identified. In five of 10 patients with AE, angiographic slow flow was identified. In patients without AE, no angiographic slow flow was identified. In the 64% of patients with AE, significant symptoms were identified. Patients without AE were either asymptomatic or had mild symptoms at the time of the MR study. CONCLUSIONS: Our data support the hypothesis that arterial slowing is the cause of AE, which appears to be an indicator of decreased brain perfusion. Such MR findings may add important supplemental information to those provided by conventional angiography.

The fuzzy Hough transform-feature extraction in medical images.

Identification of anatomical features is a necessary step for medical image analysis. Automatic methods for feature identification using conventional pattern recognition techniques typically classify an object as a member of a predefined class of objects, but do not attempt to recover the exact or approximate shape of that object. For this reason, such techniques are usually not sufficient to identify the borders of organs when individual geometry varies in local detail, even though the general geometrical shape is similar. The authors present an algorithm that detects features in an image based on approximate geometrical models. The algorithm is based on the traditional and generalized Hough Transforms but includes notions from fuzzy set theory. The authors use the new algorithm to roughly estimate the actual locations of boundaries of an internal organ, and from this estimate, to determine a region of interest around the organ. Based on this rough estimate of the border location, and the derived region of interest, the authors find the final (improved) estimate of the true borders with other (subsequently used) image processing techniques. They present results that demonstrate that the algorithm was successfully used to estimate the approximate location of the chest wall in humans, and of the left ventricular contours of a dog heart obtained from cine-computed tomographic images. The authors use this fuzzy Hough transform algorithm as part of a larger procedure to automatically identify the myocardial contours of the heart. This algorithm may also allow for more rapid image processing and clinical decision making in other medical imaging applications.

Automatic detection of myocardial contours in cine-computed tomographic images.

Quantitative evaluation of cardiac function from cardiac images requires the identification of the myocardial walls. This generally requires the clinician to view the image and interactively trace the contours. This method is susceptible to great variability that depends on the experience and knowledge of the particular operator tracing the contours. The particular imaging modality that is used may also add tracing difficulties. Cine-computed tomography (cine-CT) is an imaging modality capable of providing high quality cross-sectional images of the heart. CT images, however, are cluttered, i.e., objects that are not of interest, such as the chest wall, liver, stomach, are also visible in the image. To decrease this variability, investigators have developed computer-assisted or near-automatic techniques for tracing these contours. All of these techniques, however, require some operator intervention to confidently identify myocardial borders. The authors present a new algorithm that automatically finds the heart within the chest, and then proceeds to outline (detect) the myocardial contours. Information at each tomographic slice is used to estimate the contours at the next tomographic slice, thus allowing the algorithm to work in near-apical cross-sectional images where the myocardial borders are often difficult to identify. The algorithm does not require operator input and can be used in a batch mode to process large quantities of data. An evaluation and correction phase is included to allow an operator to view the results and selectively correct portions of contours. The authors tested the algorithm by automatically identifying the myocardial borders of 27 cardiac images obtained from three human subjects and quantitatively comparing these automatically determined borders with those traced by an experienced cardiologist.

Effect of systolic flow rate on the prediction of effective prosthetic valve orifice area.

The Gorlin equation for the hemodynamic assessment of valve area is commonly used in cardiac catheterization laboratories. A study was performed to test the prediction capabilities of the Gorlin formula as well as the Aaslid and Gabbay formula for the effective orifice area of prosthetic heart valves. Pressure gradient, flow, and valve opening area measurements were performed on four 27 mm valve prostheses (two mechanical bileaflet designs, St. Jude and Edwards-Duromedics, an Edwards pericardial tissue valve, and a trileaflet polyurethane valve) each mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a different discharge coefficient was computed for each of the four valves and three orifice area formulas. After some theoretical considerations, it was proposed that the discharge coefficient would be a function of the flow rate through the valve. All discharge coefficients were observed to increase with increasing systolic flow rate. An empirical relationship of discharge coefficient as a linear function of systolic flow rate was determined through a regression analysis, with a different relationship for each valve and each orifice area formula. Using this relationship in the orifice area formulas improved the accuracy of the prediction of the effective orifice area with all three formulas performing equally well.

Velocity and turbulence measurements past mitral valve prostheses in a model left ventricle.

Thrombogenesis and hemolysis have both been linked to the flow dynamics past heart valve prostheses. To learn more about the particular flow dynamics past mitral valve prostheses in the left ventricle under controlled experimental conditions, an in vitro study was performed. The experimental methods included velocity and turbulent shear stress measurements past caged-ball, tilting disc, bileaflet, and polyurethane trileaflet mitral valves in an acrylic rigid model of the left ventricle using laser Doppler anemometry. The results indicate that all four prosthetic heart valves studied create at least mildly disturbed flow fields. The effect of the left ventricular geometry on the flow development is to produce a stabilizing vortex which engulfs the entire left ventricular cavity, depending on the orientation of the valve. The measured turbulent shear stress magnitudes for all four valves did not exceed the reported value for hemolytic damage. However, the measured turbulent shear stresses were near or exceeded the critical shear stress reported in the literature for platelet lysis, a known precursor to thrombus formation.

Physiological pulsatile flow experiments in a model of the human aortic arch.

An experimental investigation of physiologically relevant pulsatile flow in a model of the human aortic arch has been conducted. The model aortic arch flow chamber was fabricated in clear acrylic from an in situ casting of the human aorta and was incorporated in a mock-circulatory system. The model excluded the coronary sinuses and the three major branching arteries of the mid-arch region in order to concentrate only upon the effects of the multi-dimensional curvatures and tapering in the aorta. Furthermore, a flow straightening section was placed upstream to the flow chamber to eliminate any fluid disturbances created by the prosthetic aortic valve used in these studies. The qualitative flow visualization studies in the model aorta revealed the presence of strong secondary fluid motions near the inner wall. These helical flows dissipated during diastole, being greatly affected by the dramatic flow reversals which occurred along the inner wall at the onset of diastole. Quantitative studies were conducted using a three-sensor hot-film velocity probe to determine the axial, radial and tangential velocity components at various cross-sections in the aorta. The results showed rapid reversal of axial velocity near the inner wall at the onset of diastole.

Experimental study of physiological pulsatile flow past valve prosthesis in a model of human aorta--II. Tilting disc valves and the effect of orientation.

In Part II of this two paper sequence, pulsatile flow development past a tilting disc valve in a model human aorta has been studied using quantitative laser Doppler techniques. The valve was mounted in three different orientations with respect to the aortic root in this study. Under pulsatile flow, the region of flow reversal induced near the wall of the minor flow orifice extends to more than one tissue annulus diameter downstream from the valve into the ascending aorta. In a plane perpendicular to the tilt axis, a bi-helical secondary flow is induced distal to the valve. This secondary flow is further compounded by the multiple curvatures in the aorta. Hence the valve orientation affects the velocity profiles as far downstream as the mid-arch region as well as in the brachio-cephalic arterial branch. In the mid-arch region, a flow reversal along the entire cross-section is observed in early diastole for all the three orientations of the disc valve.

Experimental study of physiological pulsatile flow past valve prostheses in a model of human aorta--I. Caged ball valves.

Pulsatile flow development past a caged ball valve in a model human aorta was studied using laser Doppler anemometry. Velocity profiles measured in the ascending aorta and in the mid-arch region were strongly influenced by the geometry of the valve at the root of the aorta. Velocity profiles distal to the valve were asymmetric with jet-like flow in the peripheral region having larger velocity magnitudes towards the left lateral wall. In early diastole, a streamwise vortex motion was observed throughout the model aorta with fluid moving towards the downstream direction along the left lateral wall and reversed flow along the right lateral wall. With the caged ball valve at the root of the aorta, no reversed flow was observed along the inner wall of curvature in the mid-arch region.

Effect of prosthetic mitral valve geometry and orientation on flow dynamics in a model human left ventricle.

Pulsatile flow dynamics through bileaflet (St Jude and Duromedics), tilting disc (Bjork-Shiley and Omniscience), caged ball (Starr-Edwards), pericardial (Edwards) and porcine (Carpentier-Edwards) mitral valves in a model human left ventricle (LV) were studied. The model human ventricle, obtained from an in situ diastolic casting, was incorporated into a mock circulatory system. Measurements were made at various heart rates and flow rates. These included the transvalvular pressure drop and regurgitation in percent and cm3 beat-1. The effect of valve geometry and the orientation of the valve with respect to the valve annulus was analyzed using a flow visualization technique. Qualitative flow visualization study indicates certain preferred orientations for the tilting disc and bileaflet valve prostheses in order to obtain a smooth washout of flow in the LV chamber.

Stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position.

In this paper, a finite element analysis of the stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position is presented. The geometry of the valve was modified from a relationship proposed by Ghista and Reul (J. Biomechanics 10, 313-324, 1977). The effects of variations in stent height, leaflet thickness and coaptation area on the stress distribution were also analyzed. Analyses were performed with both rigid and flexible stents for the trileaflet valve in order to delineate the effect of stent flexibility on the leaflet stress distribution. The results showed that regions of stress concentration were present near the commissural attachment similar to those predicted with the bioprostheses. The stresses on the leaflets were reduced by increasing the stent height with both rigid and flexible stents. Selectively increasing the leaflet thickness near the commissures and also increasing the coaptation area did not prove to reduce the leaflet stresses when the stent flexibility was taken into account. The possible effect of high stresses on the structural integrity of polyurethane leaflets and its relationship with calcification is yet to be investigated.

Steady flow development past valve prostheses in a model human aorta. I. Centrally occluding valves.

In this paper, laser-Doppler anemometry measurement of steady flow development in a model human aorta has been reported. Studies were made with uniform entry flow at the root of the aorta and our measurements showed the establishment of a pair of Dean vortices in the mid-arch region. Subsequently, the nature of flow development past centrally occluding caged ball valves in the model aorta was investigated. Our studies showed that in the ascending aorta, an asymmetric velocity profile is obtained with larger velocity gradients towards the inner wall of tertiary curvature (anatomically the left lateral wall) with centrally occluding valves. The peripheral flow past these valves prevented the development of Dean vortices in the mid-arch region. The caged ball valves at the root of the aorta had no discernible effect on the velocity profiles in the brachio-cephalic artery.

Steady flow development past valve prostheses in a model human aorta. II. Tilting disc valves.

The flow development in the model human aorta with uniform entry as well as with centrally occluding valves mounted at the root of the aorta was described in Part I of this two-paper sequence. Part II deals with the flow development in the model aorta with tilting disc valves mounted at the root of the aorta. Bjork-Shiley and Hall-Kaster tilting disc valves were mounted in three different orientations with respect to the root of the aorta. The velocity profiles and turbulent stresses were measured with laser-Doppler anemometry. Our results under steady flow conditions in the model human aorta show quantitatively that the flow development in the ascending aorta as well as in the brachio-cephalic artery are strongly dependent on the orientation of the tilting disc valves. With the valves tilting towards the outer wall of curvature, our results suggest a tendency for flow separation at the flow divider region of the brachio-cephalic artery.

Effect of wedging on the flow characteristics past tilting disc aortic valve prosthesis.

To evaluate the efficacy of implanting a tilting disc aortic valve prosthesis in an angulated (wedged) supra-annular position, an in vitro experimental study was performed. The aortic valve prosthesis was mounted in an axi-symmetric valve chamber in a wedged position and incorporated in a mock circulatory system. Measurements were obtained on the transvalvular pressure gradient, percent regurgitation as well as velocity profiles and turbulent normal stresses distal to the valve. Our results showed that there was no significant reduction in the pressure gradient in mounting a larger sized valve in the wedged supra-annular position. On the other hand, the percent regurgitation increased with increase in heart rate and wedge angle. The valve failed to function properly above 110 beats min-1 at any wedge angle with the normal flow rate. The velocity profiles also showed significant changes with an increase in the turbulent normal stress with increase in wedge angle. Hence our study suggests that implanting the tilting disc prosthesis in a wedged supra-annular position in the aorta is not advisable.

Numerical study on the effect of secondary flow in the human aorta on local shear stresses in abdominal aortic branches.

Flow in the aortic arch is characterized primarily by the presence of a strong secondary flow superimposed over the axial flow, skewed axial velocity profiles and diastolic flow reversals. A significant amount of helical flow has also been observed in the descending aorta of humans and in models. In this study a computational model of the abdominal aorta complete with two sets of outflow arteries was adapted for three-dimensional steady flow simulations. The flow through the model was predicted using the Navier-Stokes equations to study the effect that a rotational component of flow has on the general flow dynamics in this vascular segment. The helical velocity profile introduced at the inlet was developed from magnetic resonance velocity mappings taken from a plane transaxial to the aortic arch. Results showed that flow division ratios increased in the first set of branches and decreased in the second set with the addition of rotational flow. Shear stress varied in magnitude with the addition of rotational flow, but the shear stress distribution did not change. No regions of flow separation were observed in the iliac arteries for either case. Helical flow may have a stabilizing effect on the flow patterns in branches in general, as evidenced by the decreased difference in shear stress between the inner and outer walls in the iliac arteries.

Laser anemometry measurements of pulsatile flow past aortic valve prostheses.

Experimental results are presented on physiological pulsatile flow past caged ball and tilting disc aortic valve prostheses mounted in an axisymmetric chamber incorporated in a mock circulatory system. The measurements of velocity profiles and turbulent normal stresses during several times in a cardiac cycle were obtained using laser-Doppler anemometry. Our results show that with increased angle of opening for the tilting disc valves, a large but locally confined vortex is observed along the wall in the minor flow region throughout most of the cardiac cycle. The turbulent normal stresses measured downstream to the tilting disc in the minor flow region parallel to the tilt axis were found to be larger than those measured downstream to the caged ball valves. Comparison of measurements with steady flow at flow rates comparable to peak pulsatile flow rate show that the turbulent normal stresses are larger by a factor of two in pulsatile flow with a frequency of 1.2 Hz.

Pulsatile flow past aortic valve bioprostheses in a model human aorta.

Pulsatile flow development past tissue valve prostheses in a model human aorta has been studied using qualitative flow visualization and quantitative laser-Doppler techniques. Experiments were conducted both in steady and physiological pulsatile flow situations and the measurements included the pressure drop across the valve, the instantaneous flow rate as well as the velocity profiles and turbulent stresses downstream to the valves. Our study shows that the velocity profiles with pericardial valves are closer to those measured past natural aortic valves. The porcine valves with a smaller valve opening area produce a narrower and stronger jet downstream from the valve with relatively larger turbulent axial stresses in the boundary of the jet. Our study suggests that the pericardial valves with turbulent stresses comparable to those of caged ball and tilting disc valves are preferable from a hemodynamic point of view.

Numerical study on the effect of steady axial flow development in the human aorta on local shear stresses in abdominal aortic branches.

The three-dimensional flow through a rigid model of the human abdominal aorta complete with iliac and renal arteries was predicted numerically using the steady-state Navier Stokes equations for an incompressible. Newtonian fluid. The model adapted for our purposes was determined from data obtained from cine-CT images taken of a glass chamber that was constructed based on anatomical averages. The iliac arteries had a bifurcation angle of approximately 35 and a branch-to-trunk area ratio of 1.27. whereas the renal arteries had left and right branch angles of 40 and an area ratio of 0.73. The numerical tool FLOW3D (AEA Industrial Technology, Oxfordshire, UK) utilized body-fitted coordinates and a finite volume discretization procedure. Purely axial velocity profiles were introduced at the entrance of the model for a range of cardiac outputs. The four-branch numerical model developed for this investigation produced flow and shear conditions comparable to those found in other reported works. The total wall shear stress distribution in the iliac and renal arteries followed standard trends. with maximum shear stresses occurring in the apex region and lower shear stresses occurring along the lateral walls. Shear stresses and flow rate ratios in the downstream arteries were more effected by inlet Re than the upstream arteries. These results will be used to compare further simulations which take into effect the rotational component of flow which is present in the aortic arch.