Gordon Campbell

Numerical simulation of leaflet flexure in bioprosthetic valves mounted on rigid and expansile stents

Recent studies suggest that flexural stresses induced during the opening phase may be responsible for much of the mechanical failures of bioprosthetic heart valves. Sharp leaflet bending is promoted by the mounting of valves on rigid stents that do not mimic the systolic expansion of the natural aortic root. We, therefore, hypothesized that flexural stresses could be significantly reduced by incorporating a flexible or expansile supporting stent into the valve design. Using our own non-linear finite element code (INDAP) and the pre- and post-processor modules of a commercial finite element package (PATRAN), we simulated the opening and closing behaviour a trileaflet bovine pericardial valve. The leaflets of this valve were assumed to be of uniform thickness, with a non-linear elastic behaviour adapted from experimentally obtained bending stiffness data. Our simulations have shown that during maximal systolic valve opening, sharp curvatures are induced in the leaflets near their commissural attachment to the supporting stent. These areas of sharp flexure experience compressive stresses of similar magnitude to the tensile stresses induced in the leaflets during valve closure. By incorporating a stent with posts that pivot about their base, such that a 10% expansion at the commissures is realized, we were able to reduce the compressive commissural stressing from 250 to 150 kPa. This was a reduction of 40%. Conversely, a simple pliable stent with stent posts that deflect inward and outward under load did not achieve a significant reduction of compressive stresses. This numerical analysis, therefore, supports the theory that (i) high flexural and compressive stresses exist at sites of sharp leaflet bending and may promote bioprosthetic valve failure, and (ii) that proper design of the supporting stent can significantly reduce such flexural stresses.

A constrained reconstruction technique of hyperelasticity parameters for breast cancer assessment.

In breast elastography, breast tissue usually undergoes large compression resulting in significant geometric and structural changes. This implies that breast elastography is associated with tissue nonlinear behavior. In this study, an elastography technique is presented and an inverse problem formulation is proposed to reconstruct parameters characterizing tissue hyperelasticity. Such parameters can potentially be used for tumor classification. This technique can also have other important clinical applications such as measuring normal tissue hyperelastic parameters in vivo. Such parameters are essential in planning and conducting computer-aided interventional procedures. The proposed parameter reconstruction technique uses a constrained iterative inversion; it can be viewed as an inverse problem. To solve this problem, we used a nonlinear finite element model corresponding to its forward problem. In this research, we applied Veronda-Westmann, Yeoh and polynomial models to model tissue hyperelasticity. To validate the proposed technique, we conducted studies involving numerical and tissue-mimicking phantoms. The numerical phantom consisted of a hemisphere connected to a cylinder, while we constructed the tissue-mimicking phantom from polyvinyl alcohol with freeze-thaw cycles that exhibits nonlinear mechanical behavior. Both phantoms consisted of three types of soft tissues which mimic adipose, fibroglandular tissue and a tumor. The results of the simulations and experiments show feasibility of accurate reconstruction of tumor tissue hyperelastic parameters using the proposed method. In the numerical phantom, all hyperelastic parameters corresponding to the three models were reconstructed with less than 2% error. With the tissue-mimicking phantom, we were able to reconstruct the ratio of the hyperelastic parameters reasonably accurately. Compared to the uniaxial test results, the average error of the ratios of the parameters reconstructed for inclusion to the middle and external layers were 13% and 9.6%, respectively. Given that the parameter ratios of the abnormal tissues to the normal ones range from three times to more than ten times, this accuracy is sufficient for tumor classification.

Aortic Valve/Root Interactions in Porcine Hearts: Implications for Bioprosthetic Valve Sizing

Abstract The implantation of aortic allografts as well as stentless, freehand porcine xenograft valves requires proper sizing of the graft for the recipient aortic root. To visualize the aortic valve in motion and measure the cyclic expansion of the aortic root, we developed an isolated porcine heart model and a computerized three‐dimensional reconstruction technique. Dynamic and static expansions of the aortic root were obtained from beating and arrested porcine hearts, and additional static expansions at varying pressures were measured from reconstructed three‐dimensional models of valves obtained with high‐resolution magnetic resonance imaging. Measurements of aortic root expansion have shown that it is highly dependent upon the pressures imposed on the heart. Although the aortic root expanded by only 5% between systolic pressures of 60 and 100 mmHg, the total expansion was up to 40% between rest and cyclic pressurizing to 100 mmHg. This data suggest that unstented xenograft valves should be sized 30% to 40% larger than the collapsed size of the recipient aorta. Proper sizing of valves on stents should also be attempted to reduce the large amount of leaflet redundancy that current stenting techniques produce.

Automated 3-D reconstruction of vascular structures from high definition casts

A technique for producing high-contrast images from high-definition casts for use in three-dimensional reconstruction and computer modeling for studying vascular structures is presented. The methodology used for automatic contour tracing, generating a mesh of variable density, and the schemes used to reconstruct bifurcating objects are presented. With this approach. 98 Mbytes of imaging data could be reduced to 180 Kbytes of polygon vertices. and manipulated at near real-time speed on a medium performance graphics workstation. The procedures used to create a high-definition, three-dimensional computer model of any vascular structure are outlined.<<ETX>>

A computerized system for video analysis of the aortic valve

A technique was developed to study the dynamic behavior of the porcine aortic valve in an isolated heart preparation. Under the control of a personal computer, a video frame grabber board continuously acquired and digitized images of the aortic valve, and an analog-to-digital (A/D) converter read four channels of physiological data (flow rate, aortic and ventricular pressure, and aortic root diameter). The valve was illuminated with a strobe light synchronized to fire at the field-acquisition rate of the CCD (charge-coupled device) video camera. Using the overlay bits in the video board, the measured parameters were superimposed over the live video as graphical tracing, and the resultant composite images were recorded online to video tape. The overlaying of the valve images with the graphical tracings of acquired data allowed the data tracings to be precisely synchronized with the video images of the aortic valve. This technique enabled the authors to observe the relationship between aortic root expansion and valve function.<<ETX>>

Micromechanics and Mathematical Modeling: An Inside Look at Bioprosthetic Valve Function

A major contributing factor in the degeneration of glutaraldehyde‐treated porcine xenograft bioprostheses is tearing of the valve cusps near their commissural attachment to the supporting stent. We have been examining aortic valves at the micromechanical level, and have developed several sensitive techniques to evaluate the biomechanical changes produced by the glutaraldehyde fixation process. Additionally, we have developed a mathematical modeling technique that simulates valve function during the entire cardiac cycle. Our micromechanical tests have shown that compressive buckling is common to all fixed tissues, occurs at physiological bending curvatures, and is likely to be the primary mode of mechanical failure of bioprosthetic valves. We have also shown that existing glutaraldehyde fixation techniques inhibit the natural internal shearing of the valve cusps, and disable the interaction of the fibrosa and the ventricularis. With our modeling technique, we have shown that flexural stresses are indeed concentrated near the valve commissures, and that appropriate modifications of the supporting stent can reduce flexural deformations. With these new, more revealing techniques at hand, prospective valve designs can be better evaluated prior to large scale animals and clinical testing.

Development of a composite material phantom mimicking the magnetic resonance parameters of the neonatal brain at 3.0 Tesla.

Objective:Development of a composite material phantom, comprised of polyvinyl alcohol cryogel (PVA-C) and an agarose additive, to effectively mimic the magnetic resonance relaxation times (T1 and T2) of neonatal white matter (WM) and gray matter (GM) at 3.0 T. Materials and Methods:Samples of PVA-C with and without agarose were prepared with 1 cycle of freezing/thawing. Measurements of T1 and T2, at 3.0 T, were performed on the samples at temperatures ranging from 20°C to 40°C. Results:A sample temperature of 40°C was required to achieve a T1 value sufficiently long to represent neonatal WM. At this temperature, neonatal WM relaxation times required 3% PVA-C with 0.3% agarose, whereas gray matter relaxation times required 8% PVA-C with 1.4% agarose. Conclusions:By adjusting the sample temperature, polyvinyl alcohol concentration, and agarose concentration, the relaxation times of neonatal brain tissues can be obtained using this composite material.

THE USE OF LIGAMENT EFFICIENCY TO MODEL FENESTRATIONS IN THE INTERNAL ELASTIC LAMINA OF CEREBRAL ARTERIES. II-ANALYSIS OF THE SPATIAL GEOMETRY

During uniaxial extension of the latex model, the holes (both marked on the sheet and perforated) demonstrated a distinctive change in their shape, from circular to elliptical. Measurements of the axial and transverse diameters of a consistent row of holes, along with the width of the sample at distinct elongations were converted into an area, ligament efficiency (transverse and axial) and shape factor (eccentricity). Values for the expansion ratio of the holes and necking of the latex sheet were also computed. The perforations expanded more rapidly than with the holes only marked on the solid material, which translated into a more rapid increase for the axial diameter, area and eccentricity, while the axial ligament efficiency decreased more rapidly. The transverse diameter and transverse ligament efficiency remained essentially constant. Necking of the latex sheet was consistent for all of the specimens (both solid and perforated). The relative expansion ratios paralleled the relative changes for the standardized stresses presented in Part I.

An optimization procedure for the alignment of marker points to assess dimensional changes

The process of dehydration and drying associated with the preparation of tissue specimens for examination by the scanning electron microscope usually results in dimensional changes in the form of shrinkage. A new method has been devised which requires specification of the location of marker points which have been affixed in a random pattern to the surface of the specimen. A new computer programme has been developed which acquires the coordinate pairs of the marker points, optimizes the alignment between the location of the marker points for the wet (fresh or fixed) and processed (dehydration with an intermediate fluid, and/or drying) conditions of the specimen, and computes the proportional dimensional changes in two mutually-perpendicular directions, as well as along the direct path between the origin and each of the marker points. Techniques for using the programme to assess whether the shrinkage may be planar-orthotropic or planar-anisotropic are suggested.

Evaluation of mitral valve replacement anchoring in a phantom

Conventional mitral valve replacement requires a median sternotomy and cardio-pulmonary bypass with aortic crossclamping and is associated with significant mortality and morbidity which could be reduced by performing the procedure off-pump. Replacing the mitral valve in the closed, off-pump, beating heart requires extensive development and validation of surgical and imaging techniques. Image guidance systems and surgical access for off-pump mitral valve replacement have been previously developed, allowing the prosthetic valve to be safely introduced into the left atrium and inserted into the mitral annulus. The major remaining challenge is to design a method of securely anchoring the prosthetic valve inside the beating heart. The development of anchoring techniques has been hampered by the expense and difficulty in conducting large animal studies. In this paper, we demonstrate how prosthetic valve anchoring may be evaluated in a dynamic phantom. The phantom provides a consistent testing environment where pressure measurements and Doppler ultrasound can be used to monitor and assess the valve anchoring procedures, detecting pararvalvular leak when valve anchoring is inadequate. Minimally invasive anchoring techniques may be directly compared to the current gold standard of valves sutured under direct vision, providing a useful tool for the validation of new surgical instruments.