Biomedical subjects

Robin Shandas

Publications and source records attributed to Robin Shandas.

29 records · Page 2Linked to original sources

PubMed2004

Study of in vitro mitral valve filling flow.

Diastolic function is a good indicator of overall cardiac health. This study is in support of a non-invasive technique to measure diastolic function. Interpretation of mitral flow, and early diagnosis of normality or dysfunction, can be enhanced by improved understanding of the fluid dynamics. A simple in-vitro model of mitral flow is being used to illustrate vortex dynamics associated with diastolic inflow. A range of tailored mitral flow waveforms are being developed to mimic normal velocity profiles. Laser Doppler velocimetry (LDV) is being used to measure the flow speed. By combining flow visualization, particle image velocimetry and LDV, vortex ring signatures including size, circulation, and propagation speed are being examined as possible metrics for diastolic dysfunction. The detection of these signatures in the downstream flow will be compared to clinical waveforms derived from Doppler ultrasound.

Blood Flow Velocity↗

PubMed2004

Ultrasound wave propagation in tissue and scattering from microbubbles for echo particle image velocimetry technique.

Nonlinear wave propagation in tissue can be employed for tissue harmonic imaging, ultrasound surgery, and more effective tissue ablation for high intensity focused ultrasound (HIFU). Wave propagation in soft tissue and scattering from microbubbles (ultrasound contrast agents) are modeled to improve detectability, signal-to-noise ratio, and contrast harmonic imaging used for echo particle image velocimetry (Echo-PIV) technique. The wave motion in nonlinear material (tissue) is studied using KZK-type parabolic evolution equation. This model considers ultrasound beam diffraction, attenuation, and tissue nonlinearity. Time-domain numerical model is based on that originally developed by Lee and Hamilton [J. Acoust. Soc. Am 97:906-917 (1995)] for axi-symmetric acoustic field. The initial acoustic waveform emitted from the transducer is assumed to be a broadband wave modulated by Gaussian envelope. Scattering from microbubbles seeded in the blood stream is characterized. Hence, we compute the pressure field impinges the wall of a coated microbubble; the dynamics of oscillating microbubble can be modeled using Rayleigh-Plesset-type equation. Here, the continuity and the radial-momentum equation of encapsulated microbubbles are used to account for the lipid layer surrounding the microbubble. Numerical results show the effects of tissue and microbubble nonlinearities on the propagating pressure wave field. These nonlinearities have a strong influence on the waveform distortion and harmonic generation of the propagating and scattering waves. Results also show that microbubbles have stronger nonlinearity than tissue, and thus improves S/N ratio. These theoretical predictions of wave phenomena provide further understanding of biomedical imaging technique and provide better system design.

Computer Simulation↗

PubMed2004

Advantages in using multi-frequency driving ultrasound for optimizing echo particle image velocimetry techniques.

We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (echo PIV). This method takes advantage of the non-linear backscatter characteristics of ultrasound contrast microbubbles when exposed to certain ultrasonic field. Preliminary in vitro, animal and clinical studies have shown significant promise of this method for measuring multiple velocity components with good temporal and spatial resolution. However, there is still difficulty in maximizing the non-linearity of bubble backscatter using conventional Gaussian-pulse excitation techniques because significant harmonic components may not be produced at modest pressure amplitudes and the higher incident pressure amplitudes required to induce non-linear behavior may cause bubble destruction. We present here a potential solution to this problem through the use of multi-frequency excitation. A rectangular pulse with multiple harmonics is used to drive the bubble. The backscatter process is studied through a modified Rayleigh-Plesset equation. Results show that the rectangular wave is effective in improving the visibility of microbubbles with ultrasound backscattered efficiency significantly higher than the widely used Gaussian waveform. Use of rectangular pulses with 4 and 2 harmonics showed no significant difference in bubble backscatter behavior, indicating that a two-frequency excitation may be sufficient to induce non-linear behavior of the microbubbles practically at modest incident pressures.

Algorithms↗

PubMed2003

Insights into the effect of aortic compliance on Doppler diastolic flow patterns seen in coarctation of the aorta: a numeric study.

BACKGROUND: In the echocardiographic evaluation of coarctation of the aorta, the degree of antegrade diastolic flow (diastolic runoff) noted on spectral Doppler tracings traditionally was thought to be solely dependent on lesion severity. However, recent in vitro experiments suggest the presence of this spectral Doppler pattern is as much related to the severity of coarctation as it is with changes in aortic compliance. Using state-of-the-art, multidisciplinary, numeric analysis tools, the purpose of this study was to investigate the specific fluid and wall mechanics present in coarctation of the aorta to further understand these relationships. METHODS: Three computational numeric models of coarctation were developed with high, low, and no wall compliance. Flow simulations were run representing high- and low-flow states. RESULTS: In both the low- and high-flow states, the degree of diastolic runoff increased with increasing vessel compliance. The high compliance model had larger changes in aortic dilatation in the precoarctation region compared with the low compliance model. CONCLUSIONS: Increased aortic compliance brings about greater dilatation of the precoarctation aorta in systole, resulting in a persistence of stored upstream energy. This stored energy, released downstream in diastole as the precoarctation aortic walls contract, leads to increased degrees of diastolic runoff. Numeric methods offer a unique perspective into the mechanisms behind such clinical measures.

Aorta↗

PubMed2002

Use of intravascular ultrasound to measure local compliance of the pediatric pulmonary artery: in vitro studies.

BACKGROUND: The accurate measurement of local pulmonary artery compliance in pediatric pulmonary hypertension is an important step toward further understanding the biomechanical and hemodynamic aspects of the disease. The emergence of intravascular ultrasound (IVUS) imaging techniques promises the ability to make such measurements clinically. However, the use of IVUS for compliance measurements has not been validated. Furthermore, confusion exists regarding the most appropriate method to measure compliance. METHODS: This study validated IVUS measurements against a laser micrometer standard for 4 elastic tubes of varying compliance. Two methods of quantifying local compliance were explored: The pressure-strain modulus (E(p)), (E(p)(g/cm(2)) = DeltaP x R(d)/DeltaR (Where DeltaP is pulse pressure, R(d) is diastolic radius, and DeltaR is systolic minus diastolic radii) and the dynamic compliance (C(dyn)), (C(dyn)(%/100 mm Hg) = [DeltaD/(DeltaP x D(d))] x 10(4) Where DeltaD is systolic minus diastolic diameters and D(d) is diastolic diameter. RESULTS: IVUS diameter measurements agreed well with laser micrometer data although slight overestimation (mean = 3.67% +/- 2.78%) was present. Mean values of E(p) ranged from 353.3 g/cm(2) to 2676.0 g/cm(2); mean C(dyn) values ranged from 5.7% diametric change/100 mm Hg to 39.5% diametric change/100 mm Hg for all tube models. Although mean values of E(p) and C(dyn) could be distinguished among the various tubes, the extremely large measurement uncertainty for E(p) precluded statistical differentiation. The uncertainty in E(p) increased inversely with the diametric change, indicating a potential limitation of E(p) associated with stiffening arteries. CONCLUSIONS: We conclude that C(dyn) is a more robust mean of quantifying pediatric pulmonary artery compliance, especially as arteries stiffen with chronic pulmonary hypertension.

Child↗

PubMed2002

Influence of connection geometry and SVC-IVC flow rate ratio on flow structures within the total cavopulmonary connection: a numerical study.

The total cavopulmonary connection (TCPC) is a palliative cardiothoracic surgical procedure used in patients with one functioning ventricle that excludes the heart from the systemic venous to pulmonary artery pathway. Blood in the superior and inferior vena cavae (SVC, IVC) is diverted directly to the pulmonary arteries. Since only one ventricle is left in the circulation, minimizing pressure drop by optimizing connection geometry becomes crucial. Although there have been numerical and in-vitro studies documenting the effect of connection geometry on overall pressure drop, there is little published data examining the effect of SVC-IVC flow rate ratio on detailed fluid mechanical structures within the various connection geometries. We present here results from a numerical study of the TCPC connection, configured with various connections and SVC:IVC flow ratios. The role of major flow parameters: shear stress, secondary flow, recirculation regions, flow stagnation regions, and flow separation, was examined. Results show a complex interplay among connection geometry, flow rate ratio and the types and effects of the various flow parameters described above. Significant changes in flow structures affected local distribution of pressure, which in turn changed overall pressure drop. Likewise, changes in local flow structure also produced changes in maximum shear stress values; this may have consequences for platelet activation and thrombus formation in the clinical situation. This study sheds light on the local flow structures created by the various connections andflow configurations and as such, provides an additional step toward understanding the detailed fluid mechanical behavior of the more complex physiological configurations seen clinically.

Algorithms↗

PubMed2002

Designing the optimal Total Cavopulmonary Connection: pulsatile versus steady flow experiments.

BACKGROUND: The Total Cavopulmonary Connection (TCPC), used for repair of patients with single ventricle physiology, creates a passive system of blood flow into the pulmonary circulation where enhanced energy efficiency may lead to improved long term patient outcomes. Previous numerical and in vitro studies using steady flow have shown that incorporation of SVC (superior vena cava) and IVC (inferior vena cava) offsets lead to decreased energy losses. We hypothesize that the optimal TCPC offset design found in these previous steady flow experiments may not be the optimal design in pulsatile flow situations. MATERIAL/METHODS: 3-D finite volume numerical models were used to simulate flow through the total cavopulmonary connection. We ran steady and pulsatile flow experiments through 4 TCPC designs each with different SVC to IVC offsets (0.1/4, 1/2, 1 diameter offsets). The total energy (power) loss for each TCPC model was calculated. RESULTS: In steady flow experiments, % difference in energy loss between the most optimal and least optimal design was 26%. In contrast, in pulsatile flow experiments the % difference was only 8%. CONCLUSIONS: Our results demonstrate the improvements in energy loss seen using SVC-IVC offsets in steady flow experiments do not necessarily translate to pulsatile flow situations. Overall there was lower differences in efficiency between all TCPC designs in the pulsatile flow experiments. These results emphasize the need for further studies to fully define the relationship between energy losses and TCPC vessel architecture in non-steady flow physiologic situations.

Blood Flow Velocity↗

PubMed2002

Regression analysis for vortex ring characteristics during left ventricular filling.

A new technique to determine ventricular function is being developed from non-invasive measurements of centerline mitral flow, using color M-mode (CMM) ultrasound. Based on the hypothesis that vortex rings accompanying mitral flow are significant indicators of ventricular filling, we present a technique to determine characteristics of vortex rings created by mitral inflow, including diameter, position and circulation from the CMM derived centerline velocity data. The rings are modeled as vortex filament loops, which induce axial flow. A quasi-Newtonian regression was performed on the velocity data to determine model ring characteristics. Performance of the technique was determined using a synthetic data set with known vortex ring parameters. The algorithm is shown to be insensitive to initial estimates for ring parameters. However, noise in the data can lead to inaccuracy in the derived ring parameters. Further work to increase the accuracy of the regression algorithm and decrease effect of data uncertainty is ongoing.

Blood Flow Velocity↗

PubMed2002

Reverse flow in compliant vessels and its implications for the Fontan procedure: numerical studies.

The Total Cavopulmonary Connection (TCPC), a variant of the Fontan operation used for palliative cardiovascular repair of patients with single ventricle physiology, creates a passive system of blood flow into the pulmonary circulation for which energy efficiency may be critical to long term outcome. Clinical studies have shown that reverse flow in the TCPC is an indication of poor clinical status in these patients. Using numerical simulations, we demonstrate that reverse flow leads to increased energy losses in compliant vessels. Such an effect can potentially set off a series of spiraling negative events with decreased ventricular function leading to reverse flow, which causes decreased energy efficiency, which in turn leads to worsening function, and so forth, thereby suggesting one cause of progressive heart failure in this patient group.

Compliance↗

PubMedDate not supplied

Initial experience with the development and numerical and in vitro studies of a novel low-pressure artificial right ventricle for pediatric Fontan patients.

The Fontan operation, an efficient palliative surgery, is performed for patients with single-ventricle pathologies. The total cavopulmonary connection is a preferred Fontan procedure in which the superior and inferior vena cava are connected to the left and right pulmonary artery. The overall goal of this work is to develop an artificial right ventricle that can be introduced into the inferior vena cava, which would act to reverse the deleterious hemodynamics in post-Fontan patients. We present the initial design and computational analysis of a micro-axial pump, designed with the particular hemodynamics of Fontan physiology in mind. Preliminary in vitro data on a prototype pump are also presented. Computational studies showed that the new design can deliver a variety of advantageous operating conditions, including decreased venous pressure through proximal suction, increased pressure rise across the pump, increased pulmonary flows, and minimal changes in superior vena cava pressures. In vitro studies on a scaled prototype showed trends similar to those seen computationally. We conclude that a micro-axial flow pump can be designed to operate efficiently within the low-pressure, low-flow environment of cavopulmonary flows. The results provide encouragement to pursue this design to for in vitro studies and animal studies.

Animals↗

PubMedDate not supplied

Comparison of in vitro velocity measurements in a scaled total cavopulmonary connection with computational predictions.

Minimizing pressure drop through the total cavopulmonary surgical connection (TCPC), where the superior and inferior vena cavae (SVC), (IVC) are connected directly to the right and left pulmonary arteries, is an important clinical consideration. Computational fluid dynamics (CFD) models have been used to examine the impact of connection configuration on TCPC pressure drop. However, few studies have validated CFD results with experimental data. This study compares flow field measurements on two different TCPC models at varying SVC:IVC flow rate ratios using CFD and digital particle image velocimetry (DPIV). Although the primary flow fields generated by CFD and DPIV methods were similar for the majority of flow conditions, three key differences were found: (1) the CFD model did not reproduce the 3D complexity of flow interactions in the no-offset model with 50:50 flow ratio; (2) in vitro results showed consistently higher secondary flow components within the pulmonary artery segments, especially for the no-offset model; (3) recirculation areas for the 1/2 diameter offset model were consistently higher for in vitro versus CFD results. We conclude that this numerical model is a reasonable means of studying TCPC flow, although modifications need to be addressed to ensure that numerical results reproduce secondary flow characteristics.

Blood Flow Velocity↗