Time-varying Blood Flow Research Articles

Virtual Reality Visualization of CFD Simulated Blood Flow in Cerebral Aneurysms Treated with Flow Diverter Stents

Virtual reality (VR) has the potential to be a powerful tool for the visualization of simulated blood flow in cerebral aneurysms. This paper presents our study aimed at developing the VR visualization of computational fluid dynamics (CFD) simulations of cerebral aneurysms treated with flow-diverting (FD) stents. First, a spherical sidewall aneurysm located at a simplified internal carotid artery was considered for investigating the impact of stent deployment and positioning on the corresponding spatially time-varying blood flow behavior. The three-dimensional unsteady blood flow over a cardiac cycle was simulated numerically using the finite volume method, and the distributions of hemodynamic parameters inside the aneurysm sac, and on its wall, were presented with and without stent cases. Two stent positions, with and without a gap between the artery wall and stent, were considered to show the influence of correct stent position on aneurysm treatment. Second, a straightforward workflow was developed to import, process, and visualize the CFD analysis data in a VR environment by using open-source software with a high resolution. The Unity3D engine was used for displaying the processed animations in a VR environment operated on a head-mounted display (HMD). The refining process of each frame of time-varying CFD data was automated. The animated flow elements rendered in the VR environment were velocity vectors, velocity contours, streamlines, particle traces, and point clouds. CFD results showed that proper stenting facilitates thrombosis and occlusion of the aneurysm by modification of the flow patterns, which leads to lower inflow jet velocities into the aneurysm, longer turnover time, lower aneurysm-averaged kinetic energy, and lower wall shear stress. Additionally, the results indicated that a gap between the stent and the parent artery may lead to undesirable hemodynamic alterations. The VR visualization illustrated that the recognition of the potential in danger regions of aneurysms and the evaluation of the performance of FD stents in aneurysm treatment can be conducted without the need for several slices through the parent artery and aneurysm, as is required for traditional postprocessing methods. Through VR visualization, the details of the simulation results become readily available by navigating in the 3D animated flow elements using a high-degree-of-freedom headset.

Vascular Ultrasound Simulator with B-Mode Images and Real-Time Color and Spectral Doppler for Standardized Assessment of Performance and for Training

Introduction - Duplex ultrasound (DUS) is relied upon for diagnosis of vascular pathology but providers need to be proficient to ensure that patients are referred appropriately for treatment. A standardized method for assessment of competence in DUS could provide feedback during training, assist in certification of personnel, and support laboratory accreditation. We developed a simulator that enables assessment of the skills required for DUS. Methods - The simulator displays a B-mode image that updates in real-time as the user manipulates a mock transducer on the mannequin, overlaid by color-coding of velocity data. Doppler spectral waveforms are generated according to user-defined settings such as beam steering, pulse repetition frequency, Doppler angle, and sample volume size. Each simulated case is built from a series of B-mode images acquired from a patient that are reconstructed as a 3D gray-scale volume. Three-dimensional surface models of the vessels are created and computational flow modeling is used to calculate time-varying blood flow velocity for all points within the lumen. We developed a user interface that reports the user’s measurements of peak systolic velocity (PSV) and flow volume, selected parameter settings, assessment of pathology, and diagnosis. The accuracy of the simulation was assessed by comparing the velocity measured from the spectral waveform with the actual velocity at the location of the sample volume within the lumen from the flow model. Results - Four expert examiners made 43 PSV measurements on the inflow artery and outflow vein of 2 dialysis access shunt models. Three experts made 36 measurements on 2 carotid artery models (one normal, one stenosed). The mean absolute deviation from the actual PSV was 8 ± 6% and 8 ± 5%, respectively, and was similar between vessel segments and between operators. The Image Library has 15 case studies with carotid artery, dialysis shunt, and femoral artery models. Conclusion - This is the first simulator that can generate color Doppler images and Doppler spectral waveforms that respond appropriately to parameter settings and to scanning a physical mannequin in real-time. The validation results demonstrate that the user can accurately measure blood flow velocity from the spectral waveform using the settings on the simulator. The simulator enables skill in DUS to be measured in an automated and standardized manner. Test difficulty and content can be adjusted to different training levels through case selection. Feedback can be given to the learner during training, and objective and quantitative performance data can be obtained for assessing the progress of trainees and for surveying the competence of health care providers.

On the suitability of laser-Doppler flowmetry for capturing microvascular blood flow dynamics from darkly pigmented skin

Objective: To assess the performance of laser Doppler flowmetry (LDF) in measuring blood perfusion from darkly-pigmented skin, i.e. skin with high melanin concentration. LDF provides for the noninvasive monitoring of microvascular blood flow dynamics. It has been used extensively on light-skinned subjects, i.e. on skin with low melanin concentration, in both the healthy and pathological states. Because the optical properties of human skin might affect the reliability of optically-based diagnostic equipment, the effectiveness of LDF needs to be checked and evaluated on dark skin, too, if this method is to be useful in global healthcare. Approach: Thirteen dark-skinned subjects and ten light-skinned subjects were included in the study. Microvascular blood flow dynamics was measured on both the right and left ankles using LDF with a laser diode of wavelength 780 nm. The characteristics of time-varying blood flow oscillations were investigated by wavelet analysis, nonlinear mode decomposition and wavelet phase coherence. An electrocardiogram (ECG), skin temperature, and respiratory effort were measured simultaneously with the LDF for each subject. Main results: No significant differences were observed between the groups in the mean blood perfusion (p > 0.1), or wavelet power (p > 0.6). The instantaneous heart rate (IHR), extracted from the LDF at each of the recording sites, and from the ECG, did not differ significantly between the groups (p > 0.8). Nor did the wavelet power of the IHR differ (p > 0.8) between the groups. The only significant difference found between the groups lay in left/right ankle blood flow coherence near the cardiac frequency, attributable to known ethnic physiological differences. Significance: These results indicate that high melanin concentrations in skin exert no significant influence on the ability of LDF to monitor microvascular blood flow dynamics when using a laser diode of wavelength 780 nm. Hence LDF can help in the diagnosis and exploration of the pathogenesis of diseases such as diabetes, hypertension, or malaria in darkly pigmented patients across sub-Saharan Africa.

STENOSIS EFFECTS ON THE FLUID MECHANICS OF THE COMMON CAROTID ARTERY BIFURCATION FOR UNSTEADY FLOWS

The time-varying blood flow in the common carotid artery (CCA) bifurcation is numerically studied on the basis of Navier-Stokes equations for four different cases, including cases with stenoses at different sites in the vicinity of the bifurcation. The cases studied were: (a) without stenoses, (b) with one stenosis upstream the bifurcation, (c) with two opposite stenoses upstream the bifurcation and (d) with an additional stenosis on the apex of the bifurcation. The mesh was generated via a geometry reconstruction and imported into a computational fluid dynamics (CFD) solver. The numerical results of the blood flow in the CCA bifurcation gave a detailed picture of the axial velocity and presented as velocity and vorticity magnitudes. More specifically, it was observed that the appearance of stenotic regions upstream the bifurcation affect both the velocity and vorticity characteristics, whereas a stenoses on the apex of the bifurcation seems to have a small effect on the vorticity characteristics downstream the flow.

Effect of xenon-induced flow activation on xenon-enhanced computed tomography cerebral blood flow calculations.

Computer simulations of stable xenon ((S)Xe) uptake curves were used to evaluate the effect of xenon-induced flow activation on CBF calculations by xenon-enhanced computed tomography. Estimates of flow activation were based on repeated transcranial Doppler measurements of blood velocity during 4.5 minutes of (S)Xe inhalation. The synthetic curves were generated from a generalized Kety equation that included time-varying blood flow activation. In contrast to the peak 35% increase in blood flow velocity during (S)Xe inhalation, a standard analysis of the flow-varying synthetic curves revealed only minor 3% to 5% increases in calculated CBF. It is concluded that brief xenon inhalations can provide blood flow estimates that contain minimal bias from activation.

レーザースペックル法を用いた眼底血流実時間測定

Real-time measurement of blood flow in retina is achieved by developing a hardware-logic board for the Laser Flowgraphy system. The retina is illuminated by a diode laser through a fundus camera and the spot is imaged onto the area sensor. The speckle field is scanned for certain numbers of time, and the blur rate of the speckles is calculated by the logic board. The maps of blood flow are obtained in every 120 msec and are displayed successively in the color CRT screen of a personal computer. The time varying blood flow in arterioles, as well as in veinules is clearly observed; which is synchronized to the heart pulse

Application of a Heat-Transfer Model to Determine Regional Blood Flow Rate

The temporal patterns of central and skin temperature may provide important predictive and diagnostic information during the recovery period from major surgery. Experimentally it is found that large changes in toe temperature occur at a predictable time, early in the postoperative period. These changes reflect cardiovascular dynamics and are related to modern concepts of thermoregulation. In order to explore the contribution of time-varying blood flow to heat transfer, a heat-transfer model was applied to the great toe. This model allowed prediction of toe blood flow rate from measurements of the central and skin temperatures. Experimental verification of the analytically determined blood flow rates was made using a modified technique of venous occlusion plethysmography.

The Effects of Time-Varying Blood Flow on Diffusional Resistance to Oxygen Transfer in the Pulmonary Capillaries

The effects of the persistence of pulsatile blood flow in the pulmonary capillaries on the over-all diffusing capacity and alveolar-arterial oxygen tension gradient were studied. A mathematical analysis was made of the oxygen transfer process using an undamped cardiac flow pulse in the capillaries and taking into account the finite rate of reaction of oxygen with hemoglobin.In five cases of both normal and low oxygen atmospheres, combined with varying degree of exercise, it was found that the alveolar-arterial oxygen tension gradients were not affected by the time-varying blood flow, while in cases of breathing air the over-all diffusing capacity of the lung increased 10-15% over the diffusing capacity obtained with constant blood flow rate in the pulmonary capillaries.

Effect of time-varying blood flow on oxygen uptake in the pulmonary capillaries.

Effect of time-varying blood flow on oxygen uptake in the pulmonary capillaries.