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Biomedical subjects

D Liepsch

Publications and source records attributed to D Liepsch.

At least 19 recordsLinked to original sources
PubMed2004

Do contrast agents affect ultrasound flow measurements?

The use of iodine contrast agents during angiography can put patients at risk. The replacement of angiography through duplex ultrasound methods would, therefore, be of great interest. Even when duplex sonography is used however, an x-ray angiography is often performed afterwards to verify the diagnosis. This causes complications in an unacceptable number of patients. The safety of duplex ultrasound sonography diagnosis can be improved by using low-risk Doppler contrast agents. Slow blood flow through vessels with a high degree stenosis and intracerebral flow can be more easily and accurately detected using these agents. However, amplification of the Doppler echo alters the measurements during both high and low flow, so that the flow rate cannot be recorded very precisely. Our goal was to study the effect of a contrast agent on velocity measurements with color-coded Doppler ultrasonography and on pulsed, one-goal measurements. We used a laser-Doppler-anemometer (LDA) with high temporal and spatial resolution as a reference method to compare the measurements.

Blood Flow Velocity↗
PubMed2003

Localization of bypass-induced changes in flow in coronary artery models.

Right coronary artery bypass restores blood flow through heart tissues. This also induces changes in flow leading to its failure. By this work the sites which are prone to such changes are localized. The bypass models are developed from transparent silicon rubber of elastic properties similar to arterial tissues. Flow visualization is carried out by photoelasticity technique by using dilute solution of vanadium pentoxide. This analysis carried out under pulsatile flow conditions shows that the proximal stenotic region continues to contribute to the alteration in flow in the hood region of the bypass. Thus making its proximal and distal regions prone to flow-induced changes, which may lead to its blockage over the long duration.

Blood Flow Velocity↗
PubMed2002

The discrimination of stenosed carotid bifurcation models with smooth and irregular plaque surface. Part I. Laser and ultrasonic Doppler flow studies.

Irregular carotid lesion surface is considered as a factor increasing the risk of the cerebral embolism. The objective of the study was to investigate the possibility to distinguish models of stenosed carotid bifurcation with lesion irregularity on the basis of the properties of flow velocity distributions. Two groups of elastic replicas of carotid bifurcations with different stenosis degree were investigated. Each group consisted of three models with different severity of plaque surface irregularity and one with smooth wall. Velocity data were collected using a one-component laser Doppler anemometer (LDA) system and a pulsed Doppler flowmeter. The LDA velocity distributions and Doppler spectral broadening index, turbulence intensity index and coefficient of skewness were analysed. The lesion irregularity resulted in change of the size and/or shape of reversed/reduced flow areas and of the position of the jet with respect to those observed in a smooth wall model. The flow features observed in the ultrasonic Doppler spectra were generally coherent with the axial LDA velocity distributions. Doppler spectral parameters demonstrated different sensitivities to the severity of the wall irregularity, however, the complexity of curves of these indices versus time did not allow to draw decisive conclusions and implied use of a more sensitive tool of analysis.

Blood Flow Velocity↗
PubMed2002

The discrimination of stenosed carotid bifurcation models with smooth and irregular plaque surface. Part II. The multivariate statistical analysis of ultrasonic Doppler velocity data.

According to the conclusion of the first part of the paper the Doppler spectral indices were analyzed in a detailed manner to provide a means to differentiate bifurcation models with different irregularities. Autoregressive (AR) models using the Akaike FPE criterion were fitted to the curves of the spectral broadening index, turbulence intensity index and coefficient of symmetry. These curves were also submitted to the data reduction significant point extraction algorithm. The multivariate discriminant analysis was performed on parameter vectors containing coefficients of AR models, data reduction rate and data reduction error. The discriminant analysis allowed distinguishing models with the same degree of stenosis and different plaque surface characteristics on the basis of these vectors. The highest discrimination efficiency was observed for parameter vectors obtained from TBI and SKEW curves. The efficiency of discrimination was slightly higher for more severely stenosed models.

Algorithms↗
PubMed2001

Laser-Doppler-anemometry studies of flow behavior in rigid and elastic varicose vein models.

Flow behavior and velocity were studied in six varicose vein models with simulated arteriovenous anastomoses (AVAs). Experiments were performed to determine whether incoming flow entering from AV-anastomoses or small feeder veins could create either small jet streams or velocity fluctuation, which might then cause veins to bulge and get tortuous. Flow was analyzed in areas of flow change. Studies were performed in rigid Plexiglas and elastic, silicon rubber models that exactly replicated the geometry and compliance of varicose veins. Flow was visualized with dyes for steady flow and with a birefringent solution for pulsatile flow studies. Local velocity was measured with a laser-Doppler-anemometer (LDA). Very low forces and shear stresses were found on the interior wall opposite the entrance of the simulated AV anastomoses. However, these values were ten times that of an idealized flow in a straight tube without added flow from simulated AV anastomoses.

Blood Flow Velocity↗
PubMed2000

Hemodynamics in the carotid artery bifurcation: a comparison between numerical simulations and in vitro MRI measurements.

The presence of atherosclerotic plaques has been shown to be closely related to the vessel geometry. Studies on postmortem human arteries and on the experimental animal show positive correlation between the presence of plaque thickness and low shear stress, departure of unidirectional flow and regions of flow separation and recirculation. Numerical simulations of arterial blood flow and direct blood flow velocity measurements by magnetic resonance imaging (MRI) are two approaches for the assessment of arterial blood flow patterns. In order to verify that both approaches give equivalent results magnetic resonance velocity data measured in a compliant anatomical carotid bifurcation model were compared to the results of numerical simulations performed for a corresponding computational vessel model. Cross sectional axial velocity profiles were calculated and measured for the midsinus and endsinus internal carotid artery. At both locations a skewed velocity profile with slow velocities at the outer vessel wall, medium velocities at the side walls and high velocities at the flow divider (inner) wall were observed. Qualitative comparison of the axial velocity patterns revealed no significant differences between simulations and in vitro measurements. Even quantitative differences such as for axial peak flow velocities were less than 10%. Secondary flow patterns revealed some minor differences concerning the form of the vortices but maximum circumferential velocities were in the same range for both methods.

Animals↗
PubMed1998

[Optimal expansion of the multi-link stent. An in vitro study with high resolution roentgen technique].

It has been speculated that high pressure implantation may improve the results of coronary stenting. However, this method bears the risk of persistent dissection and may increase late lumen loss. Presently, there is no consensus about the optimal stent implantation technique with the regard to balloon size and pressure. To elucidate this question an experimental study was performed in a coronary stenosis model. 3.5 mm Multi-Link (ML) stents were implanted in 3.3 mm silicone rubber tubes containing 50% concentric narrowings. Three implantation techniques were applied: 1. The standard technique using the conventional ML delivery system with a compliant balloon (ML-ST). 2. A new deployment method with a high pressure delivery system (ML-HP). 3. "Focal postdilation" using the ARC catheter, which has a special balloon with an inner compliant and an outer non-compliant section (ML-ARC). For comparison, the Palmaz-Schatz stent was implanted by using a high pressure balloon. Stent expansion was imaged by magnification radiography. Minimal lumen diameter within the stent (MLD) and the lumen diameter outside the stent (BD) were measured after dilations with 6, 9, 12, 15, 18, and 21 atm. The relation of the BD to the MLD was used as an index of vessel trauma. The results lead to the following conclusions: 1. A complete apposition to the vessel wall for a balloon/vessel relation of 1.1:1 could not be reached with pressures below 9-15 atm. The increase of the pressure beyond 15 atm resulted only in a minimal additional lumen. 2. Compared to the Palmaz-Schatz stent the recoil of the ML stent was significantly lower. 3. For all three implantation techniques the ML-ARC showed the best results with the maximal dilation of the stenotic vessel-area and the minimal expansion of the vessel outside the stent.

Angioplasty, Balloon, Coronary↗
PubMed1998

Biofluid mechanics.

The flow behavior of biological fluids in living organisms plays a crucial role in determining the state of the tissue through which they flow. Biofluid mechanics, the study of the fundamentals of biological fluid flow, has been recognized to be extremely important for the understanding of how changes in the flow behavior within living tissue maybe affect both the fluid and the tissue. Fluids in living tissue include blood, water, air and bodily fluids of animals, as well as the fluids in plants. The movement and balance of forces in resting fluids and fluids in motion are among the basic subjects for research. Biofluid mechanics is a field whose importance to the field of bioengineering has increased over the last two decades as pharmaceuticals, biomaterials and non-invasive diagnostic and surgical procedures create changes in the fluid mechanics of biofluids. Biofluid mechanics is a complex field including one of the most important areas of study--blood flow and cardiovascular diseases.

Arteriosclerosis↗
PubMed1998

Flow visualization and 1- and 3-D laser-Doppler-anemometer measurements in models of human carotid arteries.

Pulsatile flow, wall distensibility, non-Newtonian flow characteristics of blood in flow separation regions, and high/low blood pressure were studied in elastic silicon rubber models having a compliance similar to human vessels and the same surface structure as the biological intima models of (1) a healthy carotid artery model, (2) a 90% stenosis in the ICA, and (3) 80% stenosis in both the internal and external carotid arteries. Flow was visualized for steady flow and pulsatile studies to localize flow separation regions and reattachment points. Local velocity was measured with a 1-, 2-, or 3-D laser-Doppler-anemometer (LDA). Flow in the unstenosed model was Re = 250. In the stenosed models, the Re number decreased to Re = 180 and 213 under the same experimental conditions. High velocity fluctuations with vortices were found in the stenosed models. The jet flow in the stenosis increased up to 4 m/s. With an increasing bifurcation angle, the separation regions in the ECA and ICA increased. Increased flow (Re = 350) led to an increase in flow separation and high velocity shear gradients. The highest shear stresses were nearly 20 times higher than normal. The 90% stenosis created high velocity shear gradients and velocity fluctuations. Downstream of the stenoses, eddies were found over the whole cross-section. In the healthy model a slight flow separation region was observed in the ICA at the branching cross-section whereas in the stenosed models, the flow separation regions extended far into the ICA. We conclude that a detailed understanding of flow is necessary before vascular surgery is performed especially before artificial grafts or patches are implanted.

Blood Flow Velocity↗
PubMed1997

Analysis of ultrasonic Doppler velocity data obtained in models of stenosed carotid bifurcations with irregular lesion surface.

In this study properties are concerned of ultrasonic Doppler spectra recorded distal to an irregularity localised in the internal carotid artery of a true-to-scale elastic model of a severely stenosed carotid bifurcation. Four models differing in irregularity were obtained by melting graded holes in the original wax cast, about 5-7 mm distal to the bifurcation. Velocity data were collected using an HP Sonos 2500 ultrasonograph working in pulsed Doppler mode. The observed effect of the irregularity with respect to the smooth wall model consisted in: (1) presence of a reduced flow zone 15 mm distal to the severe irregularity situated at the outer wall of the model in the ICA branch; (2) presence of a recirculation zone 15 mm distal to the severe irregularity situated at the inner wall of this model; (3) negative values of the measure of symmetry of the Doppler spectrum and (4) decreased values of relative width of Doppler spectra obtained 25 mm distal to the bifurcation in models with severe irregularity. The alterations of flow distribution and Doppler spectra depend on the geometry of the model under investigation and this study provides an indication as to what phenomena can happen in a model of stenosed carotid bifurcation with irregular lesion surface and how these phenomena may affect the ultrasonic Doppler spectra. Such information could be used in the evaluation of the risk of cerebral embolism.

Blood Flow Velocity↗
PubMed1995

Correlation between LDA and ultrasound heart catheter measurements in a stenosed arterial model.

Ultrasound heart catheters are used to measure the velocity in coronary arteries. However, the act of introducing a catheter into the vessel disturbs the very flow being measured. We used laser Doppler anemometry to measure the velocity distribution in an axially symmetric model, both with and without a catheter inserted. The catheter reduced the center-line velocity by as much as 60 percent at a distance of 2 mm downstream from the catheter, and by as much as 25 percent at a distance of 10 mm. This means the velocity measured with an ultrasound catheter does not show the maximum velocity of the undisturbed flow in the tube center. In the constriction, however, the measured velocities with the LDA and ultrasound catheter are almost the same. Thus, catheter measurements in the stenosis achieve accurate results. The velocity profile in the stenosed areas is flattened over nearly the whole cross section. The velocity is extremely reduced only close to the wall. The measurements outside of the stenosis lead to large differences which need to be studied carefully in the future. The disturbed flow finally disappeared 15 mm downstream of the catheter. The measurements were done at steady flow using a glycerine water solution with a dynamic viscosity of 4.35 mPas. In future studies, these experiments will be repeated for pulsatile flow conditions using non-Newtonian blood-like fluids.

Blood Flow Velocity↗
PubMed1995

The effect of wall roughness on velocity distribution in a model of the carotid sinus bifurcation--analysis of laser and ultrasound Doppler velocity data.

The purpose of this work was to find out whether a rough surface in the sinus of a carotid model affects the flow velocity distribution and the properties of the corresponding ultrasonic Doppler spectrum, in view of potential application to the evaluation of risk of cerebral embolism. True-to-scale elastic models of carotid artery were used in the experiments. Experiments were performed in three models with varying severity of roughness and a normal (physiological healthy) model. The different degrees of roughness in the three models were created by melting graded holes in the wax cast used to make the model. Velocity data were collected using 4 Mhz pulsed ultrasonic and single component laser Doppler velocimeters. The roughness resulted in modified velocity distribution, affecting to various extent ultrasonic Doppler spectral parameters, in particular the measure of symmetry of spectral distribution.

Blood Flow Velocity↗