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At least 1,135 records · Page 63Linked to original sources
PubMed1993

Volumetric flow rates in the portal venous system: measurement with cine phase-contrast MR imaging.

OBJECTIVE: The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measurements within the portal vein, (2) develop a suitable protocol for using this method to measure volumetric flow rate in the portal venous system, and (3) use this protocol, with Doppler sonography as a reference, to measure portal venous flow in healthy volunteers and in patients with protal venous hypertension. SUBJECTS AND METHODS: Flow determinations were obtained in a model of fluid movement approximating blood-flow conditions in the portal venous system. A suitable protocol was based on consideration of the theoretical effects of (1) spatial resolution, (2) obliquity of the imaging plane to the direction of flow, and (3) signal-to-noise ratio of the signed quantitative velocity images (in three volunteers) on the accuracy and precision of flow measurements. This protocol was used to obtain cine phase-contrast MR images of the portal venous system in five volunteers and six patients. RESULTS: Values obtained with a flow phantom showed good accuracy of cine phase-contrast-measured vs actual volumetric flow rate (r = .995; p = .0001; MR rate = [0.94 x actual rate] + 65.6 ml/min; standard error of the y estimate = 67.3 ml/min). Velocity encoding and section thickness substantially influenced the signal-to-noise ratio of the velocity images, whereas flip angle and matrix size had only minimal effect. In volunteers and patients, portal volumetric flow rates determined by using MR images and Doppler sonography showed good correlation (r = .94; p = .0003). CONCLUSION: Our results indicate that cine phase-contrast MR imaging is a practical noninvasive method for measuring volumetric flow rates in the portal venous system.

Adult↗
PubMed2004

Building a functional artery: issues from the perspective of mechanics.

Despite the many successes of arterial tissue engineering, clinically viable implants may be a decade or more away. Fortunately, there is much more that we can learn from native vessels with regard to designing for optimal structure, function, and properties. Herein, we examine recent observations in vascular biology from the perspective of nonlinear mechanics. Moreover, we use a constrained mixture model to study potential contributions of individual wall constituents. In both cases, the unique biological and mechanical roles of elastin come to the forefront, especially its role in generating and modulating residual stress within the wall, which appears to be key to multiple growth and remodeling responses.

Animals↗
PubMedDate not supplied

Mock circulatory system for in vitro reproduction of the left ventricle, the arterial tree and their interaction with a left ventricular assist device.

A modular physical circuit for testing monoventricular and biventricular heart assist devices (HAD) is under development. The modules now available are assembled in an open-loop circuit and reproduce the function of the left ventricle and the systemic arterial tree. The left ventricle model reproduces Starling's law of the heart and can be easily controlled to modify other parameters such as contractility and timing (i.e. heart rate and systole/diastole ratio). This circuit, in connection with a left ventricular assist device (LVAD), can be used to evaluate the LVAD performance, its effect on the circulatory system and as a training system. This paper is devoted to a description of the circuit and of its interaction with a LVAD, which is analysed after the simulation of a low contractility pathology of the ventricle. Results obtained in these experiments are reported.

Heart-Assist Devices↗
PubMed1995

Plasma skimming in vascular trees: numerical estimates of symmetry recovery lengths.

A new model of plasma skimming in vascular trees that includes the history of disturbances at previous bifurcations is developed. The model calculates the shift in the red blood cell concentration profile by mapping streamlines through the junction; it then computes the dissipation of that disturbance through random collisions among the cells. A numerical solution of this model is compared to experimental data on plasma skimming in serial bifurcations. Numerical values of symmetry recovery lengths downstream of the bifurcation are calculated using the model. Comparison of the computed symmetry recovery lengths with anatomical measurements in hamster cremaster muscle indicate that asymmetries in red blood cell concentration profiles are important in vessels with diameters below 55 microns.

Animals↗
PubMed2004

Simulation of minimally invasive vascular interventions for training purposes.

OBJECTIVE: To master the skills required to perform minimally invasive vascular interventions, proper training is essential. A computer simulation environment has been developed to provide such training. The simulation is based on an algorithm specifically developed to simulate the motion of a guide wire--the main instrument used during these interventions--in the human vasculature. In this paper, the design and model of the computer simulation environment is described and first results obtained with phantom and patient data are presented. MATERIALS AND METHODS: To simulate minimally invasive vascular interventions, a discrete representation of a guide wire is used which allows modeling of guide wires with different physical properties. An algorithm for simulating the propagation of a guide wire within a vascular system, on the basis of the principle of minimization of energy, has been developed. Both longitudinal translation and rotation are incorporated as possibilities for manipulating the guide wire. The simulation is based on quasi-static mechanics. Two types of energy are introduced: internal energy related to the bending of the guide wire, and external energy resulting from the elastic deformation of the vessel wall. RESULTS: A series of experiments were performed on phantom and patient data. Simulation results are qualitatively compared with 3D rotational angiography data. CONCLUSIONS: The results indicate plausible behavior of the simulation.

Algorithms↗
PubMedDate not supplied

Medical image interpretation: a generic approach using deformable templates.

We describe a generic approach to image interpretation, based on combining a general method of building flexible template models with genetic algorithm (GA) search. The method can be applied to a given image interpretation problem simply by training a statistical shape model, using a set of examples of the image structure to be located. A local optimization technique has been incorporated into the GA search and shown to improve the speed of convergence and optimality of solution. We present results from three medical applications, demonstrating that the new method offers significant improvements when compared with previously reported approaches to flexible template matching, particularly the ability to deal with different domains of application using a standard method and the possibility of employing complex multipart models. We also describe how the method can be simply extended to track structures in image sequences and segment three dimensional objects in volume images.

Algorithms↗
PubMed1998

Relationships among cerebral perfusion pressure, autoregulation, and transcranial Doppler waveform: a modeling study.

OBJECT: The aim of this study was to analyze how the main values extrapolated from the transcranial Doppler (TCD) waveform (systolic, mean, and diastolic velocity; velocity peak-to-peak amplitude; and pulsatility index [PI]) are affected by changes in intracranial pressure (ICP), systemic arterial pressure (SAP), autoregulation, and intracranial compliance. METHODS: The analysis was performed using a mathematical model of the intracranial dynamics. This model includes a passive middle cerebral artery, the biomechanics of large and small pial arteries subjected to autoregulatory mechanisms, a collapsing venous cerebrovascular bed, the cerebrospinal fluid circulation, and the ICP-volume relationship. The results indicate that there are approximately three distinct zones characterized by different relationships between cerebral perfusion pressure (CPP) and velocity parameters in patients with preserved autoregulation. In the central autoregulatory zone (CPP > 70 mm Hg) the mean velocity does not change with decreasing CPP, whereas the PI and velocity peak-to-peak amplitude increase moderately. In a second zone (CPP between 4045 and 70 mm Hg), in which vasodilation of small pial arteries becomes maximal, the mean velocity starts to decrease, whereas the PI and velocity amplitude continue to increase. In the third zone, in which autoregulation is completely exhausted (CPP < 40 mm Hg), arterioles behave passively, mean velocity and velocity amplitude decline abruptly, and the PI exhibits a disproportionate rise. Moreover, this rise is quite independent of whether CPP is reduced by increasing ICP or reducing mean SAP. In contrast, in patients with defective autoregulation, the mean velocity and velocity amplitude decrease linearly with decreasing CPP, but the PI still increases in a way similar to that observed in patients with preserved autoregulation. CONCLUSIONS: The information contained in the TCD waveform is affected by many factors, including ICP, SAP, autoregulation. and intracranial compliance. Model results indicate that only a comparative analysis of the concomitant changes in ultrasonographic quantities during multimodality monitoring may permit the assessment of several aspects of intracranial dynamics (cerebral blood flow changes, vascular pulsatility, ICP changes, intracranial compliance, CPP, and autoregulation).

Arteries↗
PubMed1989

Pulsatile non-Newtonian blood flow simulation through a bifurcation with an aneurysm.

Blood flow is analysed by means of computer simulation in an idealized arterial bifurcation model which is pathologically altered by a saccular aneurysm. The theoretical study of the flow pattern and the paths of fluid particles is carried out under pulsatile Newtonian and non-Newtonian flow conditions. The governing equations are solved numerically with the use of the finite element method. The results show the disturbed blood flow in the bifurcation and the relatively low intra-aneurysmal flow circulation. In addition to the study of basic flow patterns in the segment, a comparison of non-Newtonian and Newtonian results is carried out. This comparison proves that for the considered large artery model under physiological flow conditions where the yield number is relatively low there is no essential difference in the results.

Aneurysm↗
PubMed1994

In vitro pressure drop comparison between two mechanical valve prostheses.

An hemodynamic evaluation of two mechanical heart valves is presented. A tilting disc valve and a bileaflet valve were incorporated in a mock circulatory system which consists of a closed flow loop with a pneumatically driven flexible diaphragm to simulate the physiologic pulsatile flow. Comparisons between the valves were made on the aortic pressure, ventricular pressure, as well as mean pressure gradient at a systolic duration of 45% and a heart rate of 90 beats per minute. The results showed that the tilting disc valve has higher ventricular pressure and mean pressure gradient than that of the bileaflet valve. This indicates that the tilting disc valve has higher transvalvular flow resistance and energy loss than that of the bileaflet valve. From this study it is demonstrated that the mock circulatory system can be a very useful device to evaluate the prosthetic heart valves in vitro.

Heart Valve Prosthesis↗
PubMed1987

Studies of left ventricular volume estimation from gated blood pool scan with the use of regression equations.

The authors carried out experimental and clinical studies of left ventricular volume measurement using equilibrium-gated blood pool scan. Using phantoms, we experimentally obtained a regression equation between radioisotope levels and camera counts. From this equation, we deduced a regression equation between camera counts and volumes, which was then used for calculation of left ventricular volumes using count technique. In an experiment using phantom with various volumes, we obtained a favorable result that the average absolute error between the true and measured volumes was as small as 6.8%. In 11 patients, we calculated the left ventricular volume and ejection fraction employing the regression equation and compared the calculated values with the values obtained by cinemascopy. This comparison disclosed an excellent correlation (r = 0.955 for end-diastolic volume, 0.997 for end-systolic volume and 0.853 for ejection fraction). Mean absolute error was 16.2% for end-diastolic volume and 16.0% for end-systolic volume. These results indicate that this technique is clinically applicable.

Adolescent↗
PubMed1988

Determination of ejection fraction from contrast echocardiography using videodensitometry in an in vitro model.

To examine whether ejection fraction can be estimated by videodensitometric analysis of contrast washout from a chamber, a model was constructed consisting of a 60-ml syringe and a modified Harvard respirator. Ejection fraction could be adjusted mechanically. Echocardiographic images were recorded on videotape and analyzed on an offline computerized analysis system. To calibrate the relationship between target concentration and videodensity on our sector scanner-videodensity analysis system, we used a phantom with constant attenuation and eight cones with known, varying relative scatterer concentration. The time-gain-compensation curve was kept constant on the ultrasound instrument and videodensity was measured from videotapes by an offline computer system. In the image of each cone, a 10 X 70 pixel operator-designated region of interest was chosen, and mean videodensity/pixel (256 level gray scale) was calculated. A logarithmic relation was found between scatterer concentration and videodensity. This excellent log relationship existed at all three gain levels studied: at 40% of maximum gain, r = .96; at 50% gain, r = .99; and at 60% gain, r = .99 (all p less than .001). Boluses of .5 ml sonicated Hypaque were then injected into the chamber and the contrast washout was recorded at ejection fractions of 20, 30, 40, 50, 60, and 70%. Bolus delivery and complete contrast mixing was attained except at ejection fraction 20%. Using the first two to four stroke videodensity decay the ejection fraction was calculated by estimating microbubble concentration decline using the logarithmic regression equation between videodensity and scatter concentration obtained during calibration.(ABSTRACT TRUNCATED AT 250 WORDS)

Densitometry↗
PubMed1990

Testing ultrasonic pulsed Doppler instruments with a physiologic string phantom.

The spatial, temporal, and frequency resolution of conventional ultrasonic Doppler instruments and the time/space distortions in two-dimensional color Doppler imaging systems can be measured using a pulsatile moving string target. The diameter of the string is small compared with the Doppler sample volume, the velocity (speed and direction), acceleration and timing of the string motions are precisely known with reference to the R wave timing mark, and the spatial location of the string is known. A loop of surgical thread or monofilament fishline running between pulleys is driven by a motor that provides constant string speeds from 0.05 to 150 cm/s and variable string speeds programmed to mimic arterial velocity waveforms from the carotid, aortic, and femoral arteries. Constant string speeds are used to evaluate the Doppler sensitivity, frequency processing, and sample volume size; pulsatile movement of the string provides a physiologic model to evaluate the temporal performance of conventional Doppler systems and the temporal and spatial performance of two-dimensional color Doppler imaging scanners.

Blood Flow Velocity↗
PubMed1990

On the application of ultrasonic contrast agents for blood flowmetry and assessment of cardiac perfusion.

The stability, size, and ultrasonic properties of several ultrasonic microbubble contrast agents such as Albunex, SHU-454, 1% hydrogen peroxide, and sonicated solutions such as 70% and 50% dextrose, 70% sorbitol, 5% albumin, Renografin-76, and others were evaluated. Albunex was the only tested agent that was sufficiently stable over an extended period of time for in vitro ultrasonic characterization. The attenuation and backscatter coefficients of Albunex at 5 and 7.5 MHz were found to be linearly proportional to microsphere concentration at low concentrations (less than 0.01% for attenuation and less than 0.002% for backscatter). Also, Albunex was found to be more echogenic than soft tissues at 5 MHz even after being diluted to 0.0003% of the original concentration. Next, the feasibility of using this acoustic information for contrast blood flowmetry was investigated. In vitro flow estimates in a mock flow loop were made using only the ultrasonic properties of the contrast agent. Bolus injections of Albunex and indicator-dilution curves inferred from ultrasonic measurements were used to estimate calibrated flow rate ranging from 400 to 5000 mL/min. The flow estimates from attenuation measurements showed a good correlation with those from an independent method (r = 0.97), but the results from backscatter studies did not correlate well. These results demonstrate that attenuation measurement may be a feasible alternative for in vivo blood flow measurement in conjunction with the indicator dilution principle or estimation of tissue perfusion such as myocardial perfusion using a time-activity approach.

Albumins↗