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PubMed2002

[A study of the influence scan timing and depiction of blood vessel diameter in CE-3DMRA: simulation by pulsatile flow].

In imaging using contrast-enhanced three-dimensional magnetic resonance angiography (CE-3DMRA), optimizing the delay time from the start of intravenous injection of contrast medium to the start of scanning has generally been an important concern when obtaining blood vessel images of good contrast. Recent methods of mechanically assessing the attainment of contrast medium injection include Smart-Prep and others. Another method is the Test Bolus, in which a small amount of contrast medium determines the timing of scan start, in quest of the time intensity curve. Because these methods are not necessarily satisfactory, the corrected method is used in clinical cases. In terms of how scan timing affects blood vessel depiction, no study has examined pulsatile flow, which is carried out by simulation. On the other hand, there are reports on data filling of k-space using imitation blood vessels on a computer and simple experimental equipment. This research examined experimentally blood vessel depiction according to scan timing and the diameter of blood vessels by using a systemic circulation simulator that incorporated hemodynamic circulation in which pulsatile flow is the same as that of a human body, using an artificial heart developed especially for MRI. It is thought that scan timing in CE-3DMRA affects the depiction of blood vessels for which the diameter of the blood vessel differs from the experimental result. Phase-encoding k-space data filling is the contrast-to-noise ratio (CNR) of each blood vessel irrespective of sequential and centric k-space ordering. The timing shift phenomenon, in which a blood vessel is so thin that the scan timing is overdue and shows a relatively high value, can occur. Moreover, that scan timing affects not only the diameter of a blood vessel but also depiction of the narrowing of a stenotic blood vessel or a branch blood vessel was demonstrated clinically and experimentally. Therefore, blood vessel depiction changes with scan timing, and it is clinically important to determine the time of arrival of contrast medium and the duration of the enhancement effect in quest of the time intensity curve by test bolus when clearly describing the target blood vessel.

Blood Vessels↗
PubMed1993

Pulsatile flow artifacts in 3D magnetic resonance imaging.

Some of the important features of how pulsatile flow generates artifacts in three-dimensional magnetic resonance imaging are analyzed and demonstrated. Time variations in the magnetic resonance signal during the heart cycle lead to more complex patterns of artifacts in 3D imaging than in 2D imaging. The appearance and location of these artifacts within the image volume are shown to be describable as displacements along a line in a plane parallel to that defined by the phase and volume encode directions. The angle of the line in the plane depends solely upon the imaging parameters while the ghost displacement along the line is proportional to the signal modulation frequency. Aliasing of these ghosts leads to a variety of artifact patterns which are sensitive to the pulsation period and repetition time of the pulse sequence. Numerical simulations of these effects were found to be in good agreement with experimental images of an elastic model of a human carotid artery under simulated physiological conditions and with images of two human subjects.

Aged↗
PubMed2005

Differentiation from embryonic stem cells to vascular wall cells under in vitro pulsatile flow loading.

This study evaluated the possibility of differentiation from embryonic stem (ES) cells to vascular wall cells by physical (mechanical) stress loading in vitro. A cell mixture containing Flk1-positive cells (ca. 30%) derived from murine ES cells was added to a compliant microporous tube made of segmented polyurethane. The compliance of the tube was close to that of the human artery [the stiffness parameter (beta) = 57.2 (n = 5, SD < 5%)]. The luminal surface of the tube was fully covered with the cells by preincubation for two days in the presence of vascular endothelial growth factor (VEGF). After 2 days of additional incubation without VEGF under static conditions, layering of the grown cells, mostly smooth muscle actin (SMA)-positive cells, was observed only on the luminal surface of the tube. The cells were flat, polygonal, and randomly oriented. On the other hand, after a 2-day incubation under a weak pulsatile flow simulating the human venous systems [wall shear stress (WSS) from -0.98 to 2.2 dyn/cm(2); circumferential strain (CS) 4.6-9.6 x 10(4) dyn/cm(2)] without VEGF, cells in the superficial layer were regularly oriented in the direction of the pulsatile flow. The oriented cells exhibited endothelial-like appearance, indicating that they were platelet endothelial cell adhesion molecule 1 (PECAM1)-positive. In addition, the cells growing into the interstices in the deeper layer showed smooth muscle-like appearance, indicating that they were SMA-positive. Differentiation to two different cell types and segregation of incorporated ES cells may be simultaneously encouraged by the combination of WSS and CS. It is expected that the monobloc building of hierarchically structured hybrid vascular prostheses composed of several vascular wall cell types is possible by physically synchronized differentiation of ES cells.

Actins↗
PubMedDate not supplied

The usefulness of pulsatile flow detection in measuring resistive index in renal Doppler US.

OBJECTIVE: To assess the usefulness of pulsatile flow detection (PFD), a newly developed function of color Doppler US, in measuring resistive index (RI) in renal Doppler US and to compare it with conventional color Doppler (CCD). MATERIALS AND METHODS: Fifty-six kidneys in 31 patients were randomly selected and divided into two groups. In group A, RI was measured first with the aid of CCD, and then with PFD. In group B, data were obtained in the reverse order. The time required for each RI measurement was recorded in seconds. The quality of the Doppler spectral waveform was subjectively graded as 0, 1, or 2 and examination time and waveform quality were compared between PFD and CCD. RESULTS: The time required to measure RI with PFD (PFD time) was less than with CCD (CCD time) (mean 42.7 secs vs. mean 70.3 secs; p = 0.031). There was no significant difference in PFD time between group A and B, but CCD time was shorter in group B (70.3 secs vs. 24.6 secs; p = 0.0004). Spectral waveform quality was not significantly different between PFD and CCD. CONCLUSION: The time required to measure RI in kidneys can be shortened with the aid of the PFD function in color Doppler US without affecting the quality of the examination.

Blood Flow Velocity↗
PubMed2004

Pulsatile flow in fusiform models of abdoiminal aortic aneurysms: flow fields, velocity patterns and flow-induced wall stresses.

As one important step in the investigation of the mechanical factors that lead to rupture of abdominal aortic aneurysms, flow fields and flow-induced wall stress distributions have been investigated in model aneurysms under pulsatile flow conditions simulating the in vivo aorta at rest. Vortex pattern emergence and evolution were evaluated, and conditions for flow stability were delineated. Systolic flow was found to be forward-directed throughout the bulge in all the models, regardless of size. Vortices appeared in the bulge initially during deceleration from systole, then expanded during the retrograde flow phase. The complexity of the vortex field depended strongly on bulge diameter In every model, the maximum shear stress occurred at peak systole at the distal bulge end, with the greatest shear stress developing in a model corresponding to a 4.3 cm AAA in vivo. Although the smallest models exhibited stable flow throughout the cycle, flow in the larger models became increasingly unstable as bulge size increased, with strong amplification of instability in the distal half of the bulge. These data suggest that larger aneurysms in vivo may be subject to more frequent and intense turbulence than smaller aneurysms. Concomitantly, increased turbulence may contribute significantly to wall stress magnitude and thereby to risk of rupture.

Animals↗
PubMed1993

Measurement of mean velocity during pulsatile flow using time-averaged maximum frequency of Doppler ultrasound waveforms.

It has been suggested that mean velocity of flow could be estimated by the time-averaged maximum frequency over an integral number of cardiac cycles (Gill 1985). The present study verified this theory experimentally with a computer-controlled flow phantom. The effects of some parameters on the relationship between mean velocity and time-averaged maximum frequency were also studied. Parameters investigated included beam-vessel angle, diameter of tubing, pulsatility, flow rate and stenosis. The velocities measured by the Doppler system were compared with the actual velocities. A simple theoretical model was also developed to compare with the experimental results. The results showed that, in a long straight tube, the mean velocity can be estimated to within about 5% from the time-averaged maximum Doppler shift at various flow rates and pulsatilities. The error due to geometrical spectral broadening, especially for large beam-vessel angles, can be estimated to within 3% and therefore corrected.

Blood Flow Velocity↗
PubMed1998

Multiphase segmented k-space velocity mapping in pulsatile flow waveforms.

The aim of the present study was to obtain the precision of flow measurement in breath-hold segmented k-space flow sequences. The results are based on studies of pulsatile flow in a phantom tube. The ultimate purpose is to use these sequences to measure coronary flow. In abdominal and cardiothoracic magnetic resonance imaging the image quality is degraded due to respiratory motion. In the segmented k-space acquisition method, one obtains many phase-encoding steps or views per cardiac phase. This shortens imaging time in the order of phase-encoding lines and makes it possible to image in a single breath-hold, thereby eliminating respiratory artefacts and improving edge detection. With breath-hold multiframe cine flow images it is possible to evaluate flow in all abdominal and cardiothoracic areas, including the coronary arteries. Our study shows that velocity curves shift in time when the number of k-space ky-lines per segment (LPS) are varied; this shift is linear as a function of LPS. The mean velocity Vmean in the center of mass of the pulsatile peak is constant (Vmean = 40.1 +/- 2.9 cm/s) and time t = -10.1 x LPS + 268 (r = 0.993, p < 0.0001). Correlation between theoretical and experimental flow curves is also linear as a function of LPS: C = -0.977 * LPS (r = 0.987, p < 0.0001). It is concluded that velocity curves move with LPS and are smoothed when the breath-hold velocity mapping is used. The more LPS is gathered the more inaccurate results are. LPS 7 or more cannot be considered clinically relevant.

Artifacts↗
PubMed2001

Numerical investigation of physiologically realistic pulsatile flow through arterial stenosis.

Numerical simulations of pulsatile blood flow in straight tube stenosis models were performed to investigate the poststenotic flow phenomena. In this study, three axisymmetrical and three asymmetrical stenosis models with area reduction of 25%, 50% and 75% were constructed. A measured human common carotid artery blood flow waveform was used as the upstream flow condition which has a mean Reynold's number of 300. All calculations were performed with high spatial and temporal resolutions. Flow features such as velocity profiles, flow separation zone (FSZ), and wall shear stress (WSS) distributions in the poststenotic region for all models are presented. The results have demonstrated that the formation and development of FSZs in the poststenotic region are very complex, especially in the flow deceleration phase. In axisymmetric stenoses the poststenotic flow is more sensitive to changes in the degree of stenosis than in asymmetric models. For severe stenoses, the stenosis influence length is shorter in asymmetrical models than in axisymmetrical cases. WSS oscillations (between positive and negative values) have been observed at various downstream locations in some models. The amplitude of the oscillation depends strongly on the axial location and the degree of stenosis.

Arterial Occlusive Diseases↗
PubMed2005

A numerical and experimental investigation of transitional pulsatile flow in a stenosed channel.

In the present paper, a closely coupled numerical and experimental investigation of pulsatile flow in a prototypical stenotic site is presented. Detailed laser Doppler velocimetry measurements upstream of the stenosis are used to guide the specification of velocity boundary conditions at the inflow plane in a series of direct numerical simulations (DNSs). Comparisons of the velocity statistics between the experiments and DNS in the post-stenotic area demonstrate the great importance of accurate inflow conditions, and the sensitivity of the post-stenotic flow to the disturbance environment upstream. In general, the results highlight a borderline turbulent flow that sequentially undergoes transition to turbulence and relaminarization. Before the peak mass flow rate, the strong confined jet that forms just downstream of the stenosis becomes unstable, forcing a role-up and subsequent breakdown of the shear layer. In addition, the large-scale structures originating from the shear layer are observed to perturb the near wall flow, creating packets of near wall hairpin vortices.

Animals↗
PubMed2006

Encapsulated chondrocyte response in a pulsatile flow bioreactor.

We have developed a bioreactor-based millifluidic technique that allows for dynamic culture conditions and measurement of the fluid flow impinging upon a three-dimensional tissue engineering scaffold. Chondrocytes in scaffolds have been shown to require mechanical stimulation to produce an extracellular matrix that resembles native cartilage. This study investigates the effect of pulsatile flow on chondrocyte response in a model poly(ethylene glycol) dimethacrylate hydrogel. Bovine chondrocytes were encapsulated in the hydrogel and cultured for 7, 14 and 21 days at pulsatile flow frequencies of 0.5 Hz (15ml/min) and 1.5Hz (17ml/min). The scaffolds cultured under dynamic conditions were compared to those cultured under static (non-flow) conditions. Quantitative real-time reverse transcription polymerase chain reaction was used to quantify collagen type I, collagen type II and aggrecan gene copy numbers as markers for chondrocyte phenotypic expression. Histological sections stained with hematoxylin & eosin, and Alcian blue confirmed chondrocyte morphology and matrix formation. Interestingly, regulation of the collagen type II gene was particularly sensitive to the flow conditions. The understanding of the cell response to encapsulation and flow could be used to identify the appropriate culture conditions necessary to design and develop hydrogel carriers to promote the formation of extracellular matrix as well as to further our knowledge of chondrocyte mechanobiology.

Animals↗
PubMed2001

The "black hole" phenomenon in ultrasonic backscattering measurement under pulsatile flow with porcine whole blood in a rigid tube.

The "black hole" phenomenon was further investigated with porcine whole blood under pulsatile flow conditions in a straight rigid tube 120 cm long and of 0.95 cm diameter. A modified Aloka 280 commercial scanner with a 7.5 MHz linear array was used to collect the radio frequency (RF) signal of backscattering echoes from the blood inside the tube. The transducer was located downstream from the entrance and parallel to the longitudinal direction of the tube. The experimental results showed that higher hematocrits enhanced the black hole phenomenon, leading to a more apparent and larger diameter black hole. The black hole was not apparent at hematocrits below 23%. The highest hematocrit used in the experiment was 60%. Beat rates of 20, 40 and 60 beats per minute (bpm) were used, and the black hole became weaker in amplitude and smaller in diameter when the peak flow velocity was increased at each beat rate. These results are consistent with the suggestion in previous work that the black hole arises from insufficient aggregation of red blood cells (RBCs) at the center of the tube because of the low shear rate. At 20 and 40 bpm, the peak flow velocity ranges were 10 approximately 25 cm/s and 18 approximately 27 cm/s, respectively. The black hole was very clear at the minimal peak flow velocity but almost disappeared at the maximal velocities for each beat rate. At 60 bpm, experiments were only performed at one peak flow velocity of 31 cm/s and the black hole was clear. The results showed that the black hole was more pronounced at higher beat rates when the peak velocity was the same. This phenomenon cannot be explained by previous hypotheses. Acceleration seems to be the only flow parameter that varies at different beat rates when peak velocities are the same. Therefore, the influence of acceleration on the structural organization and orientation of RBC rouleaux might be another factor involved in the formation of the black hole in addition to the shear rate. As the entrance length was changed from 110 to 15 diameters (D) in seven steps at the hematocrit of 60%, it was found that a position farther downstream yielded a black hole with a greater contrast relative to the surrounding region, while the backscattering power at the central hypoechoic zone did not increase with increasing entrance length.

Animals↗
PubMedDate not supplied

Finite element modeling of three-dimensional pulsatile flow in the abdominal aorta: relevance to atherosclerosis.

The infrarenal abdominal aorta is particularly prone to atherosclerotic plaque formation while the thoracic aorta is relatively resistant. Localized differences in hemodynamic conditions, including differences in velocity profiles, wall shear stress, and recirculation zones have been implicated in the differential localization of disease in the infrarenal aorta. A comprehensive computational framework was developed, utilizing a stabilized, time accurate, finite element method, to solve the equations governing blood flow in a model of a normal human abdominal aorta under simulated rest, pulsatile, flow conditions. Flow patterns and wall shear stress were computed. A recirculation zone was observed to form along the posterior wall of the infrarenal aorta. Low time-averaged wall shear stress and high shear stress temporal oscillations, as measured by an oscillatory shear index, were present in this location, along the posterior wall opposite the superior mesenteric artery and along the anterior wall between the superior and inferior mesenteric arteries. These regions were noted to coincide with a high probability-of-occurrence of sudanophilic lesions as reported by Cornhill et al. (Monogr. Atheroscler. 15:13-19, 1990). This numerical investigation provides detailed quantitative data on hemodynamic conditions in the abdominal aorta heretofore lacking in the study of the localization of atherosclerotic disease.

Algorithms↗
PubMed1990

[The pulsatile flow index (PFI) in the diagnosis of dysfunctional kidney transplants].

In 61 patients (167 examinations) the pulsatile flow index (PFI) was used to diagnose the cause of renal transplant dysfunction. The results were correlated with histology and clinical course and outcome, angiography or quantitative radionuclide renography. Renal transplant rejection was diagnosed by PFI with a sensitivity of 85%. The specificity was 81% and the diagnostic accuracy 83%. The positive predictive value was found to be 76%, whereas the negative predictive value was 89%. In presence of acute tubular necrosis (ATN) the PFI was normal in 89% of examinations and therefore distinguishable from acute rejection.

Adolescent↗
PubMed1990

Detailed visualization of pulsatile flow fields produced by modelled arterial stenoses.

A multiple trace photochromic method was used to visualize the pulsatile flow field created by modelled arterial stenoses of 38% and 65% area reductions. Using flow parameters similar to those of a medium sized artery in man, the flow patterns at seven axial locations in relation to the stenosis were simultaneously photographed at various times throughout the flow cycle. With the 65% stenosis, the wall shear stress in the vicinity of the reattachment point was found to fluctuate quite strongly during the turbulent phase of the flow cycle, giving rise to instantaneous shear stresses that were at least eight times larger than those measured upstream. For the 38% stenosis, much smaller shear stresses were observed. These and other results are described in detail.

Arterial Occlusive Diseases↗
PubMed1987

Quantitative evaluation of arterial pulsatile flow and pressure, applying impedance plethysmography to a human arterial model incorporating anatomical branching and scale.

This paper presents the theoretical basis of a new noninvasive method for obtaining arterial pulsatile flow and pressure. The proposed technique uses a model of the human arterial system based on the anatomical branching structure of the arterial tree. Arteries are divided into segments represented by uniform thin-walled elastic tubes with realistic arterial dimensions and wall properties. A simple mathematical model equivalent to electrical transmission lines is developed which is able to fit the electrical impedance plethysmograph waveform produced by the subjects throughout the complete cardiac cycle. Ensemble averaging is suggested as an option for processing of the impedance data. This technique provides artifact-free impedance data which enables the model to be used during exercise as well as quiet breathing. The proposed model provides an enhanced capability for measuring pulsatile blood flow and pressure in both clinical and research applications.

Arteries↗
PubMed1999

[Mechanical model of ocular pulsatile flow for evaluating the ocular blood flow device with known pressure pulsations].

BACKGROUND: Ocular perfusion consists of steady-state and pulsatile components of flow. The latter can be measured clinically by means of the 'Ocular Blood Flow' (OBF)-device (O.B.F. Ltd, Crowshearst, GB). METHODS: 1) Mechanical 'eye': To mechanically simulate the effect of pulsatile flow in the eye, a mechanical 'eye' model was built: A brass chamber (9 cm3) was machined and fitted with in- and outflow connections. The front opening was covered with a taughtly fixed rubber membrane (COSANO, no. 5203.106, Migros AG, Zurich) which, as mechanical 'cornea', pulsated with changes in pressure within the mechanical chamber. 2) Mechanical 'heart': To mechanically simulate pulsatile flow (i.e. pulsations in pressure like those within the human eye), two reservoirs were constructed of acrylic plastic and mounted on an upright optical bench with a millimeter scale. The reservoirs were constantly filled to overflowing with perfusate (tap water) and were connected by rubber tubing to the 'eye'-chamber. A computer-guided valve alternated between the 'systolic' and 'diastolic' columns of different, independently adjustable elevation. Frequency and duration of each pressure phase could also be independently adjusted via dialog with the computer. Input pressure levels were measured just outside the input using a transducer. The OBF-device measured the chamber pressure at the center of the rubber 'cornea'. RESULTS: Even the slightest alterations in the parameters (frequency, amplitude, and pressure) were precisely detected by the OBF-device, both graphically and numerically. CONCLUSIONS: Challenged by the mechanical model, the OBF-device demonstrated high sensitivity and fidelity of reproduction of any and all pulsations in intra- "ocular" -pressure.

Blood Pressure↗
PubMed2001

[Finite element analysis of pulsatile flow in aortic arch].

The blood flows in the ascending aorta and the aortic arch are studied. The theoretical models, boundary conditions and calculation conditions are proposed. The numerical simulation with finite element method of the blood flow in aortic arch of dogs is carried out according to the physiologically pulsatile flow conditions. The calculation results are analyzed with scientific visualization method.

Animals↗
PubMed1993

Hemodynamic patterns in two models of end-to-side vascular graft anastomoses: effects of pulsatility, flow division, Reynolds number, and hood length.

Flow behavior in models of end-to-side vascular graft anastomoses was studied under steady and pulsatile flow conditions. Models were constructed to simulate geometries employed in experimental studies on intimal thickening in a canine model. Reynolds numbers, division of flow in the outflow tracts and the pulsatile waveform employed were taken from measurements obtained in the canine model. Flows in the scaled-up, transparent models were visualized with white, neutrally buoyant particles which were photographed under laser illumination and also recorded on video tape under bright incandescent light. Strong, three-dimensional helical patterns which formed in the anastomotic junction were prominent features of the flow fields. Regions of low wall shear, oscillatory wall shear and long particle residence time were identified from the flow visualization experiments. Comparisons with the limited qualitative data available on intimal thickening in vascular graft anastomoses suggest a relation between localization of vascular intimal thickening and those surfaces experiencing low shear and long particle residence time.

Anastomosis, Surgical↗