Biomedical subjects

X Y Xu

Publications and source records attributed to X Y Xu.

At least 19 recordsLinked to original sources

PubMed2006

Curvature and tortuosity of the superficial femoral artery: a possible risk factor for peripheral arterial disease.

Atherosclerosis in the superficial femoral artery (SFA) resulting in peripheral arterial disease is more common in men than women and shows a predilection for the region of the adductor canal. Blood flow patterns are related to development of atherosclerosis, and we investigated if curvature and tortuosity of the femoral artery differed between young men and women and if differences resulted in adverse flow patterns. Magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) were combined in 18 young adult volunteers (9 men) to assess the relationship of flow features to likely sites of future atherosclerosis formation. Subjects underwent MRI of the right SFA, three-dimensional vascular geometry was reconstructed, and measures of tortuosity and curvature were calculated. Tortuosity and curvature were significantly greater for men than women, and this was related to increased body surface area, body mass index, or weight in men. In both sexes, "tortuosity" increased from the midthigh to the popliteal fossa. The greatest curvature was found within the distal quarter of the SFA. CFD modeling was undertaken on MRI-based reconstructions of the SFA. Wall shear stresses (WSS) were extracted from the computations. WSS showed greater spatial variation in the men than in the women, and the men exhibited lower mean WSS. These data indicate that sex differences related to body size and anatomical course of the femoral artery may contribute to the enhanced risk of focal atherosclerosis in the adductor canal.

Adult↗

PubMed2006

Isolation of cytoplasts from Satsuma mandarin (Citrus unshiu Marc.) and production of alloplasmic hybrid calluses via cytoplast-protoplast fusion.

Cytoplasm of Satsuma mandarin (Citrus unshiu Marc.) is known to influence seedlessness. Transfer of cytoplasm to a seedy cultivar could possibly lead to the production of seedless citrus fruits. In the present paper cytoplasts were isolated from cell suspension-derived protoplasts of Satsuma mandarin via ultra-centrifugation in a discontinuous gradient. No nucleus could be detected in the cytoplasts by DAPI (4', 6-diamidino-2-phenylindole) staining compared with normal protoplasts. The cytoplasts, with high viability and small size, did not divide during solid embedding culture. Cytoplasts of Satsuma mandarin were electrically fused with embryogenic protoplasts of Murcott tangor (C. reticulata x C. sinensis), which led to regeneration of several cell lines. Flow cytometry (FCM) indicated that the cell lines were diploids. Simple sequence repeats (SSR) and cleaved amplified polymorphism sequence (CAPS) showed that the cell lines got their nuclear DNA from the protoplast parent, whereas the cytoplast parent donated the mtDNA, confirming transfer of mtDNA from Satsuma mandarin into Murcott tangor via cytoplast-protoplast fusion though no polymorphism was detected in chloroplast DNA between the fusion partners. This is the first report on isolation and characterization of cytoplasts, together with cytoplast-protoplast fusion in Citrus, which has a potential for citrus cultivar improvement involving cytoplasm transfer via cytoplast-protoplast fusion.

Cell Fusion↗

PubMed2005

SspA, an outer membrane protein, is highly induced under salt-stressed conditions and is essential for growth under salt-stressed aerobic conditions in Rhodobacter sphaeroides f. sp. denitrificans.

We have previously shown that an outer membrane protein, SspA, is prominently induced by salt stress in a photosynthetic bacterium, Rhodobacter sphaeroides f. sp. denitrificans IL106 (R. sphaeroides). In this study, we investigated the physiological role of SspA under various stress conditions. Using recombinant SspA expressed in Escherichia coli as an antigen, the polyclonal antiserum of SspA was prepared. Western blot analysis demonstrated that SspA was highly induced by salt stress under both anaerobic and aerobic conditions. SspA was also induced, but to a lesser extent, by osmotic and acid stress. It is reduced under heat and cold compared to non-stressed conditions. While sspA-disrupted R. sphaeroides grew normally under anaerobic conditions in either the presence or absence of stress, it displayed significantly retarded growth under aerobic conditions in the dark, especially when osmotic or salt stress were imposed. In addition, the sspA disruptant, but not the wild type, formed cell aggregates when grown under both anaerobic and aerobic conditions, and this phenotype was significantly enhanced under salt-stressed aerobic conditions. Together, our findings suggest that SspA is critical under salt-stressed, aerobic growth conditions.

Acids↗

PubMed2005

Diagnosis of human cytomegalovirus intrauterine infection using fetal cells from maternal blood.

OBJECTIVE: The sensitivity and specificity for the noninvasive prenatal diagnosis of human cytomegalovirus intrauterine infection were estimated by using isolating single fetal cells from maternal peripheral blood. METHODS: Micromanipulation techniques were employed to isolate single fetal nucleated erythroblasts from 273 maternal blood samples. SRY gene and HCMV-DNA in single fetal cells were detected by multiple primed in situ labeling (PRINS) from 76 HCMV-DNA positive samples of maternal peripheral blood. 273 samples of maternal peripheral blood were tested for SRY gene and HCMV-DNA in single fetal cells by primed extension preamplification (PEP) and polymerase chain reaction (PCR). RESULTS: The detection rate of fetal cells from maternal blood was 100% with micromanipulation techniques. The sensitivity of PRINS for SRY gene detection was 97.56% and its specificity was 100%. The sensitivity and specificity of PEP and PCR for SRY gene detection were 97.39% and 99.17%, respectively. The sensitivity of PRINS for HCMV-DNA detection was 92.68% and the specificity was 100%. The sensitivity and specificity of PEP and PCR for HCMV-DNA detection were 95.12%and 100%, respectively. CONCLUSION: The technique for noninvasive prenatal detection of intrauterine infection of HCMV using single fetal cells from maternal peripheral blood by using PRINS and PEP and PCR is more reliable than the CMV-DNA detection in peripheral maternal blood, amniocentesis or percutaneous umbilical blood sampling.

Cytomegalovirus↗

PubMed2004

Fetal origin of single nucleated erythroblasts and free DNA in the peripheral blood of pregnant women.

OBJECTIVES: To investigate the feasibility of using single fetal nucleated erythroblasts (FNRBCs) and free DNA in maternal blood for non-invasive prenatal diagnosis. METHODS: Single FNRBCs were isolated from 51 of 116 samples of maternal blood analyzed by micromanipulation after density gradient centrifugation. Furthermore, the nested polymerase chain reaction (PCR) method was used to amplify the SRY gene of single FNRBCs. Primer extension pre-amplification and nested PCR were used to amplify the SRY gene of the plasma DNA extracted from 65 samples of maternal blood. RESULTS: The detection rate of single FNRBCs was 90.20% (46/51). The concordance rates between real fetal sex and sex determined by amplification of the SRY gene from single cells and from free DNA analysis were 82.61% (38/46) and 90.77% (59/65), respectively. CONCLUSIONS: Single nucleated erythroblasts and free DNA in maternal blood are of fetal origin and can be valuable fetal material sources for non-invasive prenatal diagnosis.

Adult↗

PubMed2004

Ultrasound image-based computer model of a common carotid artery with a plaque.

Ultrasound scans were acquired from a common carotid artery in a patient with an early atherosclerotic plaque forming a mild asymmetrical stenosis. The 3D vascular geometry of the diseased arterial segment was reconstructed from a series of 2D cross-sectional images, and computational meshes for the flow and wall domains were developed. Numerical flow simulations incorporating coupled fluid-solid interaction were implemented using flow and pressure waveforms measured in vivo. The effects of wall distensibility were investigated by comparing the predictions obtained with different wall compliance, one with 'natural' compliance and another with a stiffer wall. Limited flow separation was predicted in the post-stenotic zone. The non-uniform thickness of the diseased wall restricted the wall motion locally and re-distributed the stress, giving raised concentrations at the plaque shoulders.

Blood Flow Velocity↗

PubMed2004

Image-based carotid flow reconstruction: a comparison between MRI and ultrasound.

Atherosclerosis is a major cause of morbidity and mortality. Its apparent link with wall shear stress (WSS) has led to considerable interest in the in vivo estimation of WSS. Determining WSS by combining medical images with computational fluid dynamics (CFD) simulations can be performed both with magnetic resonance imaging (MRI) and three-dimensional ultrasound (3DUS). This study compares predicted 3D flow patterns based on black blood MRI and 3DUS. Velocity fields in the carotid arteries of nine subjects have been reconstructed, and the haemodynamic wall parameters WSS, oscillatory shear index (OSI), WSS gradients (WSSG) and angle gradients (WSSAG) were compared between the two imaging techniques. There was a good qualitative agreement between results derived from MRI and 3DUS (average correlation strength above 0.60). The root mean square error between haemodynamic wall parameters was comparable to the range of the expected variability of each imaging technique (WSS: 0.411 N m(-2); OSI: 0.048; temporal WSSG: 150 N s(-1) m(-2); spatial WSSG: 2.29 N m(-3); WSSAG: 87.6 rad m(-1)). In conclusion, MRI and 3DUS are capable of providing haemodynamic parameters when combined with CFD, and the predictions are in most cases qualitatively and quantitatively similar. The relatively high cost of MRI and continuing improvement in ultrasound favour US to MRI for future haemodynamic studies of superficial arteries.

Adult↗

PubMed2004

Flow patterns and wall shear stresses in patient-specific models of the abdominal aortic aneurysm.

Abdominal Aortic Aneurysms (AAAs) are a major cause of death in Western societies. However, the factors responsible for the growth and ultimate rupture of AAA are not fully understood. Using computation fluid dynamics combined with CT images, the flow patterns and wall shear stress (WSS) in the AAA of a patient were studied. AAA geometry was obtained from images acquired during a routine CT scan, and a realistic abdominal aortic flow waveform was used as an upstream boundary condition. The computational results showed the presence of multiple flow recirculation zones and overall low WSS in the bulge of the aneurysm. This may provide some insight into the growth of thrombus in AAA.

Aged↗

PubMed2003

Reproducibility study of 3D geometrical reconstruction of the human carotid bifurcation from magnetic resonance images.

The combined magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) modeling approach is playing an increasingly important role in advancing our understanding of the relationship between hemodynamics and arterial disease. Nevertheless, such a modeling approach involves a number of uncertainties associated with various stages of the process. The present study is concerned with the reproducibility of geometry reconstruction, one of the most crucial steps in the modeling process. The reproducibility test was conducted on the right carotid bifurcation of eight normal human subjects, each of whom were scanned twice using the same MR protocol with an in-plane resolution of 0.625 mm. Models constructed from different scans of the same subject were compared and assessed using four quantitative measures: centerline distance, cross-sectional area, contour shape factors, and mean radius difference. The difference in the maximum carotid bulb area between the two scans was found to be <8.1% for all subjects. Shape factors (measuring the dissimilarity between two contours) of <10% were achieved in most of the common carotid arteries (CCAs) and internal carotid arteries (ICAs). The mean radius difference between the two scans was <0.4 mm for all subjects. Among the three vessels, the geometry of CCA was well reproduced by the reconstruction procedure in most of the cases, while the external carotid artery (ECA) showed the worst reproducibility. The impact of geometrical differences on CFD-predicted flow patterns and wall shear stress (WSS) will be investigated and discussed in a separate paper.

Adult↗

PubMed2003

Reproducibility study of magnetic resonance image-based computational fluid dynamics prediction of carotid bifurcation flow.

The importance of shear stress in the initiation and progression of atherosclerosis has been recognized for some time. A novel way to quantify wall shear stress under physiologically realistic conditions is to combine magnetic resonance imaging (MRI) and computational fluid dynamics. The present study aims to investigate the reproducibility of the simulated flow by using this combined approach. The right carotid bifurcations of eight healthy subjects were scanned twice with MRI within a few weeks. Three-dimensional geometries of the vessels were reconstructed for each scan and each subject. Pulsatile flows through these models were calculated to assess errors associated with the predicted flow parameters. This was done by comparing various wall shear stress indices, including the time-averaged wall shear stress (WSS), oscillating shear index (OSI), WSS Gradients (WSSG) and WSS Angle Deviation (WSSAD). Qualitatively, all the wall shear parameters proved to be highly reproducible. Quantitatively, the reproducibility was over 90% for OSI and WSSAD, but less impressive (60%) for other parameters. Our results indicated that WSS and WSSG values were extremely sensitive to subtle variations in local geometry and mesh design, particularly in regions around the bifurcation apex where WSS values were high and least reproducible.

Adult↗

PubMed2003

Comparative study of magnetic resonance imaging and image-based computational fluid dynamics for quantification of pulsatile flow in a carotid bifurcation phantom.

A combined magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) modeling study was carried out for pulsatile flow in a carotid bifurcation phantom. The aim of the study was to quantify differences in flow patterns between MRI measurement and MRI-based CFD simulations and to further explore the potential for in vivo applications. The computational model was reconstructed from high resolution magnetic resonance (MR) scans. Velocities derived from phase-contrast MR measurements were used as boundary conditions for the CFD calculation. Detailed comparisons of velocity patterns were made between the CFD results and MRI measurements. Good agreement was achieved for the main velocity component in both well-behaved flow (in the common carotid) and disturbed region (in the carotid sinus). Comparison of in-plane velocity vectors showed less satisfactory consistency and revealed that the MR measurements obtained were inadequate to depict the secondary flow pattern as expected. It can be concluded that the combined MRI/CFD is expected to provide more reliable information about the full three-dimensional velocity field.

Blood Flow Velocity↗

PubMed2003

Accuracy and reproducibility of CFD predicted wall shear stress using 3D ultrasound images.

Computational fluid dynamics (CFD) flow simulation techniques have the potential to enhance our understanding of how haemodynamic factors are involved in atherosclerosis. Recently, 3D ultrasound has emerged as an alternative to other 3D imaging techniques, such as magnetic resonance angiography (MRA). The method can be used to generate realistic vascular geometry suitable for CFD simulations. In order to assess accuracy and reproducibility of the procedure from image acquisition to reconstruction to CFD simulation, a human carotid artery bifurcation phantom was scanned three times using 3D ultrasound. The geometry was reconstructed and flow simulations were carried out on the three sets as well as on a model generated using computer aided design (CAD) from the geometric information given by the manufacturer. It was found that the three reconstructed sets showed good reproducibility as well as satisfactory quantitative agreement with the CAD model. Analyzing two selected locations probably representing the 'worst cases,' accuracy comparing ultrasound and CAD reconstructed models was estimated to be between 7.2% and 7.7% of the maximum instantaneous WSS and reproducibility comparing the three scans to be between 8.2% and 10.7% of their average maximum.

Carotid Arteries↗

PubMed2003

The influence of inflow boundary conditions on intra left ventricle flow predictions.

The combination of computational fluid dynamics (CFD) and magnetic resonance imaging (MRI) offers a promising tool that enables the prediction of blood flow patterns in subject-specific cardiovascular models. The influence of the model geometry on the accuracy of the simulation is well recognized. This paper addresses the impact of different boundary conditions on subject-specific simulations of left ventricular (LV) flow. A novel hybrid method for prescribing effective inflow boundary conditions in the mitral valve plane has been developed. The detailed quantitative results highlight the strengths as well as the potential pitfalls of the approach.

Blood Flow Velocity↗

PubMed2003

Carotid geometry reconstruction: a comparison between MRI and ultrasound.

Image-based Computational Fluid Dynamics (CFD) has become a popular tool for the prediction of in vivo flow profiles and hemodynamic wall parameters. Currently, Magnetic Resonance Imaging (MRI) is most widely used for in vivo geometry acquisition. For superficial arteries such as the carotids and the femoral artery, three-dimensional (3-D) extravascular ultrasound (3-DUS) could be a cost-effective alternative to MRI. In this study, nine healthy subjects were scanned both with MRI and 3-DUS. The reconstructed carotid artery geometries for each subject were compared by evaluating cross-sectional areas, centerlines, and carotid nonplanarity. Lumen areas agreed very well between the two different acquisition techniques, whereas centerlines and nonplanarity parameters showed measurable disagreement, possibly due to the different neck and head positions adopted for 3-DUS versus MRI. With the current level of agreement achieved, 3-DUS has the potential to become an inexpensive and fast alternative to MRI for image-based CFD modeling of superficial arteries.

Adult↗

PubMed2003

Various issues relating to computational fluid dynamics simulations of carotid bifurcation flow based on models reconstructed from three-dimensional ultrasound images.

Computational fluid dynamics (CFD) flow simulation techniques have the potential to enhance understanding of how haemodynamic factors are involved in atherosclerosis. Recently, three-dimensional ultrasound has emerged as an alternative to other three-dimensional imaging techniques, such as magnetic resonance angiography (MRA). The method can be used to generate accurate vascular geometry suitable for CFD simulations and can be coupled with Doppler ultrasound to provide physiologically realistic flow boundary conditions. However, there are various ways to utilize the flow data acquired, possibly leading to different results regarding both flow and wall shear stress patterns. A disadvantage of three-dimensional ultrasound for imaging the carotid bifurcation has been established as being the scanning limitation of the jawbone position. This may make artificial extensions of the internal and/or external carotid arteries necessary, which in turn may influence the predicted flow patterns. Flow simulations were carried out for three outflow calculation schemes as well as four geometries with different extensions to the carotid daughter vessels. It was found that variation of flow patterns was more strongly influenced by the outflow conditions than by the extensions of the daughter vessels. Consequently, it is recommended that for future CFD simulations of carotid flow using three-dimensional ultrasound data, the outflow boundary conditions should rely on the most accurate measurement available, and flow data recorded in the common and internal carotid are considered more reliable than data from the external carotid. Even though the extended lengths of the daughter vessels have insignificant effects on the predicted haemodynamic parameters, it would be a safer option to extend the internal carotid by approximately three times the diameter of the common carotid artery.

Blood Flow Velocity↗

PubMed2002

Inter-individual variations in wall shear stress and mechanical stress distributions at the carotid artery bifurcation of healthy humans.

Fluid shear stress and mechanical wall stress may play a role in the formation of early atherosclerotic lesions, but these quantities are difficult to measure in vivo. Our objective was to quantify these parameters in normal subjects in a clinical setting, and to define regions of low wall shear stress and high mechanical stress. The right carotid bifurcations of five healthy male volunteers were investigated using a novel non-invasive technique which integrates magnetic resonance angiography, ultrasonography, tonometry and state-of-the-art computational fluid dynamics and solid mechanics models. Significant inter-subject variations in patterns as well as magnitude of wall shear stress and mechanical stress were found. In spite of individual variabilities, this study revealed that some regions of the artery wall are exposed simultaneously to low wall shear stress and high mechanical stress and that these regions correspond to areas where atherosclerotic plaque develops. The coexistence of regions of low wall shear stress and high tensile stress may be an important determinant of the formation of atheroma in human arteries.

Adult↗

PubMed2002

Modelling of flow and wall behaviour in a mildly stenosed tube.

In the present computational analysis, pulsatile flow and vessel wall behaviour in a simplified model of a stenosed vessel were investigated. Geometry of a 45% axisymmetrically stenosed (by area) cylindrical tube and a sinusoidal inflow waveform were simulated, with the fluid being assumed to be incompressible and Newtonian. The vessel wall was treated as a thick-walled, incompressible and isotropic material with uniform mechanical properties across the normal as well as the constricted segment. The study of fluid flow and wall motion was initially carried out separately using two commercial codes CFX4.2 and ABAQUS7 respectively. Their combined effects and interactions were later investigated through an iteratively coupled algorithm. Model validations on the rigid-wall fluid and static no-flow solid models were satisfactory, with Root Mean Square deviations of around 7% in centreline axial velocity between the prediction and measurement values for the rigid wall stenosis model, and 5% in circumferential stress for a cylindrical tube model under static loading when compared with the analytical solution. Results on velocity profiles, wall shear stress, intramural strain and stress for the rigid and compliant cases were all presented. Comparison between the rigid and compliant models revealed that, the flow separation layer distal to the stenosis was thicker and longer, and wall shear stress was slightly lower in the compliant model by less than 7.2%. Results obtained from the static wall model (with uniform pressure loading) and coupled fluid/wall interaction modelling of pulsatile flow showed qualitatively similar wall strain and stress patterns but considerable differences in magnitude. The radial and axial stresses were reduced by 31 and 8%, while the circumferential stress was increased by 13% due to the presence of pulsatile flow. Under the flow and structural conditions investigated, the effects of wall compliance were small, and did not change the flow and solid behaviours qualitatively in this case.

Arterial Occlusive Diseases↗

PubMed2002

Quantification of the non-planarity of the human carotid bifurcation.

The carotid bifurcation has been a region of particular interest due to its predilection for clinically significant atherosclerosis. It has been shown that the vessel geometry is a major determinant of the local haemodynamic properties which are believed to be associated with the location of atherosclerotic lesions. Current knowledge of the geometry of the carotid bifurcation is insufficient and restricted to basic geometric parameters. To provide some means of quantifying the degree of complexity of the 3D shape of the bifurcation, we made an initial attempt by evaluating the non-planarity of an arterial bifurcation based upon the singular value decomposition theorem. In this paper we present our results obtained on the right carotid bifurcations of six normal subjects, each of whom was scanned twice using the 2D time-of-flight MR sequence. The acquired 2D cross sectional images were processed by using our in-house software which comprises 2D segmentation, 3D reconstruction and smoothing. The centroids of each transverse slices were determined and used as input data for the non-planarity analysis. Our results using the singular value decomposition method have demonstrated discernible differences in non-planarity among individuals. Comparisons with the planarity definition proposed by other investigators suggest that the singular value decomposition method offers more information about the linearity and planarity of the bifurcation. However, it is also realised that a single measure of non-planarity can never fully characterise a bifurcation owing to the great variety of geometries.

Adult↗