Doppler Optical Coherence Tomography of Retinal Circulation

Noncontact retinal blood flow measurements are performed with a Fourier domain optical coherence tomography (OCT) system using a circumpapillary double circular scan (CDCS) that scans around the optic nerve head at 3.40 mm and 3.75 mm diameters. The double concentric circles are performed 6 times consecutively over 2 sec. The CDCS scan is saved with Doppler shift information from which flow can be calculated. The standard clinical protocol calls for 3 CDCS scans made with the OCT beam passing through the superonasal edge of the pupil and 3 CDCS scan through the inferonal pupil. This double-angle protocol ensures that acceptable Doppler angle is obtained on each retinal branch vessel in at least 1 scan. The CDCS scan data, a 3-dimensional volumetric OCT scan of the optic disc scan, and a color photograph of the optic disc are used together to obtain retinal blood flow measurement on an eye. We have developed a blood flow measurement software called "Doppler optical coherence tomography of retinal circulation" (DOCTORC). This semi-automated software is used to measure total retinal blood flow, vessel cross section area, and average blood velocity. The flow of each vessel is calculated from the Doppler shift in the vessel cross-sectional area and the Doppler angle between the vessel and the OCT beam. Total retinal blood flow measurement is summed from the veins around the optic disc. The results obtained at our Doppler OCT reading center showed good reproducibility between graders and methods (<10%). Total retinal blood flow could be useful in the management of glaucoma, other retinal diseases, and retinal diseases. In glaucoma patients, OCT retinal blood flow measurement was highly correlated with visual field loss (R²>0.57 with visual field pattern deviation). Doppler OCT is a new method to perform rapid, noncontact, and repeatable measurement of total retinal blood flow using widely available Fourier-domain OCT instrumentation. This new technology may improve the practicality of making these measurements in clinical studies and routine clinical practice.     

Calf blood flow and posture: Doppler ultrasound calibrated by plethysmography.

A procedure was developed that enables measurement of rapid variations in calf blood flow during voluntary rhythmic contraction of the calf muscles in supine, sitting, and standing positions. During the exercise, maximum blood velocity is measured by Doppler ultrasound equipment in the popliteal artery. The Doppler signals are calibrated by plethysmography to enable calculation of blood flow during exercise in ml.100 ml-1.min-1. Knowledge of the cross-sectional area of the vessel and the angle of insonation is not required in this procedure. Evaluation of the calibration method with 10 healthy volunteers showed that for each subject a new calibration was necessary after a change in posture; the relationship between the blood flow and the maximum Doppler frequency averaged over one heart cycle was linear for each calibration.     

What measurements are necessary for a comprehensive evaluation of the peripheral arterial circulation?

Several methods are available to detect atherosclerotic lesions with a severe degree of stenosis (>70%), but the diagnosis of atherosclerotic lesions with no stenosis or with a minor degree of stenosis (<20%), is problematic. Hemodynamics associated with stenotic lesions are well described by the relationship of blood pressure and blood flow velocity, both as a function of time and localization (along the length and cross-section of the vessel). The use of this relationship in the clinic is difficult because no precise information is available about the geometry and branching of arteries, blood viscosity, and the velocity distribution over the cross-sectional area of the blood vessel. Besides, the invasiveness of the technique to measure arterial pressure as a function of time and localization does not allow routine application in patients. Because of these limitations, alternative methods have been developed. The degree and extensiveness of atherosclerotic disease can, for instance, be estimated from the changes in maximum blood flow velocity and in velocity profile, i.e., velocity distribution along the cross-section of the vessel. Moreover, the delay between simultaneously recorded arterial blood flow velocity tracings (pulse-wave velocity determination) is used to assess the elastic properties of the vessel. Changes in velocity profile occur at relatively slight degrees of arterial stenosis (around 20%), so that determination of these profiles along diseased arteries may contribute to the early diagnosis of atherosclerotic lesions. In man, transcutaneous information about the maximum and mean blood flow velocities over the cross-sectional area of the artery as an instantaneous function of time as well as the flow pattern can be obtained online with continuous wave Doppler flowmeters, at least when audio spectrum analysis is used as a processing technique. Velocity profiles can be determined with multichannel pulsed Doppler systems if the resolution of the system is adequate and a sufficient number of sample volumes can be obtained, limiting the interpolation between these samples. The on-line recording of velocity profiles can be facilitated by combining the pulsed Doppler device with either a velocity imaging system or a B-mode scan. In systems with a high resolution (sample distance 0.5 mm), one should be able to detect local disturbances in the velocity profile at the site of the lesion (due to local increases in shear stress) and proximal to the lesion (due to reflections), so that lesions with a minor degree of stenosis can be detected. In resistive systems (e.g., internal carotid arteries) in which the relationship between pressure and velocity changes during the cardiac cycle is relatively simple, the elasticity of the arterial wall can be determined by relating the relative diameter changes of the vessel, determined on-line with multichannel pulsed Doppler systems, to the instantaneous velocity pulse. Although the detection of atherosclerotic lesions at an early stage of the disease with sophisticated Doppler devices looks promising, further clinical evaluation is required.     

Influence of glottic aperture on the tracheal flow

The extra-thoracic mouth-throat area has a major influence on the aerosol delivery to the proximal or peripheral intra-thoracic airways. To characterize the particle deposition in this area, it is important to investigate first the flow structures in this crucial--in relation to the aerosol deposition--region. The glottis, which is delimited by the vocal cords and therefore has the narrowest passage, generates a laryngeal jet and a reverse flow downstream the glottis. It is generally assumed that the glottis has different shapes and cross-sectional areas at different moments during the respiratory cycle and also depends on the average inspiratory flow rate. Therefore, the influence of a circular glottal aperture, with a cross-sectional area of 90 mm2 and an elliptical and triangular shape, both with an area of 45 mm2, on the flow is investigated. However, the area of the circular aperture is twice as big as the area of the elliptical one, it has almost no influence on the flow structures. On the other hand, the triangular glottal aperture shifts the laryngeal jet in the direction of the posterior wall, and generates two pairs of counter rotating secondary vortices downstream the glottis, where the circular and elliptical only aperture generates one pair of vortices. The difference in pressure drop is more dominated by the cross-sectional area than by the shape of the glottis. This suggests the need for rendering geometry of future upper airway models even more realistic as the appropriate three-dimensional (3D) medical imaging techniques are becoming available.     

Size and blood flow of central and peripheral arteries in highly trained able-bodied and disabled athletes.

In a cross-sectional study, central and peripheral arteries were investigated noninvasively in high-performance athletes and in untrained subjects. The diastolic inner vessel diameter (D) of the thoracic and abdominal aorta, the subclavian artery (Sub), and common femoral artery (Fem) were determined by duplex sonography in 18 able-bodied professional tennis players, 34 able-bodied elite road cyclist athletes, 26 athletes with paraplegia, 17 below-knee amputated athletes, and 30 able-bodied, untrained subjects. The vessel cross-sectional areas (CSA) were set in relation to body surface area (BSA), and the cross-section index (CS-index = CSA/BSA) was calculated. Volumetric blood flow was determined in Sub and Fem via a pulsed-wave Doppler system and was set in relation to heart rate to calculate the stroke flow. A significantly increased D of Sub was found in the racket arm of able-bodied tennis players compared with the opposite arm (19%). Fem of able-bodied road cyclist athletes and of the intact limb in below-knee amputated athletes showed similar increases. D of Fem was lower in athletes with paraplegia (37%) and in below-knee amputated athletes proximal to the lesion (21%) compared with able-bodied, untrained subjects; CS-indexes were reduced 57 and 31%, respectively. Athletes with paraplegia demonstrated a larger D (19%) and a larger CS-index in Sub (54%) than able-bodied, untrained subjects. No significant differences in D and CS-indexes of the thoracic and abdominal aorta were found between any of the groups. The changes measured in Sub and Fem were associated with corresponding alterations in blood flow and stroke flow in all groups. The study suggests that the size and blood flow volume of the proximal limb arteries are adjusted to the metabolic needs of the corresponding extremity musculature and underscore the impact of exercise training or disuse on the structure and the function of the arterial system.     

Ultrasound imaging

Modern ultrasonic transducers mainly employ lead zirconate titanate (PZT) but vinylidene fluoride trifluoroethylene copolymer (P (VDF-TrPE)) is becoming more competitive. The static scanner is now largely replaced by mechanical or electronically controlled array real time systems; the speed of scanning is limited by the speed of sound and the resolution depends on the wavelength and so, ultimately, on the attenuation in tissue. Tissue inhomogeneities degrade the resolution. Intraoperative and intracavitary scanners have advantages in some anatomical situations and ultrasonic imaging can guide extracorporeal shock wave lithotripsy. Inexpensive battery powered scanners will soon become available. Duplex scanners are used to localize the acquisition of Doppler signals; blood flow volume rate can be estimated from measurements of blood velocity and vessel cross-sectional area, or by the attenuation-compensated technique which avoids the main sources of error. Colour flow mapping combines real time imaging with Doppler information, but has limited scanning speed. Computed tomography and acoustical microscopy are feasible. Speckle arises from the coherent nature of ultrasound and can be suppressed by summing uncorrelated images or by filtering. Image manipulation and display techniques are being developed to cope with three dimensional scan data and the approach is compatible with picture archiving and communication systems (PACS). Tissue characterization based on the measurement of properties has been disappointing but blood flow analysis and contrast agents are promising. Quality assurance programmes are crucial; ultrasonic diagnosis appears to be free from hazard and prudent use is determined by cost-benefit considerations.     

Shear-layer detection in poststenotic flow by spectrum analysis of Doppler signals

Spectrum analysis of the Doppler signals was performed 0.5 tube diameters downstream from an axisymmetric constriction with an area reduction of 80 percent in steady flow at a jet Reynolds number of 2840. Both pulsed and continuous wave (CW) Doppler spectra showed significant reverse flow components in the separated flow. The pulsed Doppler spectra exhibited sudden changes when the sample volume crossed the shear layer between the center jet and the separated flow. A power spectrum equation was theoretically derived from continuity of flow to define the Doppler shift frequency for the shear layer velocity. The CW Doppler spectrum showed a minimum spectrum density at a frequency which equalled the shear layer Doppler shift frequency derived from the equation. The pulsed spectra exhibited the sudden changes at the same frequency as well.     

Hydraulic Parameters Measured in 1-Year-Old Twigs of some Mediterranean Species with Diffuse-Porous Wood: Changes in Hydraulic Conductivity and Their Possible Functional Significance

We report measurements of hydraulic conductivity of Vitis oinifera,Oleo europaea and Populus deltoides 1-year-old twigs. Singleserial internodes were tested for the volume flow rate whichwas related to: (a) the xylem tissue cross-sectional area, (b)the vessel lumina cross-sectional area and (c) the leaf surfacearea supplied by a given stem section. From this, whole xylemhydraulic conductivity (L_x), vessel lumina hydraulic conductivity(L_xv) and leaf specific conductivity (LSC) were calculated.All the three parameters turned out to be linearly related toeach other. This is kcause: (a) the leaf surface area (A₁) waslinearly related to xylem cross-sectional area (A_x and (b) theratio of the vessel lumina cross-sectional area (A_xv) to xylemcross-sectional area (A_x) was approximately constant along thetwigs. Moreover, the hydraulic conductivity of twig segmentswhere buds grow most actively (distal internodes in V. viniferaand proximal ones in O. europaea) was much lower than in therest of the twigs. A possible role played by these ‘constricted’twig regions is discussed. Key words: Changes, Hydraulic conductivity, Stem     

Clinical evidence for myocardial derecruitment downstream from severe stenosis: pressure-flow control interaction

To verify the interaction between coronary pressure (CP) and blood flow (CBF) control, we studied nine candidates for angioplasty of an isolated lesion of the left anterior descending coronary artery [i.e. , percutaneous transluminal coronary angioplasty (PTCA)]. CBF (i.e., flow velocity x coronary cross-sectional area at the Doppler tip) and CP were monitored during washout of 2-5 mCi of (133)Xe after bolus injection into the left main artery before and after PTCA. Xe mean transit time (MTT) was calculated as the area under the time-activity curve, acquired by a gamma camera, divided by the dose obtained from a model fit of the Xe curve in the anterior wall. CBF response to intracoronary adenosine (2 mg) was also assessed. PTCA increased baseline CBF (from 14.5 +/- 9.4 to 20 +/- 8 ml/min, P < 0.01), coronary flow reserve (from 1.52 +/- 0.24 to 2.33 +/- 0.8, P < 0.01), and CP (from 64 +/- 9 to 100 +/- 10 mmHg, P < 0.05). MTT decreased from 89 +/- 32 to 70 +/- 19 s (P < 0.05) after PTCA; however, MTT and CBF changes were not correlated (r = -0.09, not significant). Inasmuch as MTT is the ratio of distribution volume to CBF, MTT x CBF was used as an index of perfused myocardial volume. Volume increased after PTCA from 23 +/- 18 to 56 +/- 30 ml. A direct correlation was observed between the percent increase in distal CP and percent increase in perfused volume (r = 0.91, P < 0.01). Thus low CP was not associated with exhaustion of flow reserve but, rather, with reduction of perfused myocardial volume. These data suggest that, in the presence of a severe coronary stenosis, derecruitment of vascular units occurs that is proportional to the decrease in driving pressure. Residual perfused units maintain a vasomotor tone, thus explaining the paradoxical persistence of coronary reserve.     

Correlation between three-dimensional power Doppler and morphometric measurement of endometrial vascularity at the time of embryo implantation in women with unexplained recurrent miscarriage

Power Doppler in combination with three-dimensional (3D-PD) ultrasonography has been used as a noninvasive tool to evaluate the vascularity. However, it is unclear whether 3D-PD can accurately reflect endometrial vascularization and replace the invasive endometrial biopsy. This study aims to investigate the correlation between 3D-PD and micro vessel morphometric measurement of endometrial vascularity. Twenty-five women with unexplained recurrent miscarriage were recruited for 3D-PD and endometrial biopsy on precisely day LH?+?7. Immunohistochemistry using vWF was employed to identify micro vessels in endometrial biopsy specimens followed by the use of morphometric technique to measure the mean vessel diameter and volume fractions. The vascularization index (VI), flow index (FI) and vascularization flow index (VFI) assessed by 3D-PD were calculated for both the endometrial and sub-endometrial regions. There were no significant correlations between any of the ultrasonographic measurements (endometrial thickness, endometrial volume, endometrial VI/FI/VFI, sub-endometrial volume, sub-endometrial VI/FI/VFI) and morphometric features (number of micro vessel, mean diameter of micro vessel and volume fraction measurement of vessel). This study indicates that endometrial vascularity assessed by 3D-PD could not be used to reflect changes in micro vessels of the endometrium at the time of embryo implantation in women with unexplained recurrent miscarriage.     

A new approach for the evaluation of the severity of coarctation of the aorta using Doppler velocity index and effective orifice area: in vitro validation and clinical implications

Early detection and accurate estimation of COA severity are the most important predictors of successful long-term outcome. However, current clinical parameters used for the evaluation of the severity of COA have several limitations and are flow dependent. The objectives of this study are to evaluate the limitations of current existing parameters for the evaluation of the severity of coarctation of the aorta (COA) and suggest two new parameters: COA Doppler velocity index and COA effective orifice area. Three different severities of COAs were tested in a mock flow circulation model under various flow conditions and in the presence of normal and stenotic aortic valves. Catheter trans-COA pressure gradients and Doppler echocardiographic trans-COA pressure gradients were evaluated. COA Doppler velocity index was defined as the ratio of pre-COA to post-COA peak velocities measured by Doppler echocardiography. COA Doppler effective orifice area was determined using continuity equation. The results show that peak-to-peak trans-COA pressure gradient significantly increased with flow rate (from 83% to 85%). Peak Doppler pressure gradient also significantly increased with flow rate (80-85%). A stenotic or bicuspid aortic valve increased peak Doppler pressure gradient by 20-50% for a COA severity of 75%. Both COA Doppler velocity index and COA effective orifice area did not demonstrate significant flow dependence or dependence upon aortic valve condition. As a conclusion, COA Doppler velocity index and COA effective orifice area are flow independent and do not depend on aortic valve conditions. They can, then, more accurately predict the severity of COA.     

Velocity field studies at surgically imposed arterial stenoses on the abdominal aorta in pigs

In order to describe velocity profiles and the size of deterministic and non-deterministic velocity disturbances at arterial stenoses, symmetrical and asymmetrical stenoses with intended area reductions of 50% (‘moderate’) and 85% (‘severe’) were applied on the abdominal aorta in six pigs. Blood velocities were registered by hot-film anemometry in 21 measuring points distributed across the vessel cross-sectional area in one pre-stenotic and three post-stenotic positions. Signal analysis included ensemble averaging, the high-pass filtering technique, and three-dimensional visualization. None of the stenoses affected the pre-stenotic velocity field. Downstream moderate stenoses flow separation and vortex formation were present. Moderate asymmetric stenoses induced turbulence in the post-stenotic velocity field. Immediately downstream of severe stenoses a prominent post-stenotic jet was present. Farther downstream, a multitude of coherent vortices and turbulence dominated the flow field. The transverse distribution of turbulence intensity parallelled with the peak systolic velocity profile, whereas transverse profiles of the relative turbulence intensity (turbulence intensity/mean velocity) revealed peak values in flow field locations with high velocity gradients. Velocity parameters for symmetric and asymmetric severe stenoses were highly comparable. However, the exact degree of stenosis was significantly higher for symmetrical (85%) than for asymmetrical (76%) stenoses. Therefore, recalling that stenosis severity strongly influences the development of velocity disturbances, this indicates that asymmetry of a stenosis is a predictor for blood velocity disturbances.     

Relationship between ultrasonic characteristics of the corpus luteum, plasma progesterone concentration and early pregnancy diagnosis in Friesian mares

The purpose of the present study was to evaluate the change in cross-sectional area of the early corpus luteum (CL) and progesterone production in relation to subsequent pregnancy diagnosis. The cross-sectional area of the CL of 75 Friesian brood mares was measured by ultrasonography on Day 1 or 2 and Day 8 or 9 after ovulation. The change in cross-sectional area was expressed in a volume ratio. Plasma progesterone concentrations were measured on Days 8 to 9, and ultrasonography to determine pregnancy status was carried out on Day 17. The data obtained were analyzed by using a multiple logistic regression model. There were significant differences in the age, volume ratio and progesterone concentration between pregnant and nonpregnant mares. Pregnancy on Day 17 was related to the change in size of the CL up to Days 8 to 9 and progesterone concentration on Days 8 to 9. These differences between pregnant and nonpregnant mares might reflect the first luteal response to pregnancy.     

Velocity profiles in the human ductus venosus: a numerical fluid structure interaction study

The veins distributing oxygenated blood from the placenta to the fetal body have been given much attention in clinical Doppler velocimetry studies, in particular the ductus venosus. The ductus venosus is embedded in the left liver lobe and connects the intra-abdominal portion of the umbilical vein (IUV) directly to the inferior vena cava, such that oxygenated blood can bypass the liver and flow directly to the fetal heart. In the current work, we have developed a mathematical model to assist the clinical assessment of volumetric flow rate at the inlet of the ductus venosus. With a robust estimate of the velocity profile shape coefficient (VC), the volumetric flow rate may be estimated as the product of the time-averaged cross-sectional area, the time-averaged cross-sectional maximum velocity and the VC. The time average quantities may be obtained from Doppler ultrasound measurements, whereas the VC may be estimated from numerical simulations. The mathematical model employs a 3D fluid structure interaction model of the bifurcation formed by the IUV, the ductus venosus and the left portal vein. Furthermore, the amniotic portion of the umbilical vein, the right liver lobe and the inferior vena cava were incorporated as lumped model boundary conditions for the fluid structure interaction model. A hyperelastic material is used to model the structural response of the vessel walls, based on recently available experimental data for the human IUV and ductus venous. A parametric study was constructed to investigate the VC at the ductus venosus inlet, based on a reference case for a human fetus at 36 weeks of gestation. The VC was found to be \(0.687\,\pm \,0.023\) (Mean \(\pm \) SD of parametric case study), which confirms previous studies in the literature on the VC at the ductus venosus inlet. Additionally, CFD simulations with rigid walls were performed on a subsection of the parametric case study, and only minor changes in the predicted VCs were observed compared to the FSI cases. In conclusion, the presented mathematical model is a promising tool for the assessment of ductus venosus Doppler velocimetry.     

Effect of endurance training on coronary artery size and function in healthy men: an invasive followup study

In eight healthy male volunteers (cardiologists; age 36 +/- 5 yr), bicycle spiroergometry, Doppler echocardiography, and quantitative coronary angiography with intracoronary Doppler measurements before and after completion of a physical endurance exercise program of >5 mo duration were performed. Maximum oxygen uptake increased from 46 +/- 6 to 54 +/- 5 ml x kg(-1) x min(-1) (P = 0.04), maximum ergometric workload changed from 3.8 +/- 0.3 to 4.4 +/- 0.3 W/kg (P = 0.001), and left ventricular mass index increased from 82 +/- 18 to 108 +/- 29 g/m(2) (P = 0.001). The right, left main, and left anterior descending coronary artery cross-sectional area increased significantly in response to exercise. Before versus at the end of the exercise program, flow-induced left anterior descending coronary artery cross-sectional area was 10.1 +/- 3.5 and 11.0 +/- 3.9 mm(2), respectively (P = 0.03), nitroglycerin-induced left coronary calibers increased significantly, and coronary flow velocity reserve changed from 3.8 +/- 0.8 to 4.5 +/- 0.7 (P = 0.001). Left coronary artery correlated significantly with ventricular mass and maximum oxygen uptake, and coronary flow velocity reserve was significantly associated with maximum workload.     

Laurel forests in Tenerife, Canary Islands

The general wood structure, vessel size and distribution along the stem xylem radius and in petioles were studied in Laurus azorica trees living in a Tenerife laurel forest. The fractions of volume occupied by dry matter, water and air in percentage of wood fresh volume were also studied. The wood showed a diffuse-porous structure, with solitary vessels or vessels somewhat clustered in small radially oriented groups. Vessels had a diameter ranging from 20 to 130 µm. This diameter was minimal close to the pith, increased more than 2-fold with age, and reached its maximum width close to the cambium. Vessel density decreased from 36 vessels mm^-2 near the pith to about 13 vessels mm^-2 near the cambium. Accordingly, the lumen area was small in young xylem close to the pith (0.0015 mm²), reaching a value 5 times larger (0.007 mm²) near the cambium than in the centre of the stem. Lumen area of vessels in petioles was about 1.5% of petiole cross-sectional area and thus much lower than in stems. Mean hydraulic diameter of these vessels was about 20 µm, and mean vessel density about 136 per petiole. There were only small differences in proportions of dry matter, water and air along stem radius. The relevance of each one of these fractions in the wood is discussed as evidence of the possible existence of a number of embolized vessels dispersed in the total functional cross-sectional area of the xylem.     

Diameter asymmetry of porcine coronary arterial trees: structural and functional implications

The coronary vasculature is characterized by highly asymmetric diameters at bifurcations, which may be an important determinant of flow distribution. To facilitate accurate reconstruction of the coronary network for hemodynamic analysis, we introduce a statistical data set of the diameter asymmetry at bifurcations based on morphometric data of the porcine coronary arterial and venous trees. The bifurcation asymmetry data were represented by the diameter ratio of the daughters relative to mother vessel and by an area expansion ratio (AER) at each bifurcation. A novel asymmetry ratio matrix was introduced to describe the diameter asymmetry of daughters to mother vessels. The relations between AER and flow velocity, and asymmetry ratio matrix and flow distribution, were considered. The results indicate that the ratio of large daughter to mother vessel has a minimum value at order 5 (mean diameter of approximately 70 microm), whereas the ratio of small daughter to mother vessel decreases monotonically with increase in order number. The AER was found to be fairly uniform for larger vessels and to increase from order 5 toward the capillaries. At order 5, we observe a transition in asymmetric bifurcation pattern that may mark a hemodynamic transition from transmural to perfusion subnetworks. The functional implications of these structural transitions are considered.     

Three-dimensional steady flow through a bifurcation

Steady flow of an incompressible, Newtonian fluid through a symmetric bifurcated rigid channel was numerically analyzed by solving the three-dimensional Navier-Stokes equations. The upstream Reynolds number ranged from 100 to 1500. The bifurcation was symmetrical with a branch angle of 60 deg and the area ratio of the daughter to the mother vessel was 2.0. The numerical procedure utilized a coordinate transformation and a control volume approach to discretize the equations to finite difference form and incorporated the SIMPLE algorithm in performing the calculation. The predicted velocity pattern was in qualitative agreement with experimental measurements available in the literature. The results also showed the effect of secondary flow which can not be predicted using previous two-dimensional simulations. A region of reversed flow was observed near the outer wall of the branch except for the case of the lowest Reynolds number. Particle trajectory was examined and it was found that no fluid particles remained within the recirculation zone. The shear stress was calculated on both the inner and the outer wall of the branch. The largest wall shear stress, located in the vicinity of the apex of the branch, was of the same order of magnitude as the level that can cause damage to the vessel wall as reported in a recent study.     

Prediction and assessment of the time-varying effective pulmonary vein area via cardiac MRI and Doppler echocardiography

Accurately estimating left atrial (LA) volume with Doppler echocardiography remains challenging. Using angiography for validation, Marino et al. (Marino P, Prioli AM, Destro G, LoSchiavo I, Golia G, and Zardini P. Am Heart J 127: 886-898, 1994) determined LA volume throughout the cardiac cycle by integrating the velocity-time integrals of Doppler transmitral and pulmonary venous flow, assuming constant mitral valve and pulmonary vein areas. However, this LA volume determination method has never been compared with three-dimensional LA volume data from cardiac MRI, the gold standard for cardiac chamber volume measurement. Previously, we determined that the effective mitral valve area is not constant but varies as a function of time. Therefore, we sought to determine whether the effective pulmonary vein area (EPVA) might be time varying as well and also assessed Marino's method for estimating LA volume. We imaged 10 normal subjects using cardiac MRI and concomitant transthoracic Doppler echocardiography. LA and left ventricular (LV) volumes were measured by MRI, transmitral and pulmonary vein flows were measured by Doppler echocardiography, and time dependence was synchronized via the electrocardiogram. LA volume, estimated using Marino's method, was compared with the MRI measurements. Differences were observed, and the discrepancy between the echocardiographic and MRI methods was used to predict EPVA as a function of time. EPVA was also directly measured from short-axis MRI images and was found to be time varying in concordance with predicted values. We conclude that because EPVA and LA volume time dependence are in phase, LA filling in systole and LV filling in diastole are both facilitated. Application to subjects in select pathophysiological states is in progress.     

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.