Some characteristics of laminar flow velocity spectra detected by a 20 MHz pulsed ultrasound Doppler

For the purpose of improving accuracy of noninvasive flow measurements in small (1–2 mm diameter) blood vessels, an existing 20 MHz pulsed ultrasound Doppler velocimeter (PUDVM) has been augmented to allow fast Fourier transformation (FFT) of its Doppler shift signal. The modified instrument was used to collect velocity spectra for a benchtop test section delivering precise Poiseuille flows at velocities in the range of physiological interest. The velocity spectra demonstrated a substantial degree of broadening, much of which was attributable to the geometry of the finite sample volume size. Several spectral indices were studied as a function of flow field variables. Results showed that the intensity-weighted mean Doppler shift frequency, when converted to its corresponding velocity v_M, agreed very closely with the theoretically predicted local fluid velocity. Measurement linearity and repeatability were evaluated for a number of system variables, indicating that FFT performance was essentially unaffected by several parameters capable of causing major degradation of (phasic) Doppler shift signals produced by conventional zero-crossing-counter circuitry. As presently configured, the augmented PUDVM instrument is fully capable of detailed flow field mapping in small subcutaneous vessels such as human digital arteries.     

The resolution of the ultrasound pulsed Doppler for blood velocity measurements

Pulsed ultrasound Doppler velocity meters (PUDVM) permit noninvasive blood velocity measurements. The emitted ultrasound beam characteristics primarily determine the resolution of the instrument when recording velocity profiles. The sample volume, the small region over which velocity information data are detected, was found to be > 2·3 mm³ depending on the transducer disk dia., distance in front of the disk, sampling time increment, and pulse length. The shape of the sample volume approximates a cylinder in the near field and a frustrum of a cone in the far field. The end surfaces of the sample volume were affected by the emitted pulse shape. Ultrasonic beam cross-sections were found to be smaller than predicted by theory due to the finite threshold levels of the PUDVM. The variation of the sample volume with range was illustrated by steady laminar flow velocity profile measurements in rigid tubes. The accuracy of velocity measurements was within 5 per cent with slightly larger deviations occurring near the walls due to the finite sample volume.     

Nontraumatic aortic blood flow sensing by use of an ultrasonic esophageal probe.

The measurement of blood velocity fields, volume flow, and arterial wall motion in the descending thoracic aorta provides essential hemodynamic information for both research and clinical diagnosis. The close proximity of the esophagus to the aorta in the dog makes it possible to obtain such data nonsurgically using an ultrasonic esophageal probe; however, the accuracy of such a probe is limited if the angle between the sound beam and the flow axis, known as the Doppler angle, is not precisely known. By use of a pulsed Doppler velocity meter (PUDVM) and a triangulation procedure, accurate empirical measurement of the Doppler angle has been obtained, allowing quantification of blood velocity scans across the aorta. Volume flow is obtained by integration of blood velocity profiles and arterial wall motion is measured with an ultrasonic echo tracking device. Accuracy of the probe was substantiated by comparison with ultrasonic and electromagnetic implanted flow cuff measurements. Use of the probe in measurement of blood velocity, volume flow and arterial wall motion at various locations along the 8- and 10-cm length of the descending thoracic aorta in adult beagle dogs is detailed. The simplicity, accuracy, and nontraumatic aspect of the technique should allow increasing use of such a probe in numerous research and clinical applications.     

Venous ultrasound catheter-tip technique for evaluation of arterial hemodynamics.

A variety of devices has been used for measuring flow properties of deep-lying arteries, but many have limitations. This paper describes a relatively nontraumatic intravenous approach which uses a catheter in connection with a pulsed ultrasonic Doppler velocity meter (PUDVM) and an ultrasound echo track. The venous ultrasound catheter (VUC) has permitted measurements of local instantaneols blood velocity, flow, and wall motion in the abdominal aorta and iliac arteries of beagle dogs; evaluation studies have been conducted to compare the VUC recordings with an independent method for measuring blood flow and wall motion. Coupling of this catheter-tip device with the PUDVM and echo track provides chronic measurements of hemodynamic parameters in these deep vessels which were virtually impossible to obtain previously. This technique may prove useful in monitoring vessel pathology longitudinally as well as in basic experimental situations requiring flow and arterial wall mechanical properties.     

Accuracy of colour Doppler ultrasound velocity measurements in small vessels

Colour Doppler ultrasound offers the possibility of imaging small vessels not visible by B-mode alone. The colour Doppler image of velocities allows the course of small vessels to be imaged in the X-Y plane of the scan provided the Doppler frequency shift is of sufficient magnitude. This permits alignments of the Doppler cursor, allowing angle correction to provide true velocity measurements from the Doppler shift obtained. Before attempting to make velocity measurements, however, it is essential to be aware of the possible error in the Z plane caused by the thickness of the Doppler sample volume. To quantify this source of error, hydrophone and flow-rig measurements were performed on an Acuson 128 colour Doppler scanner with both 5 MHz linear-array and 3.5 MHz phased-array transducers. Measurements of the transmitted pulses using a point hydrophone showed that both probes employ approximately 3.5 MHz Doppler pulses (in both colour and pulsed Doppler modes). The two transducers have the same axial resolution. In colour Doppler mode the axial length of the sample volume increases automatically with depth by up to 0.5 mm. Measurements of colour and pulsed Doppler signal strength were obtained in a controlled flow rig. Both transducers produced accurate colour flow images of the phantom at their optimum depths; flow velocity errors due to Z-plane thickness are < 5%. There was, however, substantial error outside these optimum conditions (up to 20%).     

A computational study on the influence of catheter-delivered intravascular probes on blood flow in a coronary artery model

Catheter-delivered intravascular probes are widely used in clinical practice to measure coronary arterial velocity and pressure, but the artefactual effect of the probe on the variables being measured is not well characterised. A coronary artery was simulated with a 180 degrees curved tube 3mm in diameter and the effect of catheters of different diameters was modelled numerically under pulsatile flow conditions. The presence of a catheter increased pressure by 1.3-4.3 mmHg depending on its diameter, and reduced velocity-pressure phase-lag. For an ultrasound sample volume 5mm downstream from the probe tip, the underestimation in velocity measurement attributed to catheter blockage is approximately 15-21% for an average inlet velocity of 0.1m/s. The velocity measurement error is lower at higher mean flow velocity. Accuracy of clinical velocity measurements could be improved by moving the sample volume farther downstream from the probe tip, because the centrifugal pressure gradient intrinsic to the curvature promotes re-development of flow.     

Dual-fiber laser Doppler velocimeter and its application to the measurements of coronary blood velocity

To obtain a smaller sample volume and a suitable sample position for the measurement of blood velocity, we fabricated a laser Doppler velocimeter (LDV) with a dual-fiber pickup. The two fibers (clad: 62.5 micron and core: 50 micron) were placed side by side. An He-Ne laser was introduced into the blood through one fiber and the backscattered light was collected by the other fiber. The Doppler signal was analyzed by a spectrum analyzer. The spectrum of the Doppler shift frequency showed a sharp peaked pattern for both forward and reverse flows and exhibited an excellent correlation with the known blood velocity. The blood velocity in the poststenotic portion of canine coronary artery was successfully measured by the dual-fiber LDV. These results indicate that the dual-fiber LDV is useful for measuring blood velocity accurately with a small sample volume even in disturbed flow fields.     

Spatial velocity profile in mouse embryonic aorta and Doppler-derived volumetric flow: a preliminary model

Characterizing embryonic circulatory physiology requires accurate cardiac output and flow data. Despite recent applications of high-frequency ultrasound Doppler to the study of embryonic circulation, current Doppler analysis of volumetric flow is relatively crude. To improve Doppler derivation of volumetric flow, we sought a preliminary model of the spatial velocity profile in the mouse embryonic dorsal aorta using ultrasound biomicroscopy (UBM)-Doppler data. Embryonic hematocrit is 0.05-0.10 so rheologic properties must be insignificant. Low Reynolds numbers (<500) and Womersley parameters (<0.76) suggest laminar flow. UBM demonstrated a circular dorsal aortic cross section with no significant tapering. Low Dean numbers (<100) suggest the presence of minimal skewing of the spatial velocity profile. The inlet length allows for fully developed flow. There is no apparent aortic wall pulsatility. Extrapolation of prior studies to these vessel diameters (300-350 microm) and flow velocities (~50-200 mm/s) suggests parabolic spatial velocity profiles. Therefore, mouse embryonic dorsal aortic blood flow may correspond to Poiseuille flow in a straight rigid tube with parabolic spatial velocity profiles. As a first approximation, these results are an important step toward precise in utero ultrasound characterization of blood flow within the developing mammalian circulation.     

Duplex sonography of arteriovenous fistula in chronic hemodialysis patients

Duplex sonography was used to assess functional features of arteriovenous fistula (AVF) for hemodialysis (HD). Internal diameter (ID), resistance index (RI) and blood flow (BF) velocity in feeding artery and in vein ofAVF, and venous BF volume were analyzed with purpose to determine the normal values. Presumed normal BF velocities are those of clinically well functioning shunts, allowing BF through HD lines of minimally 250 ml/min. Study included 66 nondiabetic HDpatients (30 women, 36 men), mean age 52-13 years, treated by HD for median 61 (4-252) months. Measurements in 47patients with clinically well functioning AVF were as followed: mean arterial ID 5.2 +/- 1.4 mm, median arterial RI 0.3 (0.3-0.9), median arterial BF velocity 1.5 (0.6-3.6) m/s, mean venous ID 7.6 +/- 2.2 mm, median venous RI 0.3 (0.3-0.9), mean venous BF velocity 1.6 +/- 0.7 m/s, and median venous BF volume 530 (120-1890) ml/min. Patients with poor functioning AVF had significantly less arterial ID, higher arterial RI, less venous ID, less venous BF velocity and volume. Duplex sonography findings obtained for clinically estimated well functioning shunt should be considered as normal Doppler values. Blood vessels' morphologic features depend upon age, and older patients have more pronounced changes.     

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.     

Evaluation of mammary blood flow measurements in lactating goats using the ultrasound Doppler principle

1. Non-invasive methods were developed for measuring mammary blood flow in lactating goats. 2. A Doppler principle ultrasound device was equipped with an external detector measuring maximal blood velocity (Vmax) and average blood velocity (Vav) was calculated as Vmax/2. Volume flow then depended on determination of the angle of insonation and the cross-sectional area of the milk vein (the caudal superficial epigastric or subcutaneous abdominal vein). 3. Blood velocities were measured on the milk vein of either side of the animal while clamping the pudendal veins manually. Blood velocities ranged from 7-34 cm/sec. 4. The milk vein diameter was measured by means of a slide gauge which, for clearly protruding veins, gave similar results to that measured by ultrasound scanning. In protruding veins the cross-section was circular. In non-protruding veins the cross-section was elliptical and the slide gauge significantly (P less than 0.01) overestimated the cross-sectional area. The milk vein diameter of either side measured in 10 lactating goats was 8.8 +/- 1.1 mm (means +/- SD). 5. Blood flow ranged from 90-675 ml/min in a dry and a high-yielding (3.4 l milk daily) goat, respectively. The reproducibility of the blood flow measurements was 12-16%. 6. It is concluded that the present method may be used for quantitative measurements of mammary blood flow in goats.     

Haemodynamics of coronary artery - saphenous vein bypass

The velocity distribution of a suspension of red blood cell ghosts in an idealized model of the coronary artery-saphenous vein bypass has been investigated with the aid of laser Doppler anemometry. Pulsatile flow simulated pressure variations in the ascending aorta and ghost cell velocities were determined by the Doppler shift of scattered laser light. Using four different model bypasses it was demonstrated that turbulent flow at the graft-coronary intersection can be delayed by decreasing the discontinuity in diameter between the bypass vein and coronary artery, and also by reducing the bypass vein and host coronary artery intersection angle.     

The effect of body weight on the degree of blood velocity profile skewness in the aortic annulus in domestic pigs

We investigated the blood velocity profile in the aortic annulus (AA) in two groups of domestic pigs using epicardial Doppler echocardiography. The velocity profile skewness in terms of max/mean TVI (the ratio of maximal to cross-sectional mean time-velocity integral along the diameter) was 1.107 +/- 0.01 in the small pigs (n = 10; body weight: 24.6 +/- 0.8 kg) and 1.216 +/- 0.026 in the large pigs (n = 8; body weight: 50.6 +/- 2.5 kg) (P = 0.002). The velocity profile in the AA is more skewed in large animals than in small animals and the skewness in the larger animals is similar to that in normal adult humans. This study shows the importance of choosing animals of sufficient size if flow method investigations are to be performed. This is particularly important for ultrasound Doppler investigations based on a limited sample of velocities across the flow channel.     

Doppler velocity measurements from large and small arteries of mice

With the growth of genetic engineering, mice have become increasingly common as models of human diseases, and this has stimulated the development of techniques to assess the murine cardiovascular system. Our group has developed nonimaging and dedicated Doppler techniques for measuring blood velocity in the large and small peripheral arteries of anesthetized mice. We translated technology originally designed for human vessels for use in smaller mouse vessels at higher heart rates by using higher ultrasonic frequencies, smaller transducers, and higher-speed signal processing. With these methods one can measure cardiac filling and ejection velocities, velocity pulse arrival times for determining pulse wave velocity, peripheral blood velocity and vessel wall motion waveforms, jet velocities for the calculation of the pressure drop across stenoses, and left main coronary velocity for the estimation of coronary flow reserve. These noninvasive methods are convenient and easy to apply, but care must be taken in interpreting measurements due to Doppler sample volume size and angle of incidence. Doppler methods have been used to characterize and evaluate numerous cardiovascular phenotypes in mice and have been particularly useful in evaluating the cardiac and vascular remodeling that occur following transverse aortic constriction. Although duplex ultrasonic echo-Doppler instruments are being applied to mice, dedicated Doppler systems are more suitable for some applications. The magnitudes and waveforms of blood velocities from both cardiac and peripheral sites are similar in mice and humans, such that much of what is learned using Doppler technology in mice may be translated back to humans.     

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.     

Numerical Analysis of Blood Flow through COVID-19 Infected Arteries

Computational Fluid Dynamics has become relevant in the study of hemodynamics, where clinical results are challenging to obtain. This paper discusses a 2-Dimensional transient blood flow analysis through an arterial bifurcation for patients infected with the Coronavirus. The geometry considered is an arterial bifurcation with main stem diameter 3 mm and two outlets. The left outlet (smaller) has a diameter of 1.5 mm and the right outlet (larger), 2 mm. The length of the main stem, left branch and right branch are fixed at 35 mm, 20 mm and 25 mm respectively. Viscosity change that occurs in the blood leads to different parametrical changes in blood flow. The blood flow towards the smaller branch is significantly affected by the changed blood viscosity. Extended regions of high pressure and increased velocity towards the larger outlet are obtained. The Time Averaged Wall Shear Stress (TAWSS) for the corona affected artery is found to be 10.4114 Pa at a 90° angle of bifurcation as compared to 2.45002 Pa of the normal artery. For varying angles of bifurcation, an angle of 75° was found to have a maximum Time Averaged Wall Shear Stress of 2.46076 Pa and 10.42542 Pa for normal and corona affected artery, respectively.     

Measurement of human fetal blood flow.

Real-time B-mode ultrasonography was combined with a pulsed Doppler ultrasound technique for transcutaneous measurement of human fetal blood flow in the aorta and intra-abdominal part of the umbilical vein. The target vessel was located and its diameter measured in the two-dimensional real-time image. The pulsed Doppler transducer was attached to the real-time transducer at a fixed angle. By processing the Doppler shift signals the instrument estimated the mean and maximum blood velocities and the integral under the velocity curves. This permitted calculation of the blood flow. The method was applied to 26 fetuses in normal late pregnancies. Mean blood flow in the descending part of the fetal aorta based on maximum velocity was 191 ml/kg/min. Mean flow in the intra-abdominal part of the umbilical vein was 110 ml/kg/min. This method of measurement is non-invasive and opens new perspectives in studying fetal haemodynamics.     

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.     

Differential blood flow changes between the future dominant and subordinate follicles precede diameter changes during follicle selection in mares

Diameter deviation during a follicular wave is characterized by the continued growth of the developing dominant follicle and reduced growth and regression of the subordinate follicles. This study considered the hypothesis that reduced blood flow in the future largest subordinate follicle precedes the beginning of diameter deviation. The hypothesis was tested by quantifying the daily changes in blood-flow velocities and blood-flow area within the wall of follicles before and during diameter deviation in mares (n = 7). The blood-flow end points were quantified daily by transrectal color Doppler ultrasonography. Follicles were identified retrospectively by rank as F1 (largest) and F2 according to the maximum attained diameter. Follicles were grouped into nine F1 diameter ranges of 3.0 mm each (equivalent to 1 day's growth) centered on 6.5, 9.5, 12.5, 15.5, 18.5, 21.5, 24.5, 27.5, and 30.5 mm. Diameter deviation began in the 24.5-mm group, as indicated by a smaller (P < 0.05) difference between F1 and F2 in the 24.5-mm group than in the 27.5-mm group. Based on a similar approach, peak systolic velocity and time-averaged maximum velocity of blood flow began to deviate between F1 and F2 in the 18.5-mm group (P < 0.04) and blood flow area began to deviate in the 21.5-mm group (P < 0.009). Thus, differential blood flow area between F1 and F2 began an average of 3.0 mm (equivalent to 1 day) and differential blood-flow velocities began an average of 6.0 mm before the beginning of diameter deviation. The results demonstrated that deviation between F1 and F2 in the blood flow of the follicle walls occurred 1 or 2 days before deviation in follicle diameter during follicle selection in mares.     

Dynamics of coronary circulation in rats]

The blood flow velocities in left anterior descending coronary artery and ascending aorta have been measured in anesthetized rats by high frequency Doppler technique. The measurement of coronary blood flow velocity by miniature ultrasonic probe (2.0 x 1.5 mm) was performed through myocardial surface. Two different forms of coronary blood flow curves were recorded. These forms of the curves depend on the value of the coronary blood flow velocity and are connected with the ascending aorta blood flow velocity. The dynamics of the coronary blood flow reactions under coronary artery occlusion and asphyxia in the rat is similar to the one in the cat and the dog, but less expressive. In experiments with vasodilators the direct dependence between linear and volume coronary artery velocities under the measurement through myocardial surface was found.