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PubMedDate not supplied

Transcutaneous detection of relative changes in artery diameter.

The extent of the excursions of the arterial walls during the cardiac cycle depends on both the compliance of the vessel wall and the local pressure fluctuations. Simultaneous assessment of the relative change in artery diameter in combination with the velocity distribution along the vessel cross section can reveal the cause of loss of distensibility. As will be demonstrated, a multigate pulsed Doppler system with a high spatial resolution can perform simultaneously both functions. The relative change in diameter during the cardiac cycle is obtained by taking the ratio of the distension and the diameter of the artery as observed along the ultrasound beam. It can be shown that this ratio will be angle independent. Statistical and experimental evaluations demonstrate that the system allows the assessment of the relative change in diameter of major peripheral arteries as a continuous function of time with an accuracy of about 0.5%.

Arteries↗
PubMed1988

A comparative study and assessment of Doppler ultrasound spectral estimation techniques. Part II: Methods and results.

Various alternative spectral estimation methods are examined and compared in order to assess their possible application for real-time analysis of Doppler ultrasound arterial signals. Specifically, five general frequency domain models are examined, including the periodogram, the general autoregressive moving average (ARMA) model which has the autoregressive (AR) and moving average (MA) models as special cases, and Capon's maximum likelihood spectral model. A stimulated stationary Doppler signal with a known theoretical spectrum was used as the reference test sequence, and white noise was added to enable various signal/noise conditions to be created. The performance of each method representative of each spectral model was assessed using both qualitative and quantitative schemes that convey information related to the bias and variance of the spectral estimates. Three integrated performance indices were implemented for quantitative analysis. The relative computational complexity for each algorithm was also investigated. Our results indicate that both the AR(Yule-Walker) and ARMA(singular value decomposition) models of orders (8) and (4,4), respectively, show good agreement with the theoretical spectrum, and yield estimates with variances considerably less than the Fast Fourier Transform (FFT). Preliminary results obtained with these methods using a clinical, non-stationary Doppler signal supports these observations.

Algorithms↗
PubMed1989

Doppler spectral waveform generation in vitro: an aid to diagnosis of vascular disease.

This paper describes a microcomputer controlled pump which generates pulsatile flows similar to those found in the human peripheral circulation. Continuous wave Doppler ultrasound was used to investigate the flows generated by the pump and the behaviour of diagnostic indices derived from the spectra was examined. Sonograms were recorded from elastic and rigid tubes with various degrees of axisymmetric constriction. Heart rate, stroke volume, severity of vessel constriction, vessel wall elasticity, distal resistance, and systemic peripheral impedance were varied in turn and the resulting Doppler spectra compared. Indices considered were the pulsatility index, spectral broadening index and peck Doppler frequency. In general, the indices lacked the sensitivity to detect low to moderate levels of disease.

Humans↗
PubMed1990

Doppler color flow in echocardiography: analytical and in-vitro investigations of the quantitative relationship between orifice flow and color jet dimensions.

The goal of this investigation was to explore the relationship between orifice flow rate and the dimensions of the resulting color jet. Equations were derived which describe flow rate as a function of the color jet dimensions, instrument characteristics, and a coefficient which represents the unknown velocity profile across the jet. Experiments in which fluid was injected at a variety of flow rates via an assortment of orifice sizes into a compliant, axisymmetric chamber were performed for comparison with the analytical results. During each injection, orifice flow rate and color jet dimensions were recorded. The experimental results were closely predicted (r = 0.97) by an equation which expresses flow rate as a function of the ratio of the color jet area and color jet length, and with a coefficient which approximates that of a parabolic velocity profile.

Blood Flow Velocity↗
PubMed1990

Inter- and intra-observer variability of Doppler peak velocity measurements: an in-vitro study.

To determine the variability of pulsed Doppler peak velocity measurements, four radiologists with differing experience were tested using a calibrated flow phantom. Two ultrasound units, three probes and eight velocity rates varying between 40.5 and 78 cm/sec were studied, with a total of 303 measurements. The results were normalized against a set of 106 separate measurements made under highly-controlled conditions. The residual error standard deviation (not attributable to any systematically varied factor, including the velocity rate) was 6.8 cm/sec, with most of the remaining variation due to changing transducer or machine. Observer/equipment interactions accounted for 15.8% of the observed variability. The duration of the radiologist's Doppler experience had no significant effect.

Blood Flow Velocity↗
PubMed1990

Estimation of aortic and pulmonary blood flow in a paediatric population using Doppler ultrasound: an in-vitro study.

The noninvasive measurement of blood flow using ultrasound has been attempted by numerous workers. In order to evaluate the technique of duplex scanning applied to the problem of noninvasive assessment of aortic and pulmonary blood flow in a paediatric population with ventricular septal defect, we attempted an in-vitro simulation of these flow conditions. The factors which have been considered in the design of the apparatus include the availability of a range of tube diameters, an aortic-"like" waveform, laminar flow, variable downstream peripheral resistance, a compliance factor and viscosity of the flow medium. The ultrasound probe beam characteristics, sample volume size and shape were also determined. From the results of the beam plots it was apparent that the region of sensitivity of the CW Doppler probes were inappropriate for the measurement of blood flow in neonates. In order to simulate physiological parameters in a paediatric population as closely as possible, five tubes of diameters ranging from 9-18 mm were used, in each tube a range of flow values from 0.8 L min-1 to 5 L min-1 were measured. The flow values were measured by both "bucket and stopwatch" and duplex scanner for both steady and pulsatile flow conditions. The results are presented as correlations between the direct and noninvasive pulsed Doppler assessment of flow measurement and are in the range 0.92-0.99 significant at p less than .001. A discussion of the reliability of flow measurements made using a conventional duplex scanner is given.

Aorta↗
PubMed1990

A comparison of the Doppler spectra from human blood and artificial blood used in a flow phantom.

A comparison between the Doppler signals from human blood and artificial blood used in a flow phantom is described. The artificial blood used was a suspension of Sephadex particles in a glycerol solution. The Doppler power was measured as a function of Sephadex concentration and found to peak at a concentration of about 40% by volume. The power from blood was less by a factor of 150-250 than the power from Sephadex of a similar concentration. The first and second order statistics of the Doppler spectra from Sephadex were independent of particle concentration, and were very similar to those of spectra from blood.

Blood↗
PubMed1990

Doppler waveform pulsatility index and resistance, pressure and flow in the umbilical placental circulation: an investigation using a mathematical model.

A mathematical model of the umbilical placental circulation was used to examine the effect of different physiological variables on the pulsatility index (PI) of the umbilical artery Doppler waveform. The variables include the umbilical and placental resistances, the volume flow rate and the pressure. In the model the branching structure of the placental villous tree is considered in detail, while each arterial branch is itself represented simply using a resistor and a capacitor. Placental vascular disease is modelled as obliteration of a fraction of the terminal branches of the tree. The model umbilical artery PI depends on the ratio of the placental resistance to the umbilical artery resistance. The PI increases with vascular disease, but the rate of increase is not uniform. Initially, the placental resistance and the PI increase very slowly with vessel obliteration. Once the level of vessel obliteration has reached a large enough value--typically between 60% and 90% obliteration--the PI begins to rise sharply. A larger placental vascular bed can accommodate a greater level of vessel obliteration before this rapid PI rise begins. The umbilical artery PI also depends on the pulsatility of the input (aortic bifurcation) pressure waveform, but blood pressure variations in the physically attainable range cannot account for the very high PI values associated with fetal compromise. Physically attainable pressure waveform changes would, however, enable the fetus with substantial placental vascular disease to maintain umbilical volume flow rate, and at the same time exhibit a raised umbilical artery PI value.

Blood Flow Velocity↗
PubMed1991

Quantitative study of steady flow using color Doppler ultrasound.

The use of color Doppler flow mapping systems for quantitative in vitro studies of flow fields is examined and illustrated. A 5-MHz color Doppler system was used, and the resolution was determined by comparing the results of flow-field measurement for steady parabolic pipe flow with calculated values. The velocity accuracy was about 6% of the velocity corresponding to half the pulse repetition frequency, and the spatial resolution was better than 1 mm. Frame frequency limitations permitted only partial tracking of fast temporal changes in the flow field. However, detection of vortices downstream from a small cylinder placed in the flow tube was significantly enhanced by synchronizing the frame frequency with the vortex shedding frequency and using a velocity-variance mode. Color Doppler aliasing was found to be useful to define streamlines and determine whether the flow was laminar or turbulent. The color Doppler system clearly imaged Poiseuille, transitional and turbulent flow and vortex shedding in vitro. It is concluded that color Doppler ultrasound flow mapping can enable large, complex flow fields to be quantitatively studied in vitro.

Blood Flow Velocity↗
PubMed1991

Measurement of blood perfusion in tissue using Doppler ultrasound.

A diagnostic tool for noninvasive evaluation of microcirculatory blood flow using continuous-wave CW Doppler ultrasound is presented. In this study, the properties of this method are investigated both theoretically and experimentally. The method utilizes a nondirectional CW Doppler flowmeter. Blood perfusion in tissue is shown to be proportional to the integral integral of fS (f)df where S(f) is the Doppler power spectrum and f is the Doppler frequency. The instrumentation needed to implement the method is described. Using an experimental flow model it is demonstrated that the above integral is proportional to the product between the number of scatterers in the sample volume of the Doppler probe and the mean speed of these scatterers. This is true even for low flow velocities (down to 1 mm/s). The results from in-vivo measurements on tissues in the finger, and the calf demonstrate that the method can monitor changes in the blood perfusion. It also shows the present limitations of the method due to movement artefacts.

Artifacts↗
PubMed1991

Evaluation of Doppler ultrasound for blood perfusion measurements.

The need to develop clinical methods for the noninvasive monitoring of regional blood perfusion, i.e., the blood flow through the very fine capillaries in body tissue, has long been felt. Hitherto existing methods exhibit limitations, such as insufficient measurement depth and poor time- or space-resolution, which restrict the measurements that can be performed. Dymling (1982) introduced a new CW Doppler ultrasound method for noninvasive blood perfusion measurement which might be one possible solution to this problem. Preliminary experiments indicated a correlation between blood flow and measured perfusion value. Unexpectedly large variations in the recorded perfusion values lead to further investigation of the method, both in vitro using a specially designed flow phantom and in vivo. This study indicates that at least some of the large variations recorded are the result of measurement errors caused by movement artifacts or ultrasonic signal interferences. Methods to diminish the effects of these artifacts are discussed.

Artifacts↗
PubMed1993

On the Doppler signal from a steady flow asymmetrical stenosis model: effects of turbulence.

A steady flow model with a 70% (by area) asymmetrical stenosis was used to examine how changing flow regimes (laminar to turbulent) affect the Doppler signal. Human red blood cells (RBCs) (Hct = 42%) in saline were employed at a flow rate corresponding to a Reynold's number of approximately 545. A dilute suspension of 4% fixed RBCs was also used for the purpose of backscattered power comparison. Measurements of the Doppler signal enabled the backscattered power, time domain statistics, frequency spectra, frequency domain statistics, various spectral indices, autocorrelation function and decorrelation time to be calculated as a function of distance from the stenosis. It is shown that the characteristics of the Doppler signal measured at each site provide information on the nature of the insonated flow field and these correlate well with those expected. The results demonstrate that the onset of turbulence not only affects the Doppler spectrum but also has a profound effect on the signal power, the decorrelation time and the signal statistics.

Blood Flow Velocity↗
PubMed1993

Volumetric arterial flow quantification using echo contrast. An in vitro comparison of three ultrasonic intensity methods: radio frequency, video and Doppler.

The two hypotheses presented in this paper are: (1) absolute and relative volumetric flow rates in vessels can be measured by echo contrast time-intensity curves; and (2) echo contrast time-intensity curves generated by different ultrasound backscatter intensity techniques have equivalent capability for flow measurements. A nonpulsatile flow system was built for quantitative ultrasound backscatter measurements from bolus echo contrast injections using two different volumes of mixing. A total of 49 echo contrast bolus injections were made at various flow rates (0.44-2.59 L/min). Ultrasound backscatter time-intensity curves were generated by ultrasound radio frequency, video and Doppler techniques. The rate of backscattered ultrasound intensity washout for each technique (WASHOUT RATE), and relative change in WASHOUT RATE (delta WASHOUT RATE) were compared to the volumetric flow rate (FLOW) and changes in flow rate (delta FLOW), respectively. The relationship between WASHOUT RATE, FLOW and the volume of contrast mixing was studied. A linear relationship was demonstrated between WASHOUT RATE and delta WASHOUT RATE and the corresponding FLOW and delta FLOW by all three methods (r > 0.90 for all comparisons). The WASHOUT RATE was found to depend on the FLOW and the volume of contrast mixing, but the delta WASHOUT RATE was equal to the delta FLOW and independent of the volume of mixing. Time-intensity curves can be generated from different ultrasound backscatter intensity techniques and the WASHOUT RATE correlates well with FLOW. delta FLOW can be determined directly from the corresponding delta WASHOUT RATE. Doppler ultrasound, because of its natural association with the assessment of flow in chambers and vessels, is uniquely suited to assessment of arterial volumetric blood flow in vitro.

Arteries↗
PubMed1993

Quantification of regurgitant flow through bileaflet heart valve prostheses: theoretical and in vitro studies.

A theoretical treatment using turbulent jet theory has yielded a new equation for predicting regurgitant flow through bileaflet heart valve prostheses, the most commonly implanted mechanical valve design. Previously reported techniques assuming an axisymmetric jet are not applicable to the slot-like orifices presented in these valves. The equations were therefore rederived in the context of the prosthetic valve geometry. The purpose of this study was to develop such a method and demonstrate its applicability in principle by using in vitro models. The method was validated under both steady and pulsatile flow conditions. Having derived a method geometrically specific to the orifices presented in bileaflet mechanical heart valves, it should be applicable from patient to patient due to the rigid nature of the valve. These idealized in vitro studies, along with the accompanying theoretical derivation, will guide implementation in the clinical setting.

Echocardiography, Doppler↗
PubMed1995

Atrial inflow can alter regurgitant jet size: in vitro studies.

Recent studies have attempted to predict the severity of regurgitant lesions from color Doppler jet size, which is a function of orifice momentum for free jets. Jets of mitral and tricuspid regurgitation, however, are opposed by flows entering the atria. Despite their low velocities, these counterflows may have considerable momentum that can limit jet penetration. The purpose of this study was to address the hypothesis that such counterflow fields influence regurgitant jet size. Steady flow was driven through 2.4- and 5.1-mm-diameter circular orifices at 2 to 6 m/s. At a constant orifice velocity and flow rate, the velocity of a uniform counterflow field was varied from 5 to 30 cm/s. Jet dimensions were measured by both fluorescent dye visualization and Doppler color flow mapping. The results showed that despite its relatively low velocities, counterflow dramatically curtailed jet length and area. Jet dimensions were functions of the ratio of jet to counterflow momentum. Thus, atrial inflow may participate in determining jet size and can alter the relation between jet size and lesion severity in mitral and tricuspid regurgitation.

Algorithms↗
PubMed1995

A numerical and experimental investigation of the flow acceleration region proximal to an orifice.

Attempts to quantify valvular regurgitation have recently been focused on the proximal orifice flow field. A complete description of the proximal orifice flow field is provided in this investigation. A steady state in vitro model accessible by both color Doppler ultrasound (CDU) and laser Doppler velocimetry (LDV) was utilized. Velocities for varying flow rates and orifices were calculated by finite element modeling (FEM), by LDV and by CDU. The steady flow model was composed of circular orifices of 3, 5 and 10 mm diameters at flow rates from 0.7 to 10 L/min. Regurgitant flow rates were calculated from the proximal CDU data by two separate methods. The first approach utilized angle corrected velocities while the second approach utilized only velocities which did not require angle correction (centerline velocities). Both methods correlated well with known flow rates (y = 0.97x -0.09, r = 0.98, SEE = 0.45, p < 0.0001; and y = 1.0x + 0.07, r = 0.99, SEE = 0.27, p < 0.0001, respectively) and were superior to results obtained by assuming a hemispherical geometry as is done in the aliasing technique. The methodology provides a complete analysis of the proximal flow field and involves fewer geometric assumptions than the aliasing approach. This may prove to be an advantage when analyzing in vivo flow fields with complex, uncertain geometry.

Acceleration↗
PubMed1994

Flow imaging in an end-to-side anastomosis model using two-dimensional velocity vectors.

Colour flow Doppler ultrasound images from vessels of complex geometry can be difficult to interpret, thereby limiting the effectiveness of the technique to correctly assess abnormalities and to relate the images to the underlying flow field in a quantitative manner. This paper describes progress in calculating and displaying two-dimensional (2-D) velocity vectors from a 30 degrees end-to-side anastomosis model under steady flow conditions at various Reynolds numbers. Velocity vectors were computed from colour Doppler ultrasound images obtained with a linear array for several incident beam directions, and the results were displayed either as colour-encoded magnitude images or by superimposing the vectors on one of the original colour images. Results are discussed in relation to flow visualization observations and the behaviour of flow in curved vessels.

Anastomosis, Surgical↗
PubMed1995

Cardiac ultrasound phantom using a porcine heart model.

A beating cardiac phantom has been developed using an excised porcine heart for use as a training tool in echocardiography. The heart is fixed with a formalin-based preservation method, housed in an optically transparent Lexan chamber, and undergoes hydraulic pumping to circulate a blood-mimicking fluid. The cardiac phantom has been used for a period of four months to produce images of excellent quality with ventricular wall motion typical of human subjects.

Animals↗