Volumetric Flow Measurements Research Articles

Error in MR volumetric flow measurements due to ordered phase encoding in the presence of flow varying with respiration.

Respiratory ordered phase encoding is often employed in MRI studies to reduce image artifacts due to breathing motion. The purpose of this work was to evaluate error caused by the use of respiratory ordering of phase encoding in MR cine phase-contrast (CPC) volumetric flow measurements when the flow rate is sensitive to respiration. It was hypothesized that this effect is due to the systematic biasing of a respiratory-induced phase modulation function in k-space. A theoretical model for the effects of respiration was developed and then tested in flow phantom studies and in normal volunteer studies. In phantom experiments, the use of respiratory ordering induced an error of as much as 13% in CPC volumetric flow measurements. In preliminary volunteer studies, error was as high as 26% in superior vena cava flow measurements versus less than 1% error in the ascending aorta. It is concluded that a potential for error exists in CPC volumetric flow measurements obtained with the use of respiratory ordering schemes. Volunteer studies with larger numbers are warranted. Clinical applications in which this effect may be important include flow measurements in vessels subject to variations in flow due to respiration, such as the venae cavae, pulmonary vasculature, and portal vein.

Absolute phasic blood flow measurement in the brain using digital subtraction angiography.

In this study, an angiographic method using first-pass distribution analysis was used for quantification of phasic volumetric blood flow. Studies were conducted in an angiographic perfusion phantom and in an animal model (rabbit) after intraarterial injection of contrast material. Digital subtraction angiography and first-pass distribution algorithm were used to assess the absolute volumetric flow rates. The method, based on the conservation of contrast material in successive angiographic images, uses the videodensitometric information in the arterial bed. Measurements were made by summing the pixel values in the region of interest, covering the whole perfusion bed. A contrast pass curve was generated for a sequence of images to obtain instantaneous volumetric flow rates. Instantaneous and mean absolute volumetric flow measurements made in the angiographic perfusion phantom and the common carotid artery of the animal models correlated well with validation measurements made using ultrasound flowprobes. The measured (M) and known (K) flow rates in the carotid artery were related by M = 0.87 K + 2.50 mL/minute (r = 0.96, standard error of the estimate = 3.79 mL/minute, n = 25) and M = 0.92 K - 1.00 mL/minute (r = 0.98, standard error of the estimate = 4.04 mL/minute, n = 38) using the videodensitometric and entrance vessel calibration techniques, respectively. Conclusion. Results demonstrate the potential use of the first-pass distribution method in conjunction with digital subtraction angiography for measuring phasic arterial blood flow in vivo.

A theory to correct the systematic error caused by the imperfectly matched beam width to vessel diameter ratio on volumetric flow measurements using ultrasound techniques

When a multigate procedure is used to measure volumetric flow in vessels, in addition to the flow rate result obtained from the conventional velocity profile method, a second result from an “average velocity profile method” can be obtained simultaneously. The latter method obtains the flow rate from the product of the average velocity across the profile and the cross-sectional area of the vessel. A theoretical model has been used to study the effect of the beam width to vessel diameter ratio (BWR) on these two results, as well as a third flow rate result obtained from the uniform insonation method. A theory has been established to correct the systematic error caused by the imperfectly matched BWR associated with each method. It uses a correction factor and the difference between the results from the average velocity profile method and the velocity profile method to compensate for the systematic error. The relationship between an optimal correction factor and the BWR under different flow conditions has been studied. The results using the correction therory in this model show that if the estimated BWR is within ±0.1 from the actual BWR value, the theoretical error in volumetric flow estimation can be limited to within 6.5% for the entire range of BWR.

Doppler velocity ratio measurements evaluated in a phantom model of multiple arterial disease

The objective of this study was to evaluate in vitro the accuracy of the Doppler velocity ratio (VR) (intrastenotic velocity/prestenotic or poststenotic velocity) under different geometric conditions simulating the presence of multiple stenoses. A steady flow loop model was used to test the influence of the presence of a concentric obstruction of 84% area reduction positioned at a distance of 10, 20 and 30 tube diameters, either proximal or distal to the stenosis under study. The stenosis under evaluation was either concentric or eccentric and had a percentage of area reduction ranging from 20% to 91%. An ultrasound color Doppler system was used to perform both pulsed-wave (PW) Doppler and color-flow velocity measurements. VRs were computed by dividing the maximum velocity of the jet by the velocity at 6 and 10 diameters both proximal and distal to the stenosis under study. A strong correlation was obtained between VR computed using color flow and PW Doppler velocities ( r = 0.99). Results indicated that using the prestenotic velocity as a reference velocity generally provided a more sensitive VR index to grade arterial stenosis than using the poststenotic velocity. From a curve fit model, the measured percentages of stenosis were calculated from the VR data and compared to the true percentages. The correlation coefficient, r, was 0.95. When the proximal and distal stenoses were at 10 diameters of the stenosis investigated, r was 0.81, while it increased to 0.98 when the distance was 20 diameters or more. Although VR is theoretically not influenced by hemodynamic factors, we demonstrated that, in practice, the presence of multiple stenoses reduced its sensitivity. Volumetric flow measurements are suggested to obviate this limitation.

Volumetric blood flow measurement by simultaneous Doppler signal and B-mode image processing: A feasibility study

A system is described for the estimation of volumetric flow in arteries in real time by the simultaneous processing of the audio output of a commercial ultrasound scanner carrying the Doppler signal, and the video output carrying B-mode images of the lateral view of the artery. These images are processed to provide estimates of instantaneous internal vessel diameter. Combination with the relevant estimates of spatial mean velocity provides estimates of instantaneous volumetric flow. The major limitations of the system are the low rate of production of the B-mode images (17 Hz), the limited computing power and the need to tilt the transducer to produce an acceptably low Doppler angle. This tilting diminishes the quality of the imaging echoes from the arterial walls, making diameter measurement difficult. In principle, this approach to flow measurement represents an improvement on the conventional duplex technique where a single estimate of diameter is used. In practice, its value is limited by current transducer capabilities.

Volume flow estimation using time domain correlation and ultrasonic flowmetry

A comparison of two volumetric blood flow measurement techniques, CVI-Q (based on time domain correlation) and ultrasonic flowmetry, has been performed in vitro and in vivo. A pulsatile flowpump was used to simulate carotid and femoral type waveforms which were measured simultaneously using the two methods. Five dogs had their common carotid and femoral arteries exposed, and the instantaneous maximum volume flow and the mean flow were measured. Each vessel was partially occluded halfway through the experiment, simulating a 90% stenosis. In vitro, both techniques achieved absolute errors below ±5% for flow rates.over 100 mL/min, but ultrasonic flowmetry had statistically significant larger errors for slower flow rates. In vivo correlation coefficients ranging from 0.73 to 0.95 were obtained with regression line slopes close to unity. The two techniques were in reasonable agreement, but with standard deviations of 20% to 28%. These studies indicate that noninvasive CVI-Q measurements of blood flow in the carotid and femoral arteries are linear and accurate compared to invasive ultrasonic flowmetry.

Measurement of volumetric coronary blood flow by simultaneous intravascular two-dimensional and Doppler ultrasound: Validation in an animal model

We examined the accuracy and feasibility of using simultaneous intracoronary Doppler and two-dimensional (2D) ultrasound imaging in the measurement of volumetric coronary blood flow (CBF). A femoral artery-to-circumflex coronary artery shunt was created in four dogs with tubing that incorporated an in-line transit-time ultrasound (TTU) flow probe. A 2D ultrasound catheter was introduced over a 0.014-inch Doppler guide wire into the circumflex artery with the guide wire tip positioned 2 cm beyond the imaging catheter tip. Cross-sectional area (CSA) and average peak velocity (APV) were recorded. CBF was determined from the previously validated relation CBF = CSA × 0.5 APV and the adjusted measurement formula CBF = CSA × 0.47 APV. Within the 119 measurements made, TTU CBF varied from 31 to 385 ml/min. An excellent correlation was found between 2D/Doppler-derived and TTU CBF with both constants; measurement agreement was improved and bias minimal with the 0.47 constant (slope = 0.9997; r = 0.99; p < 0.0001; SEE = −6.7 ± 8.6). We conclude that simultaneous 2D and Doppler ultrasound yields accurate estimates of volumetric CBF.

Principles and design feasibility of a Doppler ultrasound intravascular volumetric flowmeter.

A new Doppler ultrasound intravascular method is described for the measurement of volumetric flow. Based on the principles described by Hottinger and Meindl [15], it uses a novel semispherical transducer mounted at the tip of a catheter, which generates sample volumes in the form of a thin semispherical shell. Volumetric flow is calculated by using the average velocity determined from the received Doppler spectrum and the area of intersection of a sample volume that completely intersects flow across the vessel. Although a catheter-size transducer was not developed, a larger version was tested using an in vitro steady flow model. Maximum average flow error was limited to 9% for steady flows of 2 to 7 L/min. This error is believed to be a result of the nonuniform intensity generated by the prototype transducer, as well as slight variations in the received power, rather than any fundamental limitations of the flow measurement method itself. Since this study has verified the design principles and feasibility of this new approach, we believe that more detailed experimental investigations are warranted.

Validation of volumetric flow measurements by means of a Doppler-tipped coronary angioplasty guide wire

We used an in vitro model to validate volumetric flow measurements obtained with an 0.018-inch angioplasty guidewire with a 12 MHz transducer mounted on its tip. By using a modified two-head roller pump device, flow was adjusted incrementally from a minimum of 90 ml/min to a maximum of 550 ml/min. Flow was measured with the Doppler guide wire in tubing ranging from 1.9 mm to 6.0 mm internal diameter, as the product of the spectral Doppler velocity integral and the cross-sectional area of the tubing, over a 1-minute period. It was an excellent correlation between the Doppler calculated flow rates and actual flow, regardless of tubing diameter ( r = 0.99). These results suggest that the Doppler spectral output of this device might be accurately applied to estimates of volumetric flow in human coronary arteries.

Pulmonary regurgitation in the late postoperative follow-up of tetralogy of Fallot. Volumetric quantitation by nuclear magnetic resonance velocity mapping.

Pulmonary regurgitation frequently occurs after surgical correction of tetralogy of Fallot. To date, reliable quantitation of pulmonary regurgitation has not been possible, and therefore the clinical significance of pulmonary regurgitation is controversial. Nuclear magnetic resonance (NMR) velocity mapping allows accurate measurement of volumetric flow. The feasibility and accuracy of NMR velocity mapping to quantify pulmonary regurgitation volumes are studied in patients after Fallot repair. In 18 patients (mean age, 16.5 +/- 6.5 years), late (12.6 +/- 5.2 years) after Fallot surgery, forward and regurgitant volume flow was measured in the main pulmonary artery with NMR velocity mapping. To validate the measurements of pulmonary forward flow, right ventricular stroke volume was used as an internal reference standard. Pulmonary regurgitation volumes were compared with the differences between the corresponding right and left ventricular stroke volumes. Ventricular volumes were measured with a multisection gradient echo NMR method. In addition, the relation between pulmonary regurgitation and right ventricular volumes was studied. Measurements of pulmonary regurgitation volume with NMR velocity mapping closely corresponded with the tomographically determined volumes (r = .93). Forward pulmonary volume flow was nearly identical to right ventricular stroke volume (r = .98). Pulmonary regurgitation volume was significantly correlated with end-diastolic volume (r = .82, P < .0005), end-systolic volume (r = .63, P < .01), and stroke volume (r = .89, P < .0005) of the right ventricular but not with right ventricular ejection fraction (r = .41, P = NS). NMR velocity mapping is an accurate method for the noninvasive, volumetric quantification of pulmonary regurgitation after surgical correction of tetralogy of Fallot.

Analysis of systematic and random error in MR volumetric flow measurements.

The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to define acquisition strategies which will minimize error in measuring volumetric flow. Error was separated into two categories: systematic and random. Potential sources of systematic error examined were intravoxel phase dispersion (IVPD), partial volume effects, misalignment of flow axis and flow-encoding gradients, and improper choice of vessel voxels for flux calculations. Random error was addressed using analysis of propagation of variance. Analytical expressions for sources of error were derived; and computer models were used to test the analytical models. Flow phantom studies examining error in MR volumetric flow measurements were performed and compared with error predicted by the analytical models. Expected error in several clinical situations of interest was then derived to find appropriate acquisition strategies. Spatial resolution, signal to noise ratio, velocity sensitivity and the ratio of the modulus of moving isochromats to that of static isochromats were found to be the most important parameters in controlling error and were found to cause competing effects with respect to systematic and random error.

Absolute volumetric blood flow measurements using dual-energy digital subtraction angiography.

In recent years, as a solution to the well-documented problems associated with visual interpretation of coronary arteriograms, more physiologic means of assessing coronary artery stenosis are being investigated. Absolute arterial blood flow assessed as a function of time can be a valuable aid in the analysis of functional significance of arterial lesions and obstructions. An absolute volumetric blood flow measurement technique using a motion immune dual-energy subtraction technique is being investigated, where the kVp and filtration are switched at 30 Hz. The low- and high-energy images are corrected for scatter and veiling glare before subtraction. In this technique, the absolute arterial blood flow is calculated by combining the videodensitometric analysis of spatial and temporal aspects concerning the contrast propagation through the arterial bed using tissue suppressed energy subtracted images. The blood flow measurement technique was validated using a pulsatile pump and a flow chamber imaged over a Humanoid chest phantom. A 20-MHz Doppler flow probe was used to validate the measurement of phasic volumetric blood flow. The measured (M) and known (K) mean blood flow for the entrance vessel technique and the videodensitometric calibration technique were related by M = 1.14K - 0.12 ml/s (r2 = 0.98) and M = 1.12K - 0.23 ml/s (r2 = 0.90), respectively. The results indicate that phasic volumetric blood flow can be measured using a CCD camera in conjunction with real time dual-energy subtraction.

In vivo correlation of doppler waveform analysis with arterial input impedance parameters

Previous studies have confirmed that Doppler waveform analysis (DWA) offers a valid reflection of changes in peripheral vascular resistance. However, the ability of the pulsatility index (PI), a parameter of DWA, to reflect the dynamic components of the circulation, as assessed by arterial input parameters, remains uncertain. In addition, the state of the central circulation is considered an important factor influencing the accuracy of this technique. This study evaluated the ability of the aortic PI to reflect alterations of input impedance in a chronically instrumented lamb model that was subjected to pharmacologic alteration of the circulation. Pressure, volumetric flow and continuous-wave Doppler frequency shift measurements were recorded from the infrarenal abdominal aorta. The parameters of input impedance, peripheral vascular resistance ( Zpr), characteristic impedance ( Zo) and reflection coefficient ( Rc), were determined and then correlated with changes in the aortic P1. Initially, perturbations of the circulatory state were created with a vasodilator, hydralazine (HY) and a vasoconstrictor, phenylephrine (PE). During a second set of experiments, the effect of the reflex heart rate (HR) responses on the PI was evaluated. This was accomplished by inhibiting reflex HR responses to these vasoactive agents with either trimethophan (TM) or atropine methyl bromide (AMB). In response to HY and HY with TM, significant decreases in the PI and impedance parameters occurred. Administration of PE and PE with AMB resulted in significant increases in PI and each of the impedance parameters. HY and PE induced changes in PI correlated significantly with changes in volumetric flow ( r = 0.82, 0.80; p < 0.001), mean arterial blood pressure ( r = 0.64, 0.70; p < 0.001) and Zpr ( r = 0.77, 0.80; p < 0.001), but not with Zo ( r = 0.34, 0.36) and Rc ( r = 0.26, 0.31). However, when reflex HR responses were inhibited during the administration of the vasoactive agents, HY with TM and PE with AMB, induced changes in PI correlated significantly with Zo ( r = 0.93, 0.89; p < 0.001) and Rc ( r = 0.84, 0.83; p < 0.001), and the correlation with mean arterial pressure ( r = 0.78, 0.87; p < 0.05) and Zpr ( r = 0.92, 0.91; p < 0.05) was significantly greater. These findings indicate that the PI accurately assesses pharmacologically induced changes in the downstream arterial input impedance. The accuracy of this assessment is enhanced further when central circulatory factors such as changes in HR are considered.

Measurement of volumetric coronary blood flow with a doppler catheter: Validation in an animal model

Although Doppler catheter recordings are used to determine coronary flow velocity, their accuracy in the estimation of volumetric blood flow has not been validated. To address this issue, Doppler-derived coronary flow was measured in a canine model and compared with that obtained by means of an electromagnetic flowmeter. A carotid artery-to-circumflex coronary artery shunt was created in six dogs with tubing that incorporated an inline electromagnetic flow device. The circumflex artery was occlusively cannulated by means of a rigid metal stent of known internal diameter, which was placed 2 cm into the vessel, and flow was measured in the stent region by means of a 3F Doppler catheter. Analysis of Doppler shift signals was performed by means of a zero-crossing detector (ZCD) and an off-line fast-Fourier transformation (FFT) system. Flow derived from peak FFT velocities corresponded closely to electromagnetic flow (slope 1.09, r = 0.93), whereas mean FFT and ZCD velocities underestimated electromagnetic flow (with slopes of 0.47 and 0.46, respectively) despite a close correlation ( r = 0.92,0.94). Thus FFT analysis of the Doppler signal with determination of peak velocity gives the most accurate representation of flow, whereas measurements based on ZCD mean velocities may significantly underestimate coronary flow.

Vector doppler: Accurate measurement of blood velocity in two dimensions

An ultrasound Doppler system capable of determining both the magnitude and direction of complex blood velocities has been developed and demonstrated. This system uses a single transmitter and two receivers to resolve orthogonal velocity vector components. The system has been tested on a moving string phantom and on a carotid artery. The method may improve the detection of early vascular diseases and improve the accuracy of volumetric blood flow measurements in peripheral arteries.

A novel echogenic bubble-particle contrast agent: K.J. Parker, M.R. Violante, T.A. Tuthill and P. Dentinger, Rochester Center for Biomedical Ultrasound, University of Rochester, Rochester, NY 14627

A technique for volumetric blood flow measurement was developed by combining standard Doppler measurements with grey-scale decorrelation. Steered Doppler is used to determine the in-plane velocities, which are then used to extract the out-of-plane velocities from the temporal A-line decorrelation. As a result, a three-dimensional (3-D) vector flow field can be computed over the imaging plane using a single clinical transducer without knowledge of the vessel orientation. Volume flow is computed by integrating the out-of-plane flow over the vessel cross-section. The algorithm was tested using a scattering-enhanced fluid in a 6.4-mm diameter dialysis tubing. For a wide range of transducer angles, the volume flow was accurately measured to within 28% in these preliminary tests. (E-mail: [email protected])

Devices for metrological provisions for measuring the volumetric activity of uranium aerosols

It is evident that the method of direct comparison can be used for transmission of the measure of the VA unit from the reference set-up to master and working >~. Simultaneously, methods and devices have to be developed to transmit the measure of the VA unit of natural uranium aerosols to working MD under production conditions without dismounting the devices. This is extremely important for MD which form part of information-measuring systems. In accordance with GOST 8.090-79, reference radionuclide sources of alpha- and beta-radiation are used as master MD for checking master and working ~ by the method of direct measurements (by accounting for the trasient factor). Sources of this type with 2~U and 23~U radionuclides are not used since their number is very limited an does not correspond to the range of measurements. Along with this, the active surface geometry of the sources does not coincide with that of the aspirated sections of the filter and transient factors are absent. A new approach to transmission of the measure of the VA unit of artificial radioactive aerosols to working MD using special aerosol sources was proposed in [4]. These are filters on which the dispersed radioactive aerosol phase is taken using the sampling device of each specific MD in its sampling regimes. The aerosol filter material is selected such that it is identical to that in the given MD. The special aerosol source is certified on the basis of activity and volumetric activity corresponding to working regimes of sampling of aerosol samples. For transmission of the measure of the VA unit of aerosols, it is sufficient to place this source in the given type of working MD in place of the aspirated section of the filter and compare readings of the working >~ with the VA reproduced by the aerosol source. The special aerosol sources were found to be simple and effective devices for metrological certification and checking of MD under production conditions without dismounting them. An apparatus was developed for reproducing the unit of VA of uranium aerosols. It included a natural uranium aerosol generator and a device for measuring activity of aerosol samples of this uranium. The generator is based on the method of compressed air bubbling through a solution of uranium salt and it contains units for generation, purification, drying, adjustment, and measurement of volumetric flow of air and also an unit for pure air dilution of the aerosol. The generating unit is a glass vessel with cocks and a device for intake of compressed air previously purified and stabilized on filters FV-25 and stabilizers SDV-6. The vessel

Volumetric blood flow via time-domain correlation: experimental verification

A novel ultrasonic volumetric flow measurement method using time-domain correlation of consecutive pairs of echoes has been developed. An ultrasonic data acquisition system determined the time shift between a pair of range gated echoes by searching for the time shift with the maximum correlation between the RF sampled waveforms. Experiments with a 5-MHz transducer indicate that the standard deviation of the estimate of steady fluid velocity through 6-mm-diameter tubes is less than 10% of the mean. Experimentally, Sephadex (G-50; 20-80 mum dia.) particles in water and fresh porcine blood have been used as ultrasound scattering fluids. Two-dimensional (2-D) flow velocity can be estimated by slowly sweeping the ultrasonic beam across the blood vessel phantom. Volumetric flow through the vessel is estimated by integrating the 2-D flow velocity field and then is compared to hydrodynamic flow measurements to assess the overall experimental accuracy of the time-domain method. Flow rates from 50-500 ml/min have been estimated with an accuracy better than 10% under the idealized characteristics used in this study, which include straight circular thin-walled tubes, laminar axially-symmetric steady flow, and no intervening tissues.

A haemodynamic evaluation of the femoro-femoral cross-over bypass

The femoro-femoral cross-over bypass has become a popular choice for the management of unilateral iliac artery disease, being used in preference to aorto-femoral or extraperitoneal ilio-femoral bypasses. It is a relatively minor procedure and associated with a small incidence of side effects, the main one being the risk of development of a steal of blood from the donor limb by the bypass. Although this problem has been widely discussed, haemodynamic studies have been limited by the use of indirect measurements of blood flow, such as ankle systolic pressures or by the use of electromagnetic flowmetry at the time of surgery. No study employing volumetric blood flow measurements to identify and quantify blood steal in the postoperative patient has been reported to date. With aims of studying the haemodynamic effects of a femoro-femoral cross-over bypass on the circulation in both the recipient and donor limbs, and of identifying preoperatively, problems likely to lead to haemodynamic problems or to graft failure, the present study of 31 patients undergoing femoro-femoral bypass was undertaken. The patients, 18 of whom had rest pain and 13 intermittent claudication, were studied preoperatively using arteriography and a non-invasive assessment. At 3 months from the operation, all received a clinical assessment and a further non-invasive assessment, including a measurement of blood volume flow. Flow measurements were made in the bypass at rest and during a reactive hyperaemia test. In addition, flow measurements were made in the donor limb below the bypass origin at rest and during hyperaemic testing of the recipient limb in order to assess any steal effect the bypass might cause to the donor limb circulation. All 31 patients were improved by surgery, but five developed donor limb claudication which was attributed to steal in three cases. Resting blood flow in the bypasses, 161 (65-282)ml/min [median (range)], rose by 116% (5-428%) to 300 (82-1114)ml/min after hyperaemic testing. Simultaneously, bypass hyperaemia caused a fall in donor limb blood flow of 32% (0-74%). Of the preoperative non-invasive tests, only donor femoral artery pulse rise time was related to the later development of objective evidence of steal. Successful Gruntzig dilatation of four major stenoses resulted in a satisfactory outcome.

Doppler Velocimetry in Obstetrics

Doppler ultrasound velocimetry offers a noninvasive method for assessing velocities in the various components of the fetal circulation including the cardiac, umbilical, cerebral, and aortic flows. As the volumetric flow measurement in these circumstances is prone to significant errors, the major approach has been to analyze the maximum frequency shift envelope of the Doppler waveform. Numerous studies have demonstrated not only the feasibility, but also the diagnostic efficacy of this approach. The latter is particularly true in relation to the Doppler evaluation of the umbilical circulation in various complications of pregnancy. Although these findings do not establish the role of Doppler technology as a standard of practice, they clearly demonstrate its immense potential as a fetal surveillance technique.