Blood Velocity Estimation Research Articles

Sample volume shape for pulsed-flow velocity estimation using a linear array

Various definitions of the sample volume (SV) shape have been proposed, but they are mostly based on transducers with axisymmetrical geometry. We have defined the SV as that spatial region in which scatterers contribute a component to the total gated received-signal energy above a defined threshold. This definition is consistent with modern pulsed transducer arrays and accounts for the need to impose a signal/noise threshold. Based on this definition, SVs for a typical linear phased-array transducer were simulated using custom-designed software. The effects of different transmit pulses, receive gates, apertures, SV depths and lateral foci were studied using a one-dimensional (1-D) beam-forming array, with a fixed lens in the elevation direction. Based on a simplified method of analysis, the features of the beam-steered SV are qualitatively similar to those of the nonsteered SV, when compared at the same beam-flow angle. These studies have helped provide a clearer understanding of the manner in which the SV energy distribution is affected by various parameters. The results can have potentially significant implications in the use of ultrasound (US) for blood velocity estimation, specifically with respect to locating the SV within the blood vessel and the origin of the velocity spectrum.

Effects of aggregation of red cells and linear velocity gradients on the correlation-based method for quantitative IVUS blood flow at 20 mhz

Recent computer simulations suggest that the presence of aggregates of red blood cells (RBCs), at random angles and lengths, does not affect the measurements of blood flow transverse to the ultrasound (US) beam direction using a correlation-based method and an intravascular (IV) US array catheter. However, in case of aggregates of RBCs aligned with the flow, measurements of simulated blood velocity are affected. Blood velocity gradients were also shown not to influence the correlation-based method for blood velocity estimation. The objective of this study was to quantify the influence of aggregates of RBCs and blood velocity gradients on the correlation-based method during in vitro experiments. For this purpose, measurements were performed on washed RBCs (no aggregation), normal human blood, and two types of diseased blood in which a lower or a higher level of aggregation was present. The decorrelation pattern of a circular US transducer as a function of transverse blood flow was studied using a Couette system. Changing the shear rate of the Couette system modified the aggregation level of RBCs and the velocity gradient. With the exception of the results at low shear rates and abnormally high aggregation levels, agreements were found between the autoconvolution of the acoustical beam (reference curve) and the radiofrequency (RF) decorrelation patterns. For the high shear rate present in coronary arteries, the correlation-based method for blood flow estimation should not be influenced by these phenomena. (E-mail: a.vandersteen@erasmusmc.nl)

BSS-based filtering of physiological and ARFI-induced tissue and blood motion

Blind source separation (BSS) for adaptive filtering is presented in application to imaging both physiological and acoustic radiation force impulse (ARFI)-induced tissue and blood motion in the common carotid artery. The collected raw radiofrequency (RF) data includes vessel wall motion, blood flow and ARFI-induced motion. In the context of these complex motion patterns, the same BSS adaptive filtering method was employed for three diverse applications: 1. clutter filtering ensembles prior to blood velocity estimation, 2. extracting small axial velocity components from noisy velocity measurements given large flow angles and 3. reducing noise in measured ARFI-induced tissue displacement profiles to enhance differentiation of local tissue structures. The filter separated physiological vessel wall motion from axial blood flow and ARFI-induced motion; successful filter performance is demonstrated in velocity estimates, color flow images and ARFI displacement profiles. The results demonstrate the breadth of applications for BSS adaptive filtering in the clinical imaging environment. (E-mail: caterina.gallippi@duke.edu)

Simulation of RF data with tissue motion for optimizing stationary echo canceling filters

Blood velocity estimation is complicated by the strong echoes received from tissue surrounding the vessel under investigation. Proper blood velocity estimation necessitates use of a filter for separation of the different signal components. Development of these filters and new estimators requires RF-data, where the tissue component is known. In vivo RF-data does not have this property. Instead simulated data incorporating all relevant features of the measurement situation can be employed. One feature is the motion in the surrounding tissue induced by pulsation, heartbeat, and breathing. This study has developed models for the motions and incorporated them into the RF simulation program Field II, thereby obtaining realistic simulated data. A powerful tool for evaluation of different filters and estimators is then available. The model parameters can be varied according to the physical situation with respect to scan-site and the individual to be scanned. The nature of pulsation is discussed, and a relation between the pressure in the carotid artery and the experienced vessel wall motion is derived.

A new high resolution color flow system using an eigendecomposition-based adaptive filter for clutter rejection.

We present a new signal processing strategy for high frequency color flow mapping in moving tissue environments. A new application of an eigendecomposition-based clutter rejection filter is presented with modifications to deal with high blood-to-clutter ratios (BCR). Additionally, a new method for correcting blood velocity estimates with an estimated tissue motion profile is detailed. The performance of the clutter filter and velocity estimation strategies is quantified using a new swept-scan signal model. In vivo color flow images are presented to illustrate the potential of the system for mapping blood flow in the microcirculation with external tissue motion.

A new high resolution color flow system using an eigendecomposition-based adaptive filter for clutter rejection.

We present a new signal processing strategy for high frequency color flow mapping in moving tissue environments. A new application of an eigendecomposition-based clutter rejection filter is presented with modifications to deal with high blood-to-clutter ratios (BCR). Additionally, a new method for correcting blood velocity estimates with an estimated tissue motion profile is detailed. The performance of the clutter filter and velocity estimation strategies is quantified using a new swept-scan signal model. In vivo color flow images are presented to illustrate the potential of the system for mapping blood flow in the microcirculation with external tissue motion.

Blood velocity estimation by Doppler ultrasound: Problems and issues

Modern methods of estimating blood flow have advanced considerably since Dr. Satamura and his colleagues at Osaka University reported the first measurements in 1959 using CW ultrasound. Most current methods involve the use of linear or phased arrays, which make possible 2-D color flow mapping, measurements from a single sample volume, and simultaneous B-mode imaging. However, along with this technology, troubling reports have been published (including our own) concerning the accuracy with which the velocity can be estimated and this has serious potential consequences in the quantitative assessment of vascular disease. The cause has not yet been completely identified, but appears to be partially associated with the wide range of Doppler angles within the sample volume, the complexity of the ultrasound propagation process, and the stochastic nature of the red blood cells flowing in the blood vessels. These causes will provide a focus for our review of the current status of Doppler ultrasound for vascular disease assessment and suggestions for future research. Specifically, a complete model of the entire measurement system is needed, and this includes the beamforming architecture, signal processing, possible nonlinear effects, scattering process, and nature of the 3-D vector flow field being measured.

Clutter filter design for ultrasound color flow imaging

For ultrasound color flow images with high quality, it is important to suppress the clutter signals originating from stationary and slowly moving tissue sufficiently. Without sufficient clutter rejection, low velocity blood flow cannot be measured, and estimates of higher velocities will have a large bias. The small number of samples available (8 to 16) makes clutter filtering in color flow imaging a challenging problem. In this paper, we review and analyze three classes of filters: finite impulse response (FIR), infinite impulse response (IIR), and regression filters. The quality of the filters was assessed based on the frequency response, as well as on the bias and variance of a mean blood velocity estimator using an autocorrelation technique. For FIR filters, the frequency response was improved by allowing a non-linear phase response. By estimating the mean blood flow velocity from two vectors filtered in the forward and backward direction, respectively, the standard deviation was significantly lower with a minimum phase filter than with a linear phase filter. For IIR filters applied to short signals, the transient part of the output signal is important. We analyzed zero, step, and projection initialization, and found that projection initialization gave the best filters. For regression filters, polynomial basis functions provide effective clutter suppression. The best filters from each of the three classes gave comparable bias and variance of the mean blood velocity estimates. However, polynomial regression filters and projection-initialized IIR filters had a slightly better frequency response than could be obtained with FIR filters.

On velocity estimation using speckle decorrelation.

Quantitative estimation of blood velocity using Doppler techniques is fundamentally limited because only the axial component can be detected. Speckle decorrelation resulting from scatterer motion may be used to compute non-axial components and to obtain quantitative flow information. Based on both simulations and experimental results, it is shown that the decorrelation technique is feasible only for constant flows. If flow gradients are present, the correlation between two signals along the same line of observation may be significantly affected by the gradients. Therefore, the decorrelation method cannot be used for quantitative flow estimation if flow gradients are not accurately measured and effects on signal correlation are not fully compensated. Results in this paper show that accurate estimation of flow gradients is practically difficult. It is further shown that effects of signal-to-noise ratio (SNR) on the correlation must also be taken into account for quantitative flow analysis.

Modified weighting method for forward-looking ring-annular arrays.

Side-looking Intravascular Ultrasound Systems (IVUS) systems provide high resolution cross-sectional images of vascular structure. However, blood velocity estimation is hampered by the orthogonality of the primary blood flow direction to the imaging plane, and do not image in front of the catheter. A forward-looking aperture would improve the ability to guide interventions and allow Doppler processing for improved blood speed estimates; however, catheter-based systems delivered over a guide wire limit forward-looking array geometry to annular variations. Unfortunately, the imaging characteristics of annular arrays are inferior to a full aperture disk. A modified weighting method for a practical synthetic phased, ring-annular array is presented that achieves a transmit-receive aperture and on-axis performance comparable to a full aperture disk array.

FLASH correlation: a new method for 3-D ultrasound tissue motion tracking and blood velocity estimation

We report a new method for tracking tissue motion and blood flow in three dimensions (3-D) using successive volumetric ultrasound scans. The method is based on combining the concepts of feature tracking and 3-D correlation search to achieve a compromise between accuracy and computational efficiency. This paper introduces the new method and the experimental setup used for its evaluation in a tissue-mimicking material. Results are presented demonstrating that the new method has both satisfactory tracking performance and the potential for practical real-time implementation.

Echo decorrelation from displacement gradients in elasticity and velocity estimation

Several ultrasonic techniques for the estimation of blood velocity, tissue motion and elasticity are based on the estimation of displacement through echo time-delay analysis. A common assumption is that tissue displacement is constant within a short observation time used for time delay estimation (TDE). The precision of TDE is mainly limited by noise sources corrupting the echo signals. In addition to electronic and quantization noise, a substantial source of TDE error is the decorrelation of echo signals because of displacement gradients within the observation time. The authors present a theoretical model that describes the mean changes of the crosscorrelation function as a function of observation time and displacement gradient. The gradient is assumed to be small and uniform within the observation time; the decorrelation introduced by the lateral and elevational displacement components is assumed to be small compared with the decorrelation caused by the axial component. The decorrelation model predicts that the expected value of the crosscorrelation function is a low-pass filtered version of the autocorrelation function (i.e., the crosscorrelation obtained without gradients). The filter is a function of the axial gradient and the observation time. This theoretical finding is corroborated experimentally. Limitations imposed by decorrelation in displacement estimation and potential uses of decorrelation in medical ultrasound are discussed.

A new time-domain narrowband velocity estimation technique for Doppler ultrasound flow imaging. I. Theory

A significant improvement in blood velocity estimation accuracy can be achieved by simultaneously processing both temporal and spatial information obtained from a sample volume. Use of the spatial information becomes especially important when the temporal resolution is limited. By using a two-dimensional sequence of spatially sampled Doppler signal "snapshots" an improved estimate of the Doppler correlation matrix can be formed. Processing Doppler data in this fashion addresses the range-velocity spread nature of the distributed red blood cell target, leading to a significant reduction in spectral speckle. Principal component spectral analysis of the "snapshot" correlation matrix is shown to lead to a new and robust Doppler mode frequency estimator. By processing only the dominant subspace of the Doppler correlation matrix, the Cramer-Rao bounds on the estimation error of target velocity is significantly reduced in comparison to traditional narrowband blood velocity estimation methods and achieves almost the same local accuracy as a wideband estimator. A time-domain solution is given for the velocity estimate using the root-MUSIC algorithm, which makes the new estimator attractive for real-time implementation.

3D ultrasound tissue motion tracking using correlation search.

Several methods for ultrasound tissue motion tracking and blood velocity estimation have been proposed and clinically applied. While providing valuable information to the clinician that was not obtainable from B-mode anatomical imaging, these methods still suffer from various fundamental and practical limitations that compromise their performance in certain clinical situations. A significant limitation is the inability of most of these methods to estimate the complete 3D motion or velocity vectors. With the introduction of ultrasound volumetric imaging, the need for a method that is capable of obtaining the complete motion vector is even more pressing. In this paper, we investigate the implementation of a correlation search scheme to estimate the 3D motion vectors using successive volumetric ultrasound scans. We present tracking results for motion along different axes and discuss the advantages and limitations of performing the correlation search in 3D.

A new time-domain narrowband velocity estimation technique for Doppler ultrasound flow imaging. II. Comparative performance assessment

For pt.I see ibid., vol.45, no.4, pp.939-54 (1998). The statistical performance of the new 2-D narrowband time-domain root-MUSIC blood velocity estimator described previously is evaluated using both simulated and flow phantom wideband (50% fractional bandwidth) ultrasonic data. Comparisons are made with the standard 1-D Kasai estimator and two other wideband strategies: the time domain correlator and the wideband point maximum likelihood estimator. A special case of the root-MUSIC, the "spatial" Kasai, is also considered. Simulation and flow phantom results indicate that the root-MUSIC blood velocity estimator displays a superior ability to reconstruct spatial blood velocity information under a wide range of operating conditions. The root-MUSIC mode velocity estimator can be extended to effectively remove the clutter component from the sample volume data. A bimodal velocity estimator is formed by processing the signal subspace spanned by the eigenvectors corresponding to the two largest eigenvalues of the Doppler correlation matrix. To test this scheme, in vivo common carotid flow complex Doppler data was obtained from a commercially available color flow imaging system. Velocity estimates were made using a reduced form of this data corresponding to higher frame rates. The extended root-MUSIC approach was found to produce superior results when compared to both 1- and 2-D Kasai-type estimators that used initialized clutter filters. The results obtained using simulated, flow phantom, and in vivo data suggest that increased sensitivity as well as effective clutter suppression can be achieved using the root-MUSIC technique, and this may be particularly important for wideband high frame rate imaging applications.

Echo decorrelation from displacement gradients

Several ultrasonic techniques for the estimation of blood velocity and tissue motion and elasticity are based on the estimation of displacement through echo time-delay analysis. A common assumption is that tissue displacement is constant within the observation time. The precision of time delay estimation (TDE) is mainly limited by noise sources corrupting the echo signals. In addition to electronic and quantization noise, a substantial source of TDE error is the decorrelation of echo signals as a result of displacement gradients within the observation time. We present a theoretical model that describes the mean changes of the crosscorrelation function as a function of observation time and displacement gradient. The gradient is assumed to be small and uniform; the decorrelation introduced by the lateral and elevational displacement components are assumed to be small compared to the decorrelation due to the axial component. The decorrelation model predicts that the expected value of the crosscorrelation function is a low-pass filtered version of the autocorrelation (i.e., the crosscorrelation obtained without gradients). The filter is a function of the axial gradient and the observation time. This theoretical finding is corroborated experimentally. Limitations imposed by and potential uses of decorrelation in medical ultrasound are discussed.

2-D companding for noise reduction in strain imaging

Companding is a signal preprocessing technique for improving the precision of correlation-based time delay measurements. In strain imaging, companding is applied to warp 2-D or 3-D ultrasonic echo fields to improve coherence between data acquired before and after compression. It minimizes decorrelation errors, which are the dominant source of strain image noise. The word refers to a spatially variable signal scaling that compresses and expands waveforms acquired in an ultrasonic scan plane or volume. Temporal stretching by the applied strain is a single-scale (global), 1-D companding process that has been used successfully to reduce strain noise. This paper describes a two-scale (global and local), 2-D companding technique that is based on a sum-absolute-difference (SAD) algorithm for blood velocity estimation. Several experiments are presented that demonstrate improvements in target visibility for strain imaging. The results show that, if tissue motion can be confined to the scan plane of a linear array transducer, displacement variance can be reduced two orders of magnitude using 2-D local companding relative to temporal stretching.

Where to place the Doppler sample volume in the human main pulmonary artery: evaluated from magnetic resonance phase velocity maps.

To give recommendations for the placement of Doppler sample volumes for blood flow assessment in the human main pulmonary artery. In 10 healthy volunteers MR-phase velocity measurements were obtained and computing of the mean temporal blood velocity data was performed to guide single point Doppler velocity recordings. The mean temporal blood velocity profiles were consistently skewed with the lowest blood velocities towards the inferior/right vessel wall. Blood velocity indices (ratio of point to mean velocities, where a point equals a square of 4 pixels) varied considerably with the lowest indices located towards the inferior/right vessel wall. A centrally located fictive sample volume revealed an average blood velocity index value (average of all 10 subjects) of 1.08 (range 0.99-1.25; s.d. 0.08) where the central point was defined at maximum systole, and a value of 1.13 (range 0.97-1.34; s.d. 0.11) when the central point was defined in end-diastole. The mean of multiple sample volumes along the inferior/right to superior/left diameter revealed a blood velocity index of 1.01 (range 0.87-1.21; s.d. 0.09) in systole and 1.03 (range 0.87-1.19; s.d. 0.09) in diastole. For practical clinical purposes, single point estimation of the mean blood velocity in the pulmonary artery should be performed centrally. The use of multiple sample volumes placed along the inferior/right to superior/left diameter improves the mean velocity estimate in healthy volunteers. Further studies should be conducted to reinforce the basis for Doppler velocity recording in the diseased human pulmonary artery as well as to investigate other important determinants of Doppler-derived CO, namely angle of insonation and assessment of the cross-sectional area.

Ultrasonic measurement of breast tissue motion and the implications for velocity estimation

A high-resolution study of breast tissue motion during cardiac systole and respiration is presented. An experimental system was designed to achieve a velocity resolution on the order of 1 mm/s with high spatial resolution. The peak velocity of tissue motion estimated during cardiac systole ranged from 0.2 mm/s to 5.6 mm/s among the subjects studied. It is shown that motion due to the cardiac cycle is less significant when the subject is positioned on the side rather than supine. The mean tissue velocity among subjects in the supine position is 2.88 mm/s and drops to 0.81 mm/s for the side position, with a corresponding spatial displacement of 0.095 mm, dropping to 0.027 mm. The velocity profiles indicate that 100 ms is required for the entire ribcage cintraction-relaxation process to occur. Experiments using a prone biopsy table show the almost complete elimination of tissue motion due to cardiac systole, suggesting that the use of the table eliminates this motion, thus allowing for high-resolution blood velocity estimates. Features resulting from respiratory motion are also presented. we found this motion to be of a much longer time duration, and of a much higher magnitude, with velocities as high as 29 mm/s. The implications of the study on the high-resolution estimation of blood velocity and high-resolution breast imaging are discussed.

Estimation of blood velocity with high frequency ultrasound

In order to map blood velocity in small regions near the transducer, we evaluate the performance of the wideband maximum likelihood (WMLE) strategy and infinite impulse response (IIR) filters for blood velocity estimation with a transducer center frequency of 38 MHz. Using a short transmitted pulse and the narrow lateral beam width obtained using this frequency, we show that velocities smaller than 1 mm/s can be estimated reliably. In addition, using both changes in the location and magnitude of the peak of the RF correlation, vessels as small as 40 /spl mu/m can be visualized in the RF signal and distinguished from stationary tissue. The experimental system also provides the opportunity to examine changes in flow and in the vessel wall over a cardiac cycle.