L. Francalanci

An Automatic Angle Tracking Procedure for Feasible Vector Doppler Blood Velocity Measurements

Two-dimensional angle-independent blood velocity estimates typically combine the Doppler frequencies independently measured by two ultrasound beams with known interbeam angle. A different dual-beam approach was recently introduced in which one (reference) beam is used to identify the flow direction, and the second (measuring) beam directly estimates the true flow velocity at known beam-flow angle. In this paper, we present a procedure to automatically steer the two beams along optimal orientations so that the velocity magnitude can be measured. The operator only takes care of locating the Doppler sample volume in the region of interest and, through the extraction of appropriate parameters from the Doppler spectrum, the reference beam is automatically steered toward right orientation to the flow. The velocity magnitude is thus estimated by the measuring beam, which is automatically oriented with respect to the (known) flow direction at a suitable Doppler angle. The implementation of the new angle tracking method in the ULtrasound Advanced Open Platform (ULA-OP), connected to a linear array transducer, is reported. A series of experiments shows that the proposed method rapidly locks the flow direction and measures the velocity magnitude with low variability for a large range of initial probe orientations. In vitro tests conducted in both steady and pulsatile flow conditions produced coefficients of variability (CV) below 2.3% and 8.3%, respectively. The peak systolic velocities have also been measured in the common carotid arteries of 13 volunteers, with mean CV of 7%.

ACCURACY AND REPRODUCIBILITY OF A NOVEL DUAL-BEAM VECTOR DOPPLER METHOD

Conventional Doppler ultrasound (US) investigations are limited to detect only the axial component of the blood velocity vector. A novel dual-beam method has been recently proposed in which the Doppler angle is estimated through a reference US beam, and the velocity magnitude through a measuring US beam, respectively. In this study, the performance of such a method has been assessed quantitatively through in vitro and in vivo measurements made in different experimental conditions. In vitro, more than 300 acquisitions were completed using seven transducers to insonify a straight tube phantom at different Doppler angles. In steady laminar flow conditions, the velocity magnitude was measured with mean error of -1.9% (95% confidence interval: -2.33% to -1.47%) and standard deviation of 3.4%, with respect to a reference velocity. In pulsatile flow conditions, reproducibility tests of the entire velocity waveforms provided an average coefficient of variation (CV) of 6.9%. For peak velocity measurements made at five Doppler angles and three flow rates, the intrasession and intersession CVs were in the range 0.8-3.7% and 2.9-10.6%, respectively. The peak systolic velocities (PSVs) in the common carotid arteries of 21 volunteers were estimated with 95% limits of agreement of +/- 9.6 cm/s (intersession). This analysis shows that the proposed dual-beam method is capable of overcoming the Doppler angle ambiguity by producing reliable velocity measurements over a large set of experimental conditions.

Simultaneous Ultrasound Assessment of Brachial Artery Shear Stimulus and Flow-Mediated Dilation During Reactive Hyperemia

In flow-mediated dilation (FMD) studies, brachial artery diameter changes due to reactive hyperaemia are typically measured through manual or automatic analysis of high resolution B-mode images while the stimulus of diameter change, i.e., the flow change, is qualitatively estimated by measuring the mean velocity in the vessel and assuming a parabolic velocity profile. This article describes an experimental approach to simultaneously measure the wall shear rate (WSR) and the diameter variations, through multigate spectral Doppler and B-mode image processing, respectively. By using an ultrasound advanced open platform (ULA-OP), experimental results from the brachial arteries of 15 presumed healthy volunteers have been obtained. The mean increments during reflow against baseline were 105% ± 22% for the peak WSR and 8% ± 3% for the FMD. The mean time interval between the WSR peak and the beginning of plateau of diameter waveform was 38 ± 8 s. The results confirm that in young healthy subjects the postischemic vasodilation of brachial artery is largely correlated to the WSR increase.

Volumetric blood flow assessment through multigate spectral Doppler

Volumetric blood flow measurements require the assessment of both the velocity distribution and the cross-sectional area of the vessel. Unfortunately several difficulties hamper such a measurement in standard approaches. A new dual beam method has been recently introduced in which the Doppler spectral symmetry is exploited to finely orientating the first beam at 90° with respect to the flow, so that the remaining beam can perform a velocity measurement with known Doppler angle. In this paper it is shown how this special beam configuration proves particularly suitable for volume flow measurements. While the transverse beam performs a diameter measurement with optimal orientation, the velocity distribution along the second beam is measured through multigate spectral analysis. The method has been coded on a research system. In-vitro experiments produced a bias less then 7% with a coefficient of variability (CV) less then 5%. In preliminary tests on 3 volunteers CVs within 10% were obtained.

Simultaneous measurement of wall shear stress and arterial distension in FMD studies

In Flow-mediated dilation (FMD) studies, blood flow in the brachial artery is restricted for about 5 minutes by a cuff. When the restriction is removed, the subsequent increase in wall shear rate (WSR) stimulates the release of nitric oxide, a vasodilator, from the endothelial cells into the smooth muscle. Impaired FMD responses, which are considered independent predictors of possible cardiovascular events, should be reliably detected. While several efforts have been so far dedicated to measure the diameter change, the source of this change, i.e., the WSR increase, has not been measured so far. In this paper, first results of the simultaneous measurement of WSR and diameter performed during FMD studies in the brachial arteries of 15 volunteers are reported. All measurements were obtained through the ULtrasound Advanced Open Platform (ULA-OP).

Automatic tracking dual-beam method for vector Doppler applications

2-D angle independent blood velocity estimates are typically achieved by combining the Doppler frequency measurements obtained from two US beams with known interbeam angle. A recently introduced dual-beam vector Doppler method employs one (reference) beam to identify the flow direction, and the other to measure the velocity magnitude. The flow direction is found when the reference beam is set at 90deg to the flow, i.e. the corresponding Doppler spectrum is symmetrical around zero frequency. In this paper, we present an approach to automatically track the desired transverse angle. The new method has been implemented in the ULtrasound Advanced Open Platform (ULA-OP), which is connected to a linear array transducer. It is shown that, whichever is the initial beam-flow angle, the extraction of suitable parameters from the Doppler spectrum allows automatically pushing the steering towards the desired transverse orientation. The operator thus takes care only of locating the Doppler sample volume in a suitable position, as in standard duplex measurements, and the local velocity magnitude is automatically estimated. In vitro experiments conducted in both steady and pulsatile flow conditions are reported, showing that the velocity magnitude is accurately measured whichever the initial Doppler angle is over a large range. Successful preliminary application of the angle tracking procedure on the common carotid arteries of 11 volunteers is also discussed.

Combined B-Mode and Multigate Spectral Doppler-Mode Imaging for Flow-Mediated Dilation Investigation

Flow-mediated dilation (FMD) is an established non-invasive method to assess the endothelial function by ultrasound. Blood flow in the brachial artery is restricted by a cuff for about 5 min: during the reactive hyperemia following occlusion release, the consequent increase in wall shear stress stimulates the endothelial cells to release nitric oxide, a powerful vasodilator that causes relaxation of tunica media smooth muscle. By measuring the arterial diameter change induced by reactive hyperemia, a possible endothelial dysfunction can be detected. The traditional approach consists in the evaluation of arterial diameter changes, while the shear stress increase (i.e. the stimulus for dilation) has not been directly estimated so far. This paper describes an approach to simultaneously measure the wall shear rate (WSR), i.e. the blood velocity gradient near the walls, and the associated diameter changes. The WSR is measured through multigate spectral Doppler (MSD) analysis while B-Mode images are processed to estimate the instantaneous diameter. This approach was implemented in the ULtrasound Advanced Open Platform (ULA-OP), which can be programmed to arbitrarily interleave B- and PW Doppler- Modes. The method implementation and the results of a clinical validation over 15 healthy volunteers are reported.

Vector Doppler Method Based on an Automatic Transverse Angle Tracking Procedure

Traditional Doppler methods only measure the axial component of the velocity vector. The lack of information on the beam-to-flow (Doppler) angle creates an ambiguity which can lead to large errors in velocity magnitude estimates. An original approach was recently introduced, in which two ultrasound beams with known relative orientation are directed towards the same vessel, one being committed to perform a Doppler measurement, while the second beam has the specific task of detecting the beam-to-flow angle. In this paper, an angle-tracking procedure allowing the Doppler angle to be automatically determined with high accuracy is presented. The procedure is based on the real-time estimation of suitable Doppler spectrum parameters obtained from an M-line associated to a sub-aperture of a linear array probe. Such parameters are used to steer the M-line towards a direction corresponding to a desired beam-flow angle. Knowledge of this angle is finally exploited to obtain the velocity magnitude through the classic Doppler equation related to the second beam. The implementation of the method on a new ultrasound machine and its validation through in vitro and in vivo tests are reported.

An Automatic Angle Tracking Procedure for Repeatable Blood Velocity Measurements

One of the major limitations of Doppler systems consists in the difficulty of estimating the blood flow direction with respect to the ultrasound (US) beam, as needed to convert the measured Doppler frequency to velocity. The problem can be solved through an original dual-beam method, in which one of the beams is dedicated to estimate the flow direction. This goal is achieved by recognizing the beam orientation which produces symmetrical spectra, as happens uniquely at transverse Doppler angles. Although the technique has been thoroughly validated in vitro, for its practical in vivo application the needed transverse beam-flow angle should be automatically achieved.

P6B-2 Quantitative Assessment of Flow Velocity through Transverse Dual-Beam Analysis

Doppler ultrasound investigations are typically limited to detect only the axial blood velocity component, as they cannot directly estimate the flow direction. To overcome this limitation, we have recently proposed an original dual-beam method characterized by the transverse orientation of one of the two beams to the flow. In this paper, the performance of such a technique is tested through over 300 in vitro experiments made by using a set of 6 different transducers. In steady flow conditions, the velocity measurement errors featured a Gaussian distribution with mean of -1.35% (CI 95%: 0.89% to 1.82%) and 3.7% standard deviation. In pulsatile flow conditions, the coefficient of variation was evaluated below 3.7% (intra-session) and 8.1% (inter-session).