Peter J. Brands

Assessment of the distensibility of superficial arteries

Doppler signal processing cannot only be employed to detect the local blood velocity as function of time, but also to assess transcutaneously the displacement of the arterial walls during the cardiac cycle (distension waveform) and, hence, the time-dependent changes in arterial diameter relative to its initial diameter at the start of a cardiac cycle. The distension waveform normalized with respect to the local pulse pressure provides useful information about the local elasticity of the arterial wall. The displacement of the arterial wall can be obtained by processing the RF-signals within a sample volume coinciding with the arterial wall. To evaluate this method a dedicated high-speed memory system has been developed storing the RF-signal, as obtained with a conventional echo-imager in M-mode, over a number of successive sweeps covering a selected depth range. The data are transferred line after line to a personal computer (PC) and processed on the fly, thereby relieving the memory requirements of the PC. It can be concluded that a RF-signal memory in combination with a PC provides a useful tool to extract detailed diameter waveforms from the RF-signals obtained. Although the system does not process the signals in real-time the process can be considered to be on-line since the results become available within one minute after the acquisition of the data is completed.

Automated Detection of Local Artery Wall Thickness Based on M-Line Signal Processing

The Youngs modulus of an arterial segment, a measure of the elastic properties of the arterial wall, requires the simultaneous and local assessment of pulse pressure, wall thickness, diameter, and distensibility (relative increase in cross-sectional area per change in blood pressure). The diameter and relative increase in cross-sectional area can be obtained with a wall track system, processing the radiofrequency (r.f.) ultrasound signals received along a single line of observation (M-line processing). It will be demonstrated that it is feasible to combine, in a single measurement, the assessment of wall thickness and the (relative change in) diameter involving a minimum of user interaction. Phantom tests show a standard error of the estimate for intima-media thickness measurements of less than 20 microns; in vivo registrations exhibit a variation on the order of 45 microns. It is concluded that processing of the radiofrequency ultrasound signal, acquired along an M-line, provides an accurate and time-efficient alternative for videoprocessing of 2-dimensional B-mode ultrasound images to estimate artery wall thickness.

An integrated system for the non-invasive assessment of vessel wall and hemodynamic properties of large arteries by means of ultrasound

OBJECTIVES To integrate methods for non-invasive assessment of vessel wall properties (diastolic diameter, distension waveform and intima-media thickness) and hemodynamic properties (blood flow velocity and shear rate distribution) of large arteries by means of dedicated ultrasound signal processing. METHODS we have developed an arterial laboratory (ART-lab) system. ART-lab consists of software running on a standard personal computer, equipped with a data acquisition card for the acquisition of radio frequency (RF) ultrasound signals obtained with a conventional echo scanner. It operates either (1) off-line or (2) in real-time. Real-time operation is restricted to the assessment of vessel wall properties because of limitations in computational power. RESULTS This paper provides an overview of ART-lab ultrasound radio frequency data acquisition and dedicated RF-signal processing methods. The capabilities of the system are illustrated with some typical applications. CONCLUSIONS ART-lab in real-time mode is a useful tool for monitoring arterial vessel wall dynamics, while off-line it can be employed to investigate the elastic vessel wall properties in combination with hemodynamics, such as blood flow velocity and shear rate distribution.

A noninvasive method to estimate wall shear rate using ultrasound

Wall shear stress (blood viscosity x wall shear rate), imposed by the flowing blood, and blood pressure are the main mechanical forces acting on a blood vessel wall. Accurate measurement of wall shear stress is important when investigating the development of vascular disease, since both high and low wall shear stresses have been cited as factors leading to vessel wall anomalies. Furthermore, in vitro studies have shown that endothelial cells, which play a key role in the function of the underlying arterial wall, undergo a variety of structural and functional changes in response to imposed shear stress. However, there is practically no knowledge about the influence of wall shear stress on the arterial wall in vivo because of the difficulty in measuring this stress in terms of magnitude and time variation. The method presented in this article to measure the time-dependent wall shear rate in the main arteries is based on the evaluation of velocity profiles determined by means of ultrasound, using off-line signal processing. Pulsed ultrasound is well suited for this application since it is noninvasive. The processing performed in the radio-frequency (RF) domain consists of a mean frequency estimator preceded by an adaptive vessel wall filter. In a pilot study (30 measurements in the carotid artery of five healthy volunteers) we investigated the reproducibility of our method to estimate wall shear rate as compared with the reproducibility of the measurement of blood flow velocity in the middle of the vessel. The coefficient of variation was on the order of 9% for blood flow velocity estimation, and for wall shear rate estimation on the order of 5%.

Wall shear stress in the human common carotid artery as function of age and gender

OBJECTIVES It has been postulated that in the arterial system mean wall shear stress is maintained at a constant value. The present study was performed to investigate the level of wall shear stress in the common carotid artery (CCA) as function of age and possible interactions between diameter and storage capacity, defined as the absolute area change per heart beat, with mean wall shear stress. METHODS Wall shear stress (wall shear rate multiplied by whole blood viscosity) was assessed in the right CCA of 111 presumed healthy male (n = 56) and female (n = 55) volunteers, varying in age between 10 and 60 years. Wall shear rate was measured with a high resolution ultrasound system. Simultaneously, arterial diameter and storage capacity were determined. Whole blood viscosity was calculated from haematocrit, plasma viscosity and shear rate. RESULTS From the second to the sixth age decade peak wall shear stress was significantly higher in males than in females and decreased from 4.3 Pa to 2.6 Pa (r = -0.56, p < 0.001) in males and from 3.3 Pa to 2.5 Pa (r = -0.54, p < 0.001) in females. Mean wall shear stress tended to decrease from 1.5 Pa to 1.2 Pa (r = -0.26, p = 0.057) in males and decreased significantly from 1.3 Pa to 1.1 Pa (r = -0.30, p = 0.021) in females. No significant difference in mean wall shear stress was found between males and females in any age decade. The diameter of the CCA increased significantly in both males (r = 0.26, p < 0.05) and females (r = 0.40, p < 0.003). Storage capacity decreased significantly in both sexes (males: r = -0.63, p < 0.001; females: r = -0.68, p < 0.001). CONCLUSIONS These observations suggest that the reduction in mean wall shear stress with age results from the concomitant increase in diameter in an attempt of the arterial system to limit the reduction in storage capacity of the arterial system with increasing age.

A noninvasive method to estimate pulse wave velocity in arteries locally by means of ultrasound

Noninvasive evaluation of vessel wall properties in humans is hampered by the absence of methods to assess directly local distensibility, compliance, and Youngs modulus. Contemporary ultrasound methods are capable of assessing end-diastolic artery diameter, the local change in artery diameter as a function of time, and local wall thickness. However, to assess vessel wall properties of the carotid artery, for example, the pulse pressure in the brachial artery still must be used as a substitute for local pulse pressure. The assessment of local pulse wave velocity as described in the present article provides a direct estimate of local vessel wall properties (distensibility, compliance, and Youngs modulus) and, in combination with the relative change in artery cross-sectional area, an estimate of the local pulse pressure. The local pulse wave velocity is obtained by processing radio frequency ultrasound signals acquired simultaneously along two M-lines spaced at a known distance along the artery. A full derivation and mathematical description of the method to assess local pulse wave velocity, using the temporal and longitudinal gradients of the change in diameter, are presented. A performance evaluation of the method was carried out by means of experiments in an elastic tube under pulsatile pressure conditions. It is concluded that, in a phantom set-up, the assessed local pulse wave velocity provides reliable estimates for local distensibility.

Assessment of the spatial homogeneity of artery dimension parameters with high frame rate 2-D B-mode

To elicit vessel wall inhomogeneities in diameter and distension along an arterial segment, a 2-D vessel wall-tracking system based on fast B-mode has been developed. The frame rate of a 7.5-MHz linear-array transducer (length 36 mm) is enhanced by increasing the pulse-repetition frequency to 10 kHz, decreasing the number of echo lines per frame from 128 to 64, or increasing the interspacing between echo lines with a factor of two or four. Dedicated software has been developed to extract for each echo-line the end-diastolic diameter from the B-mode image and the 2-D distension waveform from the underlying radiofrequency (RF) information. The method is validated in tubes with various focal lesion sizes. Straight segments of presumably homogeneous common carotid arteries have also been tested. The temporal and spatial SD of diameter or distension reveals inhomogeneities in time or space (i.e., inhomogeneities in artery characteristics). The method can be implemented in echo systems supporting high frame rates and real-time processing of radiofrequency data.

COMPARISON OF THE PERFORMANCE OF THE RF CROSS CORRELATION AND DOPPLER AUTOCORRELATION TECHNIQUE TO ESTIMATE THE MEAN VELOCITY OF SIMULATED ULTRASOUND SIGNALS

In pulsed Doppler systems the received RF (radio frequency) signal is multiplied by a quadrature reference signal and subsequently averaged over a short depth range to obtain a sample of the complex Doppler signal. The mean frequency of the sampled Doppler signal, obtained with the autocorrelation function, reflects the mean velocity of the scatterers moving through the sample volume. An alternative is to evaluate the two-dimensional cross correlation function of a short segment of the RF signals over subsequent lines, giving the mean velocity of the scatterers. Both methods of velocity estimation were applied to computer-generated RF signals with varying RF bandwidth, signal-to-noise ratio, and mean and width of the imposed velocity distribution. The length of the RF signal segment and the number of lines for velocity estimation (package length) affects the accuracy of the velocity estimate. It can be concluded that the cross correlation technique behaves superiorly especially for a low velocity dispersion. Furthermore, the standard deviation of the velocity estimate decreases for an increasing sample volume length and package length, while the performance of the conventional Doppler technique is rather independent of the length of the sample volume. The difference between both techniques decreases for a greater package length or for signals simulating a wide velocity distribution.

Noninvasive Determination of Shear-Rate Distribution Across the Arterial Lumen

In vitro experiments have shown that the shear stress exerted by flowing blood on the endothelial surface affects the morphology of the vascular wall and the release of vasoactive substances and growth factors by that wall. It is believed that the caliber of a vessel adjusts to the local shear stress to maintain a specific value of the shear stress. The local shear stress follows from local shear rate by multiplying shear rate by the local blood viscosity. The present article describes a method in which ultrasound techniques are used to assess transcutaneously the time-dependent wall shear rate in vivo in arteries. This method is applied to the assessment of wall shear rate in the common carotid artery of volunteers, presumed to be healthy, in two age categories (young age group, 20 to 30 years old, n = 8; old age group, 60 to 70 years old, n = 6). Although the peak shear rate in the young age group is markedly higher than in the old age group, the mean shear rate averaged over a cardiac cycle has the same value of 210 s-1 for both groups, corroborating earlier observations that mean shear rate and, hence, mean shear stress are maintained at a particular value. Conversion of the observed shear rates to shear stresses, assuming a blood viscosity of 3.5 mPa.s for both age groups, gives shear stresses of approximately 0.7 Pa. This is a factor of two lower than the shear stresses estimated from the relation between volume flow and artery caliber (1.5 Pa).

A RADIO FREQUENCY DOMAIN COMPLEX CROSS-CORRELATION MODEL TO ESTIMATE BLOOD FLOW VELOCITY AND TISSUE MOTION BY MEANS OF ULTRASOUND

This article introduces a mean frequency estimator based on a radio frequency (RF) domain complex cross-correlation model (C3M). The C3M estimator differs from the real cross-correlation model (CCM) estimator in two respects; it is an unbiased estimator of blood flow velocity and/or tissue motion independent of the bandwidth of the RF ultrasound signals, and it provides an estimate of the spatial bandwidth of the RF-signal. The estimators derived from the complex cross-correlation model (mean spatial frequency, mean temporal frequency, spatial bandwidth and signal-to-noise ratio) are based on three complex cross-correlation coefficients. A full derivation and mathematical description of both estimators (C3M and CCM), starting from a Gaussian model of the complex power spectral density distribution of sampled RF signals, are presented. In addition, a thorough performance evaluation of the C3M estimator in comparison with the CCM estimator is carried out by means of simulations to document the effect of signal-to-noise ratio, bandwidth and sample frequency. In the context of the specific simulation conditions considered, the quality of the C3M estimator is shown to offer the best performance (no bias, low standard deviation of the estimate). Taking into account the computational load and the robustness of the C3M estimator, it may be concluded that the C3M estimator combines high quality and modest complexity.