Biomedical subjects

J M Willigers

Publications and source records attributed to J M Willigers.

At least 19 recordsLinked to original sources

PubMed2005

The depth of microembolic signal direction change corresponds with vessel anatomy.

BACKGROUND: Analysis of microembolic signals (MES) suggests a change of flow direction (CFD). The aim of the present study was to relate MES direction in an amplitude plot, based on the radiofrequent (RF) signal, to the vascular anatomy as seen with transcranial color-coded duplex (TCCD). METHODS: In 5 patients undergoing heart valve surgery or aortic arch replacement, preoperatively TCCD of the distal part of the internal carotid artery and the middle and anterior cerebral arteries on the right side was performed to determine potential depths of changes in flow direction. Peroperatively, a transcranial pulsed Doppler (TCD) monitoring probe was fixed over the right temporal bone. A customized RF-based system, connected to the TCD device, captured and stored the MES. Off-line, the color-coded amplitude of the clutter-filtered RF signals was plotted as a function of time (sample interval 0.17 ms) and depth (sample interval 0.05 mm). RESULTS: A total of 313 MES were recorded in 4 patients with 66 MES (21%) showing a CFD. All MES with CFD could be assigned to maximally three different depth values, six out of eight CFD depth values as seen with the RF analysis were within 1 mm from a turn in flow direction as estimated with TCCD. CONCLUSIONS: A CFD of MES occurred at a very limited number of depths and corresponded mostly with the intracranial vascular anatomy, namely a turn of the flow direction in the intracranial vessels as observed with TCCD.

Carotid Artery, Internal↗

PubMed2003

Cranial and peripheral interictal vascular changes in migraine patients.

As migraine is associated with an increased risk for ischaemic stroke and peripheral vasospastic disorders, it was hypothesized that interictal vascular changes may be present in migraine patients. Using ultrasound and applanation tonometry, the cardiovascular properties of migraine patients were compared with those of matched control subjects. Vascular parameters of the carotid arteries, cardiac output and systemic vascular resistance did not differ between both groups. Right temporal artery diameter was larger in migraine patients (mean difference 101 micro m; 95% confidence interval (CI) 9/194 micro m; P = 0.033). At the brachial artery, migraine patients displayed a smaller distension (difference -24 micro m; 95% CI -45/-4 micro m; P = 0.021) and a decreased compliance (difference -0.025 mm2/kPa; 95% CI -0.047/-0.003 mm2/kPa; P = 0.024). Thus, migraine patients display an increased peripheral arterial stiffness. The presence of these interictal vascular changes suggests that migraine might be part of a more generalized vascular disorder.

Adult↗

PubMed2002

Wall shear stress assessment in the common carotid artery of end-stage renal failure patients.

Under physiological circumstances in the common carotid artery (CCA), mean wall shear stress (WSS), defined as mean wall shear rate (WSR) times local whole blood viscosity (WBV), is maintained at approximately 1.5 Pa. In patients with end-stage renal failure (ESRF) whole blood viscosity is low and it is not unlikely that mean WSS is lower in these patients than in control subjects. Moreover, hemodialysis causes an acute increase in blood viscosity with possible effects on WSS. In this study WSS in the CCA was determined with the Shear Rate Estimating System, an apparatus based on ultrasound, in ESRF patients (n = 13) and in presumed healthy age- and sex-matched control subjects (n = 13). Prior to hemodialysis, mean WSS (0.67 +/- 0.23 Pa) was significantly lower (p < 0.05) in patients with ESRF, due to both a lower WBV (2.80 +/- 0.52 mPa.s) and mean WSR (271 +/- 109 s(-1)), than in the control subjects (mean WSS: 1.24 +/- 0.20 Pa; WBV: 3.20 +/- 0.29 mPa.s; WSR: 387 +/- 51 s(-1)). Hemodialysis induced an increase in WBV (up to 3.71 +/- 1.54 mPa.s, p < 0.01), but mean WSS did not change significantly due to a reciprocal decrease in mean wall shear rate. These findings demonstrate that WSS is lower in hemodialysis patients than in control subjects, and that mean WSS is maintained at this low level despite an acute change in blood viscosity.

Adult↗

PubMed2001

High-resolution functional imaging with ultrasound contrast agents based on RF processing in an in vivo kidney experiment.

Knowledge of the relative tissue perfusion distribution is valuable in the diagnosis of numerous diseases. Techniques for the assessment of the relative perfusion distribution, based on ultrasound (US) contrast agents, have several advantages compared to established nuclear techniques. These are, among others, a better spatial and temporal resolution, the lack of exposure of the patient to ionizing radiation and the relatively low cost. In the present study, US radiofrequency (RF) image sequences are acquired, containing the signal intensity changes associated with the transit of a bolus contrast agent through the microvasculature of a dog kidney. The primary objective is to explore the feasibility of calculating functional images with high spatial resolution. The functional images characterize the transit of the contrast agent bolus and represent distributions of peak time, peak value, transit time, peak area, wash-in rate and wash-out decay constant. For the evaluation of the method, dog experiments were performed under optimized conditions where motion artefacts were minimized and an IA injection of the contrast agent Levovist was employed. It was demonstrated that processing of RF signals obtained with a 3.5-MHz echo system can provide functional images with a high spatial resolution of 2 mm in axial resolution, 2 to 5 mm in lateral resolution and a slice thickness of 2 mm. The functional images expose several known aspects of kidney perfusion, like perfusion heterogeneity of the kidney cortex and a different peripheral cortical perfusion compared to the inner cortex. Based on the findings of the present study, and given the results of complimentary studies, it is likely that the functional images reflect the relative perfusion distribution of the kidney.

Animals↗

PubMed2001

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.

Adult↗

PubMed2000

Vascular effects of 5-HT1B/1D-receptor agonists in patients with migraine headaches.

OBJECTIVES: Second-generation triptans are believed to have fewer cardiovascular effects than sumatriptan. This was investigated in vivo by comparing the vascular effects of equipotent therapeutic dosages of selective 5-HT1B/1D-receptor agonists. METHODS: Sixteen patients with migraine headaches completed a double-blind, placebo-controlled, four-way crossover study. With ultrasonography and applanation tonometry used 1.5 hours after the oral intake of sumatriptan (50 mg), rizatriptan (10 mg), zolmitriptan (2.5 mg), or placebo arterial vessel wall properties, blood flow and pressure waveforms were measured in common carotid, brachial, and temporal arteries. At the brachial artery, flow-induced vasodilation (an endothelium-dependent process) was evaluated, and blood pressures were recorded. RESULTS: Mean arterial pressure, 91 +/- 2 mm Hg after placebo, increased (P < .05) by 4% to 6% after administration of each triptan. Each active treatment decreased (P < .001) both brachial and carotid artery diameter. Isobaric compliance of the brachial artery, 0.077 +/- 0.010 mm2/kPa after placebo, decreased (P < .01) by 11% +/- 8%, 11% +/- 11%, and 23% +/- 7% after administration of sumatriptan, rizatriptan, and zolmitriptan, respectively. Isobaric compliance of the carotid artery was 1.31 +/- 0.10 mm2/kPa after placebo (no change). Zolmitriptan was the only triptan that decreased temporal artery diameter significantly (by 12% +/- 3%, P < .001). The resistance of the temporal artery vascular bed increased after administration of sumatriptan (by 26% +/- 11%, P < .05) and zolmitriptan (by 40% +/- 9%, P = .001). Flow-induced vasodilation was unaffected. CONCLUSIONS: Selective 5-HT1B/1D-receptor agonists induce vasoconstriction and decrease compliance of conduit arteries. These effects are more pronounced at muscular (temporal, brachial) compared with elastic (carotid) arteries. Resistance is only increased at the temporal artery vascular bed, suggesting cranioselectivity for resistance vessels. Endothelial function is not differently affected by any of the triptans tested.

Adult↗

PubMed2000

Experimental investigation of the pulse inversion technique for imaging ultrasound contrast agents.

The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.

Acoustics↗

PubMed1999

Evaluation of off-line automated intima-media thickness detection of the common carotid artery based on M-line signal processing.

Intima-media thickness (IMT) measurements have gained increasing attention, because IMT is assumed to represent the endothelial adaptive response to physiological and pathophysiological processes. The main aim of the present study was to assess the intrasubject intrasession variability of a new off-line automated radio frequency (RF) IMT method in comparison with an already established off-line manual B-mode IMT method. IMT also was assessed by means of an on-line manual B-mode and an on-line manual RF IMT method. We investigated posterior wall IMT 0-1 cm proximal to the bulb in both common carotid arteries of 16 young (20-31 y; mean 25 y) female and male and 13 elderly (51-65 y; mean 56 y) female volunteers. Two commercially available ultrasound devices (Pie Medical Scanner 200 and Ultramark 9) were used to assess the effects of signal processing on the off-line automated RF IMT method. Intrasubject intrasession variability was determined using the standard deviation to evaluate and compare the various methods. Spearman rank correlation coefficients and Bland and Altman bias and limits of agreement were calculated to objectivate the comparability between the various methods. Intrasubject intrasession variation of IMT estimates was not statistically different between any of the methods. We observed a good comparability between the commonly used off-line manual B-mode IMT method and the off-line automated RF IMT method at the level of the common carotid artery.

Adult↗

PubMed1999

Angle-independent motion measurement by correlation of ultrasound signals assessed with a single circular-shaped transducer.

In medicine, pulsed ultrasound is a widespread noninvasive technique that measures motion in the direction of the ultrasound beam, i.e., axial motion. The magnitude of the actual motion can be determined only if the angle between the ultrasound beam and the direction of motion (transducer-to-motion angle) is known. For blood flow measurements, current pulsed ultrasound systems assume this angle to be equal to the angle between the ultrasound beam and the longitudinal direction of the vessel, as can be estimated from a two-dimensional brightness-mode (B-mode) image that is obtained prior to the blood flow measurement. For tissue motion measurements, current pulsed ultrasound systems are mostly unable to determine the transducer-to-motion angle. Recently, a model has been derived for the correlation of(analytic) radiofrequency (rf) signals, assessed with a circular-shaped ultrasound transducer along the same line of observation. In the present paper, this model is used to derive estimators, requiring only the calculation of a few correlation coefficients, for the motion components (axial, lateral and actual) and for some of the signal parameters (center frequency, bandwidth and signal-to-noise ratio) of the assessed rf signals. The transducer-to-motion angle can be derived from the estimated motion components. For the evaluation of the estimators, rf signals were acquired with a motion-controlled experimental arrangement. The results of the evaluation study show that the transducer-to-motion angle can be estimated with a mean standard deviation of less than 2 degrees.

Algorithms↗

PubMed1999

Non-invasive measurement of mechanical properties of arteries in health and disease.

Major conduit arteries should, by their elastic nature, be able to store blood volume temporarily during systole and release it during diastole. This reduces the systolic blood pressure required for the flow of a given volume quantity and gradually suppresses the pulsatile flow pattern. The haemodynamic characteristics of arteries have consequences for the load of the heart but also for the mechanical load of the arterial wall. The repetitive stretching of the wall (strains of up to 10 per cent) may cause fragmentation of the elastic fibres in the wall, modifying wall elasticity. To maintain wall stress the elastic arteries respond with a diameter increase in combination with an increase of arterial wall thickness. A larger diameter for a smaller distension (change in artery diameter from diastole to systole) will restrict the reduction in storage capacity. Alternatively, pulse pressure may go up increasing the mechanical load on the wall. In recent years various methods have been developed to assess and monitor the above interaction. Most of these methods are based on ultrasound techniques because of its wide availability and its non-invasive and non-traumatic nature. Presently these techniques enable the assessment of wall thickness, diastolic diameter, distension waveform, i.e., the tie-dependent change in diameter, the relative pulsatile increase in diameter, and pulse wave velocity, for elastic and muscular arteries in humans but also in small animals such as rats and mice. The present paper discusses the techniques in more detail.

Arteries↗

PubMed1998

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.

Adolescent↗

PubMed1998

Experimental verification of the correlation behavior of analytic ultrasound radiofrequency signals received from moving structures.

Conventional pulsed ultrasound systems are only able to detect motion along the ultrasound beam (i.e., axial motion). If the angle between the actual motion direction and the ultrasound beam is known, then the magnitude of the actual motion can be derived. This technique can be applied for laminar blood-flow measurements in straight vessels, but for tissue motion it is inadequate because the local tissue motion direction is unknown and may be position-dependent. Assessment of both the axial motion and the lateral motion (i.e., in the direction perpendicular to the ultrasound beam) makes angle-independent assessment of the magnitude of the actual motion feasible. Information about the axial and lateral motion is available in a set of radiofrequency (RF) signals obtained along the same line of observation (M-mode). The experiments described in the present paper show that axial and lateral motion can be estimated from the shape of the envelope of the 2-D (spatial and temporal) correlation function of analytic M-mode RF signals. Furthermore, it is demonstrated that the shape is also affected by the Band width of the received RF signals, signal-to-noise ratio, and local amplitude and phase characteristics of the ultrasound beam.

Humans↗

PubMed1998

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 Young's 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 Young's 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.

Arteries↗

PubMed1998

Modeling of the correlation of analytic ultrasound radiofrequency signals for angle-independent motion detection.

Conventional pulsed ultrasound systems are able to assess motion of scatterers in the direction of the ultrasound beam, i.e., axial motion, by determining the lag at which the maximum correlation occurs between consecutively-received radiofrequency (rf) signals. The accuracy, resolution, and processing time of this technique is improved by making use of a model for the correlation of rf signals. All previously-described correlation models only include axial motion, but it is common knowledge that lateral motion, i.e., motion in the plane perpendicular to the beam axis, reduces the correlation of rf signals in time. In the present paper, a model for the correlation of analytic rf signals in depth and time is derived and verified. It also includes, aside of some signal and transducer parameters, both axial and lateral motion. The influence of lateral motion on the correlation of (analytic) rf signals is strongly related to local phase and amplitude characteristics of the ultrasound beam. It is shown how the correlation model, making use of an ultrasound transducer with a circular beam shape, can be applied to estimate, independent of angle, the magnitude of the actual motion. Furthermore, it is shown that the model can be applied to estimate the local signal-to-noise ratio and rf bandwidth.

Blood Flow Velocity↗

PubMed1997

Reproducibility of shear rate and shear stress assessment by means of ultrasound in the common carotid artery of young human males and females.

In the present study, the reliability of an ultrasonic shear rate estimating system, in terms of intrasubject intrasession, intersubject intrasession and intersubject intersession variability coefficients for the assessment of wall shear rate (WSR) in the common carotid artery (CCA) was determined in eight presumed healthy volunteers. Measurements were performed on consecutive days (day 1, day 2 and day 7). To investigate whether there were differences in WSR due to gender, dynamic WSR in the CCA was assessed in 11 presumed healthy males (mean age 24 y) and 11 presumed healthy females (mean age 25 y). Wall shear stress (WSS) was estimated from WSR and calculated whole blood viscosity. The average intrasubject intrasession variability was about 15% for peak WSR and about 12% for mean WSR. The intersubject intrasession variability for peak WSR decreased from 19% on day 1 to 16% on day 7 and for mean WSR from 17% on day 1 to 11% on day 7. The intersubject intersession variability is on the order of 5% for peak WSR and about 4% for mean WSR. No significant differences could be detected between peak and mean WSR values on day 1, day 2 and day 7, indicating good short- and medium-term intersubject intersession reproducibilities. No differences in peak and mean WSR were found between the left and the right CCA in the male group as well as in the female group. Mean WSS was similar in males (1.3 +/- 0.3 Pa) and in females (1.2 +/- 0.2 Pa), but peak WSS was slightly, but significantly, higher in males (4.3 +/- 1.3 Pa) than in females (3.3 +/- 0.7 Pa). It can be concluded that peak and mean WSR can be reliably determined noninvasively using ultrasound.

Adult↗

PubMed1997

Automated detection of local artery wall thickness based on M-line signal processing.

The Young's 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.

Adult↗

PubMed1994

Short-term variation in the elastic properties of a muscular artery in humans.

1. Information on the interaction between mechanical strain and the elastic properties of large arteries could give insight to the origin of haemodynamic vascular injury, possibly resulting in formation of atherosclerotic plaques. Intra-individual variation in the elastic properties of muscular arteries measured using non-invasive techniques has been observed. This variation could be due to changes in blood pressure, since the elastic properties of muscular arteries are strongly non-linear. 2. The objective of the present study was to evaluate variability in the elastic properties of a muscular artery over a time period of up to 1 h. Furthermore, the relation between changes in elastic properties and alterations in blood pressure was studied. With a non-invasive high-resolution ultrasound technique, the diameter and distension of the femoral artery were assessed by repeated measurements in 10 healthy subjects (mean age 27 +/- 2.9 years) under resting conditions over 40-60 min. Intra-arterial blood pressure of the contralateral common femoral artery and heart rate were recorded simultaneously. From the distension and pulse pressure, the distension/pressure ratio was calculated, a measure of the compliance. For each parameter the variation coefficient was computed. 3. In all subjects a considerable variation was found in distension (mean variation coefficient 14.8%) but not in pulse pressure (mean variation coefficient 3.7%). The expected positive correlation between pulse pressure and distension was absent in eight subjects. As a consequence, large intra-individual variation was found in the distension/pressure ratio (mean variation coefficient 15.3%).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMedDate not supplied

Microembolic signal description: a reappraisal based on a customized digital postprocessing system.

The high variability in presence and signature of microembolic signals (MES), detected with transcranial Doppler sonography (TCD) in the middle cerebral artery (MCA), cannot be explained with the currently available published data. We applied customized postprocessing on the radiofrequency (RF) signal of a standard TCD system. The spatial resolution was on the order of 2 mm, depending only on the length of the ultrasound (US) burst emitted. The amplitude of clutter-filtered RF signals was color-coded and plotted as a function of time and depth (range 30 mm). Additionally, 128 point fast Fourier transforms (FFTs) (50% temporal overlap) were calculated, visualizing both the background Doppler spectrum and the MES. We evaluated 122 gaseous MES from two patients during cardiac surgery and 52 particulate MES from four patients after carotid endarterectomy. Both MES categories showed comparable properties: they appeared in the RF amplitude plot as rather straight lines of increased intensity, indicating that the velocity remained approximately the same while they passed the US beam. The velocity calculated from the amplitude plot never exceeded that of the background Doppler spectrum. Various "MES patterns" could be identified with respect to the depth range at which the MES were visible. A quarter of the gaseous MES changed their direction at a specific depth, suggesting that the MES entered a branch (e.g., an M2 artery or the anterior cerebral artery). In the FFT analysis, these MES contained both positive and negative frequencies. It is concluded that MES show consistent signature patterns in the amplitude-time plots and that the previously reported variability of MES appearance in conventional Doppler systems is an artefact caused by relatively large signal amplitudes and sample volumes.

Artifacts↗