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Biomedical subjects

A F van der Steen

Publications and source records attributed to A F van der Steen.

At least 19 recordsLinked to original sources
PubMed2001

Decorrelation characteristics of transverse blood flow along an intravascular array catheter: effects of aggregation of red blood cells.

A method to measure transverse blood flow, based on the correlation between consecutive radiofrequency (RF) signals, has been introduced. This method was validated for an intravascular (IVUS) rotating single element catheter. Currently, we are implementing the method for an IVUS array transducer catheter. The decorrelation characteristics during transverse blood flow using the IVUS array catheter were investigated using computer modeling. Before this, blood was simulated as a collection of randomly located point scatterers and, by moving this scattering medium transversely across the acoustical beam, blood flow was simulated. This paper presents a more realistic scattering media by simulating aggregates of red blood cells (RBCs) as strings of point scatterers. Three configurations of aggregates of RBCs were simulated. First, aggregates of RBCs were strings with different lengths and parallel to the catheter axis. Second, the strings were with a fixed length and angles of plus or minus 45 degrees with respect to the catheter axis. Third, the strings were with different lengths and random angles ranging from -45 degrees to + 45 degrees. The decorrelation characteristics for these configurations of aggregates of RBCs were investigated and compared with point scatterers. For the aggregates of RBCs parallel to the catheter axis, the decorrelation rate became slower when the aggregate length was increased. RBC aggregations with fixed and random lengths and angles resulted in a decorrelation rate that approaches the decorrelation pattern from point scatterers. Results suggests that the presence of aggregates of RBCs will probably not affect the measurements of transverse blood flow using a decorrelation-based method and an IVUS array catheter.

Blood Flow Velocity↗
PubMed2001

Advancing intravascular ultrasonic palpation toward clinical applications.

This paper describes the first reported attempt to develop a real-time intravascular ultrasonic palpation system. We also report on our first experience in the catherization laboratory with this new elastographic imaging technique. The prototype system was based on commercially available intravascular ultrasound (US) scanner that was equipped with a 20-MHz array catheter. Digital beam-formed radiofrequency (RF) echo data (i.e., 12 bits, 100 Hz) was captured at full frame rate from the scanner and transferred to personal computer (PC) memory using a fast data-acquisition system. Composite palpograms were created by applying a one-dimensional (1-D) echo tracking technique in combination with global motion compensation and multiframe averaging to several pairs of RF echo frames that were obtained in the diastolic phase of the cardiac cycle. The quality of palpograms was assessed by conducting experiments on vessel phantoms and on patients. The results demonstrated that robust and consistent palpograms could be generated in almost real-time using the proposed system. Good correlation was observed between low strain values and regions of calcification as identified from the intravascular US (IVUS) sonograms. Although the clinical results are clearly preliminary, it was concluded that the prototype system performed sufficiently well to warrant further and more in-depth clinical investigation.

Calcium↗
PubMed2000

Characterization of plaque components with intravascular ultrasound elastography in human femoral and coronary arteries in vitro.

BACKGROUND: The composition of plaque is a major determinant of coronary-related clinical syndromes. Intravascular ultrasound (IVUS) elastography has proven to be a technique capable of reflecting the mechanical properties of phantom material and the femoral arterial wall. The aim of this study was to investigate the capability of intravascular elastography to characterize different plaque components. METHODS AND RESULTS: Diseased human femoral (n=9) and coronary (n=4) arteries were studied in vitro. At each location (n=45), 2 IVUS images were acquired at different intraluminal pressures (80 and 100 mm Hg). With the use of cross-correlation analysis on the high-frequency (radiofrequency) ultrasound signal, the local strain in the tissue was determined. The strain was color-coded and plotted as an additional image to the IVUS echogram. The visualized segments were stained on the presence of collagen, smooth muscle cells, and macrophages. Matching of elastographic data and histology were performed with the use of the IVUS echogram. The cross sections were segmented in regions (n=125) that were based on the strain value on the elastogram. The dominant plaque types in these regions (fibrous, fibro-fatty, or fatty) were obtained from histology and correlated with the average strain and echo intensity. The strain for the 3 plaque types as determined by histology differed significantly (P=0.0002). This difference was mainly evident between fibrous and fatty tissue (P=0.0004). The plaque types did not reveal echo-intensity differences in the IVUS echogram (P=0.882). CONCLUSIONS: Different strain values are found between fibrous, fibro-fatty, and fatty plaque components, indicating the potential of intravascular elastography to distinguish different plaque morphologies.

Arteries↗
PubMed2000

Vascular tissue characterisation with IVUS elastography.

Knowledge about the mechanical properties of the vessel wall and plaque is important for guiding intravascular interventional procedures and detection of plaque vulnerability. Rupture of atherosclerotic plaques is associated with acute myocardial infarction and unstable angina pectoris. In a plaque with a lipid core, the stress due to the arterial pulsation will be concentrated in the cap and a thin cap may be unable to bear this stress. In this study, the potential of intravascular elastography to characterise fibrous, fibro-fatty and fatty tissue based on their mechanical properties was investigated. Using a custom-made set-up, intravascular echograms and elastograms of excised human femoral arteries were determined. High frequency r.f. data (30 MHz) were acquired using an intravascular catheter. The tissue was compressed using intravascular pressures of 80 and 100 mmHg. The cross-sections of interest were marked with a needle for matching with histology. Using cross-correlation estimation of gated echosignals, elastograms (images of the local strain) were determined. After the intravascular experiments, the specimens were fixed in formaldehyde and processed for paraffin embedding. Sections were stained with picrosirius red and alpha-actin to counterstain collagen and smooth muscle cells (SMC), respectively. Results of vessel cross-sections with fibrous and fatty plaque regions will be presented. The elastograms of these specimens show that the strain in fatty tissue is higher than the strain in fibrous material. In conclusion, these in vitro experiments on human femoral arteries indicate the potential of intravascular elastography to characterise different plaque components.

Arteriosclerosis↗
PubMed2000

Angle matching in intravascular elastography.

Intravascular elastography is a new technique to obtain mechanical properties of the vessel wall and plaque. Mechanical information of vascular tissue is important for characterisation of different plaque components, detection of plaque vulnerability and thus choosing the proper interventional technique. The feasibility of the technique is investigated using phantoms and diseased human arteries. These studies demonstrated that elastography reveals information that is unavailable or inconclusive from the echogram alone. The technique is based on the principle that tissue strain is directly related to its mechanical properties. In intravascular elastography, the tissue is compressed using different intravascular pressures. The strain is determined using cross-correlation techniques of the radio frequency (r.f.) signals. Reliable strain estimates are only obtained when signals of corresponding tissue are correlated. Owing to catheter motion, off-centre position and non-uniform rotation of the intravascular transducer, the r.f. traces at low and at high pressure may be misaligned. Four algorithms are tested to track the corresponding ultrasound signals. Three methods (l1norm, l2norm and cross-correlation) are applied on the r.f. signal and one (l1norm) on the envelope (speckle tracking). Simulations are performed to obtain a data set with a priori knowledge of the scattering particles positions in the tissue at high and low pressure. Different positions of the catheter in the lumen, compression levels of the material and signal-to-noise ratios (SNRs) are investigated. Finally, these findings are corroborated with a phantom experiment in a water tank. From the simulations, it can be concluded that the speckle tracking algorithm has the best performance, under all circumstances. The performance decreases with larger eccentricity of the catheter and larger compression of the material. The SNR is only of minor influence. The speckle tracking algorithm has also the best performance in the phantom experiment. The performance of the speckle tracking algorithm is better than the three r.f.-based algorithms. For intravascular elastography, implementation of this method may improve the quality of the elastogram.

Algorithms↗
PubMed2000

Flow estimation using an intravascular imaging catheter.

Coronary flow assessment can be useful for determining the hemodynamic severity of a stenosis and to evaluate the outcome of interventional therapy. We developed a method for measuring the transverse flow through the imaging plane of an intravascular ultrasound (IVUS) catheter. This possibility has raised great clinical interest since it permits simultaneous assessment of vessel geometry and function with the same device. Furthermore, it should give more accurate information than combination devices because lumen diameter and velocity are determined at the same location. Flow velocity is estimated based on decorrelation estimation from sequences of radiofrequency (RF) traces acquired at nearly the same position. Signal gating yields a local estimate of the velocity. Integrating the local velocity over the lumen gives the quantitative flow. This principle has been calibrated and tested through computer modeling, in vitro experiments using a flow phantom and in vivo experiments in a porcine animal model, and validated against a Doppler element containing guide wire (Flowire) in humans. Originally the method was developed and tested for a rotating single element device. Currently the method is being developed for an array system. The great advantage of an array over the single element approach would be that the transducer has no intrinsic motion. This intrinsic motion sets a minimal threshold in the detectable velocity components. Although the principle is the same, the method needs some adaptation through the inherent different beamforming of the transducer. In this paper various aspects of the development of IVUS flow are reviewed.

Animals↗
PubMed2000

A fast rotating scanning unit for real-time three-dimensional echo data acquisition.

Most three-dimensional (3-D) echocardiography (3-DE) systems today are based on off-line methods where a large number of cross-sectional 2-D scans have to be acquired sequentially before a 3-D image can be reconstructed. Because acquisition is done step-by-step based on ECG triggering plus respiratory gating, this introduces motion artefacts and takes significant acquisition time. Another 3-D approach is based on 2-D transducers and parallel beam-forming. Such a system is very complex. In this manuscript, a fast continuously-rotating scanning unit, based on a 64-element phased-array transducer, is described. Typical rotation speed of the 3-D unit is 8 rotations per s. Therefore, 16 3-D volume datasets can be acquired per s in real-time. The first clinical examples as acquired with this probe are presented.

Artifacts↗
PubMed2000

Intraluminal ultrasonic palpation: assessment of local and cross-sectional tissue stiffness.

Many intravascular therapeutic techniques for the treatment of significant atherosclerotic lesions are mechanical in nature: examples are angioplasty, stenting and atherectomy. The selection of the most adequate treatment would be advantageously aided by knowledge of the mechanical properties of the lesion and surrounding tissues. Based on the success of intravascular ultrasound (IVUS) in accurately depicting the morphology of atheromatous lesions, ultrasonic tissue characterisation has been proposed as a tool to determine the composition of atheroma. We describe the addition of local compliance information to the IVUS image in the form of a colour-coded line congruent with the lumen perimeter. The technique involves analysis of echo signals obtained at two or more states of incremental intravascular pressure. Using vessel phantoms and specimens, we demonstrate the utility of intravascular compliance imaging. The palpograms are able to identify lesions of different elasticity independently of the echogenicity contrast, because the information provided by the elastograms is generally independent of that obtained from the IVUS image. Thus, the palpogram can complement the characterisation of lesion from the IVUS image. We also describe cross-sectional measures of elasticity that are based on the elastogram. Finally, natural extensions of intravascular palpation to other endoluminal ultrasound applications are proposed.

Arteriosclerosis↗
PubMed2000

Characterization of plaque components and vulnerability with intravascular ultrasound elastography.

Intravascular ultrasound elastography is a method for measuring the local elastic properties using intravascular ultrasound (IVUS). The elastic properties of the different tissues within the atherosclerotic plaque are measured through the strain. Knowledge of these elastic properties is useful for guiding interventional procedures (balloon dilatation, ablation) and detection of the vulnerable plaque. In the last decade, several groups have applied elastography intravascularly with various levels of success. In this paper, the approaches of the different research groups will be discussed. The focus will be on our approach to the application of intravascular elastography. Elastograms were acquired in vitro and in vivo using the relative local displacements between IVUS images acquired at two levels of intravascular pressure with a 30 MHz mechanical or a 20 MHz array echo catheter. These displacements were estimated from the time shift between gated radiofrequency echo signals using cross-correlation algorithms with interpolation around the peak. Experiments on gel-based phantoms mimicking atherosclerotic vessels demonstrated the capability of elastography to identify soft and hard tissues independently of the echogenicity contrast. In vitro experiments on human arteries have demonstrated the potential of intravascular elastography to identify different plaque types based on their mechanical properties. These plaques could not be identified using the IVUS image alone. In vivo experiments revealed that reproducible elastograms could be obtained near end-diastole. Partial validation using the echogram was performed. Intravascular elastography provides information that is frequently unavailable or inconclusive from the IVUS image and which may therefore assist in the diagnosis and treatment of atherosclerotic disease.

Arteriosclerosis↗
PubMed2000

Simulation of circular array ultrasound transducers for intravascular applications.

The beam shape of a circular array transducer that is commonly used in intravascular ultrasound catheters was investigated in linear mode of operation. For this use, a simulation program which can simulate the radio frequency (rf)-response of a number of scatterers has been developed. The program is based on the impulse response method, which is implemented in the frequency domain. Due to the unusual geometry of the transducer, the far field gets peculiarly shaped for large apertures. Instead of having a far field with its maximum intensity in a single lobe on the acoustical axis, the far field splits up into a dual-lobe far field with maximum intensity in two lobes off the acoustical axis. A formula is derived that predicts the occurrence of these beam shapes.

Coronary Vessels↗
PubMed2000

Intravascular ultrasound combined with Raman spectroscopy to localize and quantify cholesterol and calcium salts in atherosclerotic coronary arteries.

Coronary intravascular ultrasound (IVUS) can assess arterial wall architecture and localize large intravascular deposits, but it does not provide quantitative chemical information, which is essential in the evaluation of atherosclerotic lesions. Previously, it has been shown that Raman spectroscopy can be used to accurately quantify the relative weights of cholesterol, calcium salts, triglycerides, and phospholipids in homogenized arterial tissue. In the present study, we explore some benefits of combining IVUS and Raman spectroscopy to evaluate the intact arterial wall. IVUS images were collected in vitro from human coronary arterial segments in various stages of disease (n=7). The images were divided into radial segments (11 to 28 per image, 332 in total), each of which was classified visually as calcified or noncalcified tissue. The arteries were opened longitudinally, and Raman spectra were collected from locations at 0. 5-mm intervals across the arterial luminal circumference. The spectra were used to calculate the chemical composition of the arterial wall at the examined locations. Generally, locations containing large amounts of calcium salts, as determined with Raman spectroscopy, were classified as calcified with IVUS. However, small calcific deposits (<6% of weight) were not readily detected with IVUS. The amounts and location of cholesterol determined with Raman spectroscopy were correlated closely with the presence of cholesterol observed by histochemistry, but these deposits could not be located accurately by IVUS. The combination of Raman spectroscopy and IVUS applied in vitro provides detailed information about the amount and location of calcific deposits and lipid pools in atherosclerotic plaques. Future advances in optical fiber technology may allow simultaneous collection of Raman spectra and IVUS images through the same catheter in vivo.

Adult↗
PubMed2000

Studies of cardiac function and myocardial tissue characterization.

The heart can be studied using ultrasound techniques. The shape of the heart, its chambers, wall thicknesses, wall tissue characteristics as well as motion of walls and valve leaflets are all diagnostic information. In addition, the blood velocity and its timing within the cardiac cycle is an important diagnostic tool. In the present paper focus will be limited to the analysis of the left ventricular function as observed with two-dimensional and three-dimensional echocardiography and the characteristics of backscattered ultrasound information from the left ventricular chamber wall. Function of the heart is often studied by observation of local wall motion or comparison of chamber volume in maximum and minimum shapes during the cardiac cycle (ejection fraction). Integrated backscatter from the wall is described in examples of cardiac transplantation and hypertrophy. Study of cyclic variation of frequency-dependent attenuation and integrated backscatter indicates that these are independent parameters.

Animals↗
PubMed1999

Echo decorrelation estimated from signal powers.

Volume flow can be estimated from the decorrelation of radiofrequency (RF) intravascular ultrasound signals. The method is based on a rather time-consuming process that measures the decorrelation slope from a time signal sequence. To improve the speed of flow processing, a more efficient way of estimating the flow velocity from the ratio between the power of the temporal averaged signal and the mean signal power is described in this paper. The relationship between the signal power-ratio index and the decorrelation slope was analyzed and tested using computer-simulated data. Volumetric flow data obtained with the power-ratio method were compared to those derived from the decorrelation slope in five patients. Results of the comparison studies indicate that no significant differences in flow measurements were found between the two methods, but the power-ratio method is able to improve the processing speed significantly.

Blood Flow Velocity↗
PubMed1999

Effect of catheter placement on 3-D velocity profiles in curved tubes resembling the human coronary system.

Novel measurement techniques based on intravenous ultrasound (IVUS) technology ('IVUS-Flowmetry') require the location of a catheter inside the coronary bed. The present study quantifies disturbances in the 3-D velocity profile induced by catheter placement inside a tube, applying computational fluid dynamics. Two curved, circular meshes (radius K = 0.025 m and K = 0.035 m) with and without a catheter inside the lumen were applied. The catheter was located at the inner curve, the outer curve and at the top position. Boundary conditions were: no slip on the wall, zero stress at the outlet, uniform inflow with entrance velocities of 0.1, 0.2 and 0.4 m/s. Curvature-associated centrifugal forces shifted the maximal velocity to the outer curve and introduced two symmetrical vortices. Additional catheter placement redistributed the 3-D axial velocity field away from the catheter, which was accompanied by the appearance of multiple low-strength vortices. In addition, peak axial velocity increased, peak secondary velocities decreased, axial pressure drop increased and shear stress increased. Flow calculations simulated to resemble IVUS-based flowmetry changed by only 1% after considering secondary velocity. In conclusion, placement of a catheter inside a curved tube resembling the human coronary system changes the velocity field and reduces secondary patterns. The present study supports the usefulness of catheter-based flowmetry during resting flow conditions. During hyperemic flow conditions, flow measurements might be accompanied by large axial pressure drops because the catheter, itself, might act as a significant stenosis.

Blood Flow Velocity↗
PubMed1998

Intravascular imaging.

Based on three-dimensional (3D) information, quantitative data such as plaque volume can be calculated. The procedure includes automatic contour detection based in image segmentation methods and greatly speeds up clinical evaluation. With the use of additional X-ray information, the true tortuous vessel geometry can be reconstructed in 3D. This allows, by numerical modelling techniques, to calculate endothelial shear stress values which in turn may indicate sites prone to stenosis. With a decorrelation technique for radio frequency (RF) echo information from sequential data in the same beam direction and integration method over the entire cross section, blood velocity can be shown colour-coded during the cardiac cycle, while even blood flow quantification seems to be possible. In vitro as well as animal experiments have shown the feasibility of the method. Intravascular imaging can be used to study the biomechanical properties of atheroma components. Local radial strain as a measure of local tissue hardness can be estimated in principle. Hard or soft plaques can be identified from the strain images independently of the echogenic contrast between plaque and vessel wall.

Angiography↗