Biomedical subjects

Radj A Baldewsing

Publications and source records attributed to Radj A Baldewsing.

9 recordsLinked to original sources

PubMed2007

Local elasticity imaging of vulnerable atherosclerotic coronary plaques.

The material composition and morphology of vulnerable atherosclerotic plaque components are considered to be more important determinants of acute coronary syndromes than the degree of stenosis. Rupture of a plaque causes thrombogenic material to contact the blood, resulting in a thrombus. Rupture-prone plaques contain an inflamed thin fibrous cap covering a large soft lipid pool. Mechanically, rupture occurs when plaques cannot withstand the internal stresses induced by the pulsating blood. These stresses concentrate within/around the cap/edge, since the lipid pool cannot bear much stress. During plaque development these stresses further increase when caps become thinner, lipid pools become larger, or the difference in stiffness (modulus) between the cap and the lipid pool increases. Intravascular ultrasound (IVUS) strain elastography/palpography and IVUS modulus elastography are imaging techniques that assess local plaque elasticity (strain and modulus) based on the principle that tissue deformation (strain) by a mechanical stress is a function of its elastic properties (modulus). Combined use of these techniques provides clinicians an all-in-one modality for detecting plaques, assessing their rupture proneness and imaging their elastic material composition. This chapter describes the terminology and pathophysiology of vulnerable plaques and discusses the techniques behind, the methods for and the validations of the elasticity imaging techniques.

Algorithms↗

PubMed2006

Intravascular palpography for vulnerable plaque assessment.

Palpography assesses the local mechanical properties of tissue using the deformation caused by the intraluminal pressure. The technique was validated in vitro using diseased human coronary and femoral arteries. Especially between fibrous and fatty tissue, a highly significant difference in strain (p = 0.0012) was found. Additionally, the predictive value to identify the vulnerable plaque was investigated. A high-strain region at the lumen vessel wall boundary has 88% sensitivity and 89% specificity for identifying these plaques. In vivo, the technique is validated in an atherosclerotic Yucatan minipig animal model. This study also revealed higher strain values in fatty than in fibrous plaques (p < 0.001). The presence of a high-strain region at the lumen-plaque interface has a high predictive value to identify macrophages. Patient studies revealed high strain values (1% to 2%) in noncalcified plaques. Calcified material showed low strain values (0% to 0.2%). With the development of three-dimensional palpography, identification of weak spots over the full length of a coronary artery becomes available. Patients with myocardial infarction or unstable angina have more high-strain spots in their coronary arteries than patients with stable angina. In conclusion, intravascular palpography is a unique tool to assess lesion composition and vulnerability. Three-dimensional palpography provides a technique that may develop into a clinically available tool for decision making to treat hemodynamically nonsignificant lesions by identifying vulnerable plaques. The clinical utility of this technique is yet to be determined, and more investigation is needed.

Animals↗

PubMed2006

Young's modulus reconstruction of vulnerable atherosclerotic plaque components using deformable curves.

Rupture, with subsequent thrombosis, of thin-cap fibroatheromas (TCFAs) is a major cause of myocardial infarction. A TCFA has two main components: these are a large, soft lipid pool and a thin, stiff fibrous cap covering it. Quantification of their morphology and stiffness is essential for monitoring atherosclerosis and quantifying the effect of plaque-stabilizing pharmaceutical treatment. To accomplish this, we have developed a model-based Young's modulus reconstruction method. From a plaque strain elastogram, measured with an intravascular ultrasound catheter, it reconstructs a Young's modulus image of the plaque. To this end, a minimization algorithm automatically varies the morphology and stiffness parameters of a TCFA computer model, until the corresponding computer-simulated strain elastogram resembles the measured strain elastogram. The morphology parameters of the model are the control-points of two deformable Bézier curves; one curve delineates the distal border of the lipid pool region, the other the distal border of the cap region. These component regions are assumed to be homogeneous and their stiffness is characterized by a Young's modulus. Reconstructions from strain elastograms that were 1. simulated using a histology-derived computer TCFA, 2. measured from a physical phantom with a soft lipid pool, and 3. simulated with a computer TCFA, where the complexity of its plaque component borders was increased, demonstrated the superior reconstruction/delineation behavior of this method, compared with a previously developed circular reconstruction method that used only circles for border delineation. Consequently, this method may become a valuable tool for the quantification of both the morphology and stiffness of vulnerable atherosclerotic plaque components.

Algorithms↗

PubMed2005

Perineal noise recording as a non-invasive diagnostic method of urinary bladder outlet obstruction: a study in polyvinyl alcohol and silicone model urethras.

AIMS: At present, an invasive pressure flow study is recommended to diagnose urinary bladder outlet obstruction. This method induces the risk of urinary tract infection and urethral trauma. We studied perineal noise recording as an alternative, non-invasive diagnostic method in three flexible/extensible model urethras and two silicone tubes. METHODS: The flexible/extensible model urethras were made of a 10% aqueous solution of polyvinyl alcohol (PVA) and differed in wall- stiffness, the silicone tubes differed in diameter and wall-thickness. Three degrees of obstruction were applied by inflating a cuff placed around the PVA-urethras and by compressing the silicone tubes with an adjustable clamp. Noise, produced during flow, was recorded at three positions distal to the obstruction using a piëzoceramic contact microphone. RESULTS: The average amplitude of the noise and the essential frequency of the power spectrum of each noise recording depended significantly on the degree of obstruction, the position of the microphone and the wall-stiffness in PVA-urethras and the diameter in silicone tubes. CONCLUSIONS: Based on the results of this study perineal noise recording shows good potential as an alternative method for diagnosing bladder outlet obstruction.

Data Interpretation, Statistical↗

PubMed2005

Robustness of reconstructing the Young's modulus distribution of vulnerable atherosclerotic plaques using a parametric plaque model.

Assessment of atherosclerotic plaque composition is crucial for quantitative monitoring of atherosclerosis and for quantifying the effect of pharmaceutical plaque-stabilizing treatments during clinical trials. We assessed this composition by applying a geometrically constrained, iterative inverse solution method to reconstruct a modulus elastogram (i.e., Young's modulus image) from a plaque strain elastogram (i.e., radial strain image) that is measured using intravascular ultrasound strain elastography. This reconstruction method is especially suited for thin-cap fibroatheromas (TCFAs) (i.e., plaques with a thin fibrous cap overlaying a lipid pool). Because a strain elastogram of a plaque depends upon the plaque material composition, catheter position within the vessel and measurement noise, this paper investigates how robust the reconstruction is when these parameters are varied. To this end, a standard plaque was defined as the modulus elastogram that was reconstructed from an in vivo measured strain elastogram of a human coronary plaque. This standard plaque was used to computer-simulate different strain elastograms, by varying the 1. geometry and material properties of its plaque components, 2. catheter position and 3. level of added strain noise. Robustness was evaluated by quantifying the correctly reconstructed size, shape and Young's modulus of each plaque component region and minimal cap thickness. The simulations showed that TCFAs can be adequately reconstructed; the thinner and stiffer the cap or the softer and larger the lipid pool, the better is the reconstruction of these components and minimal cap thickness. Furthermore, reconstructions were 1. independent of catheter position and 2. independent of strain noise. As such, this method has potential to monitor robustly and quantitatively atherosclerosis in vivo.

Algorithms↗

PubMed2005

Assessment of vulnerable plaque composition by matching the deformation of a parametric plaque model to measured plaque deformation.

Intravascular ultrasound (IVUS) elastography visualizes local radial strain of arteries in so-called elastograms to detect rupture-prone plaques. However, due to the unknown arterial stress distribution these elastograms cannot be directly interpreted as a morphology and material composition image. To overcome this limitation we have developed a method that reconstructs a Young's modulus image from an elastogram. This method is especially suited for thin-cap fibroatheromas (TCFAs), i.e., plaques with a media region containing a lipid pool covered by a cap. Reconstruction is done by a minimization algorithm that matches the strain image output, calculated with a parametric finite element model (PFEM) representation of a TCFA, to an elastogram by iteratively updating the PFEM geometry and material parameters. These geometry parameters delineate the TCFA media, lipid pool and cap regions by circles. The material parameter for each region is a Young's modulus, EM, EL, and EC, respectively. The method was successfully tested on computer-simulated TCFAs (n = 2), one defined by circles, the other by tracing TCFA histology, and additionally on a physical phantom (n = 1) having a stiff wall (measured EM = 16.8 kPa) with an eccentric soft region (measured EL = 4.2 kPa). Finally, it was applied on human coronary plaques in vitro (n = 1) and in vivo (n = 1). The corresponding simulated and measured elastograms of these plaques showed radial strain values from 0% up to 2% at a pressure differential of 20, 20, 1, 20, and 1 mmHg respectively. The used/reconstructed Young's moduli [kPa] were for the circular plaque EL = 50/66, EM = 1500/1484, EC = 2000/2047, for the traced plaque EL = 25/1, EM = 1000/1148, EC = 1500/1491, for the phantom EL = 4.2/4 kPa, EM = 16.8/16, for the in vitro plaque EL = n.a./29, EM = n.a./647, EC = n.a./1784 kPa and for the in vivo plaque EL = n.a./2, EM = n.a./188, Ec = n.a./188 kPa.

Algorithms↗

PubMed2005

Intravascular Ultrasound Elastography: A Clinician's Tool for Assessing Vulnerability and Material Composition of Plaques.

The material composition and morphology of the atherosclerotic plaque components are considered to be more important determinants of acute coronary ischemic syndromes than the degree of stenosis. When a vulnerable plaque ruptures it causes an acute thrombotic reaction. Rupture prone plaques contain a large lipid pool covered by a thin fibrous cap. The stress in these caps increases with decreasing thickness. Additionally, the cap may be weakened by macrophage infiltration. IntraVascular UltraSound (IVUS) elastography might be an ideal technique to assess the presence of lipid pools and to identify high stress regions. Elastography is a technique that assesses the local elasticity (strain and modulus) of tissue. It is based on the principle that the deformation of tissue by a mechanical excitation is a function of its material properties. The deformation of the tissue is determined using ultrasound. For intravascular purposes, the intraluminal pressure is used as the excitation force. The radial strain in the tissue is obtained by cross-correlation techniques on the radio frequency signals. The strain is color-coded and plotted as a complimentary image to the IVUS echogram. IVUS elastography, and IVUS palpography (which uses the same principle but is faster and more robust), have been extensively validated using simulations and by performing experiments in vitro and in vivo with diseased arteries from animals and humans. Strain was shown to be significantly different in various plaque types (absent, fatty, fibrous or calcified). A high strain region with adjacent low strain at the lumen vessel-wall boundary has 88% sensitivity and 89% specificity for detecting vulnerable plaques. High strain regions at the lumen plaque-surface have 92% sensitivity and 92% specificity for identifying macrophages. Furthermore, the incidence of vulnerable-plaque-specific strain patterns in humans has been related to clinical presentation (stable angina, unstable angina or acute myocardial infarction) and the level of C-reactive protein. In conclusion, the results obtained with IVUS (strain and modulus) elastography/palpography, show the potential of the technique to become a unique tool for clinicians to assess the vulnerability and material composition of plaques.

Animals↗

PubMed2004

Finite element modeling and intravascular ultrasound elastography of vulnerable plaques: parameter variation.

BACKGROUND AND GOAL: More than 60% of all myocardial infarction is caused by rupture of a vulnerable plaque. A vulnerable plaque can be described as a large, soft lipid pool covered by a thin fibrous cap. Plaque material composition, geometry, and inflammation caused by infiltration of macrophages are considered as major determinants for plaque rupture. For diagnostic purposes, these determinants may be obtained from elastograms (i.e. radial strain images), which are derived from intravascular ultrasound (IVUS) measurements. IVUS elastograms, however, cannot be interpreted directly as tissue component images, because radial strain depends upon plaque geometry, plaque material properties, and used catheter position. To understand and quantify the influence of these parameters upon measured IVUS elastograms, they were varied in a finite element model (FEM) that simulates IVUS elastograms of vulnerable plaques. MATERIALS AND METHODS: IVUS elastography measurements were performed on a vessel mimicking phantom, with a soft plaque embedded in a hard wall, and an atherosclerotic human coronary artery containing a vulnerable plaque. Next, FEMs were created to simulate IVUS elastograms of the same objects. In these FEMs the following parameters were varied: Young's modulus (E), Poisson's ratio (nu) in range 0.49-0.4999, catheter position (translation of 0.8 mm), and cap thickness (t) in range 50-350 microm. Hereby the resulting peak radial strain (PRS) was determined and visualized. RESULTS: Measured static E for phantom was 4.2 kPa for plaque and 16.8 kPa for wall. Variation of E-wall in range 8.4-33.2 kPa and/or E-plaque in range 2.1-8.4 kPa using the phantom FEM, gave a PRS variation of 1.6%, i.e. from 1.7% up to almost 3.3%; for variation in nu this was only 0.07%, i.e. from 2.37% up to 2.44%. Variation of E-lipid in range 6.25-400 kPa and E-cap in range 700-2300 kPa using the artery FEM, gave a PRS variation of 3.1%, i.e. from 0.6% up to 3.7%. The PRS was higher for lower E-lipid and E-cap; it was located at a shoulder of the lipid pool. Variation of nu gave only a variation of 0.17%. Variation of t and E-cap resulted in a PRS variation of 1.4%, i.e. from 0.3% up to 1.7%; thinner and weaker caps gave higher PRS. Catheter position variation changed radial strain value. CONCLUSIONS: Measured IVUS elastograms of vulnerable plaques depend highly upon the Young's modulus of lipid and cap, but not upon the Poisson's ratio. Different catheter positions result in different IVUS elastograms, but the diagnostically important high strain regions at the lipid shoulders are often still detectable. PRS increases when cap weakens or cap thickness decreases.

Arteriosclerosis↗

PubMed2004

A finite element model for performing intravascular ultrasound elastography of human atherosclerotic coronary arteries.

Intravascular ultrasound (US) elastography measures in an artery the arterial radial strain and displays it in an elastogram. An elastogram adds diagnostic information, such as the proneness of a plaque to rupture and its material composition. However, radial strain depends upon the material properties of an artery, including geometry and used catheter position. Therefore, there is not always a one-to-one correspondence between radial strain and rupture-proneness or material composition. Both the dependence and the correspondence can be quantified after a proper finite element model (FEM) is available. Therefore, this paper proposes a FEM and shows that it can model the arterial strain behavior. Its modelling capability was evaluated by comparing simulated with measured elastograms. Measured elastograms were processed from radiofrequency (RF) data obtained in vitro from six objects: a vessel-mimicking phantom and five excised human atherosclerotic coronary arteries. A FEM was created for each object and used to simulate an elastogram; the material properties and geometry of the FEM were obtained from the histology of the object. Comparison was performed upon high strain regions (HStR), because these regions have proven to contain plaques that show the hallmarks of vulnerable plaques. Eight HStR were automatically identified from the five arteries. Statistical tests showed that there was no significant difference between simulated and corresponding measured elastograms in location, surface area or mean strain value of a HStR. The results demonstrate that the FEM can simulate elastograms measured from arteries. As such, the FEM may help in quantifying strain-dependencies and assist in tissue characterization by reconstructing a Young's modulus image from a measured elastogram.

Coronary Artery Disease↗