| 1. |
- Smoljkić, M., et al.
(författare)
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Comparison of in vivo vs. ex situ obtained material properties of sheep common carotid artery
- 2018
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Ingår i: Medical Engineering and Physics. - : Elsevier. - 1350-4533 .- 1873-4030. ; 55, s. 16-24
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Tidskriftsartikel (refereegranskat)abstract
- Patient-specific biomechanical modelling can improve preoperative surgical planning. This requires patient-specific geometry as well as patient-specific material properties as input. The latter are, however, still quite challenging to estimate in vivo. This study focuses on the estimation of the mechanical properties of the arterial wall. Firstly, in vivo pressure, diameter and thickness of the arterial wall were acquired for sheep common carotid arteries. Next, the animals were sacrificed and the tissue was stored for mechanical testing. Planar biaxial tests were performed to obtain experimental stress-stretch curves. Finally, parameters for the hyperelastic Mooney–Rivlin and Gasser–Ogden–Holzapfel (GOH) material model were estimated based on the in vivo obtained pressure-diameter data as well as on the ex situ experimental stress-stretch curves. Both material models were able to capture the in vivo behaviour of the tissue. However, in the ex situ case only the GOH model provided satisfactory results. When comparing different fitting approaches, in vivo vs. ex situ, each of them showed its own advantages and disadvantages. The in vivo approach estimates the properties of the tissue in its physiological state while the ex situ approach allows to apply different loadings to properly capture the anisotropy of the tissue. Both of them could be further enhanced by improving the estimation of the stress-free state, i.e. by adding residual circumferential stresses in vivo and by accounting for the flattening effect of the tested samples ex vivo.
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| 2. |
- Widman, Erik, 1981-
(författare)
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Carotid Plaque Characterization in a Phantom Setup:A Comparison of Shear Wave Elastography and Pulse Wave Imaging
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Cerebrovascular disease is the second leading cause of death worldwide and determining plaque vulnerability is critical to early intervention, selecting appropriate treatment, and reducing mortality rates. Shear wave elastography (SWE) is an ultrasound-based technique to characterize the mechanical properties of tissue and pulse wave imaging (PWI) is most commonly used to measure arterial stiffness by estimating the propagation speed of the pulse wave generated from left ventricular ejection. In this study, SWE and PWI were used to characterize three homogeneous plaque mimicking inclusions in three common carotid artery phantoms by using phase velocity (PV) and group velocity (GV) analysis as well as estimating the pulse wave velocity (PWV) using PWI. Thereafter, the estimated Young’s modulus values were compared in the phantom walls. The mean wave velocities in the plaques were 1.7 ± 0.2 m/s, 1.6 ± 0.1 m/s, and 2.5 ± 0.5 m/s calculated by PV, GV, and PWI, respectively. This was lower than the mean wave speeds measured in the vessel wall (3.8 ± 0.2 m/s, 3.5 ± 0.2 m/s, and 3.3 ± 0.1 m/s by PV, GV, and PWI, respectively) showing that both techniques can detect soft vulnerable plaques. The PWV estimate was more sensitive to plaque thickness compared to the SWE GV estimate. The results indicate the ability of SWE and PWI to characterize homogeneous plaques from the arterial wall.
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| 3. |
- Widman, Erik, 1981-, et al.
(författare)
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SHEAR WAVE ELASTOGRAPHY OF THE ARTERIAL WALL – WHERE WE ARE TODAY
- 2013
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Konferensbidrag (refereegranskat)abstract
- 1. IntroductionShear Wave Elastography (SWE) is a recently developed noninvasive method for elastography assessment using ultrasound. The technique consists of sending an acoustic radiation force (pushing sequence) into the tissue that in turn generates an orthogonal low frequency propagating shear wave. The shear wave propagation is measured real time by high speed B-mode imaging. From the B-mode images, the shear wave is tracked via normalized cross-correlation and the speed is calculated, which is used to generate an elasticity map of the tissue’s shear modulus. To date, the technique has mostly been used in large homogeneous tissues such as breast and liver where it successfully detects lesions and tumors that are easily missed with normal B-mode ultrasound [1]. SWE could potentially be applied in vascular applications to assess elasticity of the arterial wall to characterize the stiffness as an early indicator of cardiac disease. Furthermore, SWE could aid in the characterization of plaques in the carotid artery, which is critical for the prevention of ischemic stroke2. Methods and ResultsAn initial study was performed using an Aixplorer SWE system (Supersonic Imagine, France) to measure the shear modulus in a polyvinyl alcohol phantom (PVA) vessel with a plaque inclusion (Figure 1). It was possible to distinguish the softer inclusion mean shear wave speed (2.1 m/s) from the arterial wall (3.5 m/s) on the SWE colour-map, but the Young’s Modulus calculation of the arterial wall (E=19.8 kPa) did not match the measured Young’s Modulus (E=53.1 kPa) from comparative mechanical testing.We have begun implementing various pushing sequences (single unfocused push, single focused push, line push, comb push) on a programmable ultrasound machine (Verasonics, USA) using a linear transducer (Philips L7-4) in a homogeneous PVA phantom. An algorithm for one dimensional cross-correlation tracking and shear wave speed estimation has been developed and initially tested in an experimental setup3. DiscussionAccording to our initial results, it is possible that SWE could be applied in vascular applications. However, the initial mechanical testing vs. SWE comparison indicated that further development to the post processing is needed before applying it on the carotid artery, which is a heterogeneous tissue with other wave propagation properties than e.g. breast tissue. The carotid artery has a difficult geometry to study for several reasons. The intima-media complex is very thin (< 1mm), and the vessel wall is not stationary. Furthermore, the cylindrical shape of the artery produces complex wave reflections within the arterial wall, which result in a polychromatic propagation of the shear wave. A few studies have applied techniques based on SWE to the arterial wall with promising results and a pilot study demonstrating the feasibility of the technique in-vivo has been published [2]. Still, a considerable effort is needed to validate and optimize the technique for the clinical vascular setting.
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| 4. |
- Widman, Erik, 1981-, et al.
(författare)
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Shear Wave Elastography Quantifies Stiffness in Ex Vivo Porcine Artery with Stiffened Arterial Region
- 2016
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Ingår i: Ultrasound in Medicine and Biology. - : Elsevier. - 0301-5629 .- 1879-291X. ; 42:10, s. 2423-2435
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Tidskriftsartikel (refereegranskat)abstract
- Five small porcine aortas were used as a human carotid artery model, and their stiffness was estimatedusing shear wave elastography (SWE) in the arterial wall and a stiffened artery region mimicking a stiff plaque. Tooptimize the SWE settings, shear wave bandwidth was measured with respect to acoustic radiation force pushlength and number of compounded angles used for motion detection with plane wave imaging. The mean arterialwall and simulated plaque shear moduli varied from 41 ± 5 to 97 ± 10 kPa and from 86 ± 13 to 174 ± 35 kPa, respectively,over the pressure range 20–120 mmHg. The results revealed that a minimum bandwidth of approximately1500 Hz is necessary for consistent shear modulus estimates, and a high pulse repetition frequency using no imagecompounding is more important than a lower pulse repetition frequency with better image quality when estimatingarterial wall and plaque stiffness using SWE.
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| 5. |
- Widman, Erik, 1981-, et al.
(författare)
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Speckle tracking strain estimation of a carotid artery plaque phantom - Validation via sonomicrometry
- 2013
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Ingår i: 2013 IEEE International Ultrasonics Symposium (IUS). - : IEEE conference proceedings. - 9781467356848 ; , s. 1757-1760
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Konferensbidrag (refereegranskat)abstract
- Current clinical ultrasound-based methods for plaque characterization are limited to visual assessment of plaque echogenicity creating demand for quantitative diagnostic tools. Our objective was to validate radial and longitudinal speckle tracking (ST) strain in phantom plaques via sonomicrometry (sono), and to compare the peak plaque and arterial wall strain. Four carotid artery gel-phantoms with a soft wall inclusion, mimicking a vulnerable plaque, were constructed. The phantoms were connected to a programmable pump simulating a carotid flow. Cineloops were acquired using a GE Vivid E9 where radial and longitudinal strain were calculated using a normalized cross-correlation ST algorithm. The region of interest was adjusted according to the plaque size. Sonomicrometry was used as a reference measurement. The correlation between estimated mean peak strain and the reference peak strain was r = 0.96 (p < 0.001) radially and r = 0.75 (p ≤ 0.005) longitudinally. The soft plaque exhibited 35.1% (SD 16.9%) greater radial (p < 0.001) and 88.6% (SD 72.0%) greater longitudinal (p < 0.001) peak strain than the arterial wall when measured with speckle tracking. It was possible to estimate plaque strain by ST and to distinguish a soft plaque from the vessel wall via strain measurements.
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| 6. |
- Widman, Erik, 1981-
(författare)
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Ultrasonic Methods for Quantitative Carotid Plaque Characterization
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cardiovascular diseases are the leading causes of death worldwide and improved diagnostic methods are needed for early intervention and to select the most suitable treatment for patients. Currently, carotid artery plaque vulnerability is typically determined by visually assessing ultrasound B-mode images, which is influenced by user-subjectivity. Since plaque vulnerability is correlated to the mechanical properties of the plaque, quantitative techniques are needed to estimate plaque stiffness as a surrogate for plaque vulnerability, which would reduce subjectivity during plaque assessment. The work in this thesis focused on three noninvasive ultrasound-based techniques to quantitatively assess plaque vulnerability and measure arterial stiffness. In Study I, a speckle tracking algorithm was validated in vitro to assess strain in common carotid artery (CCA) phantom plaques and thereafter applied in vivo to carotid atherosclerotic plaques where the strain results were compared to visual assessments by experienced physicians. In Study II, hard and soft CCA phantom plaques were characterized with shear wave elastography (SWE) by using phase and group velocity analysis while being hydrostatically pressurized followed by validating the results with mechanical tensile testing. In Study III, feasibility of assessing the stiffness of simulated plaques and the arterial wall with SWE was demonstrated in an ex vivo setup in small porcine aortas used as a human CCA model. In Study IV, SWE and pulse wave imaging (PWI) were compared when characterizing homogeneous CCA soft phantom plaques. The techniques developed in this thesis have demonstrated potential to characterize carotid artery plaques. The results show that the techniques have the ability to noninvasively evaluate the mechanical properties of carotid artery plaques, provide additional data when visually assessing B-mode images, and potentially provide improved diagnoses for patients suffering from cerebrovascular diseases.
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| 7. |
- Widman, Erik, 1981-, et al.
(författare)
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ULTRASOUND SPECKLE TRACKING STRAIN ESTIMATION IN CAROTID ARTERY PLAQUE PHANTOM WITH SONOMICROMETRY VALIDATION
- 2013
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Konferensbidrag (refereegranskat)abstract
- 1. IntroductionCarotid artery plaque characterization is critical for the prevention of ischemic events. Since plaque stiffness has shown to correlate with plaque vulnerability, quantification of plaque strain throughout the heart cycle would be a useful diagnostic tool. Our previous work encompassed the development and validation of a 2D speckle tracking (ST) algorithm to evaluate arterial stiffness by measuring strain in the carotid artery wall in silico, in vitro, and in vivo. The focus of previous studies has been to quantify plaque strain in the radial direction but lack validation against a ground truth measurement. Our objective was to validate radial and longitudinal strain in plaques via sonomicrometry (sono), and compare the measured plaque and arterial wall strain. 2. MethodThree carotid artery phantoms with soft wall inclusions, mimicking a vulnerable plaque, were constructed (10% polyvinyl alcohol (PVA), 3% graphite) by exposing the vessel and plaque to three and one freeze-thaw cycles (12h freeze, 12h thaw) respectively, see Fig. 1a. The phantoms were embedded in a tissue mimicking mixture (3% Agar, 4% graphite) at approximately 1cm depth with a pump (CompuFlow 1000 MR) connected to the phantom lumen simulating the carotid blood flow. B-mode cineloops (GE Vivid E9, 9LD linear transducer, 10 MHz, 42 fps) recorded the vessel movement at 20 and 30 mL/s peak flows. The radial and longitudinal deformation of the plaque and vessel wall was estimated by an in house 2D ST (kernel size 5x2 wavelengths) algorithm throughout two consecutive cycles. The region of interest was adjusted according to the plaque size. Sono crystals were placed on the plaque and vessel wall and used as a reference of truth. 3. ResultsFig. 1b and 1c show sample radial and longitudinal strain curves of a phantom with 20mL/s lumen flow with good agreement between sono and ST. A strong correlation was found at radial (r=0.67, p=0.03) and longitudinal peak systolic strain (r=0.84, p<0.001) between sono and ST. The plaque exhibited 47,3% (SD 27,4%) greater radial and 62,3% (SD 83,5%) longitudinal peak strain than the arterial wall when measured with ST. These preliminary data show that it is possible to measure radial and longitudinal strain in plaques; however, more extensive analysis is required as is the feasibility in vivo.
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