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  • 1. Campo, A. B.
    et al.
    Dirckx, J. J.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Waz, A. T.
    Dudzik, G.
    Abramski, K. M.
    Application of a new four-channel vibrometer for determination of atherosclerosis: Further advances towards a handheld device2016In: 2016 IEEE International Symposium on Medical Measurements and Applications, MeMeA 2016 - Proceedings, Institute of Electrical and Electronics Engineers (IEEE), 2016Conference paper (Refereed)
    Abstract [en]

    Cardiovascular diseases (CD) are the leading cause of death worldwide and their prevalence is expected to rise. Important in the etiology of CD is the stiffening of the large arteries (arteriosclerosis) and plaque formation (atherosclerosis) in the common carotid artery (CCA). Increasing evidence shows that both arteriosclerosis and atherosclerosis can be detected by assessing pulse wave velocity (PWV) in the CCA, and several techniques focus on the detection of PWV in this structure. In previous studies, laser Doppler vibrometry (LDV) was proposed as an approach to detect arterial stiffness. In the present work, a compact four-channel LDV system is introduced for PWV detection. Four phantom arteries were assessed mimicking real life cardiovascular pathologies. Due to the high sensitivity and the increased spatial and temporal resolution of the LDV system, PWV could be assessed, and even local changes in phantom architecture could be detected. The system could potentially be used to detect arteriosclerosis and arterial plaque during cardiovascular screening.

  • 2.
    Larsson, Matilda
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Kremer, F.
    Heyde, B.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Brodin, Lars-Åke
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    D'Hooge, J.
    Carotid strain estimation using an ultrasound-based speckle tracking algorithm2012In: 2012 IEEE International Ultrasonics Symposium (IUS), IEEE , 2012, p. 1394-1397Conference paper (Refereed)
    Abstract [en]

    Carotid strain imaging using ultrasound-based speckle tracking has showed potential in risk stratification of cardiovascular diseases. However, assessing strain in the artery wall and in atherosclerotic plaques is challenging because of small dimensions and low deformations in relation to the applied ultrasound wavelength. High-resolution ultrasound has potential to improve the speckle tracking performance by increasing spatial resolution. The aim of this study was to compare carotid strain estimation by speckle tracking using standard clinical ultrasound and high-resolution ultrasound in an experimental setup. Ultrasound long-axis images were obtained using a standard clinical ultrasound system (Vivid7) and a high-resolution ultrasound system (Vevo2100) in dynamic phantoms mimicking the carotid artery. Speckle tracking was performed to estimate radial and longitudinal strain whereas sonomicrometry was used as reference. The results showed a significant better performance for speckle tracking applied on images from the high-resolution system compared to the standard clinical system.

  • 3.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    Larsson, David
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Urban, Matthew W.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    ARTERIAL STIFFNESS ESTIMATION BY SHEAR WAVE ELASTOGRAPHY: VALIDATION IN PHANTOMS WITH MECHANICAL TESTING2016In: Ultrasound in Medicine and Biology, ISSN 0301-5629, E-ISSN 1879-291X, Vol. 42, no 1, p. 308-321Article in journal (Refereed)
    Abstract [en]

    Arterial stiffness is an independent risk factor found to correlate with a wide range of cardiovascular diseases. It has been suggested that shear wave elastography (SWE) can be used to quantitatively measure local arterial shear modulus, but an accuracy assessment of the technique for arterial applications has not yet been performed. In this study, the influence of confined geometry on shear modulus estimation, by both group and phase velocity analysis, was assessed, and the accuracy of SWE in comparison with mechanical testing was measured in nine pressurized arterial phantoms. The results indicated that group velocity with an infinite medium assumption estimated shear modulus values incorrectly in comparison with mechanical testing in arterial phantoms (6.7 +/- 0.0 kPa from group velocity and 30.5 +/- 0.4 kPa from mechanical testing). To the contrary, SWE measurements based on phase velocity analysis (30.6 +/- 3.2 kPa) were in good agreement with mechanical testing, with a relative error between the two techniques of 8.8 +/- 6.0% in the shear modulus range evaluated (40-100 kPa). SWE by phase velocity analysis was validated to accurately measure stiffness in arterial phantoms.

  • 4. Smoljkić, M.
    et al.
    Verbrugghe, P.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering. Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering.
    Fehervary, H.
    D'hooge, J.
    Vander Sloten, J.
    Famaey, N.
    Comparison of in vivo vs. ex situ obtained material properties of sheep common carotid artery2018In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 55, p. 16-24Article in journal (Refereed)
    Abstract [en]

    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.

  • 5.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Carotid Plaque Characterization in a Phantom Setup:A Comparison of Shear Wave Elastography and Pulse Wave ImagingManuscript (preprint) (Other academic)
    Abstract [en]

    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.

  • 6.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Ultrasonic Methods for Quantitative Carotid Plaque Characterization2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 7.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Caidahl, K.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    In vivo radial and longitudinal carotid artery plaque strain estimation via ultrasound-based speckle tracking2014In: 2014 IEEE International Ultrasonics Symposium (IUS), IEEE Computer Society, 2014, p. 523-526Conference paper (Refereed)
    Abstract [en]

    Our objective was to assess strain in common carotid artery (CCA) plaques using a previously validated speckle tracking algorithm. Radial and longitudinal strain was measured in 7 patients (77 ± 6 years) with carotid atherosclerosis and was compared with a quantitative visual assessment grading of plaques on the Gray-Weale scale by two experienced physicians. A greater pulse-pressure adjusted radial and longitudinal strain was found in echolucent plaques compared to echogenic plaques. This study shows the feasibility of ultrasound speckle tracking strain estimation in plaques and indicates the possibility to characterize plaques using speckle tracking strain in vivo.

  • 8.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Caidahl, Kenneth
    D’hooge, Jan
    Heyde, Brecht
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    ULTRASOUND SPECKLE TRACKING STRAIN ESTIMATION IN CAROTID ARTERY PLAQUE PHANTOM WITH SONOMICROMETRY VALIDATION2013Conference paper (Refereed)
    Abstract [en]
    1. 1.  Introduction

    Carotid 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.

     

    1. 2.  Method

    Three 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.

     

    1. 3.  Results

    Fig. 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.

     

  • 9.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Caidahl, Kenneth
    Karolinska Institutet.
    Heyde, Brecht
    KU Leuven.
    D’hooge, Jan
    KU Leuven.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Speckle tracking strain estimation of a carotid artery plaque phantom - Validation via sonomicrometry2013In: 2013 IEEE International Ultrasonics Symposium (IUS), IEEE conference proceedings, 2013, , p. 4p. 1757-1760Conference paper (Refereed)
    Abstract [en]

    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.

  • 10.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    Amador, Carolina
    Urban, Matthew W.
    Caidahl, Kenneth
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet, Sweden.
    Shear Wave Elastography Quantifies Stiffness in Ex Vivo Porcine Artery with Stiffened Arterial Region2016In: Ultrasound in Medicine and Biology, ISSN 0301-5629, E-ISSN 1879-291X, Vol. 42, no 10, p. 2423-2435Article in journal (Refereed)
    Abstract [en]

    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.

  • 11.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Inst, Sweden.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Carrascal, Carolina Amador
    Urban, Matthew W.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Inst, Sweden.
    Evaluating Arterial and Plaque Elasticity with Shear Wave Elastography in an ex vivo Porcine Model2015In: 2015 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IUS), IEEE , 2015Conference paper (Refereed)
    Abstract [en]

    Our objective was to use shear wave elastography (SWE) to characterize the mechanical properties of an arterial wall with a simulated calcified plaque in an ex vivo setup. A small porcine aorta was used as a model for a human carotid artery and attached to a fixture while pressurized with a water column. The stiffness of the arterial wall and a simulated plaque were estimated using SWE. The mean arterial wall and plaque shear modulus varied from 42 +/- 0 kPa to 100 +/- 1 kPa and 81 +/- 2 kPa to 174 +/- 2 kPa respectively over a pressure range of 20-120 mmHg. The results show the ability of SWE to characterize the mechanical properties of an arterial wall with a simulated plaque and take steps toward an in vivo implementation for quantitative plaque assessment.

  • 12.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Larsson, David
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Urban, M.
    Caidahl, K.
    KTH, School of Technology and Health (STH), Medical Engineering.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Feasibility of shear wave elastography for plaque characterization2014In: IEEE International Ultrasonics Symposium, IUS, 2014, p. 1818-1821Conference paper (Refereed)
    Abstract [en]

    Determining plaque vulnerability is critical when selecting the most suitable treatment for patients with atherosclerotic plaque in the common carotid artery and quantitative characterization methods are needed. In this study, shear wave elastography (SWE) was used to characterize soft plaque mimicking inclusions in three atherosclerotic arterial phantoms by using phase velocity analysis in a static environment. The results were validated with axial tensile mechanical testing (MT). SWE measured a mean shear modulus of 5.8 ± 0.3 kPa and 25.0 ± 1.2 kPa versus 3.0 kPa and 30.0 kPa measured by mechanical testing in the soft plaques and phantom walls respectively. The results show good agreement between MT and SWE for both the plaque and phantom wall.

  • 13.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Department of Molecular Medicine and Surgery, Karolinska Institutet, Solna, Sweden .
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Larsson, David
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Urban, M W
    Bjallmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Department of Molecular Medicine and Surgery, Karolinska Institutet, Solna, Sweden .
    Shear wave elastography plaque characterization with mechanical testing validation: a phantom study.2015In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 60, no 8, p. 3151-3174Article in journal (Refereed)
    Abstract [en]

    Determining plaque vulnerability is critical when selecting the most suitable treatment for patients with atherosclerotic plaque. Currently, clinical non-invasive ultrasound-based methods for plaque characterization are limited to visual assessment of plaque morphology and new quantitative methods are needed. In this study, shear wave elastography (SWE) was used to characterize hard and soft plaque mimicking inclusions in six common carotid artery phantoms by using phase velocity analysis in static and dynamic environments. The results were validated with mechanical tensile testing. In the static environment, SWE measured a mean shear modulus of 5.8±0.3kPa and 106.2±17.2kPa versus 3.3±0.5kPa and 98.3±3.4kPa measured by mechanical testing in the soft and hard plaques respectively. Furthermore, it was possible to measure the plaques' shear moduli throughout a simulated cardiac cycle. The results show good agreement between SWE and mechanical testing and indicate the possibility for in vivo arterial plaque characterization using SWE.

  • 14.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Caidahl, K.
    D'Hooge, J.
    Shear wave elastography for characterization of carotid artery plaques-A feasibility study in an experimental setup2012In: 2012 IEEE International Ultrasonics Symposium (IUS), IEEE , 2012, p. 6562400-Conference paper (Refereed)
    Abstract [en]

    Characterization of vulnerable plaques in the carotid artery is critical for the prevention of ischemic stroke. However, ultrasound-based methods for plaque characterization used in the clinics today are limited to visual assessment and evaluation of plaque echogenicity. Shear Wave Elastography (SWE) is a new tissue characterization technique based on radiation force-induced shear wave propagation with potential use in plaque vulnerability assessment. The purpose of this study was to develop an experimental setup to test the feasibility of SWE for carotid plaque characterization. A carotid artery phantom with a soft inclusion in the wall, mimicking a vulnerable plaque, was constructed (10% polyvinyl alcohol (PVA), 3% graphite) by exposing the vessel and plaque to three and one freeze-thaw cycles (6h freeze, 6h thaw) respectively. An Aixplorer SWE system (Supersonic Imagine) was used to measure the shear wave speed (cT) in the vessel wall and plaque. The Young's modulus (E) was then calculated via the Moens-Korteweg (M-K) equation. For comparison, eight cylinders (d = 4 cm, h = 4 cm) were constructed for mechanical testing from the same PVA batch, of which four were exposed to three freeze-thaw cycles (mimicking the vessel wall) and four to one freeze-thaw cycle (mimicking the plaque). The Young's moduli for the cylinders were obtained via a displacement controlled mechanical compression test (Instron 5567) by applying 5% strain. The mean shear wave speed was 2.6 (±0.7) m/s in the vessel wall, 1.8 (±0.7) m/s in the plaque, resulting in Evessel = 11.5 (±0.5) kPa, Eplaque = 4.3 (±0.5) kPa. The compression tests resulted in E = 64.2 (±11.1) kPa in the hard cylinder and E = 9.7 (±3.1) kPa in the soft cylinder. The results showed that it was possible to distinguish between the arterial wall and the plaque. The disagreement between mechanical testing and SWE can be explained by the fact that the shear wave does not propagate monochromatically in cylindrical geometry. To achieve a better calculation of the elastic modulus, the frequency dependency of the shear wave velocity must be considered.

  • 15.
    Widman, Erik
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering.
    Nordenfur, Tim
    KTH, School of Technology and Health (STH), Medical Engineering.
    Caidahl, Kenneth
    D’hooge, Jan
    SHEAR WAVE ELASTOGRAPHY OF THE ARTERIAL WALL – WHERE WE ARE TODAY2013Conference paper (Refereed)
    Abstract [en]
    1. 1.  Introduction

    Shear 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 stroke

    1. 2.  Methods and Results

    An 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 setup

    1. 3.  Discussion

    According 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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