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  • 1.
    Broomé, Michael
    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.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Frenckner, Björn
    Janerot-Sjöberg, Birgitta
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Closed-loop real-time simulation model of hemodynamics and oxygen transport in the cardiovascular system2013In: Biomedical engineering online, ISSN 1475-925X, E-ISSN 1475-925X, Vol. 12, no 1, p. 69-Article in journal (Refereed)
    Abstract [en]

    Background: Computer technology enables realistic simulation of cardiovascular physiology. The increasing number of clinical surgical and medical treatment options imposes a need for better understanding of patient-specific pathology and outcome prediction. Methods: A distributed lumped parameter real-time closed-loop model with 26 vascular segments, cardiac modelling with time-varying elastance functions and gradually opening and closing valves, the pericardium, intrathoracic pressure, the atrial and ventricular septum, various pathological states and including oxygen transport has been developed. Results: Model output is pressure, volume, flow and oxygen saturation from every cardiac and vascular compartment. The model produces relevant clinical output and validation of quantitative data in normal physiology and qualitative directions in simulation of pathological states show good agreement with published data. Conclusion: The results show that it is possible to build a clinically relevant real-time computer simulation model of the normal adult cardiovascular system. It is suggested that understanding qualitative interaction between physiological parameters in health and disease may be improved by using the model, although further model development and validation is needed for quantitative patient-specific outcome prediction.

  • 2. Broomé, Michael
    et al.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Waldenström, Anders
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Simulation of arterial hypertension and progressive arteriosclerosis with a 0-D multipurpose cardiovascular model2013In: CMBE13: 3rd International Conference on Computational & Mathematical Biomedical Engineering, 2013, p. 433-436Conference paper (Refereed)
    Abstract [en]

    The effects of systemic vascular resistance and progressive stiffening/arteriosclerosis inthe vascular tree on arterial blood pressure is explored in a 0D cardiovascular simulationmodel. Pulse pressure is both sensitive and specific for increases in stiffness and meanarterial pressure both sensitive and specific for changes in vascular resistance.

  • 3.
    Maksuti, Elira
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Imaging and modeling the cardiovascular system2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Understanding cardiac pumping function is crucial to guiding diagnosis, predicting outcomes of interventions, and designing medical devices that interact with the cardiovascular system.  Computer simulations of hemodynamics can show how the complex cardiovascular system is influenced by changes in single or multiple parameters and can be used to test clinical hypotheses. In addition, methods for the quantification of important markers such as elevated arterial stiffness would help reduce the morbidity and mortality related to cardiovascular disease.

    The general aim of this thesis work was to improve understanding of cardiovascular physiology and develop new methods for assisting clinicians during diagnosis and follow-up of treatment in cardiovascular disease. Both computer simulations and medical imaging were used to reach this goal.

    In the first study, a cardiac model based on piston-like motions of the atrioventricular plane was developed. In the second study, the presence of the anatomical basis needed to generate hydraulic forces during diastole was assessed in heathy volunteers. In the third study, a previously validated lumped-parameter model was used to quantify the contribution of arterial and cardiac changes to blood pressure during aging. In the fourth study, in-house software that measures arterial stiffness by ultrasound shear wave elastography (SWE) was developed and validated against mechanical testing.

    The studies showed that longitudinal movements of the atrioventricular plane can well explain cardiac pumping and that the macroscopic geometry of the heart enables the generation of hydraulic forces that aid ventricular filling. Additionally, simulations showed that structural changes in both the heart and the arterial system contribute to the progression of blood pressure with age. Finally, the SWE technique was validated to accurately measure stiffness in arterial phantoms.

  • 4.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering.
    Bini, Fabiano
    Fiorentini, Stefano
    Blasi, Giulia
    Urban, Matthew W
    Marinozzi, Franco
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering.
    Influence of wall thickness and diameter on arterial shear wave elastography: a phantom and finite element study.2017In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 62, no 7, p. 2694-2718Article in journal (Refereed)
    Abstract [en]

    Quantitative, non-invasive and local measurements of arterial mechanical properties could be highly beneficial for early diagnosis of cardiovascular disease and follow up of treatment. Arterial shear wave elastography (SWE) and wave velocity dispersion analysis have previously been applied to measure arterial stiffness. Arterial wall thickness (h) and inner diameter (D) vary with age and pathology and may influence the shear wave propagation. Nevertheless, the effect of arterial geometry in SWE has not yet been systematically investigated. In this study the influence of geometry on the estimated mechanical properties of plates (h  =  0.5-3 mm) and hollow cylinders (h  =  1, 2 and 3 mm, D  =  6 mm) was assessed by experiments in phantoms and by finite element method simulations. In addition, simulations in hollow cylinders with wall thickness difficult to achieve in phantoms were performed (h  =  0.5-1.3 mm, D  =  5-8 mm). The phase velocity curves obtained from experiments and simulations were compared in the frequency range 200-1000 Hz and showed good agreement (R (2)  =  0.80  ±  0.07 for plates and R (2)  =  0.82  ±  0.04 for hollow cylinders). Wall thickness had a larger effect than diameter on the dispersion curves, which did not have major effects above 400 Hz. An underestimation of 0.1-0.2 mm in wall thickness introduces an error 4-9 kPa in hollow cylinders with shear modulus of 21-26 kPa. Therefore, wall thickness should correctly be measured in arterial SWE applications for accurate mechanical properties estimation.

  • 5.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institute, Sweden .
    Broomé, Michael
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institute, Sweden .
    Modelling the heart with the atrioventricular plane as a piston unit2015In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 37, no 1, p. 87-92Article in journal (Refereed)
    Abstract [en]

    Medical imaging and clinical studies have proven that the heart pumps by means of minor outer volume changes and back-and-forth longitudinal movements in the atrioventricular (AV) region. The magnitude of AV-plane displacement has also shown to be a reliable index for diagnosis of heart failure. Despite this, AV-plane displacement is usually omitted from cardiovascular modelling. We present a lumped-parameter cardiac model in which the heart is described as a displacement pump with the AV plane functioning as a piston unit (AV piston). This unit is constructed of different upper and lower areas analogous with the difference in the atrial and ventricular cross-sections. The model output reproduces normal physiology, with a left ventricular pressure in the range of 8-130 mmHg, an atrial pressure of approximatly 9 mmHg, and an arterial pressure change between 75 mmHg and 130 mmHg. In addition, the model reproduces the direction of the main systolic and diastolic movements of the AV piston with realistic velocity magnitude (similar to 10 cm/s). Moreover, changes in the simulated systolic ventricular-contraction force influence diastolic filling, emphasizing the coupling between cardiac systolic and diastolic functions. The agreement between the simulation and normal physiology highlights the importance of myocardial longitudinal movements and of atrioventricular interactions in cardiac pumping.

  • 6.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering.
    Carlsson, Marcus
    Arheden, Hakan
    Kovacs, Sandor J.
    Broome, M.
    KTH, School of Technology and Health (STH), Medical Engineering.
    Ugander, Martin
    Hydraulic forces contribute to left ventricular diastolic filling2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 43505Article in journal (Refereed)
    Abstract [en]

    Myocardial active relaxation and restoring forces are known determinants of left ventricular (LV) diastolic function. We hypothesize the existence of an additional mechanism involved in LV filling, namely, a hydraulic force contributing to the longitudinal motion of the atrioventricular (AV) plane. A prerequisite for the presence of a net hydraulic force during diastole is that the atrial short-axis area (ASA) is smaller than the ventricular short-axis area (VSA). We aimed (a) to illustrate this mechanism in an analogous physical model, (b) to measure the ASA and VSA throughout the cardiac cycle in healthy volunteers using cardiovascular magnetic resonance imaging, and (c) to calculate the magnitude of the hydraulic force. The physical model illustrated that the anatomical difference between ASA and VSA provides the basis for generating a hydraulic force during diastole. In volunteers, VSA was greater than ASA during 75-100% of diastole. The hydraulic force was estimated to be 10-60% of the peak driving force of LV filling (1-3 N vs 5-10 N). Hydraulic forces are a consequence of left heart anatomy and aid LV diastolic filling. These findings suggest that the relationship between ASA and VSA, and the associated hydraulic force, should be considered when characterizing diastolic function and dysfunction.

  • 7.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Carlsson, Marcus
    Lund University, Skane University Hospital, Department of Clinical Sciences Lund, Clinical Physiology, Lund, Sweden..
    Arheden, Håkan
    Lund University, Skane University Hospital, Department of Clinical Sciences Lund, Clinical Physiology, Lund, Sweden..
    Kovács, Sándor J.
    Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, Missouri, USA..
    Broomé, Michael
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Ugander, Martin
    Department of Clinical Physiology, Karolinska Institutet, and Karolinska University Hospital, Stockholm, Sweden..
    Hydraulic forces contribute to left ventricular diastolic filling2016Manuscript (preprint) (Other academic)
    Abstract [en]

    Myocardial active relaxation and restoring forces are known determinants of left ventricular (LV) diastolic function. We hypothesize the existence of an additional mechanism involved in LV filling, namely, a hydraulic force contributing to the longitudinal motion of the atrioventricular (AV) plane. A prerequisite for the presence of a net hydraulic force during diastole is that the atrial short-axis area (ASA) is smaller than the ventricular short-axis area (VSA). We aimed (a) to illustrate this mechanism in an analogous physical model, (b) to measure the ASA and VSA throughout the cardiac cycle in healthy volunteers using cardiovascular magnetic resonance imaging, and (c) to calculate the magnitude of the hydraulic force. The physical model illustrated that the anatomical difference between ASA and VSA provides the basis for generating a hydraulic force during diastole. In volunteers, VSA was greater than ASA during 75-100% of diastole. The hydraulic force was the same order of magnitude as the peak driving force of LV (1-3N vs 5-10N). Hydraulic forces are a consequence of left heart anatomy and aid LV diastolic filling. These findings suggest that the relationship between ASA and VSA, and the resulting hydraulic forces, should be considered when characterizing diastolic function and dysfunction. 

  • 8.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Johnson, Jonas
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Bjällmark, Anna
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    Broomé, Michael
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Institutet Department of Physiology and Pharmacology.
    Physical modeling of the heart with the atrioventricular plane as a piston unit2013Conference paper (Other academic)
    Abstract [en]

    Cardiac models do not often take the atrioventricular (AV) interactioninto account, even though medicalimaging and clinical studies have shown that the heart pumps with minorouter volume changes throughout the cardiac cycle and with backand forthlongitudinal movements in the AVregion. We present a novel cardiac model based on physical modeling of the heart withthe AV-plane asa piston unit. Model simulationsgeneratedrealistic outputsforpressures and flows as well asAV-piston velocity, emphasizing the relevance of myocardial longitudinal movements in cardiac function

  • 9.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering.
    Larsson, David
    KTH, School of Technology and Health (STH), Medical Engineering. Karolinska Institutet, Sverige.
    Urban, M. W.
    Caidahl, K.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering. Karolinska Institutet, Sverige.
    Strain and strain rate generated by shear wave elastography in an ex vivo porcine aorta2017In: 2017 IEEE International Ultrasonics Symposium (IUS), IEEE Computer Society, 2017, article id 8091581Conference paper (Refereed)
    Abstract [en]

    In order to generate trackable shear waves in soft tissues, transmitted pulses in shear wave elastography (SWE) are longer than conventional clinical ultrasound pulses. Nevertheless, they typically obey mechanical and thermal regulatory limits. In arterial applications, specific safety concerns may arise, as acoustic radiation (ARF)-induced stresses and strain rates could potentially affect the arterial wall. The aim of this study was to assess ARF-induced strain and strain rates in ex vivo arteries. A porcine aorta (diameters 8.5 mm, wall thickness 1.2 mm) was pressurized by a saline-filled water column at 60 and 120 mmHg. A Verasonics V1 system and a L7-4 transducer were used to generate the ARF in the middle of the anterior wall (F-number = 1, push length = [100, 200, 300] μs) and to perform plane-wave imaging (10 kHz). Cumulative axial displacement was estimated using 2D auto-correlation. The axial strain rate was calculated as the time-derivative of the axial strain, obtained by spatial linear regression of the displacement inside the anterior wall. The ex vivo peak strain and strain rate were compared with peak strain and strain rate values induced by the blood pressure changes in two healthy individuals and two patients with coronary artery disease at rest and measured by a dedicated in house speckle tracking algorithm. ARF-induced ex vivo peak strains were in the range 0.3-1% and strain rates in the range 6-23 s-1. Peak values were more affected by longer push duration than pressurization level. In vivo physiological peak strain was 33% and strain rate was 2 s-1. ARF-induced strain rates in vivo are likely to be lower than those assessed in this ex vivo setup due to ultrasound attenuation and the effect of surrounding tissue. Therefore, the results of the performed study suggest that SWE could be used in a safe manner for arterial applications even though specific effects of high strain rates are to be explored.

  • 10.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering.
    Larsson, David
    KTH, School of Technology and Health (STH), Medical Engineering.
    Urban, M. W.
    Caidahl, K.
    Larsson, Matilda
    KTH, School of Technology and Health (STH), Medical Engineering.
    Strain and strain rate generated by shear wave elastography in ex vivo porcine aortas2017In: IEEE International Ultrasonics Symposium, IUS, IEEE Computer Society , 2017Conference paper (Refereed)
    Abstract [en]

    In shear wave elastography (SWE), acoustic radiation forces (ARF) are employed to generate shear waves within the tissue. Although the transmitted pulses are longer than those in conventional clinical ultrasound, they typically obey the mechanical and thermal regulatory limits. In arterial applications, specific safety concerns may arise, as ARF-induced stresses and strain rates could potentially affect the arterial wall. A previous simulation study (Doherty et al., J Biomech, 2013 Jan; 46(1):83-90) showed that stresses imposed by the ARF used in SWE are orders of magnitude lower than those caused by blood pressure. ARF-induced strain rates have not been investigated yet, therefore the aim of this study was to assess such strain rates in an ex vivo setup.

  • 11.
    Maksuti, Elira
    et al.
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Inst, Sweden.
    Westerhof, Nico
    Westerhof, Berend E.
    Broomé, Michael
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging. Karolinska Inst, Sweden.
    Stergiopulos, Nikos
    Contribution of the Arterial System and the Heart to Blood Pressure during Normal Aging: A Simulation Study2016In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 11, no 6, article id e0157493Article in journal (Refereed)
    Abstract [en]

    During aging, systolic blood pressure continuously increases over time, whereas diastolic pressure first increases and then slightly decreases after middle age. These pressure changes are usually explained by changes of the arterial system alone (increase in arterial stiffness and vascular resistance). However, we hypothesise that the heart contributes to the age-related blood pressure progression as well. In the present study we quantified the blood pressure changes in normal aging by using a Windkessel model for the arterial system and the time-varying elastance model for the heart, and compared the simulation results with data from the Framingham Heart Study. Parameters representing arterial changes (resistance and stiffness) during aging were based on literature values, whereas parameters representing cardiac changes were computed through physiological rules (compensated hypertrophy and preservation of end-diastolic volume). When taking into account arterial changes only, the systolic and diastolic pressure did not agree well with the population data. Between 20 and 80 years, systolic pressure increased from 100 to 122 mmHg, and diastolic pressure decreased from 76 to 55 mmHg. When taking cardiac adaptations into account as well, systolic and diastolic pressure increased from 100 to 151 mmHg and decreased from 76 to 69 mmHg, respectively. Our results show that not only the arterial system, but also the heart, contributes to the changes in blood pressure during aging. The changes in arterial properties initiate a systolic pressure increase, which in turn initiates a cardiac remodelling process that further augments systolic pressure and mitigates the decrease in diastolic pressure.

  • 12.
    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.

  • 13.
    Marlevi, David
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Biomedical Engineering and Health Systems. Karolinska Inst, Dept Clin Sci, Stockholm, Sweden..
    Maksuti, Elira
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Biomedical Engineering and Health Systems.
    Urban, Matthew W.
    Mayo Clin, Coll Med, Dept Radiol, Rochester, MN USA..
    Winter, Reidar
    Karolinska Inst, Dept Clin Sci, Stockholm, Sweden..
    Larsson, Matilda
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Biomedical Engineering and Health Systems.
    Plaque characterization using shear wave elastography-evaluation of differentiability and accuracy using a combined ex vivo and in vitro setup2018In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 63, no 23, article id 235008Article in journal (Refereed)
    Abstract [en]

    Ultrasound elastography has shown potential for improved plaque risk stratification. However, no clear consensus exists on what output metric to use, or what imaging parameters would render optimal plaque differentiation. For this reason we developed a combined ex vivo and in vitro setup, in which the ability to differentiate phantom plaques of varying stiffness was evaluated as a function of plaque geometry, push location, imaging plane, and analysed wave speed metric. The results indicate that group velocity or phase velocity >= 1 kHz showed the highest ability to significantly differentiate plaques of different stiffness, successfully classifying a majority of the 24 analysed plaque geometries, respectively. The ability to differentiate plaques was also better in the longitudinal views than in the transverse view. Group velocity as well as phase velocities <1 kHz showed a systematic underestimation of plaque stiffness, stemming from the confined plaque geometries, however, despite this group velocity analysis showed lowest deviation in estimated plaque stiffness (0.1 m s(-1) compared to 0.2 m s(-1) for phase velocity analysis). SWE results were also invariant to SWE push location, albeit apparent differences in signal-to-noise ratio (SNR) and generated plaque particle velocity. With that, the study has reinforced the potential of SWE for successful plaque differentiation; however the results also highlight the importance of choosing optimal imaging settings and using an appropriate wave speed metric when attempting to differentiate different plaque groups.

  • 14.
    Nordenfur, Tim
    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.
    Widman, Erik
    KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
    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.
    A Comparison of Shear Wave Elastography Pushing Sequences2013Conference paper (Refereed)
  • 15.
    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.

  • 16.
    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.

  • 17.
    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.

  • 18.
    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.

  • 19.
    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.

  • 20.
    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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