Ultrasonic methods for quantitative carotid plaque characterization

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. Thereafter, the phantom plaques were characterized throughout a simulated cardiac cycle. 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. Additionally, SWE settings were optimized to maximize the shear wave bandwidth with respect to acoustic radiation force push length and number of compounded angles used for motion detection with plane wave imaging. In Study IV, SWE and pulse wave imaging (PWI) were compared when characterizing homogeneous CCA soft phantom plaques using phase and group velocity analysis as well as estimating the pulse wave velocity.

The techniques developed in this thesis have demonstrated feasibility to characterize carotid artery plaques and assess arterial wall stiffness in vitro, ex vivo, and in vivo. 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. However, additional development of the techniques is needed including a large scale in vivo clinical study.

List of scientific papers

I. Ultrasound Speckle Tracking Strain Estimation of in vivo Carotid Artery Plaque with in vitro Sonomicrometry Validation. E. Widman, K. Caidahl, B. Heyde, J. D’hooge, M. Larsson. Ultrasound in Medicine and Biology. 2015, 41, 77-88.
https://doi.org/0.1016/j.ultrasmedbio.2014.06.013

II. Shear Wave Elastography Plaque Characterization with Mechanical Testing Validation – A Phantom Study. E. Widman, E. Maksuti, D. Larsson, M. W. Urban, A. Bjällmark, M. Larsson. Physics in Medicine and Biology. 2015, 60, 3151-74.
https://doi.org/10.1088/0031-9155/60/8/3151

III. Shear Wave Elastography Quantifies Stiffness in ex vivo Porcine Artery with Stiffened Arterial Region. E. Widman, E. Maksuti, C. Amador, M. W. Urban, K. Caidahl, M. Larsson. [Accepted]
https://doi.org/10.1016/j.ultrasmedbio.2016.05.021

IV. Carotid Plaque Characterization in a Phantom Setup: A Comparison of Shear Wave Elastography and Pulse Wave Imaging. E. Widman, I. Z. Apostolakis, P. Nauleau, M. W. Urban, K. Caidahl, M. Larsson, E. Konofagou. [Submitted]