Letter to the Editor: Shear Wave Elastography May Be Superior to Grayscale Median for the Identification of Carotid Plaque Vulnerability: A Comparison with Histology

Dear editor,

We read this article with interest. The authors report that shear wave elastography (SWE) findings regarding plaque stiffness may be used to help identify vulnerable plaque [1]. However, they found that there is a large overlap in Young’s Modulus values between stable and unstable plaques and this indistinctness requires attention to the interpretation of the ultrasound measurements for evaluating patients in routine clinical practice.

Because of the prominent role of carotid plaque in stroke incidence, their biomechanics have attracted significant scientific interest. There are several reports in the literature regarding the use of different methods to determine properties of plaque components [2] [3] [4]. In these methods, viscoelasticity is one of main parameters for clearly characterizing atherosclerotic plaques [2] [3] [4]. Elastography imaging is a novel ultrasound technique for quantifying tissue elasticity. However, studies have generally focused on the elastic properties of tissue, not on the viscosity [5]. Moreover, it has been claimed that if viscosity is not regarded, measurements of stiffness can be biased, and the results may be greater than the real values [6] [7]. Therefore, it is essential that viscosity be measured in order to fully characterize atherosclerotic plaques.

Garrard et al. could easily have analyzed the viscosity parameters of the atherosclerotic plaques because in a recent study, the viscosity in the liver was measured using post-processing techniques and the same ultrasound imaging system as Garrard et al. (Aixplorer, Supersonic Imagine, Aix-en-Provence, France) [8]. We believe that the large overlap in Young’s Modulus might be decreased by adding viscosity measurements.

• References

• 1 Garrard JW, Ummur P, Nduwayo S et al. Shear Wave Elastography May Be Superior to Greyscale Median for the Identification of Carotid Plaque Vulnerability: A Comparison with Histology. Ultraschall in Med 2015; DOI: 10.1055/s-0034-1399676.
• 2 Heiland VM, Forsell C, Roy J et al. Identification of carotid plaque tissue properties using an experimental-numerical approach. J Mech Behav Biomed Mater 2013; 27: 226-238
• 3 Gasser TC, Forsell C. The numerical implementation of invariant-based viscoelastic formulations at finite strains. Ananisotropic model for the passive myocardium. Comput Methods Appl Mech Eng 2011; 200: 3637-3645
• 4 Holzapfel GA, Gasser TC, Stadler M. A structural model for the viscoelastic behaviour of arterial walls: continuum formulation andfinite element analysis. Eur J Mech A Solids 2002; 21: 441-463
• 5 Ozkan F, Yildiz S, Duymus M et al. A new biomarker coincident with elastography: viscosity. Radiology 2015; 275: 308-309
• 6 Ozkan F, Ozel D, Bayraktarli RY et al. Re: Shear Wave Elastography for Localization of Prostate Cancer Lesions and Assessment of Elasticity Thresholds: Implications for Targeted Biopsies and Active Surveillance Protocols: K. Boehm, G. Salomon, B. Beyer, J. Schiffmann, K. Simonis, M. Graefen and L. Budaeus J Urol 2015;193:794-800. J Urol 2015; DOI: 10.1016/j.juro.2015.04.084.
• 7 Amador C, Urban MW, Chen SG et al. Shearwave Dispersion Ultrasound Vibrometry (SDUV) on Swine Kidney. IEEE Trans Ultrason Ferroelectr Freq Control 2011; 58: 2608-2619
• 8 Deffieux T, Gennisson JL, Bousquet L et al. Investigating liver stiffness and viscosity for fibrosis, steatosis and activity staging using Shear Wave Elastography. J Hepatol 2015; 62: 317-324
• 9 Chen S, Urban MW, Pislaru C et al. Shearwave dispersion ultrasound vibrometry (SDUV) for measuring tissue elasticity and viscosity. IEEE Trans Ultrason Ferroelectr Freq Control 2009; 56: 55-62 DOI: 10.1109/TUFFC.2009.1005..
• 10 Brum J, Bernal M, Gennisson JL et al. In vivo evaluation of the elastic anisotropy of the human Achilles tendon using shear wave dispersion analysis. Phys. Med. Biol 2014; 59: 505-523 DOI: 10.1088/0031-9155/59/3/505.
• 11 Couade M, Pernot M, Prada C et al. Quantitative Assessment of Arterial Wall Biomechanical Properties Using Shear Wave Imaging. Ultrasound in Medicine & Biology 2010; 36: 1662-1676
• 12 Ramnarine KV, Garrard JW, Dexter K et al. Shear Wave Elastography Assessment of Carotid Plaque Stiffness: In Vitro Reproducibility Study. Ultrasound in Medicine & Biology 2013; DOI: S0301-5629(13)01022-3. 10.1016.
• 13 Garrard J, Ramnarine KV. Shear-wave elastography in carotid plaques: comparison with Greyscale Median and histological assessment in an interesting case. Ultraschall in der Medizin. European Journal of Ultrasound 2013; DOI: 10.1055/s-0033-1350310.
• 14 Ramnarine KV, Garrard JW, Kanber B et al. Shear Wave Elastography Imaging of Carotid Plaques: Feasible, Reproducible and of Clinical Potential. Cardiovascular Ultrasound 2014; 12: 49