Current state of ultrasound elastography

Abstract

This edition of the Ultrasound includes five papers on elastography. The technique of palpation in which stiffness of tissues is evaluated by squeezing is well known. In many ways, elastography is an advanced version of palpation, providing images and measurements related to stiffness. A variety of elastography techniques have been developed in research laboratories over the last 25 years. While several techniques have involved the use of external actuators to induce tissue movement, commercial systems have adopted methods which involve the use of a single transducer. Elastography techniques may be divided into strain methods and shear-wave methods. Estimation of true elastic modulus is only possible using shear wave techniques. Shear wave techniques are technologically challenging requiring high frame rate beam-forming which currently is not widely available across different manufacturers. There are issues of measurement reproducibility and of artefacts. It is likely that a whole new class of artefacts will be reported. The testing of measurement accuracy will require careful consideration of phantom design. Though it is early days with shear wave elastography, its ability to measure true elastic modulus means that in time it will probably become the industry standard and henceforth the clinical standard Q1 . The article by Hoskins in this issue describes in detail the technology relevant to current commercial elastography systems, and discusses potential methods by which further diagnostic information may be obtained from measurements of the viscoelasticity. The article by Cournane et al. in this issue discusses phantom design for elastography. The detection of tumours has been one of the main potential clinical applications of elastography described in the literature, predominantly using strain elastography. Studies have been undertaken in the breast, prostate and thyroid. The articles in this issue by Chakraborty et al. and Munirama et al. describe other non-cancer-based clinical applications of elastography, in the brain and for anaesthesia, respectively. The other main clinical area is the measurement of liver stiffness for staging of fibrosis. Virtually all of the literature is devoted to the technique described by Sandrin et al. using the Fibroscan system (Echosens, France). This is a hand-held device which uses shear wave techniques to measure a single value of stiffness. It is a blind technique in that there is no B-mode imaging for localization. Its affordable cost and ease of use have made it very popular for clinical use. The article by Jaffer et al. in this issue critically reviews the clinical use of different liver elastography techniques, including Fibroscan. The development of elastography has taken place over 25 years. The last 10 years, and particularly the last five years, have seen commercial implementation concentrating on single-transducer techniques. In current use there are a variety of commercial techniques ranging from the simple blind hand-held Fibroscan to two-dimensional shear wave imaging of true elastic modulus by Supersonic Imagine (France). The clinical evidence base for elastography is building with the first meta-analyses being performed. The next 5–10 years is likely to see a convergence of technology, in the opinion of the authors this is likely to be based on shear wave imaging, an increasing evidence base for clinical use and widespread adoption of these techniques in clinical practice.