Introduction

The term ‘elastography’ is used to describe techniques which provide information related to the stiffness of tissues. It has long been known that diseased tissues such as tumours are stiffer than the surrounding normal tissue. Indeed one of the oldest diagnostic methods is the assessment of the stiffness of tissues by palpation; if a stiff lump is found then the lump may well be diseased. Ultrasound techniques generally work by replicating this process; the tissue is squeezed and the response to squeezing is measured using the ultrasound system.

Ultrasound elastographic techniques in this chapter are classified in two ways based on the underlying measurement principle:

• Strain techniques: these rely on the compression of the tissues, and the measurement of the resulting tissue deformation and strain using ultrasound. These may be referred to as ‘static’ methods.

• Shear-wave techniques: these rely on the generation of shear waves, and the measurement of shear-wave velocity within the tissues using ultrasound, from which elastic modulus may be estimated. These may be referred to as ‘dynamic’ methods.

The first of these techniques provides information mainly on strain; the estimation of elastic modulus is challenging and not much performed outside research labs. The second technique does provide information on elastic modulus, but is more technically challenging, especially for 2D image formation.