Many disease processes change tissue stiffness; in particular cancer, which often manifest as hard nodules relative to the surrounding normal (soft) tissues. In the last decade, researchers have started to image changes in tissue stiffness or strain using ultrasound and MRI. Imaging the elastic properties of tissues broadly falls into three groups: those applying a quasi-static compression and then estimating the local tissue strain using cross-correlation methods, those applying an external low frequency vibration to the tissue and depicting the results in color Doppler mode and, finally, those applying a high-intensity pulse to generate shear waves, which are tracked with low-intensity pulses (so called acoustic radiation force imaging or ARFI). Shear wave speed provides a quantitative measure of the elastic modulus in kPa, which enable quantitative strain images to be generated. The advantages and disadvantage of all three techniques will be discussed.
Studies have shown the feasibility of obtaining high quality strain images of liver, prostate and renal tissues in vitro. Even normal tissues demonstrate differences in elastic properties (> 4 dB), which can easily be seen in such images. Multiple manufacturers have implemented commercial elastography modes, which are being evaluated in many organs and applications e.g., characterization of breast, thyroid and prostate nodules or for the assessment of DVT or plaques. Monitoring ablative therapies (RFA or HIFU) have also been investigated. To date the most extensive clinical studies have been in breast cancer where sensitivities and specificities above 98 and 87%, respectively, have been reported. Examples of the clinical use of elastography will be presented.