Cosgrove David

David Cosgrove

Hammersmith Hospital, Imperial College, London, UK

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Freehand or strain elastography is an established clinical technique with large series documenting its value in superficial body parts, especially the breast and thyroid. With improved sensitivity, it is coming into use for abdominal organs also, especially for diffuse liver diseases. Shearwave elastography has also found wide implementation in the form of the Fibroscan （Echosens, France） for assessing liver stiffness non-invasively. The liver tissue is displaced by a push from a small mechanical transducer, which launches a shear wave along the direction of the push. Its velocity is measured by an ultrasound transducer coaxially mounted with the piston, using a form of M-mode. It is widely used by hepatologists in Europe. Limitations are that it is not an imaging technique, so that the exact site for measurements cannot be selected, and that it is less successful at discriminating mild from moderate fibrosis, though it is reliable for more severe disease.
Newer techniques that rely on acoustic radiation force impulses （Arfi） to displace the tissue, have the advantage of providing simultaneous B-mode imaging. They can also be used in other organs, both in the abdomen and increasingly in small parts, particularly the breast and thyroid. The Arfi pulses used are akin to those used for color Doppler and at the same MIs, so there is no concern about power levels. Being transverse waves, the particles move in the opposite direction to the direction of the traveling wave; thus shear waves are quite different from the compressional waves of B-mode and Doppler ultrasound. They move more slowly （1-10 m/s） and are attenuated more rapidly. They are quantitative because their velocity is directly linked to the Young’s modulus, so the display can be presented in kilopascals or in m/s.
In the S2000 system （Siemens, Mountain View, Ca）, B-mode is used to detect the tissue displacement as the shear wave passes and thus estimate its velocity in a region of interest, which can be displayed as a qualitative map or read out in m/s. The relatively slow acquisition allows stronger Arfi pushes to be used and this results in less noisy and more stable shear wave signals.
In the Aixplorer （Supersonic Imagine, Aix, France） the effect of the push pulses is amplified by sending a series of pulses successively focused at increasing depths faster than the shear wave’s velocity, so that a Mach front is generated. A high imaging frame rate is achieved by transmitting a plane wave that insonates the entire field of view in a single burst. The result is that the shear wave velocity can be measured and displayed as a quantitative color overlay image at a frame rate of around 1 per second.