Usually ultrasound equipment in medicine only use compressional waves.
But since human tissues have a high bulk modulus, the P-wave speed is
relatively constant (around 1580 m/s). Human tissues are very stiff if
you apply isotropic constraints on them (like pressure of water).
However M. Fink and his colleagues proposed a new way to investigate
human tissues by first sending a strong compressional wave in the tissue
that is able to make ripples in the body, then shear wave are produced
and they can be used for imaging fine structure, since the shear modulus
depends very strongly on their composition. For instance tumor have a
much higher shear modulus than healthy tissues.
http://mechenjoy.blogspot.fr/2012/06/shear-waves-medecine-and-brain.html