can be determined by means of a probability function displaying the similarity between consecutive signals the palpogram of fig2 a has been obtained by scanning a phantom with a soft inclusion shown in cross section by the echogram of fig2 b the phantom has the shape of a hollow tube and is made of polyvinyl alcohol cryogel the inclusion comprises a harder cap which may also be present in a naturally formed plaque the thickness of the cap varies from 2 mm to 800 u03bcm the inclusion thus has mechanical properties corresponding to those of a plaque that may be present in a natural blood vessel the phantom was kept under water and subjected to a pulsatile pressure a catheter provided with an acoustic converter was moved through the phantom at a speed of 1 0 mm s the number of acquired frames was about 30 per second i e an axial displacement of 0 03 mm per image at a beam width of about 0 6 mm this proved to be an acceptable amount in the soft part a strain until 1 was observed the strain increases with a decreasing thickness of the cap the palpograms of fig3 a 3 b and 3 c have been obtained by scanning an artherosclerotic aorta of a new zealand white rabbit at a pullback speed of 0 5 and 1 mm s respectively in this figure.