B-mode ultrasound is widely used in liver ablation. However, the necrosis zone is typically not visible under
b-mode ultrasound, since ablation does not necessarily change the acoustic properties of the tissue. In contrast,
the change in tissue stiffness makes elastography ideal for monitoring ablation. Tissue palpation for elastography
is typically applied at the imaging probe, by indenting it slightly into the tissue surface. However, in this paper
we propose an alternate approach, where palpation is applied by a surgical instrument located inside the tissue.
In our approach, the ablation needle is placed inside a steerable device called an active cannula and inserted into
the tissue. A controlled motion is applied to the center of the ablation zone via the active cannula. Since the
type and direction of motion is known, displacement can then be computed from two frames with the desired
motion. The elastography results show the ablated region around the needle.
While internal palpation provides excellent local contrast, freehand palpation from outside of the tissue via
the transducer can provide a more global view of the region of the interest. For this purpose, we used a tracked
3D transducer to generate volumetric elastography images covering the ablated region. The tracking information
is employed to improve the elastography results by selecting volume pairs suitable for elastography. This is an
extension of our 2D frame selection technique which can cope with uncertainties associated with intra-operative
elastography. In our experiments with phantom and ex-vivo tissue, we were able to generate high-quality images
depicting the boundaries of the hard lesions.