  Pressure ulcers are a severe disease affecting patients that are bedridden or
in a wheelchair bound for long periods of time. These wounds can develop in the
deep layers of the skin of specific parts of the body, mostly on heels or
sacrum, making them hard to detect in their early stages. Strain levels have
been identified as a direct danger indicator for triggering pressure ulcers.
Prevention could be possible with the implementation of subject-specific Finite
Element (FE) models. However, generation and validation of such FE models is a
complex task, and the current implemented techniques offer only a partial
solution of the entire problem considering only external displacements and
pressures, or cadaveric samples. In this paper, we propose an in vivo solution
based on the 3D non-rigid registration between two Magnetic Resonance (MR)
images, one in an unloaded configuration and the other deformed by means of a
plate or an indenter. From the results of the image registration, the
displacement field and subsequent strain maps for the soft tissues were
computed. An extensive study, considering different cases (on heel pad and
sacrum regions) was performed to evaluate the reproducibility and accuracy of
the results obtained with this methodology.The implemented technique can give
insight for several applications. It adds a useful tool for better
understanding the propagation of deformations in the heel soft tissues that
could generate pressure ulcers. This methodology can be used to obtain data on
the material properties of the soft tissues to define constitutive laws for FE
simulations and finally it offers a promising technique for validating FE
models.
