P1B-4 2D Elastographic Technique Dedicated to Large Lateral Displacements: Application to Early Detection of Pressure Ulcer

A pressure ulcer is a damaged tissue area induced by an unrelieved pressure compressing the tissue during a prolonged period of immobility. This pathology concerns subjects with either limited mobility or impaired sensitivity. The level of pressure required to produce an ulcer varies significantly from one person to another. Moreover, the lack of information and studies on the development of this pathology makes its prevention and detection difficult. However, it is known that muscles are more vulnerable to mechanical loading than skin [Nola, et al.], Thus lesions originate in the deep muscle tissues before developing on the surface. Moreover, significant stiffening of damaged areas was demonstrated [Gefen, et. al.], This makes elastography a promising tool for detecting this pathology. Unfortunately, pressure ulcers are to be found on sites with thin layered tissues in regard to a bony prominence. The axial load required in elastography therefore yields to large lateral displacements in the region of interest, which leads to biased elastograms. That´s why we propose here a 2D model, which computes the axial strain while considering lateral motion. This estimator is based on a constrained minimization strategy. For each local region selected in the pre-compression image, the principle consists in finding its corresponding deformed version in the post-compression image, and in estimating its strain. The post-compression local window is assumed to be a time-delayed and scaled replica of the initial window in the axial direction and a shifted version in the lateral direction. Parameters are then estimated thanks to the maximization of the correlation coefficient between the initial region and the deformed region. This method was then tested on simulated data mimicking a pressure ulcer at an early stage. Results demonstrate the ability of our estimator to deal with large lateral displacements and the possibility to detect the ulcer