Contrast-transfer improvement for electrode displacement elastography

Electrode displacement elastography is a strain imaging method that can be used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney. In this technique, tissue motion or deformation is introduced by displacing the same electrode used to create the lesion. Minute displacements (on the order of a fraction of a millimetre) are applied to the thermal lesion through the electrode, resulting in localized tissue deformation. Ultrasound echo signals acquired before and after the electrode-induced displacements are then utilized to generate strain images. However, these local strains depend on the modulus distribution of the tissue region being imaged. Therefore, a quantitative evaluation of the conversion efficiency from modulus contrast to strain contrast in electrode-displacement elastograms is warranted. The contrast-transfer efficiency is defined as the ratio (in dB) of the observed elastographic strain contrast and the underlying true modulus contrast. It represents a measure of the efficiency with which elastograms depict the underlying modulus distribution in tissue. In this paper, we develop a contrast-transfer efficiency formalism for electrode displacement elastography (referred to as contrast-transfer improvement). Changes in the contrast-transfer improvement as a function of the underlying true modulus contrast and the depth of the inclusion in the simulated phantom are studied. We present finite element analyses obtained using a two-dimensional mechanical deformation and tissue motion model. The results obtained using finite element analyses are corroborated using experimental analysis and an ultrasound simulation program so as to incorporate noise artifacts.