Mapping Large Strains in Phase-Sensitive OCT: Key Role of Supra-Pixel Displacement Tracking in Incremental Strain Evaluation

Displacement tracking is an important step in realization of compression optical coherence elastography (C-OCE), especially in the context of obtaining nonlinear stress-strain dependences and subsequent evaluation of the tissue Young’s modulus. The rapidly progressing phase-sensitive C-OCE, however, enables direct measurements of only rather small interframe strains (below 1%), for which displacements are also small. Obtaining stress-strain curves for larger strains (~10% and greater) in phase-sensitive C-OCE can be made via cumulation of interframe strains and particle displacements. The resultant values of the so-found cumulative displacements may significantly exceed the pixel size, whereas measurements of the phase variations in C-OCE are usually performed by comparing the signals from the same pixel in a series of compared scans. When displacements of particles in a series of acquired scans reach supra-pixel values, simple pixel-to-pixel estimation of interframe phase variations may lead to significant errors in evaluating linear and nonlinear elastic properties of tissues. Thus, for large strains, adequate accounting for the resultant supra-pixel displacements of order of several pixels and greater is of key importance for undistorted mapping of elastic properties of heterogeneous materials, as well as for correct tracking of boundaries separating tissue regions with different elastic properties. In this paper we discuss the elastographic procedures of correct tracking of supra-pixel displacements in phase-sensitive C-OCE and give real examples demonstrating the importance of such displacement tracking for undistorted reconstruction of two-dimensional maps of linear and nonlinear elastic properties of real biological tissues.