Hybrid method of strain estimation in optical coherence elastography using combined sub‐wavelength phase measurements and supra‐pixel displacement tracking

Abstract

A novel hybrid method which combines sub-wavelength-scale phase measurements and pixel-scale displacement tracking for robust strain mapping in compressional optical coherence elastography is proposed. Unlike majority of OCE methods it does not rely on initial reconstruction of displacements and does not suffer from the phase-wrapping problem for super-wavelength displacements. Its robustness is enabled by direct fitting of local phase gradients obviating the necessity of phase unwrapping and error-prone numerical differentiation. Furthermore, axial displacements significantly exceeding not only the optical wavelength, but pixel scales (i.e., multiple wavelengths) can be efficiently tracked and compensated. This feature strongly reduces errors in phase-gradient estimation and ensures high robustness with respect to both additive and decorrelation noises. Illustration of exceptionally high tolerance of the proposed method to noises: contrast of only 25% in the stiffness of the layers is clearly seen in the strain map even for equal intensities of the OCT signal and additive noise (SNR = 0 dB).

Deformation velocity imaging using optical coherence tomography and its applications to the cornea

“…In OCE, where the imaging of strain [ 49 ] is essential, improved methods have been developed to enhance performance. The strategies include: measuring local spatial phase gradient to return strain instead of displacement [ 49 ]; replacing the spatial averaging of the end result with weighted averaging [ 49 ] or earlier complex data averaging [ 50 , 51 ] to decrease noise; physically adopting a common path reference surface in the interferometer [ 50 ] to minimise path length noise; and combining the phase method with speckle tracking [ 52 ] to increase dynamic range. Some of these strategies applied to DVI may improve imaging performance further and/or give direct images of strain rate instead.…”

Does group velocity always reflect elastic modulus in shear wave elastography?

“…Similar to speckle tracking in ultrasound to detect shear waves, 34 , 35 PhS-OCT can be used to detect motion by analyzing the differential signal between successive A-scans at a single location following an applied load. 5 , 36 – 41 The displacement term is defined as the change in scatterer position between scans and is more appropriately reported by the depth-resolved vertical component (along ) of vibration speed: 5 , 15 , 33 where is the scan rate, is the optical phase difference between successive scans, is the optical wavelength, and is the medium refractive index.…”

Practical obstacles and their mitigation strategies in compressional optical coherence elastography of biological tissues

“…Inside the tissue itself, the strain is determined by comparing pixel phases in the OCT scans for the initial and deformed states and then computing the vertical gradient of the interframe phase di®erence using a weighted leastsquare procedure. 29 We recently proposed an alternate approach to estimate the compression phase gradients called the \vector method" 30,31 that also draws upon the OCE measurements with the overlying reference layer. The common and attractive feature of both realizations 17,31 is that they do not require the knowledge of the tissue refractive index.…”