Deformation-induced speckle-pattern evolution and feasibility of correlational speckle tracking in optical coherence elastography

Abstract

Feasibility of speckle tracking in optical coherence tomography (OCT) based on digital image correlation (DIC) is discussed in the context of elastography problems. Specifics of applying DIC methods to OCT, compared to processing of photographic images in mechanical engineering applications, are emphasized and main complications are pointed out. Analytical arguments are augmented by accurate numerical simulations of OCT speckle patterns. In contrast to DIC processing for displacement and strain estimation in photographic images, the accuracy of correlational speckle tracking in deformed OCT images is strongly affected by the coherent nature of speckles, for which strain-induced complications of speckle “blinking” and “boiling” are typical. The tracking accuracy is further compromised by the usually more pronounced pixelated structure of OCT scans compared with digital photographic images in classical DIC applications. Processing of complex-valued OCT data (comprising both amplitude and phase) compared to intensity-only scans mitigates these deleterious effects to some degree. Criteria of the attainable speckle tracking accuracy and its dependence on the key OCT system parameters are established.

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

“…A significant portion of such studies were based on utilization of correlational tracking of displacements . More advanced variants of this approach were developed recently , in particular progressing from correlational processing of intensity‐only images to making use of complex‐valued signals.…”

“…Assessment of correlational approaches to speckle tracking presented in recent works indicates that correlational methods of displacement tracking, while fairly successful in mechanical engineering applications (based on processing of digital photographic images), are significantly corrupted by speckle‐blinking effects intrinsic to OCT. To overcome this, super‐broadband (almost white‐light) OCT sources or very large‐scale signal processing correlation windows are needed to ensure reasonable accuracy and strain sensitivity. However, the former is technologically and practically challenging, while the latter strongly reduces the resultant resolution of the stain maps, often down to levels unacceptable in practice.…”

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

“…Two decades after the initial formulation of the basic compressional OCE concepts, 16 the problem of strain reconstruction by comparing reference and deformed scans appears to be solved (instead of initially proposed correlational principles, phasesensitive approaches have proven more practical 17,19 ). However, the expectation that simple visualization of strains throughout the OCE image would be su±cient to draw unambiguous conclusions about tissue sti®ness distribution (often approximated by a plane-layered structure) appears to be oversimpli¯ed.…”

Optical coherence elastography – OCT at work in tissue biomechanics [Invited]

“…Such methods have also been developed for volumes, so-called digital volume correlation, which was first developed for X-ray computed tomography [168]. (As an aside, "speckle" in image correlation is sometimes not the coherent speckle we observe in OCT, but often describes the effect of imaging artificial texture applied to a sample surface [167,169].) Digital volume correlation methods have recently been applied to OCT [149,166], enabling measurement of the full strain tensor.…”

“…In the meantime, other groups have been seeking to advance compression methods based on speckle tracking [46,149,166,167], largely borrowing from digital image correlation [21,22] methodologies, as described in Section 2. Such methods have also been developed for volumes, so-called digital volume correlation, which was first developed for X-ray computed tomography [168].…”