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

Abstract

Optical coherence elastography (OCE), as the use of OCT to perform elastography has come to be known, began in 1998, around ten years after the rest of the field of elastography - the use of imaging to deduce mechanical properties of tissues. After a slow start, the maturation of OCT technology in the early to mid 2000s has underpinned a recent acceleration in the field. With more than 20 papers published in 2015, and more than 25 in 2016, OCE is growing fast, but still small compared to the companion fields of cell mechanics research methods, and medical elastography. In this review, we describe the early developments in OCE, and the factors that led to the current acceleration. Much of our attention is on the key recent advances, with a strong emphasis on future prospects, which are exceptionally bright.

Keywords: (110.4500) Optical coherence tomography; (170.4500) Optical coherence tomography.

Fig. 1

(a) OCT and (b) OCE images of a phantom highlighting that optical and mechanical contrasts are not equivalent. Image sizes are 12 by 12 by 1.2 mm. Reproduced from [3].

Fig. 2

Examples of early OCT elastography. (a) Axial displacement map of a pork meat sample (left). In dark regions, SNR is too low to evaluate. At right, average displacement versus depth for two regions marked in the map (right). Adapted from [1]. (b) Upper: OCT images before and after compression loading of an artery sample. Lower: Corresponding strain (left) and displacement maps (right). (21x21-pixel kernel. No scale provided.) Adapted from [12].

Fig. 3

Typical loading methods.

Fig. 4

Wide-field compression micro-elastography (OCME) of a freshly excised malignant tumor. (a) Wide-field en face OCME overlay on OCT stitched image of the entire sample, measuring 47.5 × 47.5 mm. (b) Histology, co-registered with OCT and OCME. (c) En face OCT image showing a 1.6× magnification of the boxed region in (a). (d) Corresponding en face OCME overlay. A, adipose; C, cassette stitching artifact; D, dense tissue; NC, non-contact; S, stroma; and T, tumor. Reproduced from [151].

Fig. 5

Examples of quasi-static compression OCE using full-field, ultra-high-resolution OCT and digital volume correlation processing: in each figure part, OCT (upper) and overlaid strain map (lower). (a) cornea; (b) breast tissue. Adapted from [149].

Fig. 6

OCT images and strain elastograms of a mouse aorta taken with standard and ultra-high resolution systems: OCE and UHROCE. (a) en-face OCT images within the tunica media. (b) Corresponding en-face strain elastograms. (c) OCT B-scan images of the aorta cross-section (taken with OCE – top – and UHROCE – middle), and histology (bottom panel) from corresponding but not identical section. (d) Speckle-averaged magnified portion (top inset) of the structural B-scan image in (c), corresponding B-scan strain elastogram (middle inset), and representative histology (bottom inset). Reproduced from [171].

Fig. 7

Principles of analysis of elastic wave propagation originating from a point source.

Fig. 8

(a-d) Displacement field B-frame images at 2 ms intervals recorded from an agar phantom with mechanical inclusion. (e) B-mode structural image of the phantom. (f) Quantitative map of shear modulus computed from the dynamic shear wave visualization. (g) Linear fitting of phase delay versus position offset for two wave propagation paths. The shear modulus in the marked regions in (e) calculated using Eq. (15) is 2.9 and 17.1 kPa, respectively. Adapted from [180].

Fig. 9

Young’s modulus maps in two porcine corneas from the same animal for increasing IOP: (a) 15, (b) 20, (c) 25, and (d) 30 mmHg; and at decreasing IOP: (e) 25, (f) 20, and (g) 15 mmHg. Cornea 1 was untreated and Cornea 2 was CXL-treated. Young’s modulus scale is different for each IOP. Adapted from [187].

Fig. 10

(a) Elastic wave phase velocity versus depth indicating the distribution of corneal stiffness associated with the structural features indicated in (b), a crossectional OCT image. Region I: epithelium. Region II: anterior stroma. Region III: posterior stroma. Region IV: innermost region. (c) Combined two-dimensional depth-resolved micro-scale corneal elastography, revealing corneal layers. Adapted from [57].

Fig. 11

Elastogram of human coronary artery produced by resonant acoustic radiation force OCE. (a) OCT structural image; (b) elastogram at 500 Hz driving frequency; (c) elastogram at 800 Hz driving frequency; (d) and (e) histology. Adapted from [91].