Ultrahigh-resolution optical coherence elastography through a micro-endoscope: towards in vivo imaging of cellular-scale mechanics

In this paper, we describe a technique capable of visualizing mechanical properties at the cellular scale deep in living tissue, by incorporating a gradient-index (GRIN)-lens micro-endoscope into an ultrahigh-resolution optical coherence elastography system. The optical system, after the endoscope, has a lateral resolution of 1.6 µm and an axial resolution of 2.2 µm. Bessel beam illumination and Gaussian mode detection are used to provide an extended depth-of-field of 80 µm, which is a 4-fold improvement over a fully Gaussian beam case with the same lateral resolution. Using this system, we demonstrate quantitative elasticity imaging of a soft silicone phantom containing a stiff inclusion and a freshly excised malignant murine pancreatic tumor. We also demonstrate qualitative strain imaging below the tissue surface on in situ murine muscle. The approach we introduce here can provide high-quality extended-focus images through a micro-endoscope with potential to measure cellular-scale mechanics deep in tissue. We believe this tool is promising for studying biological processes and disease progression in vivo.This research was supported by grants and fellowships from the Australian Research Council, the National Health and Medical Research Council (Australia), the National Breast Cancer Foundation (Australia), the Department of Health, Western Australia, and the Cancer Council, Western Australia

Detecting deformation asymmetries on multiple meridians in an ex vivo keratoconic eye model

Purpose : Ocular biomechanical simulations show that air-puff induced corneal deformation imaging (APCDI) can reveal pathological asymmetric responses, as in eccentric keratoconus. Such asymmetries often go undetected when monitoring deformation on only one meridian (Birkenfeld et al., IOVS, 2019), as is the case with commercial instruments. We present a novel custom optical coherence tomography (OCT) system coupled with an air-puff module capable of detecting deformation asymmetries on multiple meridians in a keratoconic-mimicking ex vivo porcine eye model. Methods : Corneal deformation was induced by a piston-based air-puff module colinearly coupled to a custom OCT system. The puff module provided an air-puff FWHM duration of ~11 ms, reaching a maximum pressure on the corneal apex of ~13 kPa. Our OCT system used a 200 kHz 1300 nm VCSEL swept source, with an axial range of 26 mm. A freshly enucleated porcine eye globe was treated with a cross-linking (CXL) protocol with Rose Bengal (RB) photosensitizer and Green light irradiation (0.25 W/cm2, 2x200 s) of only the lower half of the cornea. We performed OCT APCDI measurements under controlled intraocular pressure (IOP, 15-30 mmHg) after application of RB (=baseline, BSL) and after partial CXL. We quantified the displaced area (DA) between the undeformed and deformed anterior cornea positions and asymmetry in displaced area (ADA), i.e. the difference between the nasal/temporal (or superior/inferior) DA referenced to the undeformed corneal apex. Results : We implemented a cross-meridian scan pattern over a lateral range of 15 mm sampled with 64 points at a repetition frequency of 1 kHz. For the BSL case, we validated the system by verifying the reduction of DA, from a maximum of ~4.3 to 1.8 mm2, with increasing IOP, from 15 to 30 mmHg. ADA was limited to below 0.2 mm2 in all cases for both meridians. For the partial CXL case and an IOP of 15 mm Hg, the ADA peaked at 0.5 mm2 for the vertical meridian, while it was below 0.2 mm2 at all times for the horizontal meridian. Conclusions : We acquired corneal deformation images with a cross-meridian scan pattern over a field of view of 15 mm at unprecedented scan rates. In keratoconus-mimicking cases, we detected deformation asymmetries up to 0.5 mm2, otherwise missed on a single meridian, that will substantially aid in corneal biomechanics diagnostics and pathology screening

Assessment of corneal dynamics with high-speed swept source Optical Coherence Tomography combined with an air puff system

We present a novel method and instrument for in vivo imaging and measurement of the human corneal dynamics during an air puff. The instrument is based on high-speed swept source optical coherence tomography (ssOCT) combined with a custom adapted air puff chamber from a non-contact tonometer, which uses an air stream to deform the cornea in a non-invasive manner. During the short period of time that the deformation takes place, the ssOCT acquires multiple A-scans in time (M-scan) at the center of the air puff, allowing observation of the dynamics of the anterior and posterior corneal surfaces as well as the anterior lens surface. The dynamics of the measurement are driven by the biomechanical properties of the human eye as well as its intraocular pressure. Thus, the analysis of the M-scan may provide useful information about the biomechanical behavior of the anterior segment during the applanation caused by the air puff. An initial set of controlled clinical experiments are shown to comprehend the performance of the instrument and its potential applicability to further understand the eye biomechanics and intraocular pressure measurements. Limitations and possibilities of the new apparatus are discussed

Swept source OCT with air puff chamber for corneal dynamics measurements

None of currently used tonometers produce estimated IOP values that are free of errors. Measurement incredibility arises from indirect measurement of corneal deformation and the fact that pressure calculations are based on population averaged parameters of anterior segment. Reliable IOP values are crucial for understanding and monitoring of number of eye pathologies e.g. glaucoma. We have combined high speed swept source OCT with air-puff chamber. System provides direct measurement of deformation of cornea and anterior surface of the lens. This paper describes in details the performance of air-puff ssOCT instrument. We present different approaches of data presentation and analysis. Changes in deformation amplitude appears to be good indicator of IOP changes. However, it seems that in order to provide accurate intraocular pressure values an additional information on corneal biomechanics is necessary. We believe that such information could be extracted from data provided by air-puff ssOCT

Imaging the small with the small: Prospects for photonics in micro-endomicroscopy for minimally invasive cellular-resolution bioimaging

Many bioimaging studies, including those in engineered tissue constructs, intravital microscopy in animal models, and medical imaging in humans, require cellular-resolution imaging of structures deep within a sample. Yet, many of the current approaches are limited in terms of resolution, but also in invasiveness, repeatable imaging of the same location, and accessible imaging depth. We coin the term micro-endomicroscope to describe the emerging class of small, cellular-resolution endoscopic imaging systems designed to image cells in situ while minimizing perturbation of the sample. In this Perspective, we motivate the need for further development of micro-endomicroscopes, highlighting applications that would greatly benefit, reviewing progress, and considering how photonics might contribute. We identify areas ripe for technological development, such as micro-scanners and small lens systems, that would advance micro-endomicroscope performance. With the right developments in photonics, many possibilities exist for new minimally invasive translatable imaging tools across the scientific, pre-clinical, and clinical spectrum: from longitudinal studies of engineered tissue constructs, to tracking disease progression in animal models, to expanding the ability to diagnose and develop treatments for diseases without the need for invasive medical procedures

OCT with air puff stimulus

Peer Reviewe

Jones matrix-based speckle-decorrelation angiography using polarization-sensitive optical coherence tomography

We show that polarization-sensitive optical coherence tomography angiography (PS-OCTA) based on full Jones matrix assessment of speckle decorrelation offers improved contrast and depth of vessel imaging over conventional OCTA. We determine how best to combine the individual Jones matrix elements and compare the resulting image quality to that of a conventional OCT scanner by co-locating and imaging the same skin locations with closely matched scanning setups. Vessel projection images from finger and forearm skin demonstrate the benefits of Jones matrix-based PS-OCTA. Our study provides a promising starting point and a useful reference for future pre-clinical and clinical applications of Jones matrix-based PS-OCTA

Estimation of vibration amplitude in Fourier domain optical coherence tomography interferometric signals from Doppler spectrum

In presented method combination of Fourier and Time domain detection enables to broaden the effective bandwidth for time dependent Doppler Signal that allows for using higher-order Bessel functions to calculate unambiguously the vibration amplitudes