Convolutional neural network-based common-path optical coherence tomography A-scan boundary-tracking training and validation using a parallel Monte Carlo synthetic dataset.

We present a parallel Monte Carlo (MC) simulation platform for rapidly generating synthetic common-path optical coherence tomography (CP-OCT) A-scan image dataset for image-guided needle insertion. The computation time of the method has been evaluated on different configurations and 100000 A-scan images are generated based on 50 different eye models. The synthetic dataset is used to train an end-to-end convolutional neural network (Ascan-Net) to localize the Descemet's membrane (DM) during the needle insertion. The trained Ascan-Net has been tested on the A-scan images collected from the ex-vivo human and porcine cornea as well as simulated data and shows improved tracking accuracy compared to the result by using the Canny-edge detector.

Demonstration of Optical Coherence Tomography Guided Big Bubble Technique for Deep Anterior Lamellar Keratoplasty (DALK).

Deep anterior lamellar keratoplasty (DALK) is a highly challenging procedure for cornea transplant that involves removing the corneal layers above Descemet's membrane (DM). This is achieved by a "big bubble" technique where a needle is inserted into the stroma of the cornea down to DM and the injection of either air or liquid. DALK has important advantages over penetrating keratoplasty (PK) including lower rejection rate, less endothelial cell loss, and increased graft survival. In this paper, we successfully designed and evaluated the optical coherence tomography (OCT) distal sensor integrated needle for a precise big bubble technique. We successfully used this sensor for micro-control of a robotic DALK device termed AUTO-DALK for autonomous big bubble needle insertion. The OCT distal sensor was integrated inside a 25-gauge needle, which was used for pneumo-dissection. The AUTO-DALK device is built on a manual trephine platform which includes a vacuum ring to fix the device on the eye and add a needle driver at an angle of 60 degrees from vertical. During the test on five porcine eyes with a target depth of 90%, the measured insertion depth as a percentage of cornea thickness for the AUTO-DALK device was 90 . 05 % ± 2 . 33 % without any perforation compared to 79 . 16 % ± 5 . 68 % for unassisted free-hand insertion and 86 . 20 % ± 5 . 31 % for assisted free-hand insertion. The result showed a higher precision and consistency of the needle placement with AUTO-DALK, which could lead to better visual outcomes and fewer complications.

Common-path optical coherence tomography guided vertical pneumodissection for DALK.

We reported a design and evaluation of an optical coherence tomography (OCT) sensor-integrated 27 gauge vertically inserted razor edge cannula (VIREC) for pneumatic dissection of Descemet's membrane (DM) from the stromal layer. The VIREC was inserted vertically at the apex of the cornea to the desired depth near DM. The study was performed using ex vivo bovine corneas (N = 5) and rabbit corneas (N = 5). A clean penumodissection of a stromal layer was successfully performed using VIREC without any stomal blanching on bovine eyes. The "big bubble" was generated in all five tests without perforation. Only micro bubbles were observed on rabbit eyes. The results proved that VIREC can be an effective surgical option for "big bubble" DALK.

Downward viewing common-path optical coherence tomography guided hydro-dissection needle for DALK.

Deep anterior lamellar keratoplasty (DALK) is a partial-thickness cornea transplant procedure in which only the recipient's stroma is replaced, leaving the host's Descemet's membrane (DM) and endothelium intact. This highly challenging "Big Bubble" procedure requires micron accuracy to insert a hydro-dissection needle as close as possible to the DM. Here, we report the design and evaluation of a downward viewing common-path optical coherence tomography (OCT) guided hydro-dissection needle for DALK. This design offers the flexibility of using different insertion angles and needle sizes. With the fiber situated outside the needle and eye, the needle can use its' full lumen for a smoother air/fluid injection and image quality is improved. The common-path OCT probe uses a bare optical fiber with its tip cleaved at the right angle for both reference and sample arm which is encapsulated in a 25-gauge stainless still tube. The fiber was set up vertically with a half-ball epoxy lens at the end to provide an A-scan with an 11-degree downward field of view. The hydro dissection needle was set up at 70 degrees from vertical and the relative position between the fiber end and the needle tip remained constant during the insertion. The fiber and needle were aligned by a customized needle driver to allow the needle tip and tissue underneath to both be imaged within the same A-scan. Fresh porcine eyes (N = 5) were used for the studies. The needle tip position, the stroma, and DM were successfully identified from the A-scan during the whole insertion process. The results showed the downward viewing OCT distal sensor can accurately guide the needle insertion for DALK and improved the average insertion depth compared to freehand insertion.

Convolutional Neural Network-based Optical Coherence Tomography (OCT) A-scan Segmentation and Tracking Platform using Advanced Monte Carlo Simulation.

We reported a parallel Monte Carlo simulation platform for generating OCT cornea images and training the convolutional neural network. The trained network showed improved segmentation results when applied to the ex-vivo cornea A-scan images.

Optimized OCT-based depth-resolved model for attenuation compensation using point-spread-function calibration.

Optical coherence tomography (OCT) with a robust depth-resolved attenuation compensation method for a wide range of imaging applications is proposed and demonstrated. The proposed novel OCT attenuation compensation algorithm introduces an optimized axial point spread function (PSF) to modify existing depth-resolved methods and mitigates under and overestimation in biological tissues, providing a uniform resolution over the entire imaging range. The preliminary study is implemented using A-mode numerical simulation, where this method achieved stable and robust compensation results over the entire depth of samples. The experiment results using phantoms and corneal imaging exhibit agreement with the simulation result evaluated using signal-to-noise (SNR) and contrast-to-noise (CNR) metrics.

Selective retina therapy monitoring by speckle variance optical coherence tomography for dosimetry control.

SIGNIFICANCE: Selective retina therapy (SRT) selectively targets the retinal pigment epithelium (RPE) and reduces negative side effects by avoiding thermal damages of the adjacent photoreceptors, the neural retina, and the choroid. However, the selection of proper laser energy for the SRT is challenging because of ophthalmoscopically invisible lesions in the RPE and different melanin concentrations among patients or even regions within an eye. AIM: We propose and demonstrate SRT monitoring based on speckle variance optical coherence tomography (svOCT) for dosimetry control. APPROACH: M-scans, time-resolved sequence of A-scans, of ex vivo bovine retina irradiated by 1.7-µs duration laser pulses were obtained by a swept-source OCT. SvOCT images were calculated as interframe intensity variance of the sequence. Spatial and temporal temperature distributions in the retina were numerically calculated in a 2-D retinal model using COMSOL Multiphysics. Microscopic images of treated spots were obtained before and after removing the upper neural retinal layer to assess the damage in both RPE and neural layers. RESULTS: SvOCT images show abrupt speckle variance changes when the retina is irradiated by laser pulses. The svOCT intensities averaged in RPE and photoreceptor layers along the axial direction show sharp peaks corresponding to each laser pulse, and the peak values were proportional to the laser pulse energy. The calculated temperatures in the neural retina layer and RPE were linearly fitted to the svOCT peak values, and the temperature of each lesion was estimated based on the fitting. The estimated temperatures matched well with previously reported results. CONCLUSION: We found a reliable correlation between the svOCT peak values and the degree of retinal lesion formation, which can be used for selecting proper laser energy during SRT.

Imaging Chemical Kinetics of Radical Polymerization with an Ultrafast Coherent Raman Microscope.

Numerous mechanisms have been proposed for polymerization to provide qualitative and quantitative prediction of how monomers spatially and temporally arrange into the polymeric chains. However, less is known about this process at the molecular level because the ultrafast chemical reaction is inaccessible for any form of microscope so far. Here, to address this unmet challenge, a stimulated Raman scattering microscope based on collinear multiple beams (COMB-SRS) is demonstrated, which allows label-free molecular imaging of polymer synthesis in action at speed of 2000 frames per second. The field of view of the developed 2 kHz SRS microscope is 30 × 28 µm2 with 50 × 46 pixels and 7 µs dwell time. By catching up the speed of chemical reaction, COMB-SRS is able to quantitatively visualize the ultrafast dynamics of molecular vibrations with submicron spatial resolution and sub-millisecond temporal resolution. The propagating polymer waves driven by reaction rate and persistent UV initiation are observed in situ. This methodology is expected to permit the development of novel functional polymers, controllable photoresists, 3D printing, and other new polymerization technologies.

Analysis and evaluation of BC-mode OCT image visualization for microsurgery guidance.

Optical coherence tomography (OCT) has been gaining acceptance in image-guided microsurgery as a noninvasive imaging technique. However, when using B-mode OCT imaging, it is difficult to continuously keep the surgical tool in the imaging field, and the image of the tissue beneath the tool is corrupted by shadow effects. The alternative using C-mode OCT imaging is either too slow in imaging speed when operating in a high-resolution mode, or provides a poor image resolution in a high-speed mode, with the sweep rate less than one million hertz. Moreover, the 3-dimensional rendering of C-mode OCT image makes it difficult to visualize the tissue structure and track the surgical tool beneath the tissue surface. To solve these problems, we propose a BC-mode OCT image visualization method. This method uses a sparse C-scanning scheme, which provides a set of high-resolution B-mode OCT images at sparsely spaced cross sections. The final BC-mode OCT image is obtained by averaging the image set, with inter frame variance processing to enhance the signal of the surgical tool and tissue layers. The performance of BC-mode OCT images, such as image resolution, signal to noise ratio (SNR), imaging speed, and surgical tool tracking accuracy, is analyzed theoretically and verified experimentally. The feasibility of the proposed method is evaluated by guiding the insertion of a 30-gauge needle into the cornea of an ex-vivo human eye freehand. The results show that this provides better visualization of both the surgical tool and the tissue structure than the conventional B- or C- mode OCT image.

Intraoperative Speckle Variance Optical Coherence Tomography for Tissue Temperature Monitoring During Cutaneous Laser Therapy.

Background: Tissue temperature monitoring during cutaneous laser therapy can lead to safer and more effective treatments. In this study, we investigate the use of speckle variance optical coherence tomography (svOCT) to monitor real-time temperature changes in the excised human skin tissue sample during laser irradiation. Methods: To accomplish this, we combined the pulse laser system with a reference-based svOCT system. To calibrate the svOCT, the ex-vivo human skin samples from three individuals with tissues collected from the arm, face, and back were heated with 1-degree increments. Additionally, linear regression was used to extract and evaluate the linear relationship between the temperature and normalized speckle variance value. Experiments were conducted on excised human skin sample to monitor the temperature change during laser therapy with a svOCT system. Thermal modeling of ex-vivo human skin was used to numerically simulate the laser-tissue interaction and estimate the thermal diffusion and peak temperature of the tissue during the laser treatment. Results and Conclusion: These results showed that normalized speckle variance had a linear relationship with the tissue temperature before the onset of tissue coagulation (52°) and we were able to measure the rapid increase of the tissue temperature during laser therapy. The result of the experiment is also in good agreement with the numerical simulation result that estimated the laser-induced peak temperature and thermal relaxation time.