John A. Rogers

Topography and volume measurements of the optic nerve using en-face optical coherence tomography

A special imaging instrument was developed which can acquire optical coherence tomography (OCT) en-face images from the eye fundus, and simultaneously a confocal image. Using this instrument we illustrate for the first time the application of en-face OCT imaging to produce topography and perform area and volume measurements of the optic nerve. The procedure is compared with the topography, area and volume measurements using a confocal scanning laser ophthalmoscope.

Simultaneous low coherence interferometry imaging at two depths using an integrated optic modulator

Abstract A Mach–Zehnder unbalanced LiNbO3 integrated modulator with independent control of the phase of each arm is incorporated into the reference arm of a low coherence interferometer set-up. Using different RF modulation frequency and processing electronics tuned to these frequencies, the system can be used for simultaneous interrogation of the signal reflected from two different depths in tissue or from two different axial positions in profilometry. When a pair of XY scanning mirrors are incorporated into the sensing arm, then two en-face images from different axial positions can be simultaneously produced. The depth separation between the axial positions of the points or layers interrogated is equal to half of the modulator path difference. The operation of the system is illustrated by displaying simultaneously two images from a coin.

Simultaneous multiple-depths en-face optical coherence tomography using multiple signal excitation of acousto-optic deflectors

We present a novel low-coherence interferometer configuration, equipped with acousto-optic deflectors that can be used to simultaneously acquire up to eight time domain optical coherence tomography en-face images. The capabilities of the configuration are evaluated in terms of depth resolution, signal to noise ratio and crosstalk. Then the configuration is employed to demonstrate simultaneous en-face optical coherence tomography imaging at five different depths in a specimen of armadillidium vulgare.

Visualization and measurement methods using transversal OCT images of the eye fundus

Presented here are methods of visualization of the retina, specifically the optic nerve, based on transversal OCT imaging and on the operation of a standalone dual channel OCT transversal/confocal system purpose built for the eye. We also demonstrate that enhanced measurement capabilities of parameters in the fundus are possible using these two channels. This is made possible owing to the unique guidance capability of the confocal channel along with the information it provides about the transversal eye movements. A large number of transversal OCT images are collected from the eye. Then, by software, longitudinal (B-scan) or transversal cuts (C-scan) may be made, post-examination, on the stack of transversal images. The software inferred B-scans are shown to have a similar level of resolution to the hardware generated images after movement errors are corrected. We hope that the system equipped with this 3D software processing could reduce the irradiation time required to collect the necessary data from the eye. We show that 3D visualization of transversal OCT images along with the simultaneous confocal imaging allows easier alignment of the patient eye than is possible with longitudinal OCT imaging alone. We also demonstrate that by this method of transversal imaging, direct comparison may be made between quantitative SLO acquired measurements made on the eye fundus and OCT measurements. For the first time, measurements of volumes and areas in the optic nerve area are demonstrated using transversal OCT imaging, similar to procedures utilized by scanning laser ophthalmoscope users.

3D OCT images from retina and skin

Using a versatile system, transversal and longitudinal OCT images from the retina and skin are presented. Using stacks of transversal OCT images, 3D profiles of retina and skin in vivo are constructed.

Polarization maintaining multiple-depth en face optical coherence tomography system using active re-circulation loops in the non-stationary state

Multiple path optical coherence tomography using re-circulating loops has previously been presented as a means of simultaneously acquiring images from multiple depths in multiple imaging channels. The configurations reported so far present the drawback that the strength of the signal from one channel to the next, reduced as the number of circulations increased. A decay of signal of not better than 4 dB from one channel to the next was reported. We present a technique to reduce this attenuation by using polarization maintaining fiber, and modulation of the drive current of the semiconductor optical amplifiers contained in each arm. The effect of these improvements resulted in a decay less than 20 dB from the 1st channel to the 10th channel.

Quasi-simultaneous OCT en-face imaging with two different depth resolutions

We report a system capable of acquiring two quasi-simultaneous en-face optical coherence tomography (OCT) images of different depth resolution (one better than 20 μm and the other between 80 and 330 μm) at a frame rate of 2 Hz. The larger depth resolution image makes it ideal for target positioning in the OCT imaging of moving organs, such as eye fundus and cornea, as well as in the alignment of stacks of en-face OCT images. This role is similar to that of the confocal channel in a previously reported dual channel OCT/confocal imaging instrument. The system presented operates as a dual channel imaging instrument, where both channels operate on the OCT principle. We illustrate the functionality of the system with examples from a coin, skin from a finger and optic nerve in vivo.

Versatile optical coherence tomography system applied for imaging of teeth

The utility of a versatile multifunctional standalone Optical Coherence Tomography (OCT)/confocal system for imaging dental tissue was investigated. The system can collect A-scan (reflectivity versus depth graph), longitudinal (B-scan) and en-face (C-scan) OCT images, simultaneously with a confocal image. The power to the sample was 250(mu) W, wavelength (lambda) =850 nm and the depth resolution in air was 16 micrometers . The OCT images showed caries lesions as volumes of reduced reflectivity. Transversal images (C-scan) showed the en-face slices of the tooth tissue like in confocal microscopy. Longitudinal images showed the depth of the lesion into the tooth tissue as well as the different structural layers of sound tooth in the same way as seen in ultrasound images. A-scans performed in locations selected in the en-face images provided quantitative data about the reflectivity versus depth. The confocal channel was extremely useful for guidance and it has also shown the integral of the intensity over depth at transversal locations. We concluded that OCT proved capable to detect an early caries lesion, to show the depth of the lesion into the tissue, and quantitatively assess the degree of demineralization.

Swept source optical coherence tomography Gabor fusion splicing technique for microscopy of thick samples using a deformable mirror

Abstract. We present a swept source optical coherence tomography (OCT) system at 1060 nm equipped with a wavefront sensor at 830 nm and a deformable mirror in a closed-loop adaptive optics (AO) system. Due to the AO correction, the confocal profile of the interface optics becomes narrower than the OCT axial range, restricting the part of the B-scan (cross section) with good contrast. By actuating on the deformable mirror, the depth of the focus is changed and the system is used to demonstrate Gabor filtering in order to produce B-scan OCT images with enhanced sensitivity throughout the axial range from a Drosophila larvae. The focus adjustment is achieved by manipulating the curvature of the deformable mirror between two user-defined limits. Particularities of controlling the focus for Gabor filtering using the deformable mirror are presented.

En-face OCT imaging of the anterior chamber

We report a system capable of collecting pairs of en-face OCT and confocal images from the anterior chamber. Pairs of such images are collected from up to 7 mm deep in the anterior chamber measured from the top of the cornea. The wavelength is 85micrometers and the power 0.3 mW. The system offers: (i) versatility, being capable of displaying both C-scan OCT images (constant depth, oriented perpendicularly on the optic axis) as well as B-scan OCT images (containing the optic axis or longitudinal); (ii) eye alignment using the Purkinje reflections in the confocal channel; (iii) overall eye guidance, on the confocal image; (iv) correction for the en-face movement in the B-scan images generated by en-face imaging using the confocal image. Animations of such pairs of images demonstrate the utility of the system for in vivo imaging of the anterior segment of the eye.