Mark Hathaway

Simultaneous OCT/SLO/ICG imaging.

PURPOSE To evaluate how information from combined coronal optical coherence tomography (OCT) and confocal laser scanning ophthalmoscopy (SLO) with integrated simultaneous indocyanine green (ICG) dye angiography can be used in the diagnosis of a variety of macular diseases. METHODS A compact chin-rest-based OCT/confocal imaging system was used to produce the OCT image and excite the fluorescence in the ICG dye. The same eye fundus area can be visualized with coronal (C-scans, en face) OCT and ICG angiography simultaneously. Fast T scanning (transverse scanning, en face) was used to build B- or C-scan OCT images along with confocal SLO views, with and without ICG filtration. The OCT, confocal SLO and ICG fluorescence images were simultaneously presented in a three-screen format. A live mixing channel overlaid the ICG sequence on the coronal OCT slices in a fourth panel for immediate comparison. RESULTS Thirty eyes were imaged. The pathologic conditions studied included classic and occult neovascular membranes, vascularized RPE detachments, polypoidal choroidal vasculopathy, traumatic choroidal rupture, diabetic maculopathy, central serous retinopathy, and macular drusen. Images were evaluated with special attention toward identifying novel relationships between morphology and function revealed by the superimposition of the studies. CONCLUSIONS Simultaneous visualization of an en face (coronal, C-scan) OCT image and of an ICG angiogram, displayed side by side and superimposed, permits more precise correlations between late fluorescence accumulation with structures deep to the retinal surface at the retina-choroid interface. The multiplanar scanning also permits immediate B-scan OCT cross-sectional views of regions of abnormal fluorescence. The paper demonstrates the synergy between the two types of studies, functional and anatomic, in providing a more complete view of the pathologic condition.

Multidimensional en-Face OCT imaging of the retina

Fast T-scanning (transverse scanning, en-face) was used to build B-scan or C-scan optical coherence tomography (OCT) images of the retina. Several unique signature patterns of en-face (coronal) are reviewed in conjunction with associated confocal images of the fundus and B-scan OCT images. Benefits in combining T-scan OCT with confocal imaging to generate pairs of OCT and confocal images similar to those generated by scanning laser ophthalmoscopy (SLO) are discussed in comparison with the spectral OCT systems. The multichannel potential of the OCT/SLO system is demonstrated with the addition of a third hardware channel which acquires and generates indocyanine green (ICG) fluorescence images. The OCT, confocal SLO and ICG fluorescence images are simultaneously presented in a two or a three screen format. A fourth channel which displays a live mix of frames of the ICG sequence superimposed on the corresponding coronal OCT slices for immediate multidimensional comparison, is also included. OSA ISP software is employed to illustrate the synergy between the simultaneously provided perspectives. This synergy promotes interpretation of information by enhancing diagnostic comparisons and facilitates internal correction of movement artifacts within C-scan and B-scan OCT images using information provided by the SLO channel.

Simultaneous OCT/SLO/ICG system

The authors report preliminary clinical results using an unique instrument which acquires and displays simultaneously an OCT image, a confocal image similar to that of a scanning laser ophthalmoscope and an indocyanine green fluorescence image. The three images are produced by three channels, an OCT and a confocal channel operating at 793 nm and a confocal channel tuned on the ICG fluorescence spectrum, which peaks at 835 nm. The system is based on our previously described ophthalmic Optical Coherence Tomography (OCT)/confocal imaging system, where the same source is used to produce the OCT image and excite fluorescence in the ICG dye. The system is compact and assembled on a chin rest and it enables the clinician to visualise the same area of the eye fundus in terms of both en-face OCT slices and ICG angiograms, displayed at the same time. The images are collected by fast T-scanning (en-face) which are then used to build B-scan or C-scan images.

Evaluation of the signal noise ratio enhancement of SS-OCT versus TD-OCT using a full field interferometer

Comparative evaluation of signal-to-noise ratio (SNR) is presented using a Full-field (FF)-OCT configuration, which is adapted to work in either Swept-Source (SS)-Full-field OCT or Time Domain (TD)-Full-field OCT regime. We implement the two regimes in the same set-up, using the same CCD camera and the same samples. We describe the experimental set-up and the procedure implemented to verify the theory which says that Spectral Domain (SD)-OCT is superior to TD-OCT. A simple theoretical analysis of the signal-to-noise ratio is presented to evaluate the improvement from TD-OCT to SD-OCT in FF configuration. Experimental results demonstrate that the SNR is indeed better in the SS-OCT regime, however not to the level predicted by theory. More work is required to understand why the experimental set-up does not achieve the improvement predicted by theory. We also show how to perform the measurements and imaging in the two regimes of operation. The system can deliver B-Scan OCT images in the SS-OCT regime and C-scan OCT images in the TD-OCT regime.

Active axial eye motion tracking by extended range, closed loop OPD-locked white light interferometer for combined confocal/en face optical coherence tomography imaging of the human eye fundus in vivo

We report an active tracking device based on white light coherence ranging using a spectrally interrogated Michelson interferometer, which is used to monitor and correct for the axial displacement of the eye and head of the subject in a confocal scanning ophthalmoscope/ en face OCT system (SLO/OCT). The Nyquist limit range of the spectrometer in the tracking interferometer is ~5.4 mm, which is adequate for monitoring the axial position of axially extended layered objects like the human eye fundus. Both the tracking and imaging interferometers share the eye interface optics and the sample and also an optical path (OPD) changing device in the reference (fast voice coil mounted retroreflector), that keeps them locked at constant OPD values. As a consequence, the sensitivity of the tracking interferometer is not affected by the spectrometer sensitivity roll-off with increased OPD and mirror term ambiguity tracking errors close to OPD = 0 are eliminated. Moreover, the axial tracking range is only limited by the voice coil stage travel range. A real time data acquisition processor board is used to digitize the spectrometer signal and calculate the correction signal applied to the voice coil with an update time better than 5 ms. We demonstrate axial motion corrected combined confocal/ en face OCT imaging of the human eye fundus in vivo.

Variable lateral size imaging of the human retina in vivo by combined confocal/en face optical coherence tomography with closed loop OPD-locked low coherence interferometry based active axial eye motion tracking

We reported recently an active tracking device based on white light coherence ranging using a spectrally interrogated Michelson interferometer, which was used to monitor and correct for the axial displacement of the eye and head of the imaged subject in a confocal scanning ophthalmoscope/ en face OCT system (SLO/OCT) by tracking the axial position of the eye fundus. Both the tracking and imaging interferometers share the eye interface optics and the patient eye and also an optical path difference (OPD) changing device in the reference (fast voice coil mounted retroreflector), that keeps them locked at constant OPD values. As a consequence, the sensitivity of the tracking interferometer is not affected by the spectrometer sensitivity roll-off with increased OPD and mirror term ambiguity tracking errors close to OPD = 0 are eliminated. Moreover, the axial tracking range is only limited by the voice coil stage travel range and the tracking system has an update time better than 5 ms. We investigate the potential of the new configuration for acquiring volumetric data free of axial eye motion artifacts for two different lateral field sizes. Sets of SLO and en face OCT images at progressively deeper locations in the retina are simultaneously acquired for two lateral sizes, 15°x15° and 3.5°x3.5°. The large lateral field size provides a means of navigating the retina, while the high magnification small lateral size imaging reveals interesting microscopic details of the retinal morphology.

Progress in the en-face optical coherence tomography applied to eye imaging

A review is presented of the research on high resolution imaging of the eye based on en-face OCT. This can provide a dual display of images with different depth resolutions, where the two images are OCT and the other confocal. Two applications are presented: (i) OCT/ICG systems where the confocal channel is tuned to the fluorescence of indocyanine green and (ii) aberration corrections in both OCT and confocal channels using closed loop adaptive optics for enhanced contrast and transversal resolution.

Combined confocal/en face optical coherence tomography imaging of the human eye fundus in vivo in the 1050 nm spectral region

The 1020-1080 nm spectral region appears as a viable alternative to the 700-900 nm spectral band for in vivo eye fundus OCT imaging due to a local absorption minimum of water (main constituent of the eye aqueous and vitreous). Light at these wavelengths also experiences less attenuation due to lower scattering and absorption by melanin in the retinal pigment epithelium and choroid, which results in deeper penetration of the probe beam in the choroid. T-scan based en face OCT is a modification of the OCT technique that has the unique capability of acquiring both longitudinal (B-scans) and tranversal (C-scans) OCT images of the eye fundus in real time and allows the addition of a confocal scanning ophthalmoscope channel to the OCT instrument. We report for the first time a combined T-scan based en face OCT and confocal scanning opthalmoscopy system for imaging the human eye fundus in vivo in the 1050 nm region. The instrument allows the visualization of choroidal blood vessels in both the confocal and OCT channels without the use of contrast agents such as indocyanine green (ICG) dye and could prove an alternative tool for diagnosing eye conditions like age related macular degeneration that are preceded by choroidal neovascularisation.

Adaptive optics assisted Fourier domain OCT with balanced detection

Two factors are of importance to optical coherence tomography (OCT), resolution and sensitivity. Adaptive optics improves the resolution of a system by correcting for aberrations causing distortions in the wave-front. Balanced detection has been used in time domain OCT systems by removing excess photon noise, however it has not been used in Fourier domain systems, as the cameras used in the spectrometers saturated before excess photon noise becomes a problem. Advances in camera technology mean that this is no longer the case and balanced detection can now be used to improve the signal to noise ratio in a Fourier domain (FD) OCT system. An FD-OCT system, enhanced with adaptive optics, is presented and is used to show the improvement that balanced detection can provide. The signal to noise ratios of single camera detection and balanced detection are assessed and in-vivo retinal images are acquired to demonstrate better image quality when using balance detection.