Justin Pedro

En-face optical coherence tomography - a novel application of non-invasive imaging to art conservation

Optical Coherence Tomography (OCT) is an optical interferometric technique developed mainly for in vivo imaging of the eye and biological tissues. In this paper, we demonstrate the potential of OCT for non-invasive examination of museum paintings. Two en-face scanning OCT systems operating at 850 nm and 1300 nm were used to produce B-scan and C-scan images at typical working distances of 2 cm. The 3D images produced by the OCT systems show not only the structure of the varnish layer but also the paint layers and underdrawings (preparatory drawings under the paint layers). The highest ever resolution and dynamic range images of underdrawings are presented and for the first time it is possible to find out non-invasively on which layer the underdrawings were drawn.

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.

Combined confocal/en face T-scan-based ultrahigh-resolution optical coherence tomography in vivo retinal imaging

Combined confocal scanning ophthalmoscopy/en face T-scan-based ultrahigh-resolution optical coherence tomography (OCT) of the human retina in vivo is reported for the first time to our knowledge. The system uses a superluminescent diode-based broadband source, which gives an axial resolution of 3.2 microm in the retina. We demonstrate acquisition of T-scan-based OCT B-scan and simultaneous confocal/C-scan images of the human retina of large lateral size (covering a field of up to 20 degrees ) at a frame rate of 2Hz.

Multiscan time-domain optical coherence tomography for retina imaging

A versatile time-domain optical coherence tomography system is presented that can generate cross-sectional images by using either transverse priority or depth priority scanning. This is made possible by using a transmissive scanning delay line compatible with balance detection operating at a speed similar to that of the transverse scanner used to scan the beam across the target. In vivo images from the retina are generated and shown using the same system switched to either transverse or depth priority scanning regime, by using the scanning delay line either in slow or fast scanning modes, respectively. A comparative analysis of different scanning regimes depending on image size to fit different areas to be imaged is presented. Safety thresholds due to the different continuous irradiation time per transverse pixel in different scanning regimes are also considered. We present the maximum exposure level for a variety of scanning procedures, employing either A scanning (depth priority) or T scanning (transverse priority) when generating cross-sectional images, en face images, or collecting 3D volumes.

Investigations of the eye fundus using a simultaneous optical coherence tomography/indocyanine green fluorescence imaging system.

We develop a dual-channel optical coherence tomography/indocyanine green (OCT/ICG) fluorescence system based on our previously reported ophthalmic OCT/confocal imaging system. The confocal channel is tuned to the fluorescence wavelength range of the ICG dye and light from the same optical source is used to generate the OCT image and to excite the ICG fluorescence. The system enables the clinician to visualize simultaneously en face OCT slices and corresponding ICG angiograms of the ocular fundus, displayed side by side. C-scan (constant depth) and B-scan (cross section) images are collected by fast en face scanning (T-scan). The pixel-to-pixel correspondence between the OCT and angiography images enables the user to precisely capture OCT B-scans at selected points on the ICG confocal images.

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.

Polarization-sensitive OCT system using single-mode fiber

Polarization sensitive optical coherence tomography (PS-OCT) takes into account the vector nature of light waves (state of polarization). The most complete information about the polarization properties of a biological target is given by depth resolved Mueller matrix elements. However, the construction of a Mueller matrix PS-OCT easy to use for in-vivo imaging is a difficult task. We designed and assembled a simpler and more versatile system that provides only essential depth-resolved information about the tissue polarization properties (linear birefringence retardation). The interferometer is a hybrid configuration of bulk optic and single mode (SM) optical fiber components. No polarization maintaining (PM) fiber is used. The reference and sample beams interfere in two single mode fiber couplers. The system can display either a pair of OCT images corresponding to linear orthogonal polarization components or a pair of images consisting of a polarization insensitive (pure reflectivity) image and a linear birefringence retardation map. All images are 8 bit grayscale and are acquired by fast en face scanning (T-scan) at 2 frames/s. We demonstrate in vivo en face images of the optic nerve region and the peripheral cornea.

Simultaneous OCT/confocal-OCT/ICG system for imaging the eye

En-face OCT acquired simultaneously with paired confocal ophthalmoscopic (CO) images provides unprecedented point-to-point correlation between surface and subsurface anatomy of the retina. An advanced prototype of a dual channel OCT/CO instrument was developed in terms of signal to noise ratio and image size. The system can operate in A, B and C-scan regimes. The design is such that there is a strict pixel to pixel correspondence between the OCT and confocal images. An extensive array of clinic pathologies were studied including macular degeneration, central serous retinopathy (CSR), macular hole, macular pucker, cystoid macular edema (CME), diabetic maculopathy, and macular trauma. We report observation of reoccurring patterns in the en-face OCT images which could be identified with different diseases. The system can also simultaneously produce en-face OCT and indocyanine green (ICG) fluorescence images where the same source is used to produce the OCT image and excite the ICG. 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 side by side. The images are collected by fast en-face scanning (T-scan) followed by slower scanning along a transverse direction and depth scanning. The system is capable of providing chosen OCT B-scans at selected points from the ICG image.

Optical-coherence tomography, extension to imaging of low coherence interferometry

Results are presented in using low coherence interferometry in quantifying the reflectivity and imaging of different objects, such as tissue, paintings and fruits. All images have been obtained using en-face flying spot technology. This allows simultaneous generation of optical coherence tomography and confocal scanning images.