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.

3-D optical coherence tomography of the laryngeal mucosa.

Laryngeal carcinoma is one of the commonest primary head and neck malignancy and the need for early identification is very important for successful treatment. Outpatient fibreoptic examination of the larynx is unreliable in differentiating benign, pre-malignant and malignant lesions, and therefore surgeons have to rely on biopsies for a definitive diagnosis. This is an invasive procedure requiring general anaesthesia and may have a detrimental effect on the patient's voice. Conventional imaging modalities (ultrasound, computed tomography and magnetic resonance imaging) have a limited resolution and hence cannot give sufficient information on the extent or nature of laryngeal lesions. The aim of our study is to investigate the feasibility of optical coherence tomography (OCT) in imaging the normal larynx, to lay the foundations for an investigation of its ability to differentiate between benign and malignant disease. Ten tissue specimens from normal larynges were imaged with an 850 nm OCT system that was capable of providing both B-scan (longitudinal or cross-section) images as well as C-scan (en-face or images at constant depth). The en-face OCT mode allowed us to reconstruct 3-D OCT images of the tissue examined. Imaged specimens were processed with standard histopathological techniques and sectioned in the plane of the B-scan OCT images. Haematoxylin-eosin stained specimens were compared with the OCT images thus collected. Preliminary results showed good correlation between OCT images and histology sections in normal tissue.

Topography and volume measurements of the optic nerve usingen-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.

En-face coherence imaging using galvanometer scanner modulation.

We introduce a novel optical path-modulation technique for a low-coherence interferometric imaging system based on transverse scanning of the target with a galvanometric scanning-mirror pair. The path modulation arises when the beam that is incident upon one of the scanning mirrors does not fall on its axis of rotation. The method is demonstrated by the production of en-face low-coherence images of different objects such as a fiber-optic tip and a human retina invivo .

Transversal and longitudinal images from the retina of the living eye using low coherence reflectometry.

An optical coherence tomography (OCT) system to produce both longitudinal and transversal images of the in vivo human eye is presented. For the first time, OCT transversal images collected from the living eye at 50-µm depth steps show details unobtainable with the state-of-the-art scanning laser ophthalmoscope. Images of up to 3×3 mm are produced from the retina in less than a second. For images larger than 1.6×1.6 mm, a path modulation is introduced by the galvanometric scanning mirror and is used as an effective phase modulation method.© 1998 Society of Photo-Optical Instrumentation Engineers.

Simultaneous en-face imaging of two layers in the human retina by low-coherence reflectometry.

We show that with a fiberized multiple Michelson-interferometer-type configuration, transverse images from several layers in the human eye can be simultaneously obtained. We demonstrate the principle by producing simultaneous 100x100 pixel en-face images of a 4 mmx4 mm region on a postmortem retina for two depth positions 250 mum apart.

Coherence imaging by use of a Newton rings sampling function.

We show that, with suitable optics in the arm of a Michelson interferometer, orthogonal galvo-scanning mirrors build a sampling function in the form of Newton rings when the two interferometer arms are matched. Using a low-coherence source, one can obtain transversal depth-resolved images. A fast display procedure using a storage oscilloscope was devised based on this method.