A technique called optical coherence tomography (OCT) has been developed for noninvasive cross-sectional imaging in biological systems. OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. OCT has longitudinal and lateral spatial resolutions of a few micrometers and can detect reflected signals as small as approximately 10(-10) of the incident optical power. Tomographic imaging is demonstrated in vitro in the peripapillary area of the retina and in the coronary artery, two clinically relevant examples that are representative of transparent and turbid media, respectively.

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Optical coherence tomography

Optical coherence tomography (OCT) has developed rapidly since its first realisation in medicine and is currently an emerging technology in the diagnosis of skin disease. OCT is an interferometric technique that detects reflected and backscattered light from tissue and is often described as the optical analogue to ultrasound. The inherent safety of the technology allows for in vivo use of OCT in patients. The main strength of OCT is the depth resolution. In dermatology, most OCT research has turned on non-melanoma skin cancer (NMSC) and non-invasive monitoring of morphological changes in a number of skin diseases based on pattern recognition, and studies have found good agreement between OCT images and histopathological architecture. OCT has shown high accuracy in distinguishing lesions from normal skin, which is of great importance in identifying tumour borders or residual neoplastic tissue after therapy. The OCT images provide an advantageous combination of resolution and penetration depth, but specific studies of diagnostic sensitivity and specificity in dermatology are sparse. In order to improve OCT image quality and expand the potential of OCT, technical developments are necessary. It is suggested that the technology will be of particular interest to the routine follow-up of patients undergoing non-invasive therapy of malignant or premalignant keratinocyte tumours. It is speculated that the continued technological development can propel the method to a greater level of dermatological use.

https://link.springer.com/content/pdf/bfm%3A978-3-319-06419-2%2F1.pdf

Optical Coherence Tomography

This paper reviews the state of the art of optical coherence tomography (OCT), an interferometric imaging technique that provides cross-sectional views of the subsurface microstructure of biological tissue. Following a discussion of the basic theory of OCT, an overview of the issues involved in the design of the main components of OCT systems is presented. The review concludes by introducing new imaging modes being developed to extract additional diagnostic information.

Optical coherence tomography (OCT): a review

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

Confocal optical microscopy is a technique for increasing the contrast of microscope images, particularly in thick specimens. By restricting the observed volume, the technique keeps overlying or nearby scatterers from contributing to the detected signal. The price for this is that the instrument must observe only one point at a time (in the scanning laser version) or a group of separated points with very little light (in the disc version). This paper describes how the confocal advantage comes about and how it is implemented in actual instruments.

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Confocal optical microscopy

We have constructed a correlation microscope based on the Mirau interferometer configuration using a thin silicon nitride film beam splitter. This microscope provides the amplitude and phase information for the reflected signal from a sample located on the microscope-object plane. The device is remarkably insensitive to vibrations and is self-correcting for spherical and chromatic range aberrations of the objective. An imaging theory for the correlation microscope has been derived, which predicts accurately both the transverse resolution at a sharp edge and the range resolution for a perfect plane reflector. The range resolution is slightly better than that for a scanning optical microscope using a lens with the same aperture.

Mirau correlation microscope.

We describe an efficient algorithm based on the Hilbert transform for reconstructing cross-sectional or three-dimensional images from the input images acquired by an interference microscope. First the design of this filter is presented, and cross-sectional images of an integrated circuit constructed with this algorithm are demonstrated. It is shown that this Hilbert transform algorithm can be easily implemented with a low-cost frame grabber so that the computation time required for image reconstruction is drastically reduced.

Three-dimensional image realization in interference microscopy.

A method of processing the interference signals form a correlation microscope to provide images of a surface being examined. The interference signals for a plurality of x,y locations at a plurality of z positions are transformed to provide an intensity envelope of the signals along the z axis for each x,y location. The intensity envelope can then be processed to provide desired images.

Method of providing images of surfaces with a correlation microscope by transforming interference signals

A new algorithm for extracting amplitude and phase information from the data collected by the Mirau correlation microscope is presented. The accuracy of the phase measurements is first determined by measuring phase steps and vertical resolutions in the nanometer range. Experimental results with metal gratings show good agreement with Fourier theory. An inverse filter is then used to improve the resolution of the system. The filtered image has sharper edges and better phase contrast.

Phase measurements using the Mirau correlation microscope

A method of processing images obtained with a microscope along a line on an object, in which the object, having trenches or lines, is moved in the z direction to form a cloud plot; signals in the z direction are scanned to form a linescan vector; the linescan vectors are clustered and classified; a centroid is defined; a similarity curve is created from the linescan vectors and the centroids; and the distance between adjacent points on the similarity curve are measured to provide a measure of line or trench width.

Stanley S. C. Chim

Papers

Mirau correlation microscope.

Three-dimensional image realization in interference microscopy.

Method of providing images of surfaces with a correlation microscope by transforming interference signals

Phase measurements using the Mirau correlation microscope

Method and apparatus of measuring line structures with an optical microscope by data clustering and classification