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

There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

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Optical coherence tomography - principles and applications

In the past two decades, optical coherence tomography (OCT) has been established as an adjunct diagnostic technique for noninvasive, high-resolution, cross-sectional imaging in a variety of medical fields. The rapid development of ultrabroad bandwidth light sources has recently enabled a significant improvement in OCT imaging resolution, demonstrating the potential of OCT to accomplish its original goal of performing noninvasive optical biopsies, i.e., the in vivo visualization of microstructural morphology in situ, which had previously only been possible with histopathology. In addition, these novel light sources might also enable the use of spectroscopic OCT, an extension of ultrahigh-resolution OCT, for enhancing image contrast as well as detecting spatially resolved functional, biochemical tissue information. State-of-the-art-light sources that now permit ultrahigh-resolution OCT covering the whole wavelength region from 500 to 1600 nm are reviewed and fundamental limitations of OCT image resolution are discussed. Ex vivo ultrahigh-resolution OCT tomograms are compared with histological results; first clinical in vivo ultrahigh-resolution OCT and preliminary spectroscopic OCT results are presented and their impact for future clinical and research applications is discussed.

Ultrahigh Resolution Optical Coherence Tomography

This paper gives an introduction to optical coherence tomography (OCT), explains its basic principles, and
discusses the information content of OCT images. Various interferometric techniques used in OCT are reviewed
and a short survey of results obtained so far in different fields of application and possible future
developments are presented.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-1/issue-02/0000/Optical-coherence-tomography/10.1117/12.231361.pdf

Optical coherence tomography.

Background: The precision of intraocular lens (IOL) calculation is essentially determined by the accuracy of the measurement of axial length. In addition to classical ultrasound biometry, partial coherence interferometry serves as a new optical method for axial length determination. A functional prototype from Carl Zeiss Jena implementing this principle was compared with immersion ultrasound biometry in our laboratory. Patients and methods: In 108 patients attending the biometry laboratory for planning of cataract surgery, axial lengths were additionally measured optically. Whereas surgical decisions were based on ultrasound data, we used postoperative refraction measurements to calculate retrospectively what results would have been obtained if optical axial length data had been used for IOL calculation. For the translation of optical to geometrical lengths, five different conversion formulas were used, among them the relation which is built into the Zeiss IOLMaster. IOL calculation was carried out according to Haigis with and without optimization of constants. Results: On the basis of ultrasound immersion data from our Grieshaber Biometric System (GBS), postoperative refraction after implantation of a Rayner IOL type 755U was predicted correctly within ±1 D in 85.7% and within ±2 D in 99% of all cases. An analogous result was achieved with optical axial length data after suitable transformation of optical path lengths into geometrical distances. Conclusions: Partial coherence interferometry is a non- contact, user- and patient-friendly method for axial length determination and IOL planning with an accuracy comparable to that of high-precision immersion ultrasound.

Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis

If the pupil of the eye is illuminated by a collimated laser beam the light returning from the eye exhibits Newton's interference fringes. This phenomenon can be used (1) to measure fundus tissue pulsations and (2) to measure the optical length of the eye. A first series of clinical measurements of patients with angiological problems has shown a clear correlation of the interferometrically measured fundus pulsations with circulation conditions. First measurements of the optical length of human eyes indicate a very high potential precision of this new technique.

E. Roth

Papers

Ophthalmic Laser Interferometry