Optical Coherence Tomography

Email: alessandro.invernizzi@gmail.com Optical coherence tomography (OCT) has dramatically changed the understanding and management of uveitis and other ocular conditions. Currently, OCT angiogra- phy (OCTA) combines structural information with the visualization of blood flow within the imaged area. The aim of this review is to present the basic principles of OCT and OCTA interpretation and to investigate the role of these imaging tech- niques in the diagnosis and management of uveitis. Common complications of intraocular inflammation such as macular oedema and inflammatory choroidal neo- vascularization are often diagnosed and followed with OCT/OCTA scans. How- ever, uveitis specialists can obtain much more information from tomographic scans. This review provides a comprehensive description of typical OCT/OCTA findings characterizing different ocular structures in uveitis, proceeding from the cornea to the choroid. A careful interpretation of OCT/OCTA images can help in the differential diagnosis, the prediction of clinical outcomes, and the follow-up of patients with uveitis.

Koch, „Influence and compensation of autocorrelation terms in depth resolved spectroscopic Fourier-Domain optical coherence tomography” , Applied Optics 2010.. Outlook[r]

The development of Optical Coherence Tomography (OCT) systems started in the 1990's. The technology early found its application in Ophthalmology. At the beginning of the 2000's, the agreement for the reimbursement of OCT exams was obtained in Europe and in the USA. The OCT market started to grow quickly with a CAGR between 15 and 20 %. Around ten years later, the growth of the OCT ophthalmology market slowed down because of saturation in the Western countries.

flow, such as increasing the intraocular pressure, result in a fluorescein angiogram that shows a lobular pattern to the filling, but the actual vessels in any given lobule are not seen ( Singh Hayreh, 1974 ). Although indocyanine green dye shows less leakage than fluorescein, it still leaks from the choriocapillaris and stains Bruch's membrane and the chor- oidal stroma ( Chang et al., 1998 ). Optical coherence tomography (OCT) has high axial resolution, but without adaptive optics, the lateral re- solution is insu fficient to visualize the choriocapillaris clearly in the posterior pole. While OCTA has comparable lateral resolution to structural OCT, it can detect choriocapillaris blood flow, producing contrast between the RPE and choriocapillaris ( Choi et al., 2013a; Motaghiannezam et al., 2012; Sohrab et al., 2012; Spaide et al., 2015b ). When the inner most thickness of the choroid is imaged with OCTA a granular image is obtained that is suggestive of the choriocapillaris ( Fig. 32 ). Since OCTA has similar resolution to structural OCT, it may seem paradoxical that flow can be detected. In the early days of radio it was common to for enterprising individuals to make crystal radios, which could passively detect radio signals, at the time often originating from a radio station in the local town. The selectivity of the crystal radio was typically limited, which means the ability to differentiate closely spaced stations on the radio frequency spectrum would be im- paired. In this case, both stations would be heard, and it would not be possible with the primitive tuning used in early crystal sets to separate them. In an analogous manner even though structural OCT may not be able to separate closely placed capillaries, an OCTA flow signal may still be detected. This signal is used to create an image of the chor- iocapillaris.

Keywords: optical coherence tomography, non-invasive imaging, high resolution, tissue, medical imaging. 1. INTRODUCTION Optical coherence tomography (OCT) was invented by combining low coherence reflectometry with lateral scanning of an investigating optical beam [1]. The number of reports on optical coherence tomography (OCT) continues to double every three years. Although the subject was initially confined to a dedicated conference at the Biomedical Optics Photonics West, it has spread to other related conferences. There is practically no conference on biomedical optics without OCT. Optical waves do not penetrate much into tissue and OCT is the method that makes the most out of the little depth interval, which in retina and skin is below 2 mm. However, the number of resolved points along the axial coordinate continued to grow given the attention given to enlargement in the optical spectrum of OCT sources. With the recent advances in ultra-wide bandwidth sources, axial resolution was improved to below the level of 1 micron, within a resolution interval achievable so far by confocal microscopy only. The last decade has also witnessed increase in the rate of delivery of depth resolved information, which has exceeded several MHz for sufficient signal to noise ratio [2]. Acquisition speed increase has enabled recent reports of high resolution delivery of several volumes a second useful in real time display of surgery intervention and ablation. Another relevant progress was that in multimodality, with OCT integrated with other optical techniques, such as fluorescence and optoacoustics and the tendency of OCT groups to identify OCT applications outside the traditional field of eye imaging.

(Paper JBO IB-005 received Aug. 4, 1997; revised manuscript received Nov. 10, 1997; accepted for publication May 5, 1998.) A BSTRACT A new noninvasive technique that reveals cross sectional images of scattering media is presented. It is based on a continuous wave frequency modulated radar, but uses a tunable laser in the near infrared. As the full width at half maximum resolution of 16 mm is demonstrated with an external cavity laser, the chirp optical coherence tomography becomes an alternative to conventional short coherence tomography with the advan- tage of a simplified optical setup. The analysis of two-layer solid phantoms shows that the backscattered light gets stronger with decreasing anisotropic factor and increasing scattering coefficient, as predicted by Monte Carlo simulations. By introducing a two-phase chirp sequence, the combination of lateral resolved perfusion and depth resolved structure is shown. © 1998 Society of Photo-Optical Instrumentation Engineers. [S1083-3668(98)00503-6]

The optic disk and the RNFL are the principal sites of apparent glaucomatous damage which precedes glaucomatous visual field alterations. RNFL defects are known to precede detection of visual field defects by approximately 6 years. Accurate early detection and monitoring of ONH and RNFL defects has become the prime focus of effective management of glaucoma. Optical coherence tomography employs low-coherence interferometry to obtain cross-sectional images of the ocular tissues.

licenses/by-nc/3.0) which permits unrestricted non- commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Optical coherence tomography (OCT) has been recently applied to investigate cor- onary artery disease in interventional cardiology. Compared to intravascular ultra- sound, OCT is able to visualize various vascular structures more clearly with high- er resolution. Several validation studies have shown that OCT is more accurate in evaluating neointimal tissue after coronary stent implantation than intravascular ultrasound. Novel findings on OCT evaluation include the detection of strut cover- age and the characterization of neointimal tissue in an in-vivo setting. In a previous study, neointimal healing of stent strut was pathologically the most important fac- tor associated with stent thrombosis, a fatal complication, in patients treated with drug-eluting stent (DES). Recently, OCT-defined coverage of a stent strut was pro- posed to be related with clinical safety in DES-treated patients. Neoatherosclerosis is an atheromatous change of neointimal tissue within the stented segment. Clini- cal studies using OCT revealed neoatherosclerosis contributed to late-phase lumi- nal narrowing after stent implantation. Like de novo native coronary lesions, the clinical presentation of OCT-derived neoatherosclerosis varied from stable angina to acute coronary syndrome including late stent thrombosis. Thus, early identifica- tion of neoatherosclerosis with OCT may predict clinical deterioration in patients treated with coronary stent. Additionally, intravascular OCT evaluation provides additive information about the performance of coronary stent. In the near future, new advances in OCT technology will help reduce complications with stent thera- py and accelerating in the study of interventional cardiology.

In the first part of this chapter, we introduced spectral-domain optical coherence tomography (SD-OCT). The SNR of a shot-noise-limited SD-OCT system was earlier defined in Eq. 13 as η P sample τ i /E v . The SNR performance of an SD-OCT system improves with increasing sample arm power or increased integration time. Most importantly, Eq. 13 shows that the SNR performance of an SD-OCT system is independent of the bandwidth of the source. In theory, by combining SD-OCT with an ultra-broadband source, high-resolution imaging at high acquisition rates should become within reach. This new technique may facilitate the diagnosis and monitoring of several ocular diseases, such as glaucoma, diabetic retinopathy, cancer and age- related macular degeneration. The first in vivo retinal SD-OCT images were presented by Wojtkowski et al. 11 Earlier in this chapter, we demonstrated an SD-OCT system suitable for in vivo video-rate ophthalmic imaging. 3, 4 This system had a sensitivity of 98.4 dB, an acquisition rate of 29,300 A-lines per second and an axial resolution of 6 µm in the eye at a safe ocular exposure level of 600 µW. Here we will quantify the dynamic range within a structural intensity image and the axial resolution of an SD- OCT system equipped with an ultra-broadband source and identify previously unseen features in the retina.

Optical coherence tomography (OCT) is an optical acquisition method to examine biological tissues. In recent years, OCT has become an important imaging technology used in diagnosing and following macular pathologies. Further development enabled application of optical coherence tomography in evaluation of the integrity of the nerve fiber layer, optic nerve cupping, anterior chamber angle, or corneal topography. In this manuscript we overview the use of OCT in the clinical practice to enable corneal, iris, ciliary body, and angle evaluation and diagnostics.

T. G. Van Leeuwen & M. Van Beurden & T. J. M. Ruers Received: 31 January 2013 / Accepted: 19 February 2013 # The Author(s) 2013. This article is published with open access at Springerlink.com Abstract Optical coherence tomography (OCT) is an optical technique that measures the backscattering of near-infrared light by tissue. OCT yields in 2D and 3D images at micrometer-scale resolution, thus providing optical biopsies, approaching the resolution of histopathological imaging. The technique has shown to allow in vivo differentiation between benign and malignant epithelial tissue, through qualitative assessment of OCT images, as well as by quantitative evalu- ation, e.g., functional OCT. This study aims to summarize the

1. INTRODUCTION Optical-coherence tomography (OCT) is a non-invasive high-resolution imaging modality which employs non-ionising optical radiation. OCT derives from low-coherence interferometry. This is an absolute measurement technique which was developed for high- resolution ranging and characterisation of optoelectronic components 1 . The first application of low-coherence interferometry in the biomedical optics field was for the measurement of eye length 2 . Adding lateral scanning to a low-coherence interferometer, allows depth resolved acquisition of 3D information from the volume of biologic material 3 . The concept was initially employed in heterodyne scanning microscopy. OCT has the potential of achieving high-depth resolution, which is determined by the coherence length of the source. Optical sources are now available with coherence lengths below 1 µm. When combined with confocal microscopy, OCT adds improved depth resolution and sensitivity.

Received: 2 February 2014; in revised form: 18 April 2014 / Accepted: 22 April 2014 / Published: 5 May 2014 Abstract: Optical coherence tomography (OCT) is a real-time optical imaging technique that is similar in principle to ultrasonography, but employs light instead of sound waves and allows depth-resolved images with near-microscopic resolution. Endoscopic OCT allows the evaluation of broad-field and subsurface areas and can be used ancillary to standard endoscopy, narrow band imaging, chromoendoscopy, magnification endoscopy, and confocal endomicroscopy. This review article will provide an overview of the clinical utility of endoscopic OCT in the gastrointestinal tract and of recent achievements using state-of-the-art endoscopic 3D-OCT imaging systems.

Fig. 2. Typical SD-OCT (left) from (Wang, 2007) with permission and SS-OCT system (right). 1.2 Principles of Swept Source Optical Coherence Tomography (SS-OCT) Detailed aspects of traditional time-domain OCT theory and applications, such as resolution, sensitivity, noise, signal-to-noise ratio, etc, could be found in many literatures (Schmitt, 1999; Fercher et al., 2003), and will not be covered in this chapter. For FD-OCT, the optical paths of light beam are the same as those in TD-OCT techniques. The light beam from the source is split by a fiber coupler or beam splitter and comes through and back in the reference arm and the sample arm. The beam is then recombined at the coupler or the beam splitter and forms an interferometric signal. In SD-OCT a spectrogram is obtained by using a dispersive spectrometer with a CCD camera in the detector arm, and in SS-OCT a spectrogram is obtained by using a frequency swept laser or tunable laser with just a single detector and without dispersion components.

Keywords: retinal, optical coherence tomography, quinine toxicity Introduction Acute quinine poisoning is a devastating ocular event that can result in severe loss of vision. 1 Quinine is an antimalarial agent that is often used for the treatment of restless legs syndrome. The drug has a narrow therapeutic range and carries the risk of multiple side effects such as cinchonism, hypoglycemia, arrhythmias, hemolysis, gastrointestinal intolerance, fever, and nephritis. 2 In early 2007, the US Food and Drug Administration restricted the use of quinine solely to the treatment of malaria.

Key words: Optical Coherence Tomography, Surface Measurements, Optical Instruments, Three- Dimensional Image Acquisition, Visual Optics, Ophthalmic Instrumentation. RESUMEN: Presentamos en este artículo la cuantificación de los aspectos geométricos del segmento anterior del ojo empleando Tomografía de Coherencia Óptica del segmento anterior (AS-OCT). Esta cuantificación se logró mediante el desarrollo de nuestra metodología que incluye el desarrollo de dispositivos y la calibración de sus distorsiones (Fan y refracción) para imagen 3-D, así como el desarrollo de herramientas de procesado automáticas. La comparación estadística de las topografías corregidas de la superficie anterior de la córnea comparada con aquellos topógrafos estándares en el ámbito clínico, supone una contribución hacia los sistemas topográficos basados únicamente en OCT.

Optical coherence tomography angiography for the cornea The healthy human cornea is avascular and transpar- ent [2]. It is believed that the balance of angiogenic and anti-angiogenic factors is important to ensure this avascularity and is maintained by the inhibition of immune and inflammatory response [1, 12]. Corneal vascularisation is a pathological condition whereby the normal avascular cornea loses transparency due to the ingrowth of blood vessels [12]. It can result from diverse aetiologies, including chemical injury, chronic hypoxic conditions from contact lens use, limbal stem cell deficiency and infections such as herpes and trachoma [12, 14, 16]. Detecting and treating corneal vascularisation is critical as it can lead to adverse consequences, such as persistent inflammation, cor- neal oedema, scarring, significant loss in visual acuity or even blindness [16]. In addition, corneal vascular- isation can cause disruption of the cornea’s immuno- logically privileged state, and thus increases the risk of graft rejection from corneal transplant procedures [1, 14]. Qualitative and quantitative assessments are needed for optimal disease monitoring, treatment plan- ning and prognostic evaluation [4]. Studies have shown Table 1 Comparison of currently available OCTA systems for imaging the anterior segment of the eye

The authors report a case of severe systemic methanol toxicity and relatively mild optic neuropathy demonstrating unique retinal changes on optical coherence tomography (OCT). Case presentation: A previously healthy student developed ataxia, difficulty breathing and loss of consciousness hours after drinking homemade alcohol while traveling in Indonesia. She was found to have a serum pH of 6.79 and elevated methanol levels. She was treated with intravenous ethanol, methylprednisolone and sodium bicarbonate.

Main body: Optical coherence tomography (OCT) is a non‑invasive technology that acquires cross‑sectional images of retinal structures allowing neural fundus integrity assessment. Several previous studies demonstrated that both peripapillary retinal nerve fiber layer and macular thickness measurements assessed by OCT were able to detect neu‑ ronal loss in AD. Moreover, recent advances in OCT technology, have allowed substantial enhancement in ultrastruc‑

19. Optical coherence tomography and its applications in glaucoma The RNFL represents a highly reflective band whose anterior limit begins at the vitreo- retinal interface, and the posterior limit is detected on the basis of signal thresholding. OCT provides both qualitative and quantitative information regarding the optic nerve and the RNFL. The images in this chapter are based on Cirrus 3D HD domain, therefore, may differ from older versions of the OCT. The measurement of the RNFL thickness marks one of the most important applications of OCT in the field of ophthalmology. The authenticity of pathological specificity of OCT RNFL measurements correlates with histological measurements in primate and human studies, and this has been correlated with visual field changes as well (Chen et al., 2006). An OCT RNFL report generated by Cirrus-3D HD OCT in a normal person reads as following: Figure 30.