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Since the introduction of OCT, the non-invasive technique has been mainly applied for imaging in ophthalmology and optometry [67, 68]. Advances in OCT technology have made it possible to be used in a wide variety of applications during the past decade. The micrometre spatial resolution allows high precision investigations of both interior imaging and surface topography of specimen. The superior interferometric detection sensitivity enables optical radiation to penetrate reasonably deep into biomedical tissues and industrial materials [4, 69]. This section only presents a brief overview of some OCT applications, covering the main biomedical and non-biomedical applications.

a. Biomedical Applications

Ophthalmology is still the dominating field of the biomedical OCT. The ocular media generally exhibit high transmittance and low scattering in the visible and NIR region, which allows effective interferometric sensitivity and precision in the OCT imaging [3]. Furthermore, imaging of the fundus of the eye, especially the retinal structure can be perfectly fulfilled by OCT with high depth resolution at a lower beam NA. [4] (i.e. better penetration depth). Retinal OCT has been reigning in ophthalmology studies for the measurement of details of retinal pathologies [14, 56, 67, 70] and diagnosis of retinal disorders, such as macular degeneration [71–74] and macular oedema [75–79]. The evaluation of retinal nerve fibre layer, optic nerve head and macular thickness using OCT helps the detection and management of glaucoma [80–84]. Recently, the OCT technology is enhanced to be able to penetrate into the choroid [85–88] and measure choroidal thickness in normal eyes [89, 90].

In addition, OCT is helpful to investigate anterior segment [14, 91, 92], i.e. to image and measure corneal pathologies and structural changes of the chamber angle and iris. Anterior segment OCT provides comparable results with the conventional ultrasound biomicroscopy (UBM) method, but OCT is featured with a higher spatial resolution [93, 94]. Furthermore, OCT has also been applied to monitor the corneal laser ablation [95, 96] and cataract surgery [97].

OCT is also an important analytical tool for other biomedical disciplines such as cardiology [47, 98] and gastroenterology [98]. Both of the two subjects require intraluminal imaging into the small coronary arteries and the gastrointestinal (GI) tract, respectively. These conditions are accessible to endoscopic OCT, which uses a catheter integrated with fibre optics to detect disorders at the innermost layer [4].

Intracoronary OCT has been demonstrated as a safe and effective modality for characterising coronary atherosclerosis and vulnerable plaques [99–102], which are primary indications of acute coronary events including acute myocardial infarction (AMI) [103] and acute coronary syndromes (ACS) [104]. OCT delivers consistent image features with intravascular ultrasound (IVUS), which is the conventional standard for intracoronary imaging [99, 103]. However, OCT images turn out to have much better depth resolution than high frequency IVUS [105–107].

Likewise, endoscopic OCT with a catheter probe is applied to investigate GI track with several millimetres penetration depth and micrometre-scale spatial resolution. The GI wall structure is of a multiple layer architecture. OCT characterisation of the GI wall permits an accurate evaluation of the mucosa, lamina propria, muscularis mucosae, and part of the submucosa [4, 108, 109]. OCT has been used to predict the presence of many disorders of the GI tract, including dysplasia [110–114], metaplasia [115, 116], Barrett’s esophagus [111, 112, 117, 118], intramucosal carcinoma [112], adenocarcinoma [113, 117], malignancy [114, 119, 120], etc. Furthermore, the pancreatico-biliary ductal system can be explored by a side-view endoscopic OCT catheter probe [113]. The OCT resolution can be 10 times better (about 10 µm) than with the emerging catheter-probe endoscopic ultrasonography (CPEUS), but OCT’s depth of penetration is up to 3 mm [113]. Hence, it is potential that OCT and CPEUS can become complementary techniques for high-resolution endoscopic imaging of the GI tract [121].

Further biomedical applications using OCT techniques are in developmental biology, dermatology, dentistry, laryngology, pulmonary medicine, etc. [122–124]. Their histological architectures are evaluated by OCT, functioning as the optical biopsy to make an instant diagnosis at endoscopy. Previously, this was only possible by using histological or cytological analysis, which requires removal of a tissue specimen and processing for microscopic examination [2, 32, 125, 126].

b. Non-biomedical Applications

OCT enables a non-contact, non-invasive, and high-resolution imaging of subsurface features. It is already a well-established diagnostics technique in biomedical areas. However, OCT has been receiving attention as a non-destructive testing (NDT) tool for non-biomedical applications, with more and more research groups being engaged in further exploring possible applications of OCT [127, 128].

First, the use of OCT technique has grown in art and cultural heritage artefacts studies, e.g. non-destructive investigations of varnish layer, paint layer and underdrawings in oil

paintings [5, 129, 130]. Second, layer structures (or thicknesses) of moving plastic foils can be monitored with an in-line OCT system along with the foil production [131], which is applicable in the food packaging industry. Third, the potential of OCT has been demonstrated to non-destructively monitor coating structures of pharmaceutical tablets [7, 132, 133]. This helps control the manufacturing procedure and maintain the drug effectiveness in a more precise manner than the conventional ways, such as the weight-gain method.

Besides, OCT has also been used for contactless and faster measurements of textile roughness or surface topography, e.g. in printed electronics products quality [6] and in paper industry [134, 135]. Furthermore, OCT has been introduced for the characterisation of laser-drilled holes and micro-machined devices [136], organic solar cells [137], aerospace materials [138, 139], etc. Hence, OCT techniques help NDT of a wide variety of material systems and processes. The use of faster and more robust OCT devices in industrial environments is desired to enable thein situand real-time monitoring of industrial processes [127].