## 3 Optical Imaging Modalities to Develop and Verify Diagnosis/Treatment Techniques of Stroke
### Optical Coherence Tomography  
Optical coherence tomography (OCT) is an imaging technique that employs an infrared interferometer to obtain optical interference information in depth using light scattering materials and scans it to acquire two- or three-dimensional images of optical coherence information [78, 79, 80]. An imaging resolution of OCT is typically in micron units, and OCT is generally used as a tool and a diagnostic device for obtaining biomedical information such as eyes (retina) and skin [81, 82, 83, 84]. A more advanced form of OCT, which can acquire bio-information in deeper areas through light sources with a longer wavelength and can be used for acquiring functional information such as vascular distributions through high-speed signal measurements and post-processing techniques, has been developed, which increases a scope of its applications [85, 86, 87, 88]. In this section, a series of studies that conducted studies of cerebrovascular stroke by analyzing information about blood vessels and blood flow rates using advanced OCT instruments are discussed.  
[PERSON] et al. investigated a Doppler OCT to measure blood circulation in ischemic stroke-induced mice [89]. The study employed a 532 nm LASER to induce an ischemic stroke and changes in blood flow in a localized area, where stroke was generated, were observed using spectral Doppler OCT. [PERSON] et al. analyzed structural brain damages induced by artificial stroke [90]. The rat brain was extracted after different time periods after stroke was induced, and areas of brain damages were imaged using OCT. The results suggested a possibility of OCT as a pathological analysis equipment for ex vivo brain tissues with an acute or chronic stroke. As a follow-up study, an application of an image analysis based on artificial neural networks was additionally applied to classifications of stroke injuries in ex vivo brain tissues [91]. [PERSON] et al. employed a depth-resolved OCT angiographic imaging, which was integrated in a conventional upright microscope, to analyze acute cerebral ischemic stroke, which was induced by distal MCAO, and a chronic restoration of blood vessels over time in in vivo small animal studies, as illustrated in Figure 4 [92]. [PERSON] et al. applied depth-coded visible OCT angiography for making accurate analyses of microvascular blood flow variations when a localized ischemic stroke was induced through photothromobis [93; 94]. In this study, the difference in the blood oxygen saturation caused by a localized ischemic stroke induction was also observed simultaneously by calculating oxygen saturations using the spectroscopic images from visible OCT angiography.  
Figure 3: (a) Near-infrared images of in vivo fluorescence using fibrin deposition in stroke-induced mice at 6, 8, 20, and 96 h after stroke was triggered. Reprinted with permission from Ref. [56]. Copyright 2012 Public Library of Science. (b) Near-infrared optical diffusion tomographic rat brain images of oxygenate/deoxygenate/total hemoglobin concentrations 30 min after an ischemic stroke. Reprinted with permission from Ref. [63]. Copyright 2014 Elsevier B.V. (c) _f_NIRS probe arrays designed for an analysis of acute ischemic stroke patients in clinical tests and computed tomographic images used to determine a measurement possibility for areas with and without brain lesions. Reprinted with permission from Ref. [65]. Copyright 2019 International Society for Optics and Photonics (SPIE).