## 3 Optical Imaging Modalities to Develop and Verify Diagnosis/Treatment Techniques of Stroke
### Photoacoustic Imaging and Tomography  
Photoacoustic imaging is a label-free bio-imaging technique in which minute thermal expansion and its consequent pressure changes, which are caused by excitations of pulsed light to a specific biomaterial, are measured in a range of acoustic or ultrasonic waves, and are scanned in multiple dimensions to obtain information on a distribution of a particular biomaterial [95,96]. In detail, photoacoustic imaging consists largely of five steps to obtain images: pulsed light irradiation to excite a specific material, excitation \(\rightarrow\) relaxation, increasing a pressure, measurement of the increased pressure, and finally, image reconstruction [97]. This technique is widely employed in in vivo studies due to its advantages such as deeper imaging depth compared to that of optical imaging, which gives and receives optical signals, and it is label-free [98, 99, 100]. It also exhibits potential for application in minimally invasive clinical diagnosis of several diseases such as breast cancer and tumors [102,103]. As a research tool for stroke, several studies were conducted to analyze photoacoustic signals generated by oxy-/deoxy-hemoglobin for understanding processes of stroke and verifying diagnostic/therapeutic techniques.  
[PERSON] et al. developed a high-resolution photoacoustic imaging system that allows precise, three-dimensional observation of blood vessels in mice brains [104]. The system consists of a nanosecond pulsed LASER with excitation wavelength ranges between 590 nm and 889 nm, and an interferometric detection film as a photoacoustic signal sensor. Moreover, it provides images of brain blood vessels with a high sensitivity, precision and greater image acquisition depth. [PERSON] et al. investigated a high-speed photoacoustic imaging system to acquire time-varying distributions of blood vessels in mice brains [105]. The photoacoustic microscopy measured a loss of vessels caused by micro-hematoma and  
Figure 4: Schematic of an optical coherent tomographic imaging system for in vivo brain angiography and optical coherence tomography angiographic images of acute ischemic mice before inducing stroke and after one week. Reprinted with permission from Ref. [92]. Copyright 2013 Public Library of Science.  
laser irradiation using precise photoacoustic brain blood vessels image acquisition, and an analysis of shrinkage response of cerebral blood vessels, which was carried out by injection of phenylephrine, was also performed using photoacoustic imaging. [PERSON] et al. employed a multispectral optoacoustic imaging to acquire oxy-/deoxy-hemoglobin distributions of mice brains when an artificial stroke was induced by a MCAO, as illustrated in Figure 5 [106]. The imaging method using this proposed optoacoustic imaging system is meaningful in that it provides functional hemodynamic images with enhanced depth ranges while ensuring noninvasiveness. [PERSON] et al. monitored responses of blood vessel distributions and blood oxygen saturations in a rat brain after an induction of acute stroke using a multimodal imaging system, which consisted of laser speckle and photoacoustic imaging [107]. [PERSON] et al. developed a multispectral photoacoustic imaging system for a noninvasive analysis of intracerebral hematoma locations induced by collagenase [108]. Images computed using photoacoustic signals at 532 nm, 750 nm, and 875 nm as excitation wavelengths offered location and size information of a lesion in hematoma and perihematomal regions. [PERSON] et al. applied multispectral photoacoustic imaging to acquire noninvasive measurements of a matrix-metalloproteinase, a unique group of enzymes associated with acute stroke and brain damages, in in vivo mouse model of an ischemic stroke [109].