# Photoacoustic Imaging in Oxygen Detection
## 2 Photoacoustic Imaging in sO\({}_{2}\) Measurement
Unlike conventional spectral-based methods that give only oxygen saturation levels, PAI can yield simultaneous 3D imaging information including anatomical, functional, histological, and metabolic parameters. For example, together with vascular diameter and blood flow, PAI is a unique technique for measuring metabolic rate of oxygen (MRO\({}_{2}\)), which is helpful for fundamental pathophysiological studies, and even early diagnosis and treatment of diseases. By using a high-frequency ultrasonic transducer with a large bandwidth and high numerical aperture, the detected acoustic waves can be reconstructed and be localized into a very small volume with high spatial resolution to give information about the optical absorption distribution inside small samples. Specifically, under the condition that the illumination and ultrasonic focal volumes assembled well confocally, a PAM system is capable of providing a signal-to-background contrast of 3500% with high sensitivity and specificity in vivo [21]. With a single small-vessel-resolution photoacoustic microscopy (PAM) system, [PERSON] et al. [22] calculated the metabolism rate of oxygen as well as its changes with environmental thermal conditions, and for the first time obtained five quantitative parameters simultaneously. As shown in Figure 2a, hemodynamic responses to a hyperthermic challenge (red bars) and the relative changes of the skin temperature, vessel diameter, total hemoglobin concentration, sO\({}_{2}\), blood flow speed, oxygen extraction fraction (OEF), and metabolic rate of oxygen (MRO\({}_{2}\)) are measured by PAM. It must be noted, however, these parameters in the aforementioned study were measured in different spatial scales, which affected the resolution of imaging MRO\({}_{2}\). Later on,Ning et al. [23] simultaneously quantified microvascular diameter, oxygen saturation of hemoglobin, and blood flow at the same spatial scale and improved the resolution down to a capillary level in vivo. As can be seen from Figure 2b, Simultaneous PAM of these factors (from i to iii) and their dynamic changes (from iv to vii) revealed by vessel segmentations are shown. Apart from resolution, fast imaging speed is also pursued to capture the blood oxygenation process, which reflects the acceleration of sO\({}_{2}\) change. For example, [PERSON] et al. [24] developed a high-speed functional PAM system for mouse brain imaging, and demonstrated capillary-level-resolution blood oxygenation with a one-dimensional imaging rate of 100 kHz. As shown in Figure 2c, the left hindlimb of a mouse is electrically stimulated, sO\({}_{2}\) in different conditions are imaged, and fractional changes in the cerebral blood flow (CBF), oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO\({}_{2}\)) in the core responding region are quickly recorded. [PERSON] et al. [25] trialed dynamic hemoglobin oxygenation in a pregnant mouse using spectral photoacoustic imaging, which is the first step towards functional imaging of the maternal-fetal environment (Figure 2d). Figure 1: Absorption spectra of major endogenous contrast agents in biological tissue at normal concentrations (reproduced with permission from [16]. Copyright Elsevier, 2014). Figure 2: \(Cont.\)  
Figure 2: _Cont._  
Not only as a reflection of blood-oxygen supply, sO\({}_{2}\) is also used to infer tissue oxygen condition in most of the work done with PAI. The hypothesis is that pO\({}_{2}\) of tissue has a close relationship with sO\({}_{2}\) in blood, and pO\({}_{2}\) may increase with increased oxygen delivery and decrease with insufficient blood oxygen [26]. That said, sO\({}_{2}\) reflects oxygen supply and delivery, but does not reflect the oxygen consumption in tissue, thus yielding an inaccurate estimation of oxygen level inside soft tissue.