# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 4 Current PAI Technology Used in Brain Imaging
#### Brain Studies Utilising PAI
VSD was delivered through the BBB, opened by pharmacological modulation using adenosine receptor signaling. Normalized time-frequency analysis presented in vivo VSD response in the seizure group which was significantly distinguishable from the control groups at sub-mm spatial resolution. This gives a promising non-invasive tool for in vivo monitoring of brain activities. Finally, it is known that copper enrichment in the brain is highly related to AD pathogenesis, [PERSON] et al. developed contrast agents with low molecular weights, which can specifically chelate with Cu\({}^{2+}\) to form radicals with turn-on PAsignals in the NIR region [118]. The agents significantly enhanced the radical stability and PA intensity in PAT. The best agent showed a fast response to Cu\({}^{2+}\) with high selectivity and a low PA detection limit of 90.9 nm. Owing to the low molecular weight and amphiphilic structure, the agents could effectively cross BBB and thus allowed to visualize Cu\({}^{2+}\) in vivo via PA imaging in the brains of AD mice. Other than small animal imaging, PAI has been used in human soft tissue study, imaging from initial human breast tumors to vasculature or tissue abnormalities in arm, palm, finger and abdomen. However, photoacoustic imaging of the human adult brain is much more difficult due to much thicker human adult skull [33] which greatly attenuates incident light into and photoacoustic wave out of the brain. It is known that human adult skull has a thickness of 7 mm to 11 mm, compared with less than 1 mm for small animals such as mouse [34]. Available research shows that, for example, in the optical tissue window near 1064 nm, there is only 2.1% light transmittance through the human skull [35]. Meanwhile, the skull produces 20 dB/cm acoustic attenuation at 1 MHz for the PA signal [138]. A so-called photon recycle was developed to increase light transmittance through skull, which improved the PA SNR by a factor of 2.4 and used to study functional activities of the human cerebral [35]. The attenuation and distortion of the skull for photoacoustic wave due to acoustic impedance mismatch between the skull and underlying tissue layers still need to further study [139]. In recent years, PAT systems have been developed for mapping cerebral blood oxygenation in human adults and neonates [36; 140]. The systems provided noninvasive, transcranial PA imaging in the near-infrared tissue window. The systems measured blood oxygenation in neonates and adults at different lateral sites from the superior sagittal sinus (SSS), allowing for the mapping of cerebral blood oxygenation. Due to the skull thickness of a neonatal infant is only about 1.3 mm [37], much thinner than that of an adult, transcranial PA imaging of the human brain was therefore mainly focused on neonatal infant [128; 141; 37; 142; 143]. For example, [PERSON] et al. [37] used PAT to image blood vessels through infant skull with 50 um axial resolution and 420 um lateral resolution. The imaging depth can be as deep as 21 mm or more. However, even though neonatal skull is thin, an accurate skull aberration correction algorithm [139; 140; 141] still needs in noninvasively PA imaging of the neonatal brain. At last, it is worth mentioning that some researchers used big animal head to simulate child head in brain study by PAT, as both sizes are similar [PERSON] and [PERSON] applied PAT to image the brain cortex of monkey through the intact scalp and skull ex vivo [144]. [PERSON] et al. [145] investigated neonatal piglets noninvasively, and to monitor HbO saturation in the cerebral SSS in vivo under varying conditions of hypoxia and ischemia.