Hemodynamic responses of the brain to hypoxia or ischemia are one of the major interests in neurosurgery and
neuroscience. In this study, we performed real-time transcutaneous PA imaging of the rat brain that was exposed to a
hypoxic stress and investigated depth-resolved responses of the brain, including the hippocampus. A linear-array 8ch
10-MHz ultrasonic sensor (measurement length, 10 mm) was placed on the shaved scalp. Nanosecond, 570-nm and 595-
nm light pulses were used to excite PA signals indicating cerebral blood volume (CBV) and blood deoxygenation,
respectively. Under spontaneous respiration, inhalation gas was switched from air to nitrogen, and then reswitched to
oxygen, during which real-time PA imaging was performed continuously. High-contrast PA signals were observed from
the depth regions corresponding to the scalp, skull, cortex and hippocampus. After starting hypoxia, PA signals at 595
nm increased immediately in both the cortex and hippocampus for about 1.5 min, showing hemoglobin deoxygenation.
On the other hand, PA signals at 570 nm coming from these regions did not increase in the early phase but started to
increase at about 1.5 min after starting hypoxia, indicating reactive hyperemia to hypoxia. During hypoxia, PA signals
coming from the scalp decreased transiently, which is presumably due to compensatory response in the peripheral tissue
to preserve blood perfusion in the brain. The reoxygenation caused a gradual recovery of these PA signals. These
findings demonstrate the usefulness of PA imaging for real-time, depth-resolved observation of cerebral hemodynamics.