A schematic of the PA imaging system with the chamber on the monkey’s head is shown in Fig. 1. During the imaging process, the cover of the chamber was removed to expose the dural surface, and saline solution was used to fill the chamber. The detection surface of PA detector was then immersed in saline solution. The illumination light source was a Q-switched 30-Hz pulsed Nd:YAG laser (Surelite OPO PLUS; Continuum, Santa Clara, CA) producing light at a 532-nm wavelength. The light was delivered through an optical fiber to the dural surface overlying motor cortex. The incident energy density of the laser that reached the dural surface was less than , which complies with the safety limit for human skin exposure.12 The PA signal from the cortex was collected with a 20-MHz focused ultrasound transducer (V316, focal distance: 0.5 in., Olympus-NDT, Waltham, MA). The detected PA signal was amplified (5072PR, Olympus-NDT, Waltham, MA) and collected using a data acquisition board (CS21G8, Gage, Lockport, IL). Two automated stepper motors [Fig. 1(a)] were used to allow two-dimensional scanning of the brain. The scanning step size was 100 μm for both directions in the scanning area. A third manual translation stage was used to adjust the height of the sensor and laser source above the dural surface. At each scan position, A-line data (2000 samples, 250-MHz sampling rate) was acquired.