A method for obtaining a 3-D OR-PAM image of microvasculature within a region of interest of a subject is provided. The method includes: focusing a first light pulse at a first depth beneath a first surface position within the region of interest; receiving a first PA signal in response to the first light pulse; focusing a second light pulse at a second depth beneath the first surface position within the region of interest; receiving a second PA signal in response to the second light pulse; and forming the 3-D OR-PAM image by combining the first PA signal and the second PA signal.
Single-cell label-free photoacoustic flowoxigraphy in vivo