Acoustic-resolution photoacoustic microscopy (AR-PAM) with dark-field confocal illumination enables unique
high-resolution visualization of chromophores in tissue, such as microvasculatures, within depths of a few millimeters.
However, most current systems are bulky and use complex optical components for illumination, thus requiring highly
sensitive alignment. In this study, we developed a compact alignment-free acoustic-resolution photoacoustic imaging
system with simple fiber illumination. Four optical fibers were placed in four directions around a high-frequency
(30-MHz) ultrasound sensor attached with the high-numerical-aperture acoustic lens. The setting angle of the fibers were
determined to form a dark field on the tissue surface under the acoustic lens and for the four light beams from the fibers
to be combined near the focal point of the acoustic lens, i.e., at a depth of around 1.2 mm in the tissue. The acoustic lens
and output ends of the fibers were capped with an acoustically and optically transparent engineering plastic sheet, whose
surface can be directly placed and scanned on the tissue surface with ultrasound gel. The diameter and height of this
imaging head were as small as 32 mm and 27 mm respectively. The phantom study showed that the lateral signal
spreading was 120 μm, which agreed well with the theoretical value of 112 μm. With the system, we attempted to image
vasculatures in the rat skin, demonstrating high-contrast visualization of the blood vessels of a few hundred micrometers
in diameter in the tissue.