Abstract
We developed a reflection-mode subwavelength-resolution photoacoustic microscopy system capable of imaging optical absorption contrast in vivo. The simultaneous high-resolution and reflection-mode imaging capacity of the system was enabled by delicately configuring a miniature high-frequency ultrasonic transducer tightly under a water-immersion objective with numerical aperture of 1.0. At 532-nm laser illumination, the lateral resolution of the system was measured to be ~320 nm. With this system, subcellular structures of red blood cells and B16 melanoma cells were resolved ex vivo; microvessels, including individual capillaries, in a mouse ear were clearly imaged label-freely in vivo, using the intrinsic optical absorption from hemoglobin. The current study suggests that, the optical-absorption contrast, subwavelength resolution, and reflection-mode ability of the developed photoacoustic microscopy may empower a wide range of biomedical studies for visualizing cellular and/or subcellular structures.
Highlights
Photoacoustic imaging is capable of noninvasively mapping the optical absorption properties of biological tissue, by detecting transient thermo-elastic expansion induced acoustic waves from the absorption of pulsed optical energy [1,2]
We developed a reflection-mode subwavelength-resolution photoacoustic microscopy system capable of imaging optical absorption contrast in vivo
The simultaneous high-resolution and reflection-mode imaging capacity of the system was enabled by delicately configuring a miniature high-frequency ultrasonic transducer tightly under a water-immersion objective with numerical aperture of 1.0
Summary
Photoacoustic imaging is capable of noninvasively mapping the optical absorption properties of biological tissue, by detecting transient thermo-elastic expansion induced acoustic (photoacoustic) waves from the absorption of pulsed optical energy [1,2]. In transmission-mode OR-PAM, the photoacoustic excitation (laser illumination) and ultrasonic detection are at the opposite sides of the imaging target, which limits its application to only in vitro cell samples or thin biological tissue of body extremities (e.g., the ear) [3,5]. OR-PAM in reflection mode configures its laser illumination and ultrasonic detection at the same side of the imaging target [4,6] As a result, it allows the imaging of many more anatomic sites, including the brain and the eye. It allows the imaging of many more anatomic sites, including the brain and the eye It is quite challenging for reflection-mode systems to realize fine subwavelength resolution due to the very limited working distance of optical objectives with high numerical aperture (NA) [3,13]. The reported highest resolution of reflection-mode OR-PAM is about 500 nm with an optical objective of 0.63 NA at 532-nm laser illumination [6]
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