December 2014
Spotlight Summary by Summer Gibbs
Reflection-mode in vivo photoacoustic microscopy with subwavelength lateral resolution
Interest in photoacoustic microscopy has grown tremendously since its first publication in 2005. Photoacoustic imaging utilizes the strengths of both optical excitation and acoustic detection to break through both the optical diffusion and diffraction limits. As photons travel through the sample, a percentage are absorbed by molecules and their initial energy is partially or completely converted into heat. When utilizing stand-alone optical imaging techniques, this is in fact the portion of the light that is not recovered at the detector. The photons absorbed by the sample as heat induce an initial pressure rise, which propagates as an acoustic wave. An ultrasonic transducer or transducer array can be utilized to detect this acoustic wave and form a photoacoustic microscopy (PAM) image. Of note PAM has extremely high optical absorption contrast and provides the ability to noninvasively map the optical absorption properties of biological tissue. Optical-resolution PAM (OR-PAM) utilizes tightly focused light for photoacoustic excitation, which enables optical-diffraction-limited lateral resolution down to micron or even submicron scale. OR-PAM instruments have been configured in both transmission and reflectance-based modes, where the reflectance-based mode provides access to many more anatomical sites than transmission-mode instruments. However, to date it has been challenging for reflectance-based-mode OR-PAM systems to obtain fine subwavelength resolution due to the very limited working distance of optical objectives with high numerical aperture (NA) lenses.
In the current work by Song et al., this limitation of reflectance-based OR-PAM imaging systems has been overcome by configuring a miniature high-frequency ultrasonic transducer tightly under a water-immersion objective with an NA of one. Using a 532-nm laser for optical excitation, the lateral resolution of the system was measured to be ~320 nm with an axial resolution of ~29 nm. To date, OR-PAM has demonstrated the capability of providing optical-absorption-based anatomical, functional, and molecular information with broad applications in vascular biology, neurology, and ophthalmology. To that end, the investigators in this work demonstrate the resolution of their OR-PAM imaging system on red blood cells (RBCs), cancer cell lines, and mouse vasculature. Due to the strong optical absorption of hemoglobin in RBCs, the OR-PAM system offered high contrast for RBCs where the donut-like shape could be readily visualized. Compared with the RBC images from a conventional bright field optical microscope the OR-PAM system demonstrated ~25 dB improved contrast to noise ratio. OR-PAM images of B16 melanoma cancer cell line demonstrated high-resolution speckled images where the bright spots corresponded to melanin, which can produce high-amplitude photoacoustic signals due to the strong absorption of melanin. Example mouse vasculature in the ear of a murine model was imaged with the OR-PAM system. Excitingly, the submicron resolution of the novel OR-PAM system enabled visualization of individual RBCs in small capillaries.
In summary, the investigators have built and validated a subwavelength-resolution OR-PAM system with a compact optical-acoustic configuration, which allows laser focusing at high NA and photoacoustic signal detection in reflectance-based mode with improved resolution over other OR-PAM systems. In principle, the compact design of this OR-PAM system should enable integration with other widely used optical imaging techniques such as optical coherence tomography, confocal microscopy and multi-photon microscopy, facilitating visualization of complimentary optical contrast mechanisms.
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In the current work by Song et al., this limitation of reflectance-based OR-PAM imaging systems has been overcome by configuring a miniature high-frequency ultrasonic transducer tightly under a water-immersion objective with an NA of one. Using a 532-nm laser for optical excitation, the lateral resolution of the system was measured to be ~320 nm with an axial resolution of ~29 nm. To date, OR-PAM has demonstrated the capability of providing optical-absorption-based anatomical, functional, and molecular information with broad applications in vascular biology, neurology, and ophthalmology. To that end, the investigators in this work demonstrate the resolution of their OR-PAM imaging system on red blood cells (RBCs), cancer cell lines, and mouse vasculature. Due to the strong optical absorption of hemoglobin in RBCs, the OR-PAM system offered high contrast for RBCs where the donut-like shape could be readily visualized. Compared with the RBC images from a conventional bright field optical microscope the OR-PAM system demonstrated ~25 dB improved contrast to noise ratio. OR-PAM images of B16 melanoma cancer cell line demonstrated high-resolution speckled images where the bright spots corresponded to melanin, which can produce high-amplitude photoacoustic signals due to the strong absorption of melanin. Example mouse vasculature in the ear of a murine model was imaged with the OR-PAM system. Excitingly, the submicron resolution of the novel OR-PAM system enabled visualization of individual RBCs in small capillaries.
In summary, the investigators have built and validated a subwavelength-resolution OR-PAM system with a compact optical-acoustic configuration, which allows laser focusing at high NA and photoacoustic signal detection in reflectance-based mode with improved resolution over other OR-PAM systems. In principle, the compact design of this OR-PAM system should enable integration with other widely used optical imaging techniques such as optical coherence tomography, confocal microscopy and multi-photon microscopy, facilitating visualization of complimentary optical contrast mechanisms.
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Article Information
Reflection-mode in vivo photoacoustic microscopy with subwavelength lateral resolution
Wei Song, Wei Zheng, Ruimin Liu, Riqiang Lin, Hongtao Huang, Xiaojing Gong, Shousheng Yang, Rui Zhang, and Liang Song
Biomed. Opt. Express 5(12) 4235-4241 (2014) View: Abstract | HTML | PDF