Imaging melanin quantitatively using label-free third-harmonic-generation enhancement-ratio microscopy
Melanin is the most important pigment in human and acts as a physical barrier to protect keratinocytes and vessels from UV irradiation. Various label-free high resolution imaging technologies had shown their capability for clinical melanin imaging in human skin, but all failed to provide absolute quantitative information in term of mass or mass density. Here we review clinical in vivo microscopical imaging of the melanin distribution in human skin with absolute quantities on mass density using label-free third-harmonic-generation-enhancement-ratio (erTHG) microscopy. By adopting the resonant enhancement ratio to deal with the background calibration beacon issue, here we propose to retrieve the erTHG parameter in situ and thus recover the absolute quantity of melanin within a sub-femtoliter volume. In vivo clinical studies were conducted to compare the absolute quantitative measure obtained by erTHG with those obtained by diffuse reflectance spectroscopy with excellent agreement found. This quantitative imaging tool was further applied for treatment assessment of Asian volunteers with solar lentigines on face. Abnormal melanosome dispersion in diseased cytosols were observed for the first time and treatment does not heal this abnormality. Based on our findings, we conclude the high potential of slide-free label-free erTHG microscopy for absolutely-quantitative melanin imaging in vivo with a subfemtoliter 3D spatial resolution in human skin.
Chi-Kuang Sun is a Life Distinguished Professor in the College of Electrical Engineering and Computer Science (EECS) and the College of Medicine at Taiwan University. He received his S.B. in Electrical Engineering from Taiwan University in 1987, and S.M. and Ph.D. in Applied Physics from Harvard University in 1990 and 1995 respectively. He was an Assistant Researcher in the NSF Center for Quantized Electronics Structures (QUEST) at UCSB between 1995 and 1996. He joined the Taiwan University faculty in 1996 and is currently the Life Distinguished Professor of Electrical Engineering and Photonics at Taiwan University. He served as Chairman of the Photonics Program, Ministry of Science and Technology, Taiwan and was Deputy Dean of the EECS College, Taiwan University. He is the founder and a chief investigator of the Molecular Imaging Center, one of the university-level excellence centers at Taiwan University. Chi-Kuang Sun’s research involves optical molecular imaging, nonlinear microscopy, ultrafast phenomena, nano-ultrasonics, THz health care, advanced femtosecond laser technologies, and applications in virtual biopsy diagnosis, treatment and therapy assessment, surgical guidance, wearable monitoring device, neural science, virus epidemic control, paleontology, interfacial water imaging, and boson peak studies. He leads the advancement and clinical applications of third harmonic generation microscopy for noninvasive differential diagnosis of skin lesions. He is a pioneer for the development of dielectric THz fibers and THz-fiber-based imaging systems, including THz fiber-endoscope, THz near-field microscope, and THz mammography for noninvasive breast cancer and blood examination. He proposes and demonstrates the generation and detection of femtosecond acoustic pulses by using piezoelectric nanolayers, combined with the slow sound velocity and high temporal resolution, for noninvasive atomic-level resolution ultrasound imaging. Chi-Kuang Sun is a Fellow of OSA, IEEE, SPIE, and RMS (London).