Optical coherence tomography
Optical coherence tomography (OCT) is an optical interference based high-speed, high-resolution, cross-sectional imaging technology with wide-spread applications in both fundamental biomedical investigations and clinical diagnoses / management of various diseases. We focus on developing OCT technologies for hemodynamic imaging. Using visible light, instead of near-infrared light, illumination in OCT and combining with our sophisticated inverse algorithms, we are able to image the complete metabolic parameters, including blood flow, hemoglobin oxygen saturation, and metabolic rate of oxygen, in microcirculations. We apply our functional OCT technology to better understand blinding diseases, visual functions, and brain circuitry.
Visible-light OCT images of a rat eye showing the detailed reconstruction procedure to measure retinal metabolic rate of oxygen.
Photoacoustic microscopy utilizes endogenous light-absorption contrast from, for example, hemoglobin and melanin for high-resolution biological imaging. When energy from a pulsed laser illumination is absorbed, partial energy is transformed into heat. The transient heating process leads to a rapid local volume expansion and collapse and further produces mechanical waves—ultrasound. Such a photoacoustic effect (i.e. energy transformation from light to sound) can be detected by an external ultrasound transducer; these detected ultrasonic waves can then be used to construct an image. We are mostly interested in ophthalmic applications of photoacoustic microscopy and how to make photoacoustic microscopy more accessible to biological researchers.
Micro vascular network imaged by photoacoustic microscopy in a mouse ear.
In collaborating with Prof. Cheng Sun from Northwestern Mechanical Engineering, we design and fabricat novel micro-photonic devices for imaging and sensing. One photonic device we use is the optical micro-ring resonator. When fabricated in compressible polymer materials, optical micro-ring resonators can be used as ultra-sensitive ultrasonic detectors for photoacoustic microscopy. Moreover, we fabricate these micro-photonic devices on optically-transparent substrate to enable easy integration of photoacoustic microscopy with other established optical microscopic technologies such as OCT and confocal microscopy.
We develop novel retinal imaging system to study early metabolic variation and retinal pigment epithelium dysfunctions to achieve early diagnosis and better understanding of ischemia-driven retinopathy and age-related retinal degeneration. We explore multimodal and functional imaging technologies for more comprehensive retinal imaging. Our technologies include photoacoustic ophthalmoscopy (PAOM), optical coherence tomography (SD-OCT), autofluorescence scanning laser ophthalmoscopy (AF-SLO), and fluorescein angiogram (FA)
In vivo multimodal retinal imaging of an albino rat (top row) and a pigmented rat (bottom row). Panels (a) and (d) are PAOM fundus images, panels (b) and (e) are FA-SLO images; and panels (c) and (f) are en face SD-OCT images. RV: retinal vessels; CV: choroidal vessels. Bar: 500 µm.
Numerical modeling and image processing
Using Monte Carlo simulation, we try to understand the fundamental limits of existing optical imaging technologies such as funds photograph based retinal oximetry. We are also interested in new image processing methods for quantitative information extraction, better segmentation, and faster reconstruction.
Left: a fast retinal vessel segmentation algorithm that can automatically segment all the major retinal vessels based on OCT phase variation contrast. Right: a Monte Carlo simulation of optical absorption cross section in a vessel and diffusive optical reflection measured from outside the eye.