High-resolution biophotonic tomography

We develop novel biophotonic imaging for early-cancer detection, a grand challenge in cancer research, using non-ionizing electromagnetic and ultrasonic waves. Unlike ionizing x-ray radiation, non-ionizing electromagnetic waves such as optical waves are safe for biomedical applications and reveal new contrast mechanisms and functional information. For example, our spectroscopic oblique- incidence reflectometry can detect skin cancers based on functional hemoglobin parameters and cell nuclear size with 95% accuracy. Unfortunately, electromagnetic waves in the non-ionizing spectral region do not penetrate biological tissue in straight paths as x-rays do. Consequently, high-resolution tomography based on non-ionizing electromagnetic waves alone, as demonstrated by confocal microscopy and two-photon microscopy as well as our Mueller optical coherence tomography, is limited to superficial tissue imaging within similar to1 mm depth. Ultrasonic imaging, on the contrary, furnishes good imaging resolution but has poor contrast in early-stage tumors and has strong speckle artifacts as well. We developed ultrasound-mediated imaging modalities by combining electromagnetic and ultrasonic waves synergistically. The hybrid modalities yield speckle-free electromagnetic-contrast information at ultrasonic resolution in relatively large biological tissue. In photo-acoustic tomography, a diverged laser pulse induces ultrasonic waves in biological tissue due to thermoelastic expansion as a result of a small but rapid temperature rise. The short-wavelength ultrasonic waves are detected to form wavelength-limited high-resolution tomographic images of the tissue.