Ultrafast time-gated approach in optical biomedical imaging

IMAGING an object located in a highly scattering medium is a challenging problem with a wide range of applications in different fields: medicine, industry, defence and space. A better understanding of photon migration in turbid media has resulted in a surge of research interest in optical biomedical imaging. Limitations of X-ray imaging in detecting small lesions in tissues, in distinguishing malignant tumors from benign ones, and the concern regarding potentially harmful effects of ionizing radiation have led to a renewed interest in the development of noninvasive and non-ionizing optical diagnostic and imaging methods since the early 1980s. Advances in ultrashort light pulse generation and high-speed optical detection systems have provided the technological support for the development of various optical imaging modalities. A number of different experimental schemes based on cw, time-resolved, and frequency-domain methods, as well as space gating, and polarization gating have evolved over the years for selectively choosing different components of scattered light (ballistic, snake or diffuse) for imaging in a highly scattering medium. In this paper we briefly describe photon migration in a highly scattering random medium in the diffusion approximation. We illustrate some of the time-resolved imaging modalities – streak camera, Kerr–Fourier imaging and picosecond electronic time-gated imaging – developed at the Institute for Ultrafast Spectroscopy and Lasers, City College of the City University of New York, for biomedical optical imaging.