Kwong Mow Yoo

Ultrafast time-gated imaging in thick tissues: a step toward optical mammography

With an ultrafast time-gated optical detection method, a thin translucent strip of fat (2.5 mm thick) hidden inside a 4-cm-thick tissue is located with millimeter spatial resolution.

Imaging of a translucent object hidden in a highly scattering medium from the early portion of the diffuse component of a transmitted ultrafast laser pulse.

A translucent object hidden in a highly scattering medium is shown to be visible when the early portion of the transmitted diffuse pulse (snake photons) is detected. The use of the snake scattered photons to image objects depends on the scattering characteristics along a quasi-straight-line path that the photons traverse across the medium. A translucent object with different scattering characteristics compared with its surrounding medium will change the intensity of the snake photons. By scanning the medium across the laser beam and detecting only the snake photons, a translucent object hidden in a highly scattering medium could be located.

Coherent backscattering of light from biological tissues

The angular and temporal distributions of light scattered around the backward direction from biological tissues are measured. An enhancement of scattered light intensity around the backward direction, which is associated with the weak photon localization, is observed. The transport mean free path l(t) and the absorption length l(a) of the light in biological tissue can be obtained from the angular line shape of the coherent peak. The values l(t) and l(a) can also be obtained from the temporal profile of the scattered pulse.

Effect of multiple light scattering and self-absorption on the fluorescence and excitation spectra of dyes in random media

The absorption, fluorescence, and excitation spectra of a dye in a highly scattering random medium were studied experimentally. The intrinsic absorption spectrum of the dye does not change in the presence of scatterers, but the presence of scatterers in the media will change the observed fluorescence spectra. The observation is accounted for by the change in the photon trajectory path length for the fluorescence emission.

Conical emission by four-photon parametric generation by using femtosecond laser pulses

Conical continuum Stokes and anti-Stokes emissions are observed when intense 100-fs/620-nm ultrafast laser pulses propagate through an ethylene glycol medium. The angle of the anti-Stokes conical emission is modeled by class-II Raman and four-photon parametric generations in a small-scale filament with a nonlinear index change.

Transmitted photon intensity through biological tissues within various time windows

The intensity of the early-arriving snake portion of 100-fs ultrashort laser pulses transmitted through biological tissue of increasing thickness was measured by a streak camera. The snake photon intensity within the first arrival time interval Deltat was found to decrease exponentially with tissue thickness (z) as I(Deltat) = I(0)A exp[-b(Deltat)z/l(t)], where I(0) is the incident laser pulse intensity, l(t) is the transport mean free path of the medium, and the parameters b and A depend on the time interval Deltat. This result shows that the intensity of snake photons decays significantly more slowly than that of ballistic photons as tissue thickness increases.

Time dynamics of photon migration in semiopaque random media

The temporal behavior of femtosecond laser pulses propagating and scattering in random media was studied using an optical fiber probe and a streak camera. Information on the temporal profile of laser pulse scattering at 90 degrees in semiopaque media was analyzed. Apparent ballistic transport was observed for both single and multiple scattering regimes. A phantom (hidden foreign object) was identified in a semiopaque environment using time resolved techniques with a total optical density of ~3.

Imaging of diffusing media by a progressive iterative backprojection method using time-domain data

A method for the reconstruction of 3-D images of the interior of dense scattering media, based on the analysis of time-resolved backscattered signals is described. The method evaluates a linear perturbation equation by a progressive iterative backprojection scheme. A key feature of the method is the use of weighting functions which estimate the impact that absorption of photons in the interior have on the response of detectors located at the surface. Examples of reconstructed images shown are based on the analysis of simulated data for multilayered media and simulated and experimental data for media containing finite-volume absorbers. These results contain features which indicate that images having high resolution are obtainable even in the limiting case where the view angle is restricted to only backscattered signals and the absorption contrast across an interior boundary is 1%. A general scheme, similar to a layer- stripping approach, is described for the case where signals emerging about a target are measured.

Dynamic effect of weak localization on the light scattering from random media using ultrafast laser technology

Angle and time resolved experiments on the backscattering of light from model random media are presented. Weak localization was observed using 30-fs laser pulses, where this time scale is shorter than the scattering mean free time of the light in the random medium. This observation implies that the coherent peak is due to interference between the scattered light at the point of observation. The time resolved experiment directly shows that the light rays which undergo longer scattering paths contribute to the narrow coherent peak. A proper description of the temporal profile pulses scattered from random media requires the effect of weak localization to be taken into account.

Spectral optical-density measurements of small particles and breast tissues

The optical density of breast tissues without blood is found to be relatively constant from 320 to 800 nm, indicating a relatively independent scattering cross section over this wavelength region.