Numerical method for studying the detectability of inclusions hidden in optically turbid tissue

Reconstruction of optical scanned images of inhomogeneities in biological tissues by Monte Carlo simulation

The optical imaging of inhomogeneities located in phantoms of biological tissues, prepared from goat's isolated heart as control tissue and embedded with spleen and adipose tissues representing tumors, by Monte Carlo simulation, is carried out. The proposed scanning probe consists of nine units. Each unit is equipped with one photon injection port and three ports arranged in a straight line to collect backscattered photons emerging from various depths, and one port, placed coaxially to the source on the opposite side of the phantom, to collect the transmitted component. At each position of the grid, superposed on the tissue phantom, photons are introduced through source port into the phantoms and backscattered and transmitted photons are collected by respective ports. Based on the data collected from the entire grid surface the respective gray-level images are reconstructed. The inhomogeneity located at certain depth (2, 4, 6mm) is visualized in three images formed by the backscattered data collected by three ports. Increase or decrease in normalized backscattered intensity (NBI) observed in their scans corresponds to that of high scattering (adipose) or absorbing (spleen) inhomogeneity compared to that of control tissue and also their location as determined by NBI variation as received at various ports. The images constructed from the transmitted data are associated with decrease in intensity. The scans of these images through their centers show that normalized transmitted intensity (NTI) attains its maximum value when the inhomogeneities are at depth 6mm. These scans are of higher amplitude for spleen compared to that of adipose tissues. Thus the data received by backscattering and transmission complement each other in identifying the location and type of inhomogeneities.

Monte carlo procedure for investigating light propagation and imaging of highly scattering media

A Monte Carlo procedure has been developed to study photon migration through highly scattering nonhomogeneous media. When two scaling relationships are used, the temporal response when scattering or absorbing inhomogeneities are introduced can be evaluated in a short time from the results of only one simulation carried out for the homogeneous medium. Examples of applications to the imaging of defects embedded into a diffusing slab, a model usually used for optical mammography, are given. Comparisons with experimental results show the correctness of the results obtained.

Role of tissue structure in photon migration through breast tissues

Photon migration has been investigated experimentally in vitro on human breast tissues, bovine liver, tissue phantoms, and theoretically by Monte Carlo simulations and diffusion theory. The spatial intensity profiles have been measured at the output surface of a sample illuminated by a collimated beam. Experimental results have then been compared with simulations that assume the sample to be homogeneous. Measurements on phantoms, i.e., fat emulsion and microspheres suspension, and on liver are in good agreement with theory. On the other hand, the width of the intensity profiles measured on breast tissues (adipose and fibrous) are systematically larger than those measured on phantoms or calculated by simulations. The structure of these samples, not considered in simulations and not present in phantoms, explains these differences.

Absorptivity contrast in transillumination imaging of tissue abnormalities

We calculate the time-resolved flux of photons transmitted across an optically turbid slab containing a partially absorbing inclusion. An analytical expression is obtained for the flux at a detector positioned opposite a point source (at a distance equal to the thickness of the slab) when the center of the inclusion lies on the line connecting those points. The calculation employs a discrete-time lattice random-walk model of photon transport. The resulting expression is used to assess the affects of time resolution on the detectability of the inclusion.

Image quality in time-resolved transillumination of highly scattering media

Using a photon-counting setup and a streak-camera arrangement with time resolutions of 35 and 6 ps, respectively, we have investigated the spatial resolution of a time-gated transillumin tion technique applied to turbid media. In the case of large relative amounts of unscattered light, it is found that small detection angles improve the spatial resolution. For large concentrations of scatterers and large sample thicknesses, i.e., when the amount of unscattered light is negligible, the best time-gate position is found to be at times that are later than the minimum transit time. In this case (minimum transit time), temporal resolutions from small values up to approximately 50 ps yield almost the same image resolution. The only advantage of measuring systems with a higher than 50-ps temporal resolution is their ability to distinguish the diffused from the unscattered light, when a significant amount of the latter is present.

Concentration measurements of a light absorber localized in a scattering medium

Using a time-resolved technique, we have studied two methods of quantifying the concentration of a light absorber localized in a scattering medium. Because some of the transmitted photons pass through the absorber region many times because of complicated photon propagation in the scattering medium, we used the following methods. First, early-arriving photons passing through the absorber approximately once were selected. Using these photons, we quantified the concentration of the absorber by an ordinary spectroscopic method based on the Beer-Lambert law. Second, for later-arriving photons, after empirically determining the distribution of the number of passes through the absorber as a function of detection time, we obtained the concentration by applying this function to the Beer-Lambert law.