Pierre Beaudry

Fabrication and characterization of a solid polyurethane phantom for optical imaging through scattering media

A phantom based on a polyurethane system that replicates the optical properties of tissue for use in near-infrared imaging is described. The absorption properties of tissue are simulated by a dye that absorbs in the near infrared, and the scattering properties are simulated by TiO2 particles. The scattering and absorption coefficients of the plastic were measured with a new technique based on time-resolved transmission through two slabs of materials that have different thicknesses. An image of a representative phantom was obtained from time-gated transmission.

Inclusion characterization in a scattering slab with time-resolved transmittance measurements: perturbation analysis

A procedure for the time-domain optical characterization of an inclusion in a scattering slab is investigated theoretically and experimentally. The method relies on the measurement of a contrast function, which is defined as the time-dependent relative change in the transmitted signal resulting from the presence of the inclusion. Analytical expressions for the contrast functions of absorptive and diffusive inclusions are obtained through a perturbation solution of the diffusion equation. This procedure is used successfully to determine the optical properties of absorptive, diffusive, and mixed inclusions located at midplane in a scattering slab by use of time-resolved transmittance measurements.

TIME-DOMAIN OPTICAL IMAGING : DISCRIMINATION BETWEEN SCATTERING AND ABSORPTION

A technique for discriminating between scattering and absorbing inclusions located in the center of a scattering slab is presented. The technique is based on an empirical model that provides a simple mathematical expression to describe the change in the time-resolved transmission resulting from the presence of an inclusion. Experimental results from various configurations show that the technique allows for proper recognition of the type of an inclusion whether it is scattering or absorbing. This technique is a significant step toward tissue differentiation.

Hybrid Monte Carlo for photon transport through optically thick scattering media

A Monte Carlo simulation code developed to model time-domain transillumination measurements with small-area detectors through an optically thick scattering slab is presented. A hybrid approach has been implemented to reduce calculation times. Most of the scattering slab is treated stochastically, albeit with variance reduction techniques and the isotropic diffusion similarity rule. The contribution to the output signal per unit area and time of photon packets propagating in a thin slice near the output face of the slab is calculated analytically after each propagation step. This approach drastically reduces the calculation time but produces spikes in the temporal signals.

Multiport time-domain laser mammography: results on solid phantoms and volunteers

A prototype for laser mammography based on a time-domain technique has been developed. The system uses a streak camera and a Titanium:sapphire laser which provides ultrashort pulses at a repetition rate of 80 MHz. A multi-port scanning head which includes optical fibers scans the breast in a point-by-point scanning procedure. Time-resolved transmission is measured at 15000 locations in 7 minutes. The breast is slightly compressed in both the cranio-caudal and the mediolateral projections. Amplitude calibration of the streak camera has been performed allowing for absolute measurement of time- resolved transmission. In addition to the shape of the time-resolved transmission, the absolute amplitude is relevant in properly evaluating the absorption and scattering coefficients. Promising results on solid phantoms and in vivo have been obtained. Both breasts of 10 volunteers have been scanned to date and a larger pilot study is planned in the near future. In addition to the usual time-gating processing, images of the scattering and absorption contributions are also extracted using an original data processing technique.

Time-domain perturbation analysis of a scattering slab

Propagation of light in a homogeneous scattering slab is conveniently modelled with a diffusion equation. This approach can be extended to a heterogeneous slab through a perturbation analysis. Within BornOs approximation, the effect of an inclusion on the transmitted light is described by space-time integrals. Closed-form time integration is possible, which reduces the perturbation expressions to volume integrals over the inclusion. These can be useful to model small inclusions over which the integrand can be considered as constant. In the case of cubic inclusions with sides parallel and perpendicular to the boundaries of the surrounding slab, closed-form volume integration over the inclusion can be performed instead. Only time integrals are left, which reduces the numerical work. Numerical examples are presented. It is shown that inclusions with different volume and contrast with regards to the surrounding medium can produce the same effect on the transmitted light and are thus indistinguishable. The perturbation analysis has been used to assess the possibility of obtaining some longitudinal localization of an inclusion by using source beams and detectors of different sizes. Calculation results are also compared to experimental measurements to illustrate the validity of this analysis in the presence of small perturbations.

Time-resolved transmission through homogeneous scattering media: time-response effects.

An analytical expression has been obtained that describes the time variation of time-resolved signals transmitted through or reflected by a homogeneous scattering slab as measured with a detection system having a square-impulse response. This expression can be used to improve the match between theoretical and experimental time-resolved signals measured with a system having a finite response time. It can also be used to assess the effect of a finite detection response time on the time-domain characterization of a turbid medium. The expression can be adapted to detection systems that are not time invariant.

Dual-spatial integration for longitudinal localization of inclusions in turbid media.

We introduce a technique called dual-spatial integration (DSI) that is used to isolate and enhance inclusions that differ only by their longitudinal placement within a scattering medium. DSI uses three different source-detector configurations to section a scattering medium into three longitudinal zones. This sectioning permits the extraction of structures close to surfaces and the enhancement of those structures located in the central part of the medium. Both the simulation and the experimental results indicate that DSI has potential interest for applications in biomedical imaging such as optical mammography.

Time-domain laser mammography: separation of scattering and absorption contributions

ABSTRACT A technique for properly separating the scattering and absorption contributions in laser mammography is proposed. Thetechnique is based on an empirical model obtained from a series of experiments performed on homogeneous scattering slabs containing a single inclusion. The scattering and absorption contributions are obtained by performing a Fit of an Inhomogeneous Diffusion Model (FIDM). The performance of this new technique is compared to that involving a curve fit ofthe solution of the diffusion model for a homogeneous slab. The FIDM technique allows a very good discrimination betweenscattering and absorption inclusions, better than that obtained with a curve fit of the homogeneous diffusion model. Themathematical expressions of the empirical model are extremely simple and allow for a fast calculation (about 1 second forcomputing two 44 1 pixel images compared to about 6 minutes with the previous technique). A perturbation analysis of thediffusion model will provide theoretical support to the FIDM technique and should allow its refinement. Although it has notbeen demonstrated that the separation between absorption and scattering is totally correct when applied to real breastscanning, the method associates some structures to absorption and some other to scattering, which could result in a betterspecificity of laser mammography.Keywords: Turbid medium, photon migration, time-resolved, mammography, breast cancer, perturbation analysis.