Experimental tests of different solutions to the diffusion equation for optical characterization of scattering media by time-resolved transmittance

Laser interstitial thermotherapy (LITT) for breast cancer: dosimetry optimization and numerical simulation

Surgical treatment is standard for the treatment of small breast cancers. Due to the pain and esthetic sequelae that can follow surgery, minimally invasive treatments are under investigation. Our aim was to conduct a dosimetry study of laser interstitial thermotherapy. Turkey tissue was used as an ex vivo model, and mammary glands from ewes were used as in vivo models. We used two different wavelength lasers (805 nm and 980 nm). Two types of fiber from two different manufacturers were used: bare fibers with a diameter of 600 μm and diffusing fiber. The diffusing fibers were 5 mm and 10 mm in length. We also used a computerized model to predict thermal damage and to correlate with the ex vivo and in vivo procedures using a constant and variable coefficient. The mathematical model was based on the finite element method for solving light distribution, bio-heat, and thermal damage equations. Based on our ex vivo and in vivo experiments, we found that the optimal configuration for this treatment was the use of the 980-nm laser at 4 W with bare fibers for a minimum treatment time of 150 s. We also developed a predictive mathematical model that showed good predictability of necrosis in line with the experimental data. Laser treatment is a promising therapy for small breast lesions. However, further development of treatment guidance is necessary to support its use in clinical practice.

Optical Characterization of Homogeneous and Heterogeneous Intralipid-Based Samples

Different scattering processes take place when photons propagate inside turbid media. Many powerful experimental techniques exploiting these processes have been developed and applied over the years in a large variety of situations from fundamental and applied research to industrial applications. In the present paper, we intend to take advantage of Static Light Scattering (SLS), Dynamic Light Scattering (DLS), and Time-Resolved Transmittance (TRT) for investigating all the different scattering regimes by using scattering suspensions in a very large range of scatterer concentrations. The suspensions were prepared using Intralipid 20%, a material largely employed in studies of the optical properties of turbid media, with concentrations from 10−5% to 50%. By the analysis of the angular and temporal dependence of the scattered light, a more reliable description of the scattering process occurring in these samples can be obtained. TRT measurements allowed us to obtain information on the reduced scattering coefficient, an important parameter largely used in the description of the optical properties of turbid media. TRT was also employed for the detection of inclusions embedded in Intralipid suspensions, by using a properly designed data analysis. The present study allowed us to better elucidate the dependence of scattering properties of Intralipid suspensions in a very large concentration range and the occurrence of the different scattering processes involved in the propagation of light in turbid media for the first time to our knowledge. In so doing, the complementary contribution of SLS, DLS, and TRT in the characterization of turbid media from an optical and structural point of view is strongly evidenced.

Study of a simple model for the transition between the ballistic and the diffusive regimes in diffusive media

A Monte Carlo simulation was utilized to investigate a simple model for the transition between the ballistic and the diffusive regimes in diffusive media. The simulation focuses on the propagation of visible and near-infrared light in biological tissues. This research has mainly two findings: (1) the transition can be described, as was found experimentally, with good accuracy by only two terms (ballistic and diffusive). (2) The model can be utilized for cases where the absorption coefficient is not negligible compared to the scattering coefficient by adding a power-law prefactor to the diffusive term.

Non-tissue-like features in the time-of-flight distributions of plastic tissue phantoms

We measure high-temporal-resolution time-of-flight distributions of picosecond laser pulses in the visible and near-infrared, scattered in the forward direction by solid and liquid phantoms, and compare them to those obtained by using ex vivo tissues. We demonstrate that time-of-flight distributions from solid phantoms made of Delrin, Nylon, and Teflon are modulated by ripples that are absent in the biological samples and disappear when the temporal and/or angular resolution of the measuring apparatus is decreased. This behavior prevents the use of such materials as tissue phantoms when spatial mode and time selection are required, such as in imaging methods exploiting early arriving photons.

Measuring optical temperature coefficients of Intralipid®

The temperature sensitivities of absorption and reduced scattering coefficients in the range 700-1000 nm are determined for the liquid phantom Intralipid using spatially resolved continuous wave measurements. The measurements were conducted on a 10 L heated volume of 1% Intralipid subjected to a 40-30 degrees C cooling regime. The temperature sensitivities of the absorbance coefficients are similar to that expected for pure water. However, the reduced scattering coefficients are more sensitive than can be explained by temperature related density changes, and show an unexpected relationship with wavelength. We have also found that temperature perturbations provide a useful means to evaluate instrument model performance.

Experimental determination of photon propagation in highly absorbing and scattering media

Optical imaging and tomography in tissues can facilitate the quantitative study of several important chromophores and fluorophores. Several theoretical models have been validated for diffuse photon propagation in highly scattering and low-absorbing media that describe the optical appearance of tissues in the near-infrared (NIR) region. However, these models are not generally applicable to quantitative optical investigations in the visible because of the significantly higher tissue absorption in this spectral region compared with that in the NIR. We performed photon measurements through highly scattering and absorbing media for ratios of the absorption coefficient to the reduced scattering coefficient ranging approximately from zero to one. We examined experimentally the performance of the absorption-dependent diffusion coefficient defined by Aronson and Corngold [J. Opt. Soc. Am. A 16, 1066 (1999)] for quantitative estimations of photon propagation in the low- and high-absorption regimes. Through steady-state measurements we verified that the transmitted intensity is well described by the diffusion equation by considering a modified diffusion coefficient with a nonlinear dependence on the absorption. This study confirms that simple analytical solutions based on the diffusion approximation are suitable even for high-absorption regimes and shows that diffusion-approximation-based models are valid for quantitative measurements and tomographic imaging of tissues in the visible.

Depth dependence of the analytical expression for the width of the point spread function (spatial resolution) in time-resolved transillumination

Simple analytical expressions for the point spread function (PSF) at different depths can save computation time and improve the performance of inverse algorithms for optical imaging. In particular, application of such formulas simplifies quantification of the optical characteristics of tissue abnormalities inside highly scattering media. Earlier it was shown within the random walk theory framework that the PSF for time-resolved transillumination imaging through a highly scattering slab is well represented by a Gaussian. We have experimentally validated a simple formula of the random walk model for the effective width of this Gaussian, as a function of time delay, at different depths of the target. Presented analysis of published experimental data, concerning effective width of the PSF, for a slab of considerably smaller thickness also demonstrates good agreement between the data and predictions of our model. This PSF width determines spatial resolution of the time-resolved imaging and is widely discussed in the literature.

Accuracy of a perturbation model to predict the effect of scattering and absorbing inhomogeneities on photon migration

The accuracy of the perturbation model to predict the effect of scattering and absorbing inhomogeneities on photon migration has been investigated by comparisons with experimental and numerical results. Comparisons for scattering inhomogeneities showed that the model gives satisfactory results both for the intensity and for the temporal profile of the perturbation over a large range of values for the scattering properties of the defect. As for absorbing inhomogeneities, the model provides an excellent description for the temporal profile, but the results for the intensity are accurate only when the perturbation is small. For absorbing inhomogeneities an empirical model that has a significantly more extended application range has been proposed. The model is based on an expression for the time-resolved mean path length that detected photons have followed inside the inhomogeneity. The application range of the proposed model covers the values expected for the optical properties and for the volumes of inhomogeneities of practical interest for optical mammography.