Evaluation of the in vivo and ex vivo optical properties in a mouse ear model.

Determination of in vivo optical properties is a challenging problem. Absorption and scattering measured ex vivo are often used for in vivo applications. To investigate the validity of this approach, we have obtained and compared the optical properties of mouse ears in vivo and ex vivo in the spectral range from 370 to 1650 nm. Integrating sphere spectrophotometry in combination with the inverse Monte Carlo technique was employed to determine absorption coefficients, mu(a), scattering coefficients, mu(s), and anisotropy factors, g. Two groups of mice were used for the study. The first group was measured in vivo and ex vivo within 5-10 min post mortem. The second group was measured in vivo and ex vivo every 24 h for up to 72 h after sacrifice. Between the measurements the tissues were kept at 4 degrees C wrapped in a gauze moistened with saline solution. Then the specimens were frozen at -25 degrees C for 40 min, thawed and measured again. The results indicate that the absorption coefficients determined in vivo and ex vivo within 5-10 min post mortem differed considerably only in the spectral range dominated by hemoglobin. These changes can be attributed to rapid deoxygenation of tissue and blood post mortem. Absorption coefficients determined ex vivo up to 72 h post mortem decreased gradually with time in the spectral regions dominated by hemoglobin and water, which can be explained by the continuing loss of blood. Absorption properties of the frozen-thawed ex vivo tissues showed increase in oxygenation, which is likely caused by the release of hemoglobin from hemolyzed erythrocytes. Scattering of the ex vivo tissues decreased gradually with time in the entire spectral range due to the continuing loss of blood and partial cell damage. Anisotropy factors did not change considerably.

Fluorescence polarization imaging for delineating nonmelanoma skin cancers.

We present a method for detecting nonmelanoma skin cancers using exogenous fluorescence polarization. We built an automated system that permits exogenous fluorescence polarization imaging. It includes a tunable linearly polarized monochromatic light source and a CCD camera equipped with a rotating linear polarizer and a filter to reject excitation light. Two fluorophores that are retained in tumors, toluidine blue and methylene blue, are employed. We demonstrate that fluorescence polarization imaging can be used for accurate delineation of nonmelanoma cancers. The results suggest that this optical technique may be suitable for real-time noninvasive demarcation of epithelial cancers.

Optical properties of selected native and coagulated human brain tissues in vitro in the visible and near infrared spectral range.

Medical laser applications require knowledge about the optical properties of target tissue. In this study, the optical properties of selected native and coagulated human brain structures were determined in vitro in the spectral range between 360 and 1100 nm. The tissues investigated included white brain matter, grey brain matter, cerebellum and brainstem tissues (pons, thalamus). In addition, the optical properties of two human tumours (meningioma, astrocytoma WHO grade II) were determined. Diffuse reflectance, total transmittance and collimated transmittance of the samples were measured using an integrating-sphere technique. From these experimental data, the absorption coefficients, the scattering coefficients and the anisotropy factors of the samples were determined employing an inverse Monte Carlo technique. The tissues investigated differed from each other predominantly in their scattering properties. Thermal coagulation reduced the optical penetration depth substantially. The highest penetration depths for all tissues investigated were found in the wavelength range between 1000 and 1100 nm. A comparison with data from the literature revealed the importance of the employed tissue preparation technique and the impact of the theoretical model used to extract the optical coefficients from the measured quantities.

A scattering phase function for blood with physiological haematocrit.

Though the optics of red blood cells as well as whole blood has been studied extensively, an effective scattering phase function for whole blood is still needed. The interference of waves scattered by neighbouring cells cannot be neglected in highly concentrated suspensions such as whole blood. As a result, the phase function valid for single erythrocytes may fail to describe a single scattering process in whole blood with physiological haematocrit (Hct approximately 0.4). In this study we compared the results obtained in goniophotometric measurements of blood samples with the results of angle-resolved Monte Carlo simulations. The results show that a Henyey-Greenstein phase function with an anisotropy factor of 0.972 is an adequate approximation for the effective scattering phase function of whole blood with high haematocrit at a wavelength of 514 nm.

Scattering delay time of Mie scatterers determined from steady-state and time-resolved optical spectroscopy.

We have determined the scattering delay time of Mie scatterers (r = 255 nm quartz spheres in polyester resin) from a combination of steady-state (integrating-sphere) and time-resolved (frequency-domain) measurements performed in the multiple-scattering regime. The effective transport velocity of light was derived from intensity and phase measurements at four different wavelengths by using the time-integrated microscopic Beer-Lambert law. We could demonstrate a systematic underestimation of the effective transport velocity compared with the phase velocity in the medium. Assuming that this discrepancy was caused entirely by the transient nature of a single-scattering process, the data presented resulted in time delays of between 18 fs (lambda = 678 nm) and 177 fs (lambda = 1,064 nm) per scattering event. For three out of four wavelengths investigated, the measured values are in excellent agreement with values predicted by a theoretical model for the scattering delay time based on Mie theory.

Influence of the scattering phase function approximation on the optical properties of blood determined from the integrating sphere measurements.

We investigated the impact of the scattering phase function approximation on the optical properties of whole human blood determined from integrating sphere measurements using an inverse Monte Carlo technique. The diffuse reflectance Rd and the total transmittance Tt(λ=633 nm) of whole blood samples (Hct=38%) were measured with double-integrating sphere equipment. The scattering phase functions of highly diluted blood samples (Hct=0.1%) were measured using a goniophotometer. We approximated the experimentally determined scattering phase functions with either Henyey-Greenstein (HGPF), Gegenbauer kernel (GKPF), or Mie (MPF) phase functions to preset the anisotropy factor µ¯ for the inverse problem. We have employed HGPF, GKPF, and MPF approximations in the inverse Monte Carlo procedure to derive the absorption coefficient µa and the scattering coefficient µs. To evaluate the obtained data, we calculated the angular distributions of scattered light for optically thick samples and compared the results with goniophotometric measurements. The data presented in this study demonstrate that the employed approximation of the scattering phase function can have a substantial impact on the derived values of µs and µ¯, while µa and the reduced scattering coefficient µs' are much less sensitive to the exact form of the scattering phase function. © 1999 Society of Photo-Optical Instrumentation Engineers.

Treatment planning for MRI-guided laser-induced interstitial thermotherapy of brain tumors--the role of blood perfusion.

MR techniques have been demonstrated to allow a reliable monitoring of laser-induced interstitial thermotherapy (LITT). However, an adequate on-line control of this coagulation technique requires an exact therapy planning. The latter is mandatory to interpret the MR-monitoring data correctly to guarantee a precise laser irradiation. Moreover, it is a prerequisite for on-line decisions if modifications of the therapeutic regimen are required. In this work, we present a new simulation technique for LITT planning. The model accounts for the specific geometry of the treatment site, the exact configuration of the applicator, and the optical and thermal properties of the tissue, including changes during the heating process. The simulation results were compared with MR scans of laser-induced lesions in three patients with World Health Organization (WHO) grade II astrocytoma. Special interest was directed toward the role of blood perfusion, which was studied parametrically. Good agreement between the simulation results and the MR data was found if the appropriate blood perfusion rates were taken into account. Thus, the model can generate valid therapy plans allowing a precise on-line control of laser irradiation using MR techniques. Neglecting adequate perfusion parameters resulted in substantial errors with respect to the prediction of the final laser lesion.

Effect of the scattering delay on time-dependent photon migration in turbid media.

We modified the diffusion approximation of the time-dependent radiative transfer equation to account for a finite scattering delay time. Under the usual assumptions of the diffusion approximation, the effect of the scattering delay leads to a simple renormalization of the light velocity that appears in the diffusion equation. Accuracy of the model was evaluated by comparison with Monte Carlo simulations in the frequency domain for a semi-infinite geometry. A good agreement is demonstrated for both matched and mismatched boundary conditions when the distance from the source is sufficiently large. The modified diffusion model predicts that the neglect of the scattering delay when the optical properties of the turbid material are derived from normalized frequency- or time-domain measurements should result in an underestimation of the absorption coefficient and an overestimation of the transport coefficient. These observations are consistent with the published experimental data.

Inverse hybrid technique for determining the optical properties of turbid media from integrating-sphere measurements.

We have combined the Monte Carlo method with the small-angle approximation of the radiative transfer theory to derive the optical properties (the absorption coefficient, the scattering coefficient, and the anisotropy factor) of turbid materials from integrating-sphere measurements (the total transmittance and the diffuse reflectance) and the collimated transmittance. Unlike one-dimensional models, the technique accounts for the side losses of light at the edges of the sample. In addition, it enables the correction of the measured collimated signal for the contribution of multiply scattered light. On the other hand, the hybrid technique allows a significant reduction in calculation time compared with inverse methods based on a pure Monte Carlo technique. Numerical tests and experimental results from a phantom material (milk) as well as samples of biological tissue (porcine myocardium) confirmed the feasibility of applying this technique to the determination of the optical properties of turbid media.

Combined elastic and Raman light scattering of human eye lenses.

The distribution of the scattering coefficient (as defined in the appendix) at a wavelength of 647.1 nm along the visual axis of human eye lenses was investigated using a specially designed set-up for spatially resolved measurements of the intensity of the scattered light. For the same lenses, the distribution of the protein content was measured using confocal Raman microspectroscopy. Data collected by both methods were processed in terms of a recently developed theory of short-range, liquid-like order of crystallin proteins that accounts for eye lens transparency. Seven fresh intact human lenses of varying age have been investigated. In addition, elastic and Raman scattering measurements have been performed on fixed lenses. The main results and conclusions are: (1) Fixation significantly affects the light scattering properties of the eye lens. The average level of scattering increases and a change in the distribution of scattered light intensity along the visual axis occurs. Protein content and average distribution were not altered by fixation. (2) There are significant differences between the distribution of the scattering coefficient for lenses of different ages. For young lenses (18 and 20 years) regions with a low protein content (anterior and posterior cortex) show a higher level of elastic light scattering, while for older lenses (42-78 years old) there is no obvious correlation between the scattering level and protein content. (3) Changes in the level of light scattering along the visual axis of the lens cannot be explained by protein concentration effects.(ABSTRACT TRUNCATED AT 250 WORDS)