Evaluation of higher-order time-domain perturbation theory of photon diffusion on breast-equivalent phantoms and optical mammograms

Time-domain perturbation theory of photon diffusion up to third order was evaluated for its accuracy in deducing optical properties of breast tumors using simulated and physical phantoms and by analyzing 141 projection mammograms of 87 patients with histology-validated tumors that had been recorded by scanning time-domain optical mammography. The slightly compressed breast was modeled as (partially) homogeneous diffusely scattering infinite slab containing a scattering and absorbing spherical heterogeneity representing the tumor. Photon flux densities were calculated from densities of transmitted photons, assuming extended boundary conditions. Explicit formulas are provided for second-order changes in transmitted photon density due to the presence of absorbers or scatterers. The results on phantoms obtained by perturbation theory carried up to third order were compared with measured temporal point spread functions, with numerical finite-element method (FEM) simulations of transmitted photon flux density, with results obtained from the diffraction of diffuse photon density waves, and from Pade approximants. The breakdown of first-, second-, and third-order perturbation theory is discussed for absorbers and a general expression was derived for the convergence of the Born series in this case. Taking tumor optical properties derived by the diffraction model as reference we conclude that estimates of tumor absorption coefficients by perturbation theory agree with reference values within +/-25% in only 65% (first order), 66% (second order), and 77% (third order) of all mammograms analyzed. In the remaining cases tumor absorption is generally underestimated due to the breakdown of perturbation theory. On average the empirical Pade approximants yield tumor absorption coefficients similar to third-order perturbation theory, yet at noticeable lower computational efforts

Evidence for seasonal shift in the reproduction of Aldabra giant tortoises (Aldabrachelys gigantea) in managed care in the Northern hemisphere compared to the natural habitat in the Southern hemisphere

Ex situ breeding constitutes an important tool for species conservation; however, many reptile species are not managed sustainably under human care due to poor fecundity in ex situ settings. In this study, we tested whether the translocation of a seasonally reproducing species to a different environment results in decoupling of extrinsic signals and intrinsic conditions. The endocrinological patterns of plasma steroid sex hormones, follicular development, and mating behaviour of two female and two male sexually mature Aldabra tortoises (Aldabrachelys gigantea) in a zoological institution in the Northern hemisphere was aligned with enclosure climate data (mean monthly daylight duration, temperature, and precipitation) and compared with respective hormone patterns of wild individuals and climate conditions in the native habitat on the Aldabra Atoll in the Southern hemisphere. Whereas occurrence of mating behaviour was not considered a limiting factor, lack of ovulation and subsequent follicular atresia was the main reason for the lack of reproductive output. While it was impossible to elucidate the triggering factors of ovulation and the multifactorial complexity of reproduction was not fully addressed, this study indicates suboptimal temperature conditions and relative temporal shifts of interacting external triggers (temperature and photoperiod) in the zoo setting

Active Membrane Fluctuations Studied by Micropipet Aspiration

We present a detailed analysis of the micropipet experiments recently reported in J-B. Manneville et al., Phys. Rev. Lett. 82, 4356--4359 (1999), including a derivation of the expected behaviour of the membrane tension as a function of the areal strain in the case of an active membrane, i.e., containing a nonequilibrium noise source. We give a general expression, which takes into account the effect of active centers both directly on the membrane, and on the embedding fluid dynamics, keeping track of the coupling between the density of active centers and the membrane curvature. The data of the micropipet experiments are well reproduced by the new expressions. In particular, we show that a natural choice of the parameters quantifying the strength of the active noise explains both the large amplitude of the observed effects and its remarkable insensitivity to the active-center density in the investigated range. [Submitted to Phys Rev E, 22 March 2001]Comment: 14 pages, 5 encapsulated Postscript figure

Temporal structure of stimulated-Brillouin-scattering reflectivity considering transversal-mode development

The time-resolved reflectivity of optical phase conjugation by stimulated Brillouin scattering ~SBS! is investigated both theoretically and experimentally. A three-dimensional and transient model of SBS is developed to compare the experimental and theoretical results. Noise initiation of the SBS process is included in the model to simulate the shot-to-shot variation in the reflectivity and the Stokes temporal profile.Shahraam Afshaarvahid, Axel Heuer, Ralf Menzel, and Jesper Munc

Nature of intensity and phase modulations in stimulated Brillouin scattering

The nature of stimulated Brillouin scattering (SBS) temporal modulations for a focused beam in a finite-length cell with homogeneous medium is examined numerically. The finite phonon lifetime produces deterministic oscillations at the threshold while the inclusion of the random noise as an initiation source of SBS leads to stochastic fluctuations in Stokes intensity and phase. A unified study of both modulations under different parameters is presented. The results indicate a large useful parameter space for excellent Stokes beam quality.Shahraam Afshaarvahid, Vladimyros Devrelis, and Jesper Munc

Trichomanes polypodioides

Pteridophyte