Anisotropy of light propagation in biological tissue

We investigated the propagation of light in biological tissues that have aligned cylindrical microstructures (e.g., muscle, skin, bone, tooth). Because of pronounced anisotropic light scattering by cylindrical structures (e.g., myofibrils and collagen fibers) the spatially resolved reflectance exhibits a directional dependence that is different close to and far from the incident source. We applied Monte Carlo simulations, using the phase function of an infinitely long cylinder, to explain quantitatively the experimental results. These observations have consequences for noninvasive determination of the optical properties of tissue as well as for the diagnosis of early tissue alterations.     

Comparison of polarized-light propagation in biological tissue and phantoms

We demonstrate significant differences in the propagation of polarized light through biological tissue compared with two common tissue phantoms. Depolarization of linearly and circularly polarized light was measured versus propagation distance by use of two independent measurement techniques. The measurements were performed on adipose and myocardial tissues and on tissue phantoms that consisted of polystyrene microsphere suspensions and Intralipid. The results indicate that, in contrast with results obtained in tissue phantoms, linearly polarized light survives through longer propagation distances than circularly polarized light in biological tissue.     

A numerical formulation for nonlinear ultrasonic waves propagation in fluids

A finite-difference algorithm is developed for analysing the nonlinear propagation of pulsed and harmonic ultrasonic waves in fluid media. The time domain model allows simulations from linear to strongly nonlinear plane waves including weak shock. Effects of absorption are included. All the harmonic components are obtained from only one solving process. The evolution of any original signal can be analysed. The nonlinear solution is obtained by the implicit scheme via a fast linear solver. The numerical model is validated by comparison to analytical data. Numerical experiments are presented and commented. The effect of the initial pulse shape on the evolution of the pressure waveform is especially analysed.     

Variational formulation of covariant eikonal theory for vector waves

The eikonal theory of wave propagation is developed by means of a Lorentz-covariant variational principle, involving functions defined on the natural eight-dimensional phase space of rays. The wave field is a four-vector representing the electromagnetic potential, while the medium is represented by an anisotropic, dispersive nonuniform dielectric tensor D^μν(k,x). The eikonal expansion yields, to lowest order, the hamiltonian ray equations, which define the lagrangian manifold k(x), and the wave-action conservation law, which determines the wave-amplitude transport along the rays. The first-order contribution to the variational principle yields a concise expression for the transport of the polarization phase. The symmetry between k-space and x-space allows for a simple implementation of the Maslov transform, which avoids the difficulties of caustic singularities.     

The numerical and experimental study of photon diffusion inside biological tissue using boundary integral method

In this study, the diffusion of photons in turbid media, like biological tissue has been studied. Due to scattering and absorption of photons in such media, the study of photon propagation in biological tissue is complicated. The several numerical methods have been presented to simulate the behavior of diffused photons. Recently, Boundary Integral Method (BIM) has been offered to simulate photon migration inside biological tissues. This method has advantage, e.g. lower computational time in compared with other numerical methods. In this study, the accuracy and precision of BIM compares with another numerical method like Monte Carlo technique and finite difference method, and also the calculated results obtained by BIM and Monte Carlo method evaluate with measured results. Furthermore, the effects of scattering and absorption coefficient of tissue on the measured signal are studied.     

Soft photon spectrum in orthopositronium and vector quarkonium decays

QED gauge invariance, when combined with analyticity, leads to constraints on the low energy end of the emitted photon spectra. This is known as Low's theorem. It is shown that the Ore–Powell result, as well as further developments for the orthopositronium differential decay rate, are in contradiction with Low's theorem, i.e., that their predicted soft photon spectra are incorrect.A solution to this problem is presented. The implications for the orthopositronium lifetime puzzle, the charmonium ρ–π puzzle, the prompt photon spectrum in inclusive quarkonium decays and the extraction of α_S from quarkonium annihilation rates are briefly commented.     

原子激光传输的有效ABCD形式研究

利用含适量子系统传播子的ABCD形式的理论，将品质因子守恒系统中的ABCD定律，推广到有效品质因子守恒的系统——原子激光的传输中，通过引入有效品质因子和有效参数q_eff，利用Heisenberg图像得到传播子的有效ABCD形式.由于原子激光内部原子间相互作用的存在，原有的品质因子不再守恒，有效品质因子是描述原有品质因子和原子间相互作用综合作用效果的物理量. 关键词： 原子激光 传输 有效ABCD参数 传播子     

Kinetics of photon generation and propagation in a scintillation layer

Experimental results of an investigation of the kinetics of photon generation and propagation in a scintillation layer (n-terphenyl in polystyrene) are presented in this paper for the case of relativistic μ-meson passage through the layer. The duration of the photon emission Δt_em has been measured as a function of the scintillation layer thickness (l=0.05–0.5 m). The total duration of the photon emission and exit from the scintillation layer Δt_tot measured with a side coating having diffuse light reflection coefficient ρ=0.9 and 0.95 is given. A kinetic model of the photon generation and propagation in the scintillation layer is described. Satisfactory agreement of the expected values of Δt_em and Δt_tot with the exeeriment is indicated. Ivanovo State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 73–77, June, 1999.     

A single-pole model for the propagation of ultrasound in soft tissue

A minimum-phase function, which characterizes the velocity dispersion in tissue was calculated from measured attenuation. This function was incorporated into a causal tissue model. Predictions of attenuation using the minimum-phase function with just a single pole matched measured attenuation in the 1- to 10-MHz range within a few percent. Dispersion of phase velocity predicted by the single-pole model was comparable to measured dispersion. The frequency associated with the single pole, which is a relaxation frequency, decreased with hemoglobin concentration and collagen content but increased with temperature. The electrical equivalent circuit for this model is a delay coupled with a low-pass filter which can be configured as a resistance in series with a parallel combination of resistance and capacitance.     

Noise signal propagation in soft biological tissues

Mathematical models are formulated that discribe linear and nonlinear wave propagation in biological tissues. The basis of the method is evolutionary integro-differential equations with a kernel that takes into account the specific properties of tissue. An equation is obtained for the correlation function of acoustic noise in a medium with memory. The procedure for calculating the correlation function by the given type of kernel and noise spectrum at the entrance to the medium is described. It is shown that in different tissue, there is a difference in the laws of decrease in full intensity of a wideband signal with distance. It is demonstrated that the nonlinear equation in the limiting cases of ??short-?? and ??long-term?? memory reduces to equations that have been well studied in statistical nonlinear acoustics.     

Time-domain impedance formulation for transmission line matrix modelling of outdoor sound propagation

Outdoor sound propagation modelling has attracted considerable attention in the past and has lead to many analytical and numerical models. More recently, the increase of computational power has led to consider time-domain methods that enable to consider transient phenomena. Among these models, the transmission line matrix (TLM) method has been proposed, but the sound absorption at boundaries appears to be a somewhat underdeveloped aspect of this approach. In this paper, a specific implementation of impedance boundary condition is proposed. The method is based on the approximation of the impedance as a sum of linear systems, which allows the formulation of an equivalent impedance model in the time-domain. The proposed implementation is applied for two common impedance models of porous material. Numerical simulations have been carried out in the case of sound propagation over a flat ground with and without an impedance discontinuity, and for several values of specific airflow resistivity. TLM numerical predictions expressed in terms of excess attenuation relative to free field show a good agreement with analytical solutions.     

Effective photon spectra for the photon colliders

The luminosity distribution in the effective mass at a photon collider usually has two peaks which are well separated: a high energy peak with mean energy spread about 5–7% and a wide low energy peak. The low energy peak strongly depends on the details of the design and is unsuitable for the study of new physics phenomena. We find a simple approximate form for the spectra of colliding photons for and colliders, whose convolution describes the high energy luminosity peak with a good accuracy in most of the essential preferable region of the parameters. Received: 14 September 1999 / Published online: 17 February 2000     

An axisymmetric hypersingular boundary integral formulation for simulating acoustic wave propagation in supercavitating flows

Flow supercavitation begins when fluid is accelerated over a sharp edge, usually at the nose of an underwater vehicle, where a phase change occurs and causes a low density gaseous cavity to gradually envelop the whole object (supercavity) thereby allowing for higher speeds of underwater vehicles. The supercavity may be maintained through ventilated cavitation caused by injection of gases into the cavity, which causes fluctuations at the vapor–water interface. A major issue that concerns the efficient operation of an underwater object’s guidance system (which is achieved by high frequency acoustic sensors mounted within the nose region), is the hydrodynamic noise produced due to the fluctuating vapor–water interface. It is important to carry out a detailed study on the effect of self-noise at the vehicle’s nose that is generated by the ventilating gas jet impingement on the supercavity wall. For this purpose, the present study uses a boundary element method which is more versatile compared to other numerical techniques such as the finite element/finite difference methods. The variation of acoustic pressure at the vehicle nose for various shapes of cavitators, boundary conditions and jet impact diameters are presented. Comparisons are made with the semi-analytical procedure of Howe et al. (Howe et al., On self-noise at the nose of a supercavitating vehicle. Journal of Sound and Vibration, 322 (2009a), 772–784) and finite element based COMSOL commercial package. Several issues pertaining to the behaviour of analytical and numerical results are highlighted. Finally, the proposed boundary element technique is used to study arbitrary shapes of supercavities which may encountered at various stages of supercavity development.     

复数折射率介质中光束传输的Schrodinger形式理论研究

将复数折射率介质的Helmholtz方程化为具有复数势能的Schredinger方程的形式,用进化算子的逆算子代替相应的共轭算子,定义了与实数折射率系统光束传输参数具有相似性质的力学量的复数平均值,证明复数光束质量因子守恒是复数ABCD定律成立的充要条件.利用推广的Heisenberg图象,从复数光束传输参数的进化方程得到了位置算子和动量算子的线性进化方程,进一步利用量子力学的表象和表示理论得到了一般复数ABCD系统的Huygens积分.     

A semi-analytical finite element formulation for modeling stress wave propagation in axisymmetric damped waveguides

A semi-analytical finite element (SAFE) method is presented for analyzing the wave propagation in viscoelastic axisymmetric waveguides. The approach extends a recent study presented by the authors, in which the general SAFE method was extended to account for material damping. The formulation presented in this paper uses the cylindrical coordinates to reduce the finite element discretization over the waveguide cross-section to a mono-dimensional mesh. The algorithm is validated by comparing the dispersion results with viscoelastic cases for which a Superposition of Partial Bulk Waves solution is known. The formulation accurately predicts dispersion properties and does not show any missing root. Applications to viscoelastic axisymmetric waveguides with varying mechanical and geometrical properties are presented.