Scattering Media Research Articles

Estimation of dose enhancement to soft tissue due to backscatter radiation near metal interfaces during head and neck radiothearpy - A phantom dosimetric study with radiochromic film

The objective of this study was to investigate the dose enhancement to soft tissue due to backscatter radiation near metal interfaces during head and neck radiotherapy. The influence of titanium-mandibular plate with the screws on radiation dose was tested on four real bones from mandible with the metal and screws fixed. Radiochromic films were used for dosimetry. The bone and metal were inserted through the film at the center symmetrically. This was then placed in a small jig (7 cm × 7 cm × 10 cm) to hold the film vertically straight. The polymer granules (tissue-equivalent) were placed around the film for homogeneous scatter medium. The film was irradiated with 6 MV X-rays for 200 monitor units in Trilogy linear accelerator for 10 cm × 10 cm field size with source to axis distance of 100 cm at 5 cm. A single film was also irradiated without any bone and metal interface for reference data. The absolute dose and the vertical dose profile were measured from the film. There was 10% dose enhancement due to the backscatter radiation just adjacent to the metal-bone interface for all the materials. The extent of the backscatter effect was up to 4 mm. There is significant higher dose enhancement in the soft tissue/skin due to the backscatter radiation from the metallic components in the treatment region.

Measurement of the Time-Resolved Reflection Matrix for Enhancing Light Energy Delivery into a Scattering Medium

Multiple scatterings occurring in a turbid medium attenuate the intensity of propagating waves. Here, we propose a method to efficiently deliver light energy to the desired target depth in a scattering medium. We measure the time-resolved reflection matrix of a scattering medium using coherent time-gated detection. From this matrix, we derive and experimentally implement an incident wave pattern that optimizes the detected signal corresponding to a specific arrival time. This leads to enhanced light delivery at the target depth. The proposed method will lay a foundation for efficient phototherapy and deep-tissue invivo imaging in the near future.

Focused Lasing Through Dynamic Scattering Media

The ability to control light through dynamically varying heterogeneous media could be important for applications ranging from free-space communication to laser therapy. The underlying challenge is to control an optical wavefront with a large number of degrees-of-freedom (DOF) faster than the medium dynamics.

Propagation of a Collimated Beam in the Refractive Scattering Medium

We consider the problem of propagation of the radiation from a point unidirectional source in a scattering medium with a spatially inhomogeneous refractive index. A scheme for numerical solution of the radiative transfer equation with isolated singularities of forward and backward fluxes is proposed. It is demonstrated that analytical solution of the problem is possible in the small-angle approximation, within the framework of which the numerical calculations of the angular distribution of intensity are also performed with allowance for the variances of the scattered-radiation propagation path lengths.

The Combined Radiative Integral Equations and Finite-Element Method for Radiation in Anisotropic Scattering Media

A combined procedure of the radiative integral equation and finite-element method (IEFEM) is proposed for handling radiative heat transfer in linearly anisotropic scattering media. The IEFEM can eliminate the angular discretization and easily handle irregular geometries. The present work provides a solution of radiative transfer in rectangular and irregular quadrilateral enclosures containing participating media. The influences of emissivities, albedos, and anisotropy on the boundary fluxes or incident intensity have been analyzed. Compared with the results in published references, the present IEFEM has no limitation to geometry and can predict the radiative heat transfer in linearly anisotropic scattering media accurately.

Photoacoustic Tomography System

Basados en el efecto fotoacústico pulsado, se desarrolló un dispositivo experimental para generar imágenesbidimensionales de muestras ópticamente opacas ocultas en un medio turbio. El medio turbio se hizo de agar ynanopartículas monodispersas, con estos materiales indujimos una absorción y esparcimiento óptico semejante al delos tejidos humanos. La señal PA se genera por la absorción óptica del la muestra; una vez producida viaja a travésdel medio esparsor. Las propiedades de ópticas de la muestra así como la forma de éste definen la amplitud y formade la señal PA. En el sistema fotoacústico se utilizaron pulsos láser de Nd: YAG de 10 ns, con frecuencia derepetición de 10Hz, y longitud de onda de 1064 nm. Las señales generadas fueron registradas por un transductorultrasónico con frecuencia de corte de 10 MHz. Tanto la muestra como el medio esparsor se rotaron para obtener 36proyecciones, las cuales fueron utilizadas para alimentar un algoritmo de reconstrucción de imágenes basado en latransformada de Radon. Como resultado se obtuvieron imágenes tomográficas 2D de tres muestras distintas.

The impact of cataract on the quantitative, non-invasive assessment of retinal blood Flow

The aim of the study was to determine the impact of cataract on the quantitative, non-invasive assessment of retinal blood flow assessed by bidirectional laser Doppler flowmetry and simultaneous vessel densitometry. Ten patients scheduled for extracapsular cataract extraction using phacoemulsification and intraocular lens implantation between the ages of 61 and 84 (mean age 73 years, SD ± 8) were prospectively recruited. Two visits were required to complete the study; one visit prior to extracapsular cataract extraction and one at least 6 weeks after the surgery to allow for sufficient postoperative recovery. The severity of cataract was documented using the Lens Opacity Classification System (LOCS, III) at the first visit. Retinal arteriolar hemodynamics were measured at both visits using the high-intensity setting of the Canon Laser Blood Flowmeter. All eyes showed no clinical signs of postoperative intraocular inflammation. The quantitative assessment of retinal arteriolar diameter and blood flow were reduced following extracapsular cataract extraction (Wilcoxon signed-rank test, p = 0.022 and p=0.028, respectively); however, centreline blood velocity was not significantly changed (Wilcoxon signed-rank test, p=0.074). Intraocular pressure was unchanged pre- and postcataract extraction. Retinal vessel densitometry assessment in the presence of cataract results in the erroneous elevation of the diameter measurement and thereby the calculation of blood flow. The bidirectional Doppler assessment of blood velocity appears to be more robust to light scatter induced by cataract. Care needs to be exercised in the interpretation of studies of retinal vessel diameter or blood flow that utilize similar densitometry techniques.

Properties of spark plasma sintered nanostructured Zn1+xSb

AbstractEngineering materials with specific physical properties has recently focused on the effect of nanoscopic inhomogeneities at the 10 nm scale. Such features are expected to scatter medium and long‐wavelength phonons lowering thereby the thermal conductivity of the system without simultaneously decreasing the charge transport (phonon–glass electron–crystal concept). A new Zn1+xSb nanophase obtained by a wet chemical approach was densified by spark plasma sintering (SPS). Investigations on compounds subsumed as “Zn4Sb3” always suffer from its low thermal stability and the contamination of the nanoparticles with solvents and additives used in the synthesis. In order to gain insight into this compound's electronic properties we investigated a material free from remnants of the synthesis but contaminated with a small amount of well‐characterized decomposition product, i.e., ZnSb. To investigate the influence of the sintering process on the densified samples, different SPS conditions were applied. Four different conditions were used with heating rates between 160° and 230 °C/min, sintering temperatures between 130 and 190 °C and sintering times between 3 and 6 min. Powders from the surface of the pellets were subject to powder X‐ray diffraction (XRD) yielding information about the surface composition. Small pieces of the pellets were also characterized using high‐energy synchrotron radiation scattering in order to reveal the phase compositions inside the pellets. Small changes in the sintering conditions of the samples were found to have a large influence on the resulting sample compositions. In addition, the phase compositions on the surface differ significantly from the ones inside the pellets which show a much higher grade of decomposition. The density and morphology of the obtained pellets have been investigated by means of laser microscopy and scanning electron microscopy (SEM). The low density and porosity of the different pellets is a result of the graphite pressing tool which has to be used to ensure the temperature control during the SPS process.

Experimental Determination of Conditions for Existence of a Bimodal Temporal Distribution of a Laser Pulse Passing through a Homogeneous Layer of a Highly Scattering Biological Medium

Experimental research into conditions for observation of separation of ballistic and scattered components of tem-poral distribution (bimodality effect) of ultrashort laser pulses passed through a homogeneous layer of model high scattering biological medium is discussed. The range of existence of bimodal temporal distribution for three values of layer thickness was determined.

COHERENT BACKSCATTERING OF CIRCULARLY POLARIZED LIGHT FROM A DISPERSE RANDOM MEDIUM

To describe propagation of polarized electromagnetic wave within a disperse random medium a new Monte Carlo based technique with an adopted vector formalism has been developed. The technique has been applied for simulation of coherent backscattering of circularly polarized optical radiation from a random scattering medium. It has been found that the sign of helicity of circular polarized light does not change for a medium of point-like scatterers and can change signiflcantly for the scatterers with the higher anisotropy. We conclude that the helicity ∞ip of the circular polarized light can be observed in the tissue-like media. We flnd that this phenomenon manifests itself in case of limited number of scattering events and, apparently, can be attributed to the pulse character of incident radiation rather than to the speciflc form of scattering particles.

Solution Synthesis of a New Thermoelectric Zn1+xSb Nanophase and Its Structure Determination Using Automated Electron Diffraction Tomography

Engineering materials with specific physical properties have recently focused on the effect of nanoscopic inhomogeneities at the 10 nm scale. Such features are expected to scatter medium- and long-wavelength phonons thereby lowering the thermal conductivity of the system. Low thermal conductivity is a prerequisite for effective thermoelectric materials, and the challenge is to limit the transport of heat by phonons, without simultaneously decreasing charge transport. A solution-phase technique was devised for synthesis of thermoelectric "Zn(4)Sb(3)" nanocrystals as a precursor for phase segregation into ZnSb and a new Zn-Sb intermetallic phase, Zn(1+delta)Sb, in a peritectoid reaction. Our approach uses activated metal nanoparticles as precursors for the synthesis of this intermetallic compound. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. Both phases were identified and structurally characterized by automated electron diffraction tomography combined with precession electron diffraction. An ab initio structure solution based on electron diffraction data revealed two different phases. The new pseudo-hexagonal phase, Zn(1+delta)Sb, was identified and classified within the structural diversity of the Zn-Sb phase diagram.

Quantification of Absorber through a Scattering Medium of Different Thickness Using Evanescent Light Piping

A non-invasive method has been developed for analyte quantification in fluids surrounded by optically-scattering, opaque walls. This method is based on steady state, visible wavelength reflectance measurements made simultaneously at multiple positions on the surface of a sample. Previous work has shown that reflectance measurements contain information about underlying scattering layers in layered scattering samples. We hypothesise that similar information about an absorbing layer below a scattering layer can be obtained from evanescent wave effects. Principal component analysis showed the data to be composed of three components, which were refined by a multivariate curve resolution alternating least squares (MCR-ALS) approach with non-negativity constraints. The first component is related to the scattering layer thickness, the second is associated with analyte concentration and the third is due to a minor back reflection within the sample cell. Both MCR and stagewise multi-linear regression (SMLR) approaches were taken to estimate analyte concentration and scattering layer thickness, for samples having thicknesses between 1 mm and 8 mm. Results demonstrate that a simple experimental configuration can easily predict optical properties of unknown samples. With the adoption of a multi-wavelength approach to this method, it is expected that improved absorption coefficient (μa) estimation accuracy can be realised in a variety of application areas such as in analysis through opaque containers, in vivo measurements and in-line monitoring of reactions.

Ultrasound-modulated optical imaging using a high-power pulsed laser and a double-pass confocal Fabry–Perot interferometer

We report the use of short ultrasonic bursts and high-peak-power laser pulses to detect absorbing objects in thick scattering media (SMs). The detection of ultrasound-tagged photons is performed with a double-pass confocal Fabry-Perot interferometer. Photons shifted by the fundamental and harmonic frequencies of the ultrasonic bursts were observed. Absorbing objects were detected in 30- and 60-mm-thick SMs including a sample of biological tissue.

Diffusive Waves in a Dilating Scattering Medium

We consider the propagation of a diffusive wave in a scattering medium submitted to a homogeneous expansion. The light multiply scattered by a glass spheres sample is measured. We analyze the variations of the scattered light when the material and the optical wavelength are dilated. We experimentally show that an isotropic expansion of the material is equivalent to a contraction of the wavelength. Moreover, the effect of an expansion of the material on the scattered wave may be canceled by a proportional increase of the wavelength, keeping the phase of the scattered wave unchanged. Applications to the characterization of deformation of disordered materials are outlined.

TU‐E‐304A‐02: Characterizing Metal Oxide Semiconductor Field Effect Transistor MOSFET) Radiation Detectors In a Scatter Medium Using CT Radiation Beam Delivery System

Purpose: To study MOSFET radiation response characteristics for CT organ dosimetry applications in a scatter medium (phantom), using multi slice CT as radiation source. Method & Materials: We used five Thomsen‐Nielsen MOSFETs with reader and wireless communication system to a laptop computer, GE Light Speed VCT‐64 scanner, Radcal 9095 dosimeter with 0.6 cc ionization chamber and kVp sensor, 10 cm Acrylic phantom with Acrylic rods. Each MOSFET was placed separately at isocenter, inside the acrylic phantom for characterization. An exposure of approximately 1000 mR (1006.6 mR to 1011.8 mR) was delivered to each MOSFET. Exposures were determined using 0.6 cc ionization chamber placed inside the phantom, at isocenter with CT tube in rotate mode as well as parked at various angles. MOSFET energy dependence, linearity of response and sensitivity with CT X‐ray tube in rotate mode were studied. Angular dependency of MOSFETS response was studied with tube parked at various angles in 45 degree increments. Results: MOSFET response (in mV, averaged for 5 MOSFETS) changed by 14% when kilovoltage varied from 80 through 140 kVp. Linearity of response was very good for all the energies selected. Sensitivity was obtained in the range of 25.6 mV/R to 28.8 mV/R for all the MOSFETs studied. The averaged maximum variation of all MOSFETs as a function of tube angle was about 5% and no statistically significant angular dependence was observed. Conclusion: Calibrating MOSFETs inside scatter media with CT X‐ray tube in rotate mode is recommended for MOSFET use in organ dosimetry applications with multislice CT scanners.

Characterization of a water‐equivalent fiber‐optic coupled dosimeter for use in diagnostic radiology

This work reports on the characterization of a new fiber-optic coupled (FOC) dosimeter for use in the diagnostic radiology energy range. The FOC dosimeter was constructed by coupling a small cylindrical plastic scintillator, 500 microm in diameter and 2 mm in length, to a 2 m long optical fiber, which acts as a light guide to transmit scintillation photons from the sensitive element to a photo-multiplier tube (PMT). A serial port interface on the PMT permits real-time monitoring of light output from the dosimeter via a custom computer program. The FOC dosimeter offered excellent sensitivity and reproducibility, allowing doses as low as 0.16 mGy to be measured with a coefficient of variation of only 3.64%. Dose linearity was also excellent with a correlation coefficient of 1.000 over exposures ranging from 0.16 to 57.29 mGy. The FOC dosimeter exhibited little angular dependence from axial irradiation, varying by less than 5% over an entire revolution. A positive energy dependence was observed and measurements performed within a scatter medium yielded a 10% variation in sensitivity as beam quality changed due to hardening and scatter across a 16 cm depth range. The current dosimetry system features an array of five PMTs to allow multiple FOC dosimeters to be monitored simultaneously. Overall, the system allows for rapid and accurate dose measurements relevant to a range of diagnostic imaging applications.

Passive Sensor Imaging Using Cross Correlations of Noisy Signals in a Scattering Medium

It is well known that the travel time or even the full Green's function between two passive sensors can be estimated from the cross correlation of recorded signal amplitudes generated by ambient noise sources. It is also known that the direction of the energy flux from the noise sources affects the estimation of the travel time. Using the stationary phase method, we show here that the travel time can be effectively estimated when the ray joining the two sensors continues into the noise source region. We extend this analysis to passive sensor imaging of reflectors with different ambient noise source configurations by suitably migrating the cross correlations. If in addition there is multiple scattering in the medium, then reflectors can be imaged with passive sensor networks or arrays by migrating suitable fourth-order cross correlations. Fourth-order cross correlations can also be used with auxiliary passive sensors in order to enhance travel time estimation in a scattering medium.

Ray-Tracing Method for Radiative Heat Transfer in a Two-Layer Anisotropic Scattering Medium

Transient radiation heat transfer in a two-layer anisotropic scattering medium is investigated. Each layer of the medium can have different thermal and radiative properties. The two surfaces are assumed to be semitransparent or opaque, and the interface between the two layers is assumed to be semitransparent. Specular reflection is considered at the surfaces and the interface. A general criterion for total reflection at the surfaces and the interface is proposed to overcome the integral singularity problem above the critical angle. A fully implicit control-volume method is used to solve the transient energy equation of coupled radiation and conduction, and a ray-tracing/nodal-analyzing method is applied in the solution of the radiative transfer. Effects of emissivities of the surfaces, extinction coefficients, and optical boundary conditions on transient radiative heat transfer are examined. Results show that for the medium layer having a larger refractive index, its average temperature level may be higher, mainly due to the contribution of total reflection at the interface facing the layer, even if its absorption coefficient is relatively small compared to the medium layer having a larger absorption coefficient and a smaller refractive index.

Influence of size parameter and refractive index of the scatterer on polarization-gated optical imaging through turbid media

The influence of incident polarized light, refractive index, and size parameter of the scatterer on achievable resolution and contrast (image quality) of polarization-gated transillumination imaging in turbid media is reported here. Differential polarization detection led to significant improvement of image quality of an object embedded in a medium of small-sized scatterers (diameter D<<lambda, isotropic scattering medium, anisotropy parameter g<or=0.2), especially using circular polarization. In contrast, for anisotropic scattering media composed of larger-sized scatterers (D>or=lambda,g>or=0.7), the improvement in image quality was less pronounced using either linear or circular polarization gating when the refractive index of the scatterer was high (ns=1.59), but for a lower value of refractive index (ns=1.37), image quality improved with the differential circular polarization gating. We offer a plausible explanation for these observations.

Least-Squares Finite-Element Method of Multidimensional Radiative Heat Transfer in Absorbing and Scattering Media

A least-squares finite-element method (LSFEM) is developed to simulate the radiative transfer in absorbing and scattering media. This model is based on the discrete ordinates method (DOM) and the least-squares finite-element method. It can be presented as an alternative to the traditional Galerkin finite-element method (GFEM). This method is used to overcome the spurious oscillation which be found in the GFEM for radiative transfer. In addition, the resulting coefficients matrix produced by the LSFEM is symmetric and positive-definite. Only half of a sparse matrix needs to be stored. Some efficient iterative algorithms for symmetrical systems of equations can be used successfully. In order to validate this method, two 2-D problems and a 3-D problem of radiative transfer are examined. The computational results indicate that the present model can simulate the radiative transfer in the multidimensional complex geometric enclosure efficiently and accurately. More important, the present model is proved to be wiggle-free.