Search

A four level chiral medium is considered to analyze and investigate theoretically the reflection/transmission coefficients of right circularly polarized (RCP) beam and left circularly polarized (LCP) beam as well as their corresponding GH-shifts under the effect of compton scattering. Density matrix formalism is used for calculation of electric and magnetic probe fields coherence. The polarization and magnetization are calculated from probes coherence terms in the chiral medium. The electric and magnetic susceptibilities as well as chiral coefficients are related with polarization and magnetization. The refractive indices of RCP and LCP beams under compton scattering effect is modified from the electric/magnetic susceptibilities, chiral coefficients, mass and charge of electron as well as compton scattering angle. The giant positive and negative birefringent Goos–Hänchen (GH) shifts in reflection and transmission beams are investigated in this manuscript under Compton scattering effect. The RCP and LCP beams obey the normalization condition | R (+ , -) | + | T (+ , -) | = 1 at the interface of a lossy chiral medium of | A (+ , -) | ≃ 0 and a thin sheet of balsa wood under the effect of compton scattering angle, incident angle, probe field detuning, control field Rabi frequency, phases of electric and magnetic fields and phase of superposition states. Significant positive/negative giant GH-shifts in reflection and transmission beams are investigated. The results show potential applications in modification of cloaking devices, image coding, polarizing filters and LCD displays. [ABSTRACT FROM AUTHOR]

Background: Silicon (Si) is a possible sensor material for photon counting detectors (PCDs). A major drawback of Si is that roughly two‐thirds of x‐ray interactions in the diagnostic energy range are Compton scattering. Because Compton scattering is an energy‐insensitive process, it is commonly assumed that Compton events retain little spectral information. Purpose: To quantify how much information can be recovered from Compton scattering events in models of Si PCDs. Methods: We built a simplified model of Si interactions including two interaction mechanisms: photoelectric effect and Compton scattering. We considered three different binning options that represent strategies for handling Compton events: in Compton censoring, all events under 38 keV (the maximum energy possible from Compton scattering for a 120 keV incident photon) were discarded; in Compton counting, all events between 1 and 38 keV were placed into a single bin; in Compton binning, all events were placed into energy bins of uniform width. These were compared to the ideal detector, which always recorded the correct energy (i.e., 100% photoelectric effect). Every photon was assumed to interact once and only once with Si, and the energy bin width was 5 keV. In the primary analysis, the Si detector was irradiated with a 120 kV spectrum filtered by 30 cm of water, with 99.5% of the arriving spectrum above 38 keV so that there was good separation between photoelectric effect and Compton scattering, and the figures of merit were the Cramér–Rao lower bound (CRLB) of the variance of iodine and water basis material decomposition images, as well as the CRLB of virtual monoenergetic images (i.e., linear combinations of material images) that maximize iodine CNR or water CNR. We also constructed a local linear estimator that attains the CRLB. In secondary analyses, we applied other sources of spectral distortion: (1) a nonzero minimum energy threshold; (2) coarser, 10 keV energy bins; and (3) a model of charge sharing. Results: With our chosen spectrum, 67% of the interactions were Compton scattering. Consistent with this, the material decomposition variance for the Compton censoring model, averaged over both basis materials, was 258% greater than the ideal detector. If Compton events carried no spectral information, the Compton counting model would show similar variance. Instead, its basis material variance was 103% greater than the ideal detector, implying that Compton counts indeed carry significant spectral information. The Compton binning model had a basis material variance 60% greater than the ideal detector. The Compton binning model was not affected by a 5 keV minimum energy threshold, but the variance increased from 60% to 107% when charge sharing was included and to 78% with coarser energy bins. For optimized CNR images, the average variance was 149%, 12%, and 10% higher than the ideal detector for the Compton censoring, counting, and binning models, reinforcing the hypothesis that Compton counts are useful for detection tasks and that precise energy assignments are not necessary. Conclusions: Substantial spectral information remains after Compton scattering events in silicon PCDs. [ABSTRACT FROM AUTHOR]

Sensitivity and detection limit of X‐ray detectors are crucial for security checks, medical diagnoses, and industrial inspections. In this study, it is reported that introducing some cations containing lone‐pair electrons is beneficial for enhancing the Compton scattering effect and thus improving X‐ray detection performance. As an example, SnTe3O8 is selected and grown as a novel high‐temperature X‐ray detection crystal. Because of the high resistivity of 2 × 1014 Ω cm and high mobility lifetime product of 3.22 × 10−4 cm2 V−1, SnTe3O8 X‐ray detector exhibits a high sensitivity of 436 µC Gyair−1 cm−2 under 120 keV hard X‐ray, a low dark current drift of 2.44 × 10−9 nA cm−1 s−1 V−1 and a record low detection limit of 8.19 nGyair s−1 among all oxide X‐ray detectors. Furthermore, the high‐temperature sensitivity of SnTe3O8 X‐ray detector is enhanced to 617 µC Gyair−1 cm−2 at 175 °C, which is ≈31 times larger than that of the commercial α‐Se. The high thermal stability and stable high‐temperature sensitivity of SnTe3O8 single crystal X‐ray detectors have potential applications in high‐temperature environments. The results not only provide an excellent high‐temperature X‐ray detection crystal but also propose an effective method to explore X‐ray detector materials with excellent performances. [ABSTRACT FROM AUTHOR]

The process of Compton scattering by a free electron with subsequent reemission of one or two photons is considered in the assumption of finite interaction time. The corresponding cross sections are obtained in the framework of relativistic quantum electrodynamics using a modified form of fermion propagator with complex transmitted momentum. It is shown that finite time effects can be observable at sufficiently low energies of scattered photons. The proposed method also regularizes arising infrared divergence in the cross section of the double Compton effect. Possible experimental verification of considered theoretical approach is discussed. [ABSTRACT FROM AUTHOR]

We theoretically investigated the sub-luminal and super-luminal light propagation by introducing compton scattering and Kerr non-linear effects to a four level atomic system. Slow light propagation is observed at a group index of 3918 in the transparency region. The slow light propagation is tuned to fast light propagation at group index 0.2, using compton scattering angle of φ = π / 6 . The surperluminality is further enhanced with compton scattering angle at the resonance frequency. Under the Kerr nonlinearity, the slow light propagation at a group index of 3918 is enhanced to fast light propagation at group index of 2 × 10 6 at the resonance frequency. The group index is reduced to 6.56281 × 10 - 5 in the presence of both Kerr non-linearity and compton scattering. In the presence of compton scattering, the Phase shift at π / 2 without Kerr nonlinearity is 7 × 10 - 8 radian and with Kerr nonlinearity its value is 1.7 × 10 - 9 radian at resonance frequency. This demonstrate that introducing Kerr effect and compton scattering lead to significant modifications in the phase shift. [ABSTRACT FROM AUTHOR]

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view (FOV), which requires multiple detectors to cover the whole required FOV or machinery to rotate the detector’s narrow FOV detector. We propose a novel $4\pi $ directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and $4\pi $ detection ability. Instead of the traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on Gd3Al2Ga3O12 (GAGG) (Ce) scintillator coupled to silicon photomultiplier. An additional advantage of the proposed detector is obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such an arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is an efficient $4\pi $ radiation detection for high-energy gamma rays without the use of heavy collimators. [ABSTRACT FROM AUTHOR]

An isotropic electromagnetic field in a plasma of thermalized electrons undergoes changes in energy as a result of Compton scattering and an Einstein–Hopf drag force on the electrons, eventually approaching a Bose–Einstein photon distribution at the electron temperature. The rate of change of field energy due to the combined effects of Compton scattering and the drag force is shown to be described by the Kompaneets equation for photon diffusion in frequency space. A similarity is noted between this approach and Einstein's derivation of the Planck spectrum based on the recoil of atoms as they absorb and emit radiation. [ABSTRACT FROM AUTHOR]

We propose a simple generalization of axion-electrodynamics (A-ED) for the general case in which both scalar and pseudoscalar axion-like fields are present in the (scalar) Lagrangian of the system. We make some remarks on the problem of finding solutions to the differential equations of motion characterizing the propagation of coupled axion fields and electromagnetic (EM) waves. Our primary goal (which is not explored here) is to understand and predict novel phenomena that have no counterpart in pseudoscalar A-ED. With this end in view, we discuss on very general grounds possible processes related to scalar (and pseudoscalar) axions, e.g., the Primakoff effect; the Compton scattering; and, notably, the EM two-photon axion decay. [ABSTRACT FROM AUTHOR]

Radiative energy losses are very important in regulating the cosmic ray electron and/or positron (CRE) spectrum during their propagation in the Milky Way. Particularly, the Klein–Nishina (KN) effect of the inverse Compton scattering (ICS) results in less efficient energy losses of high-energy electrons, which is expected to leave imprints on the propagated electron spectrum. It has been proposed that the hardening of CRE spectra around 50 GeV observed by Fermi-LAT, AMS-02, and DAMPE could be due to the KN effect. We show in this work that the transition from the Thomson regime to the KN regime of the ICS is actually quite smooth compared with the approximate treatment adopted in some previous works. As a result, the observed spectral hardening of CREs cannot be explained by the KN effect. It means that an additional hardening of the primary electrons spectrum is needed. We also provide a parameterized form for the accurate calculation of the ICS energy-loss rate in a wide energy range. [ABSTRACT FROM AUTHOR]

It is shown that, although Einstein and Compton's arguments were fundamental for the acceptance of the corpuscular character of electromagnetic radiation, there are other approaches able to explain the photoelectric and Compton effects, without using the photon concept. In this context, radiation is treated as a classic electromagnetic wave that interacts with the electron. [ABSTRACT FROM AUTHOR]

A Lorentz-covariant system of wave equations is formulated for a quantum-mechanical two-body system in one space dimension, comprised of one electron and one photon. Manifest Lorentz covariance is achieved using Dirac's formalism of multi-time wave functions, i.e., wave functions Ψ (2) (x ph , x el ) where x el , x ph are the generic spacetime events of the electron and photon, respectively. Their interaction is implemented via a Lorentz-invariant no-crossing-of-paths boundary condition at the coincidence submanifold { x el = x ph } , compatible with particle current conservation. The corresponding initial-boundary-value problem is proved to be well-posed. Electron and photon trajectories are shown to exist globally in a hypersurface Bohm–Dirac theory, for typical particle initial conditions. Also presented are the results of some numerical experiments which illustrate Compton scattering as well as a new phenomenon: photon capture and release by the electron. [ABSTRACT FROM AUTHOR]

Liquid Scintillation Counting (LSC) is the most commonly used technique for quantification of 14C in the environmental samples. An optimization study was carried out for the analysis of 14C with the direct carbon dioxide absorption method using LSC. The absorption capacity of CO2 in varying amounts of CO2 absorber Carbo-Sorb E to Permafluor scintillation cocktail volumes is found to be 5.33 ± 0.053 mmol/mL of Carbo-Sorb E. The optimum volume ratio of Carbo-Sorb E to Permafluor scintillation cocktail is found to be 1:1 based on the minimum detection activity (MDA) values. The effect of CO2 loading in the optimized absorption mixture shows that with an increasing CO2 amount (up to saturation) there is an increase in tSIE values which is due to an increase of Compton scattering effect in the CO2 loaded samples. The region of interest (ROI) for 14C measurement is found to be 10–96 keV based on the figure of merit values and the efficiency for detecting 14C is 83.45 % in the optimized ROI window. [ABSTRACT FROM AUTHOR]

Since the decomposition of positron lifetime spectra requires solving an ill-posed and inverse problem, the accurate knowledge of the spectrometer's instrument response function is crucial for extracting the true underlying physical information of the phenomenon under investigation. In general, the instrument response function is modelled by a superposition of Gaussian functions, since an analytical solution for the convolution with an exponential distribution function exists and, hence, the characteristic lifetimes and its corresponding contributions can be obtained by non-linear least-squares fitting. In contrast, the iterative least-squares re-convolution approach determines the best fit of the recorded lifetime spectrum by re-convoluting a sum of N expected exponential decays with the numerical data of the experimentally obtained instrument response function. For a laboratory setup, two methods exist to estimate the shape of the instrument response function from experiment: (1) the direct method using a 60Co isotope and an indirect method by graphically deconvoluting the monoexponential lifetime spectrum obtained from 207Bi. For both variants, the energies of the incident gamma-rays are considerably different to the energies accompanying the creation (1274 keV) and annihilation (511 keV) of a positron using 22Na: 60Co (1173 keV, 1333 keV), 207Bi (570 keV, 1064 keV). Here we present a detailed study on the basis of plastic scintillators regarding the spectra decomposability by using the re-convolution technique with experimentally obtained instrument response functions. We can clearly show that beyond incident gamma-ray energy differences, the Compton scattering effects and pile-up events represent the limiting factors in this approach. [ABSTRACT FROM AUTHOR]

The leading role of the bremsstrahlung at resonant Compton scattering of a hard X-ray photon by a multielectron atom has been theoretically predicted. This result can be important in the interpretation of Kα, β spectra of X-ray emission of stars, galaxies, and clusters of galaxies. [ABSTRACT FROM AUTHOR]

Dual-energy information in computed tomography can be obtained through different technical approaches. Most available scanner designs acquire examination with two different X-ray spectra. Recently, the first detector-based approach became clinically available. Upfront, physical principles of dual-energy CT are reviewed, including the interaction of photons with matter in terms of the Photoelectric effect and Compton scattering. In addition, available concepts to dual energy computed tomography are described. Afterwards, the spectral detector CT system is described in detail. The design of the of the stacked detector design and its inherent technical advantages and disadvantages are discussed. Further, the principles of image reconstruction, their possibilities and limitations are referred. The increase in reconstructions and data pose some challenges to both, clinical and technological workflow which are hereafter addressed. Finally, the detector-based approach is discussed in light of other, emission-based DECT approaches. [ABSTRACT FROM AUTHOR]

Purpose: Xtrim‐PET is a newly designed Silicon Photomultipliers (SiPMs)‐based prototype PET scanner dedicated for small laboratory animal imaging. We present the performance evaluation of the Xtrim‐PET scanner following NEMA NU‐4 2008 standards to help optimizing scanning protocols which can be achieved through standard and reliable system performance characterization. Methods: The performance assessment was conducted according to the National Electrical Manufacturers Association (NEMA) NU‐4 2008 standards in terms of spatial resolution, sensitivity, counting rate performance, scatter fraction and image quality. The in vivo imaging capability of the scanner is also showcased through scanning a normal mouse injected with 18F‐FDG. Furthermore, the performance characteristics of the developed scanner are compared with commercially available systems and current prototypes. Results: The volumetric spatial resolution at 5 mm radial offset from the central axis of the scanner is 6.81 µl, whereas a peak absolute sensitivity of 2.99% was achieved using a 250–650 keV energy window and a 10 ns timing window. The peak noise‐equivalent count rate (NECR) using a mouse‐like phantom is 113.18 kcps at 0.34 KBq/cc with 12.5% scatter fraction, whereas the NECR peaked at 82.76 kcps for an activity concentration level of 0.048 KBq/cc with a scatter fraction of 25.8% for rat‐like phantom. An excellent uniformity (3.8%) was obtained using NEMA image quality phantom. Recovery coefficients of 90%, 86%, 68%, 40% and 12% were calculated for rod diameters of 5, 4, 3, 2 and 1 mm, respectively. Spill‐over ratios for air‐filled and water‐filled chambers were 35% and 25% without applying any correction for attenuation and Compton scattering effects. Conclusion: Our findings revealed that beyond compactness, lightweight, easy installation and good energy resolution, the Xtrim‐PET prototype presents a reasonable performance making it suitable for preclinical molecular imaging‐based research. [ABSTRACT FROM AUTHOR]

The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated, synthetic, phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous, rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in both the size and the temperature of the emitting hotspot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER. [ABSTRACT FROM AUTHOR]

Here, we consider the problem of separating the relative contributions of kinematics and dynamics to the differential Klein-Nishina electronic cross section using graphical and numerical analysis. We show that the values of the energy of scattered photons, and hence the kinetic energy of recoiled electrons calculated from Compton's quantum theory of scattering of radiation, show a degree of matching that increases with the increase in incident photon energy as quantified by chi-square goodness of fit test, with the calculated differential Klein-Nishina electronic cross section per electron per unit solid angle for the scattering of an unpolarized photon by a stationary free electron, when appropriate normalization procedures are invoked. There is a high degree of matching in a regime where the total electronic Klein-Nishina cross section for the Compton scattering on a free stationary electron scales as the inverse of the incident photon energy and the contribution of the electro-magnetic interaction to differential electronic cross section diminishes. Hence the third level explanation of Compton effect by quantum electrodynamics has a degree of matching with the first level of Compton's quantum theory. The degree of mismatch is an indicator of the relative contribution of dynamics to differential Klein-Nishina electronic cross section compared to kinematics. For incident photon energies less than 1 MeV, we obtain the values of the scattering angles at which calculated differential cross section is nonzero but is kinematically limited which may lead to broadening of Compton profile. At the scattering angle where the differential cross section value is minimum for a given incident photon energy, we obtain the relative contribution of dynamics to the differential cross section compared to kinematics. Therefore, these predictions which need to be confirmed experimentally have significance to the understanding of the mechanisms of photon-electron interactions in the Compton scattering. [ABSTRACT FROM AUTHOR]

Abstract The AXSIS project (Attosecond X-ray Science: Imaging and Spectroscopy) aims to develop a THz-driven compact X-ray source for applications e.g. in chemistry and biology by using ultrafast coherent diffraction imaging and spectroscopy. The key components of AXSIS are the THz-driven electron gun and THz-driven dielectric loaded linear accelerator as well as an inverse Compton scattering scheme for the X-rays production. This paper is focused on the prototype of the THz-driven electron gun which is capable of accelerating electrons up to tens of keV. Such a gun was manufactured and tested at the test-stand at DESY. Due to variations in gun fabrication and generation of THz-fields the gun is not exactly operated at design parameters. Extended simulations have been performed to understand the experimentally observed performance of the gun. A detailed comparison between simulations and experimental measurements is presented in this paper. [ABSTRACT FROM AUTHOR]

Abstract The purpose of this study is to investigate the hidden corrosion at selected sites in corrosion simulated Mild Steel using information from the Compton scattered gamma rays. The proposed non-contact method employs a 137Cs source and an HPGe detector as part of the practical framework and it is integrated with the MCA system. The scattered gamma rays reflecting the corrosion is mapped in real time during the scanning of the specimen. Point by point scanning is carried out to acquire the pulse height spectrum and datas are used to determine the thickness of a known test object and hence thickness calibration is attained. With the calibration as a basis, hidden corrosion at the sub surfaces is detected and analysed. A reconstruction algorithm is explicated for this proposed work with a good precision of corrosion quantification in the subsequent layers. The precision is comparable with the radiography and transmission results. Experimental measurements and their results strongly confirm hidden corrosion to a depth of 4 mm (with a range of corrosion from 1.76 mm to 2.29 mm) in Mild Steel could be detected. Highlights • Compton photon scattering is explored as complementary technique for hidden corrosion inspection in Mild Steel. • A complete automated assembly of source, manipulator and detector are used. • Comprehensive evaluation of hidden corrosion in Mild Steel is provided. • The proposed method is validated by Radiography and Transmission technique. • Potential method for sub-surface and multilayered defect detection in complicated and inaccessible locations. [ABSTRACT FROM AUTHOR]

Wide-angle Compton scattering (WACS) is discussed within the handbag approach in which the amplitudes are given by products of hard subprocess amplitudes and form factors, specific to Compton scattering, which represent 1/ x -moments of generalized parton distributions (GPDs). The quality of our present knowledge of these form factors and of the underlying GPDs is examined. As will be discussed in some detail the form factor $ R_A$ and the underlying GPD ˜ are poorly known. It is argued that future data on the spin correlations $ A_{LL}$ and/or $ K_{LL}$ will allow for an extraction of $ R_A$ which can be used to constrain the large - t behavior of ˜ . [ABSTRACT FROM AUTHOR]

The processes of the nonlinear Compton and the nonlinear Thomson scattering in a field of intense plane electromagnetic wave in terms of photon yield have been considered. The quantum consideration of the Compton scattering process allows us to calculate the probability of a few successive collisions k of an electron with laser photons accompanied by the absorption of n photons (nonlinear regime) when the number of collisions and the number of absorbed photons are of random quantities. The photon spectrum of the nonlinear Thomson scattering process was obtained from the classical formula for intensity using the Planck's law. The conditions for which the difference between the classical and the quantum regimes is manifested was obtained. Such a condition is determined by a discrete quantum radiation mechanism, namely, by the mean number of photons k ¯ emitted by an electron passing through the laser pulse. [ABSTRACT FROM AUTHOR]

The Compton scattering in a dielectric medium is adopted. We have found that the momentum and energy of the photon inside the medium are related by the formula E ′ = ncp ′, where n is the refractive index of the medium. This formula is shown to stem from the invariance of the Einstein relativity relation, p = v ( E / c 2 ) inside and outside the medium. The Minkowski and Abraham models of refractive index p ′ = nE / c and p ′ = E /( nc ), respectively, require that E ′ = E and E ′ = n 2 E , respectively. The former model treated the photon as a wave (massless) and the latter treated the photon as a particle (matter) with an effective mass. The Snell's law is obtained by further imposing that the horizontal component of the light momentum remains the same. [ABSTRACT FROM AUTHOR]

Analytical properties of Compton profiles are used in order to simplify the analysis of neutron Compton scattering experiments. In particular, the possibility to fit the difference of Compton profiles is discussed as a way to greatly decrease the level of complexity of the data treatment, making the analysis easier, faster and more robust. In the context of the novel method proposed, two mathematical models describing the shapes of differenced Compton profiles are discussed: the simple Gaussian approximation for harmonic and isotropic local potential, and an analytical Gauss–Hermite expansion for an anharmonic or anisotropic potential. The method is applied to data collected by VESUVIO spectrometer at ISIS neutron and muon pulsed source (UK) on Copper and Aluminium samples at ambient and low temperatures. [ABSTRACT FROM AUTHOR]

In an effort to deal with many ionizing radiation imaging mechanisms involving the Compton effect, we study a Radon transform on circular cone surfaces having a fixed axis direction, which is called here conical Radon transform (CRT). Concretely, we seek to recover a density functioninfrom its integrals over such circular cone surfaces or its conical projections. Although the existence of the inverse CRT has been established, it is the aim of this work to use this result to extent the concept of back-projection, well known in Computed Tomography (CT) to this type of cone surfaces. We discuss in some details the features of back-projection in relation to the corresponding CRT adjoint operator as well as the filters that arise naturally from the exact solution of the inversion problem. This intuitive approach is attractive, lends itself to efficient computational algorithms and may provide hints and guide for more general back-projection methods on other classes of cone surfaces, for example, occurring in Compton camera imaging. Comprehensive numerical simulations results are presented and discussed to illustrate and validate this approach based on the concept of back-projection. [ABSTRACT FROM AUTHOR]

Purpose: In digital breast tomosynthesis (DBT), scatter correction is highly desirable, as it improves image quality at low doses. Because the DBT detector panel is typically stationary during the source rotation, antiscatter grids are not generally compatible with DBT; thus, a software-based scatter correction is required. This work proposes a fully iterative scatter correction method that uses a novel fast Monte Carlo simulation (MCS) with a tissue-composition ratio estimation technique for DBT imaging. Methods: To apply MCS to scatter estimation, the material composition in each voxel should be known. To overcome the lack of prior accurate knowledge of tissue composition for DBT, a tissue-composition ratio is estimated based on the observation that the breast tissues are principally composed of adipose and glandular tissues. Using this approximation, the composition ratio can be estimated from the reconstructed attenuation coefficients, and the scatter distribution can then be estimated by MCS using the composition ratio. The scatter estimation and image reconstruction procedures can be performed iteratively until an acceptable accuracy is achieved. For practical use, (i) the authors have implemented a fast MCS using a graphics processing unit (GPU), (ii) the MCS is simplified to transport only x-rays in the energy range of 10-50 keV, modeling Rayleigh and Compton scattering and the photoelectric effect using the tissue-composition ratio of adipose and glandular tissues, and (iii) downsampling is used because the scatter distribution varies rather smoothly. Results: The authors have demonstrated that the proposed method can accurately estimate the scatter distribution, and that the contrast-to-noise ratio of the final reconstructed image is significantly improved. The authors validated the performance of the MCS by changing the tissue thickness, composition ratio, and x-ray energy. The authors confirmed that the tissue-composition ratio estimation was quite accurate under a variety of conditions. Our GPU-based fast MCS implementation took approximately 3 s to generate each angular projection for a 6 cm thick breast, which is believed to make this process acceptable for clinical applications. In addition, the clinical preferences of three radiologists were evaluated; the preference for the proposed method compared to the preference for the convolution-based method was statistically meaningful (p < 0.05, McNemar test). Conclusions: The proposed fully iterative scatter correction method and the GPU-based fast MCS using tissue-composition ratio estimation successfully improved the image quality within a reasonable computational time, which may potentially increase the clinical utility of DBT. [ABSTRACT FROM AUTHOR]

Purpose: For accurate tissue inhomogeneity correction in radiotherapy treatment planning, the author previously proposed a simple conversion of the energy-subtracted computed tomography (CT) number to an electron density (ΔHU–ρe conversion), which provides a single linear relationship between ΔHU and ρe over a wide ρe range. The purpose of the present study was to reveal the relation between the ΔHU image for ρe calibration and a virtually monochromatic CT image by performing numerical analyses based on the basis material decomposition in dual-energy CT. Methods: The author determined the weighting factor, α0, of the ΔHU–ρe conversion through numerical analyses of the International Commission on Radiation Units and Measurements Report-46 human body tissues using their attenuation coefficients and given ρe values. Another weighting factor, α(E), for synthesizing a virtual monochromatic CT image from high- and low-kV CT images, was also calculated in the energy range of 0.03 < E < 5 MeV, assuming that cortical bone and water were the basis materials. The mass attenuation coefficients for these materials were obtained using the xcom photon cross sections database. The effective x-ray energies used to calculate the attenuation were chosen to imitate a dual-source CT scanner operated at 80–140 and 100–140 kV/Sn. Results: The determined α 0 values were 0.455 for 80–140 kV/Sn and 0.743 for 100–140 kV/Sn. These values coincided almost perfectly with the respective maximal points of the calculated α(E) curves located at approximately 1 MeV, in which the photon-matter interaction in human body tissues is exclusively the incoherent (Compton) scattering. Conclusions: The ΔHU image could be regarded substantially as a CT image acquired with monoenergetic 1-MeV photons, which provides a linear relationship between CT numbers and electron densities. [ABSTRACT FROM AUTHOR]

Compton cameras have become an instrument of interest for long-range localization of radioactive materials in nuclear-nonproliferation applications. In this work, a specialized simulation tool was developed for the optimization of a Compton camera through realistic Monte Carlo simulations. This tool can be used for Compton cameras with different geometries. The MCNPX-PoliMi code was used to simulate Compton scatters and photoelectric absorptions in the camera's detectors. The imaging capability is derived from the physics of Compton scattering and the full-energy information obtained in photoelectric absorptions. The simulated system is a 1 × 1 m 2 Compton camera consisting of two planar arrays of photon detectors. Several scintillators were evaluated: LaBr 3 , CaF 2 , and NaI(Tl) (all inorganic scintillators), and C 9 H 10 (plastic organic scintillator). The investigation was carried out with a 137 Cs source. A minimum detectable activity (MDA) was defined and used to assess the performance of cameras based on different detectors. The simulation results show that C 9 H 10 is a reliable, low-cost, scatter-plane material, increasing the MDA by ∼0.2 mCi when compared to a CaF 2 scatter plane. On the other hand, the intrinsic background of LaBr 3 undermines its above-average energy resolution as the MDA increased by ∼0.5 mCi when compared to a NaI(Tl) absorption plane. [ABSTRACT FROM AUTHOR]

Purpose: Spectral information of the output of x-ray tubes in (dual source) computer tomography (CT) scanners can be used to improve the conversion of CT numbers to proton stopping power and can be used to advantage in CT scanner quality assurance. The purpose of this study is to design, validate, and apply a compact portable Compton spectrometer that was constructed to accurately measure x-ray spectra of CT scanners. Methods: In the design of the Compton spectrometer, the shielding materials were carefully chosen and positioned to reduce background by x-ray fluorescence from the materials used. The spectrum of Compton scattered x-rays alters from the original source spectrum due to various physical processes. Reconstruction of the original x-ray spectrum from the Compton scattered spectrum is based on Monte Carlo simulations of the processes involved. This reconstruction is validated by comparing directly and indirectly measured spectra of a mobile x-ray tube. The Compton spectrometer is assessed in a clinical setting by measuring x-ray spectra at various tube voltages of three different medical CT scanner x-ray tubes. Results: The directly and indirectly measured spectra are in good agreement (their ratio being 0.99) thereby validating the reconstruction method. The measured spectra of the medical CT scanners are consistent with theoretical spectra and spectra obtained from the x-ray tube manufacturer. Conclusions: A Compton spectrometer has been successfully designed, constructed, validated, and applied in the measurement of x-ray spectra of CT scanners. These measurements show that our compact Compton spectrometer can be rapidly set-up using the alignment lasers of the CT scanner, thereby enabling its use in commissioning, troubleshooting, and, e.g., annual performance check-ups of CT scanners. [ABSTRACT FROM AUTHOR]

Purpose: In hybrid medical imaging devices combining positron emission tomography (PET) with magnetic resonance imaging, PET attenuation correction remains challenging. Known approaches to estimating attenuation (μ-)maps from PET emission data, especially maximum-likelihood reconstruction of activity and attenuation (MLAA), take into account true coincidences only and exhibit two kinds of ambiguities: First, the attenuation sinogram can only be determined up to a constant offset (sinogram ambiguity). Second, the attenuation sinogram is unknown outside of the support of the activity sinogram and does not completely define a μ-map (image-space ambiguity). In this work, the authors aim at using additional information from scattered coincidences to resolve these ambiguities-information that is unavailable using true coincidences. Methods: The authors propose a two-level scheme for combining measurements of true and scattered coincidences. On the top level, scatter-to-attenuation (S2A) reconstruction recovers the μ-map from a measurement of scattered coincidences and results available from trues-based algorithms. On the lower level, S2A reconstruction is implemented by iterative scatter simulation and a proposed (simplified) S2A back-projection. The S2A back-projection is based on determining possible scattering locations in image space from energy measurements (via Compton-scattering angles) and summing contributions from scattered coincidences in image space. S2A back-projection is validated with GATE simulations of activity-attenuation configurations with both sinogram and image-space ambiguities. The authors further evaluate the impact of asymmetric source and activity distributions, extended source distributions, and energy uncertainty to demonstrate the limitations of the simplified noniterative S2A back-projection approach. Feasibility of the iterative S2A reconstruction is evaluated in a low-resolution, analytical 2D problem. Results: S2A back-projection of scattered coincidences with scattered-photon energies in the range of 248-478 keV provides image-space information about the attenuation distribution, even in challenging cases of perfect spherical symmetry of attenuation and activity distributions as well as attenuation outside of the activity support. Realistic energy uncertainties (5% and 10% full width at half maximum at 511 keV) deteriorate spatial image resolution in the proposed noniterative method. The iterative S2A reconstruction is able to recover the full μ-map (errors less than 3.7×10-5/cm) as well as the unknown scaling factor (error smaller than 0.0005%) from scattered coincidences and an activity distribution with unknown scaling. Conclusions: Scattered coincidences provide information to complement existing PET attenuationcorrection approaches such as MLAA. The proposed scatter-to-attenuation back-projection and reconstruction may constitute a missing piece for resolving ambiguities in the simultaneous reconstruction of activity and attenuation, and improving the quality of μ-maps reconstructed from PET emission data. [ABSTRACT FROM AUTHOR]

We report here a theoretical proof of the limiting value of the velocity of target/particle responsible for various photon–electron interactions in the Compton scattering experiment which may be carried out in various applications such as gamma-ray spectroscopy. Compton scattering deals with the collision of photons with a target particle where both energy and momentum are transferred to the particle while the photon moves with reduced energy and a change of momentum. Here, the changed energy of a photon depends on the angle between the incident axis and scattered photon (θ), the rest mass of the electron, and the frequency of the photon before the collision. However, when some initial velocity is provided to an electron that is at rest initially, then in such cases, the particle obeys limiting value on velocity for a particular energetic photon to follow Compton scattering. In such cases, the changed frequency of photons also depends on the initial and final velocity of nonstationary electrons during a collision, including the angle (φ ′) between the recoil electron and incident axis. Here, we have presented the derivations to calculate the recoil velocity of the nonstationary electron. These terms are the accurate solution of the Compton shift. We have tried to calculate the upper limit or limiting values of the initial velocity of the target particle in Compton scattering. Further, we have computationally demonstrated that there is a dependence of scattered photon frequency on initial velocity of target particle (u) and mass of the target particle (m) at a higher scattering angle. Finally, numerical simulations were carried out with particles such as an electron, pion (+/-, 0) to verify the greater degree of accuracy of the particles in spectroscopic detectors such as gamma-ray spectroscopy and other high energy spectroscopic techniques. [ABSTRACT FROM AUTHOR]