Article
Evaluation of scattertoprimary ratio, grid performance and normalized average glandular dose in mammography by Monte Carlo simulation including interference and energy broadening effects.
Departamento de Física e Matemática, FFCLRP, Universidade de São Paulo, 14040901, Ribeirão Preto, São Paulo, Brazil.
Physics in Medicine and Biology (Impact Factor: 2.7). 08/2010; 55(15):433559. DOI:10.1088/00319155/55/15/010 Source: PubMed

Conference Proceeding: Annealing of plasma charging damage and residual degradation in MOS transistors
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ABSTRACT: This paper presents data on an important mode of plasmainduced damage, namely residual charging damage in MOS devices. The antenna dependence of residual degradation is observed in the device threshold voltage, transconductance, and subthreshold behaviour. The residual damage is observed due to incomplete annealing of oxide charge and interface states, and its level depends on the intensity of plasma charging stress and on the gate oxide thickness. Auxiliary experiments with electrical stressing of NMOS devices and their subsequent anneal confirm the dependence of residual damage on the degree and polarity of previous degradation and indicate the importance of the annealing conditionsPlasma ProcessInduced Damage, 1999 4th International Symposium on; 02/1999  [show abstract] [hide abstract]
ABSTRACT: One of the benefits of photon counting (PC) detectors over energy integrating (EI) detectors is the absence of many additive noise sources, such as electronic noise and secondary quantum noise. The purpose of this work is to demonstrate that thresholding voltage gains to detect individual x rays actually generates an unexpected source of white noise in photon counters. To distinguish the two detector types, their point spread function (PSF) is interpreted differently. The PSF of the energy integrating detector is treated as a weighting function for counting x rays, while the PSF of the photon counting detector is interpreted as a probability. Although this model ignores some subtleties of real imaging systems, such as scatter and the energydependent amplification of secondary quanta in indirectconverting detectors, it is useful for demonstrating fundamental differences between the two detector types. From first principles, the optical transfer function (OTF) is calculated as the continuous Fourier transform of the PSF, the noise power spectra (NPS) is determined by the discrete space Fourier transform (DSFT) of the autocovariance of signal intensity, and the detective quantum efficiency (DQE) is found from combined knowledge of the OTF and NPS. To illustrate the calculation of the transfer functions, the PSF is modeled as the convolution of a Gaussian with the product of rect functions. The Gaussian reflects the blurring of the xray converter, while the rect functions model the sampling of the detector. The transfer functions are first calculated assuming outside noise sources such as electronic noise and secondary quantum noise are negligible. It is demonstrated that while OTF is the same for two detector types possessing an equivalent PSF, a frequencyindependent (i.e., "white") difference in their NPS exists such that NPS(PC) > or = NPS(EI) and hence DQE(PC) < or = DQE(EI). The necessary and sufficient condition for equality is that the PSF is a binary function given as zero or unity everywhere. In analyzing the model detector with Gaussian blurring, the difference in NPS and DQE between the two detector types is found to increase with the blurring of the xray converter. Ultimately, the expression for the additive white noise of the photon counter is compared against the expression for electronic noise and secondary quantum noise in an energy integrator. Thus, a method is provided to determine the average secondary quanta that the energy integrator must produce for each x ray to have superior DQE to a photon counter with the same PSF. This article develops analytical models of OTF, NPS, and DQE for energy integrating and photon counting digital xray detectors. While many subtleties of real imaging systems have not been modeled, this work is illustrative in demonstrating an additive source of white noise in photon counting detectors which has not yet been described in the literature. One benefit of this analysis is a framework for determining the average secondary quanta that an energy integrating detector must produce for each x ray to have superior DQE to competing photon counting technology.Medical Physics 12/2010; 37(12):648095. · 2.91 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: Purpose: The proliferation of conebeam CT (CBCT) has created interest in performance optimization, with xray scatter identified among the main limitations to image quality. CBCT often contends with elevated scatter, but the wide variety of imaging geometry in different CBCT configurations suggests that not all configurations are affected to the same extent. Graphics processing unit (GPU) accelerated Monte Carlo (MC) simulations are employed over a range of imaging geometries to elucidate the factors governing scatter characteristics, efficacy of antiscatter grids, guide system design, and augment development of scatter correction.Methods: A MC xray simulator implemented on GPU was accelerated by inclusion of variance reduction techniques (interaction splitting, forced scattering, and forced detection) and extended to include xray spectra and analytical models of antiscatter grids and flatpanel detectors. The simulator was applied to small animal (SA), musculoskeletal (MSK) extremity, otolaryngology (Head), breast, interventional Carm, and onboard (kilovoltage) linear accelerator (Linac) imaging, with an axistodetector distance (ADD) of 5, 12, 22, 32, 60, and 50 cm, respectively. Each configuration was modeled with and without an antiscatter grid and with (i) an elliptical cylinder varying 70280 mm in major axis; and (ii) digital murine and anthropomorphic models. The effects of scatter were evaluated in terms of the angular distribution of scatter incident upon the detector, scattertoprimary ratio (SPR), artifact magnitude, contrast, contrasttonoise ratio (CNR), and visual assessment.Results: Variance reduction yielded improvements in MC simulation efficiency ranging from ∼17fold (for SA CBCT) to ∼35fold (for Head and Carm), with the most significant acceleration due to interaction splitting (∼6 to ∼10fold increase in efficiency). The benefit of a more extended geometry was evident by virtue of a larger air gape.g., for a 16 cm diameter object, the SPR reduced from 1.5 for ADD = 12 cm (MSK geometry) to 1.1 for ADD = 22 cm (Head) and to 0.5 for ADD = 60 cm (Carm). Grid efficiency was higher for configurations with shorter air gap due to a broader angular distribution of scattered photonse.g., scatter rejection factor ∼0.8 for MSK geometry versus ∼0.65 for Carm. Grids reduced cupping for all configurations but had limited improvement on scatterinduced streaks and resulted in a loss of CNR for the SA, Breast, and Carm. Relative contribution of forwarddirected scatter increased with a grid (e.g., Rayleigh scatter fraction increasing from ∼0.15 without a grid to ∼0.25 with a grid for the MSK configuration), resulting in scatter distributions with greater spatial variation (the form of which depended on grid orientation).Conclusions: A fast MC simulator combining GPU acceleration with variance reduction provided a systematic examination of a range of CBCT configurations in relation to scatter, highlighting the magnitude and spatial uniformity of individual scatter components, illustrating tradeoffs in CNR and artifacts and identifying the system geometries for which grids are more beneficial (e.g., MSK) from those in which an extended geometry is the better defense (e.g., Carm head imaging). Compact geometries with an antiscatter grid challenge assumptions of slowly varying scatter distributions due to increased contribution of Rayleigh scatter.Medical Physics 05/2013; 40(5):051915. · 2.91 Impact Factor
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