Scatter Contribution Research Articles

Physics of the Signal Formation in Photopletysmography: Assessment of the Contribution of Light Absorption and Scattering to the Registered Flux of Optical Radiation

The paper is devoted to the study of physical mechanisms of photoplethysmography (PPG) signal formation using Monte Carlo simulations of light transport in biological tissue. The problem of estimating the contribution of absorption and scattering variations to the registered PPG signal is solved. Based on a three-layer skin model, changes in the optical properties of the dermal layer (absorption and scattering) were sequentially simulated and their contributions to the total signal were estimated. Calculations were carried out for two wavelengths, 525 nm and 810 nm. It was found that for green light the main contribution to the signal formation is made by absorption (88 % versus 12 % scattering, respectively). While for the near infrared light, scattering predominates over absorption. In this case, the contributions of absorption and scattering are 28 % and 72 %. Thus, for the green wavelength range the classical volumetric model of signal formation is valid. Whereas for the near-infrared range, the predominant factor in signal formation is scattering of the medium, which can change due to processes such as changes in orientation, aggregation and deformation of red blood cells, their concentration in the diagnostic volume of tissue, etc.

Development of a Monte Carlo-based scatter correction method for total-body PET using the uEXPLORER PET/CT scanner

Objective. This study presents and evaluates a robust Monte Carlo-based scatter correction (SC) method for long axial field of view (FOV) and total-body positron emission tomography (PET) using the uEXPLORER total-body PET/CT scanner. Approach. Our algorithm utilizes the Monte Carlo (MC) tool SimSET to compute SC factors in between individual image reconstruction iterations within our in-house list-mode and time-of-flight-based image reconstruction framework. We also introduced a unique scatter scaling technique at the detector block-level for optimal estimation of the scatter contribution in each line of response. First image evaluations were derived from phantom data spanning the entire axial FOV along with image data from a human subject with a large body mass index. Data was evaluated based on qualitative inspections, and contrast recovery, background variability, residual scatter removal from cold regions, biases and axial uniformity were quantified and compared to non-scatter-corrected images. Main results. All reconstructed images demonstrated qualitative and quantitative improvements compared to non-scatter-corrected images: contrast recovery coefficients improved by up to 17.2% and background variability was reduced by up to 34.3%, and the residual lung error was between 1.26% and 2.08%. Low biases throughout the axial FOV indicate high quantitative accuracy and axial uniformity of the corrections. Up to 99% of residual activity in cold areas in the human subject was removed, and the reliability of the method was demonstrated in challenging body regions like in the proximity of a highly attenuating knee prosthesis. Significance. The MC SC method employed was demonstrated to be accurate and robust in TB-PET. The results of this study can serve as a benchmark for optimizing the quantitative performance of future SC techniques.

Robustness of Single- and Dual-Energy Deep-Learning-Based Scatter Correction Models on Simulated and Real Chest X-rays

Chest X-rays (CXRs) represent the first tool globally employed to detect cardiopulmonary pathologies. These acquisitions are highly affected by scattered photons due to the large field of view required. Scatter in CXRs introduces background in the images, which reduces their contrast. We developed three deep-learning-based models to estimate and correct scatter contribution to CXRs. We used a Monte Carlo (MC) ray-tracing model to simulate CXRs from human models obtained from CT scans using different configurations (depending on the availability of dual-energy acquisitions). The simulated CXRs contained the separated contribution of direct and scattered X-rays in the detector. These simulated datasets were then used as the reference for the supervised training of several NNs. Three NN models (single and dual energy) were trained with the MultiResUNet architecture. The performance of the NN models was evaluated on CXRs obtained, with an MC code, from chest CT scans of patients affected by COVID-19. The results show that the NN models were able to estimate and correct the scatter contribution to CXRs with an error of <5%, being robust to variations in the simulation setup and improving contrast in soft tissue. The single-energy model was tested on real CXRs, providing robust estimations of the scatter-corrected CXRs.

Requirements for dose calculation on an active scanned proton beamline for small, shallow fields

Introduction:A growing interest in using proton pencil beam scanning in combination with collimators for the treatment of small, shallow targets, such as ocular melanoma or pre-clinical research emerged recently. This study aims at demonstrating that the dose of a synchrotron-based PBS system with a dedicated small, shallow field nozzle can be accurately predicted by a commercial treatment planning system (TPS) following appropriate tuning of both, nozzle and TPS. Materials:A removable extension to the clinical nozzle was developed to modify the beam shape passively. Five circular apertures with diameters between 5 to 34mm, mounted 72cm downstream of a range shifter were used. For each collimator treatment plans with spread-out Bragg peaks (SOBP) with a modulation of 3 to 30mm were measured and calculated with GATE/Geant4 and the research TPS RayStation (RS11B-R). The dose grid, multiple coulomb scattering and block discretization resolution were varied to find the optimal balance between accuracy and performance. Results:For SOBPs deeper than 10mm, the dose in the target agreed within 1% between RS11B-R, GATE/Geant4 and measurements for aperture diameters between 8 to 34mm, but deviated up to 5% for smaller apertures. A plastic taper was introduced reducing scatter contributions to the patient (from the pipe) and improving the dose calculation accuracy of the TPS to a 5% level in the entrance region for large apertures. Conclusion:The commercial TPS and GATE/Geant4 can accurately calculate the dose for shallow, small proton fields using a collimator and pencil beam scanning.

Decoupling of Light Scattering and Absorption Contributions to Extinction in Pickering Emulsions

Decoupling of Light Scattering and Absorption Contributions to Extinction in Pickering Emulsions

Auto-commissioning of a Monte Carlo electron beam model with application to photon MLC shaped electron fields

Objective. Presently electron beam treatments are delivered using dedicated applicators. An alternative is the usage of the already installed photon multileaf collimator (pMLC) enabling efficient electron treatments. Currently, the commissioning of beam models is a manual and time-consuming process. In this work an auto-commissioning procedure for the Monte Carlo (MC) beam model part representing the beam above the pMLC is developed for TrueBeam systems with electron energies from 6 to 22 MeV. Approach. The analytical part of the electron beam model includes a main source representing the primary beam and a jaw source representing the head scatter contribution each consisting of an electron and a photon component, while MC radiation transport is performed for the pMLC. The auto-commissioning of this analytical part relies on information pre-determined from MC simulations, in-air dose profiles and absolute dose measurements in water for different field sizes and source to surface distances (SSDs). For validation calculated and measured dose distributions in water were compared for different field sizes, SSDs and beam energies for eight TrueBeam systems. Furthermore, a sternum case in an anthropomorphic phantom was considered and calculated and measured dose distributions were compared at different SSDs. Main results. Instead of the manual commissioning taking up to several days of calculation time and several hours of user time, the auto-commissioning is carried out in a few minutes. Measured and calculated dose distributions agree generally within 3% of maximum dose or 2 mm. The gamma passing rates for the sternum case ranged from 96% to 99% (3% (global)/2 mm criteria, 10% threshold). Significance. The auto-commissioning procedure was successfully implemented and applied to eight TrueBeam systems. The newly developed user-friendly auto-commissioning procedure allows an efficient commissioning of an MC electron beam model and eases the usage of advanced electron radiotherapy utilizing the pMLC for beam shaping.

Collinear factorization at subasymptotic kinematics and validation in a diquark spectator model

We revisit the derivation of collinear factorization for Deep Inelastic Scattering at subasymptotic values of the four-momentum transfer squared, where the masses of the particles participating in the interaction cannot be neglected. By using an inclusive jet function to describe the scattered quark final state, we can restrict the needed parton kinematic approximations just to the four-momentum conservation of the hard scattering process, and explicitly expand the rest of the diagram in powers of the unobserved parton transverse momenta rather than neglecting those. This procedure provides one with more flexibility in fixing the virtuality of the scattered and recoiling partons than in the standard derivation, and naturally leads to scaling variables that more faithfully represent the partonic kinematic at subasymptotic energy than the Bjorken's ${x}_{B}$ variable. We then verify the validity of the obtained factorization formula by considering a diquark spectator model designed to reproduce the main features of electron-proton scattering at large ${x}_{B}$ in Quantum Chromodynamics. In the model, the Deep Inelastic Scattering contribution to the cross section can be explicitly isolated and analytically calculated, then compared to the factorized approximation. Limiting ourselves to the leading twist contribution, we then show that use of the new scaling variables maximizes the kinematic range of validity of collinear factorization, and highlight the intrinsic limitations of this approach due to the unavoidably approximate treatment of four-momentum conservation in factorized diagrams. Finally, we briefly discuss how these limitations may be overcome by including higher-twist corrections to the factorized calculation.

Development of a scatter correction technique for planar 99mTc-MAA imaging to improve accuracy in lung shunt fraction estimation

PurposePrior to 90Y selective internal radiation therapy (SIRT) treatment, 99mTc-MAA scintigraphy imaging is used in the estimation of the lung shunt fraction (LSF). Planar imaging is recommended for determining a LSF ratio. However, the estimate may be affected by scatter contributions, attenuation and respiratory motion. The objective of this study was to correct for the effects of scatter in the LSF, towards the determination of a more accurate estimation method of LSF derived from planar scintigraphy imaging, which is recommended by international guidelines. MethodsThe open access SIMIND Monte Carlo modelling software was used to estimate an optimum scatter window (SW) for scatter correction. The uncertainties associated with scatter and scatter contributions from the liver on the LSF were evaluated using an anthropomorphic thorax phantom and a virtual Vox-Man phantom. A brief retrospective examination of patient scans and tumour location investigated the impact that the inclusion of the simulated scatter corrections had on the LSF estimation. ResultsThe percentage overestimation of the manufacturer recommended method of LSF estimation was 192%. SW corrections improved the uncertainty to within 19% for the range of known LSFs. Similar findings were observed for our patient and tumour location studies. ConclusionThe incorporated scatter corrections can significantly improve the accuracy of the LSF estimation, thereby providing a robust gamma camera, patient and tumour depth specific correction which is easily implementable. This is supported by Monte Carlo, phantom and preliminary patient studies.

Effect of Sub-energy Windows’ Parameters on the Triple Energy Window Scatter Correction Method Accuracy in 99mTc SPECT Imaging

Scatter correction in SPECT quantification is of major importance to compensation for the scatter contribution under the photopeak. The triple energy windows method (TEW) is one of the suggested ways for scatter correction that are widely used in clinical routine. However, it can be a source of additional noise if the width or the number of sub-energy windows is not accurately chosen. To determine the precise scatter estimation windows settings under the 99m Tc photopeak, scatter fraction was calculated for different sub-energy widths and numbers through GATE Monte Carlo simulation, for a main energy window of 15 %, centered at 140 keV. Four different acquisitions, with cold or hot inserts in a warm or a cold background, were studied. The estimation was done by two methods. The first method was the extraction of the number of detected Compton photons under the photopeak, therefore considered as the true scattered photons. The second method was the application of TEW method to the simulated energy spectra. The comparison of results corresponding to both methods shows a good agreement in two cases: simultaneous 7 % and 5 % sub-energy windows, respectively, positioned on the left and the right of the main energy window, and the second case is a 3 % left sub-energy window without a right sub-energy window. These sub-energy windows were then applied to experimental tomographic acquisitions to assess their impact on contrast, relative noise of the background (RNB), signal‑to‑noise ratio (SNR), integral uniformity (IU), and tomographic spatial resolution. Good results for these quantitative parameters were acquired with simultaneous 7 % and 5 % sub-energy windows. However, there was very little enhancement for tomographic spatial resolution. Normal 0 21 false false false FR X-NONE AR-SA /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Tableau Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Cambria","serif"; mso-ascii-font-family:Cambria; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Cambria; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:Arial; mso-bidi-theme-font:minor-bidi;} Normal 0 21 false false false FR X-NONE AR-SA /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Tableau Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Cambria","serif"; mso-ascii-font-family:Cambria; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Cambria; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:Arial; mso-bidi-theme-font:minor-bidi;}

Ship Detection Method Based on Scattering Contribution for PolSAR Image

Due to the differentiation of polarimetric scattering mechanisms between ships and sea surface, designing the ship detection method in polarimetric synthetic aperture radar (PolSAR) is a potential promising technique and has been paid extensive attention. The complexity of sea clutter and weak scattering of small ships result in a great challenge for high-precision ship detection. In this letter, we investigate the scattering mechanisms of ships to improve the detection performance and propose a novel ship detection method based on the principal contribution of scattering mechanisms. First, the seven-component model-based decomposition (SCMD) is used to analyze the scattering mechanisms of ships. Second, the primary scattering contribution and local contrast (SCLC) mechanism are used to enhance ships, especially small ships. Finally, the threshold segmentation is used to realize the extraction of ships. Experimental results by real PolSAR data not only verify the rationality and effectiveness of the constructed detection metric but also show the clear superiority of the proposed detection method, which can encourage further application of polarimetric scattering mechanisms in ship detection.

An Improved Two-Scale Model for Sea Surface Scattering in the Transition Range of Incidence Angles

An improved two-scale model (I-TSM) for the incidence angle transition range (5-11°) is proposed by analyzing the contributions of both quasi-specular and Bragg scatterings to the total sea surface scattering. With normalized radar cross section (NRCS) products of surface wave investigation and monitoring (SWIM) instrument, the mean square slope (MSS) and reflection coefficient of the geometrical optics (GO) model are modified to simulate the quasi-specular scattering in I-TSM, and a Bragg scattering contribution factor adaptive to the incidence angle and wind speed is fitted to provide the Bragg scattering component for I-TSM. At different incidence angles and wind speeds, I-TSM combines the quasi-specular and Bragg scatterings in variable proportions by the fitting factor. I-TSM avoids the critical angle selection and separation problems of the traditional TSM in the incidence angle transition range. Compared with the GO model and traditional TSM, I-TSM has significantly higher accuracies with root mean square error (RMSE) of 0.309dB, relative error (RE) of 2.019% and scattering index (SI) of 0.03 when SWIM NRCS products are the standard data in the validation. I-TSM will provide high-precision NRCS simulation data in the incidence angle transition range for scattering characteristics analysis and echo simulation of SWIM.

Out-of-field dose and its constituent components for a 1.5 T MR-Linac

This study aims to quantify the relative contributions of phantom scatter, collimator scatter and head leakage to the out-of-field doses (OFDs) of both static fields and clinical intensity-modulated radiation therapy (IMRT) treatments in a 1.5 T MR-Linac. The OFDs of static fields were measured at increasing distances from the field edge in an MR-conditional water phantom. Inline scans at depths of dmax (14 mm), 50 and 100 mm were performed for static fields of 5 × 5, 10 × 10 and 15 × 15 cm2 under three different conditions: full scatter, with phantom scatter prevented, and head leakage only. Crossline scans at isocenter and offset positions were performed in full scatter condition. EBT3 radiochromic films were placed at 100 mm depth of solid water phantom to measure the OFD of clinical IMRT plans. All water tank data were normalized to Dmax of a 10 × 10 cm2 field and the film results were presented as a fraction of the target mean dose. The OFD in the inline direction varied from 3.5% (15 × 15 cm2, 100 mm depth, 50 mm distance) to 0.014% (5 × 5 cm2, dmax, 400 mm distance). For all static fields, the collimator scatter was higher than the phantom scatter and head leakage at a distance of 100–400 mm. Head leakage remained the smallest among the three components, except at long distances (>375 mm) with small field size. Compared to the inline scans, the crossline scans at the isocenter showed higher doses at distances longer than 80 mm. All crossline profiles at longitudinal offset positions showed a cone shape with laterally shifted maxima. The OFD of IMRT deliveries varied with different target size. For prostate stereotactic body radiation therapy (SBRT) treatment, the OFD decreased from 2% to 0.03% at a distance of 50–500 mm. The OFDs have been measured for a 1.5 T MR-Linac. The presented dosimetric data are valuable for radiation safety assessments on patients treated with the MR-Linac, such as evaluating carcinogenic risk and radiation exposure to cardiac implantable electronic devices.

Response of Multi-Incidence Angle Polarimetric RADARSAT-2 Data to Herbaceous Vegetation Features in the Lower Paraná River Floodplain, Argentina

Wetland ecosystems play a key role in hydrological and biogeochemical cycles. In emergent vegetation targets, the occurrence of double-bounce scatter is indicative of the presence of water and can be valuable for hydrological monitoring. Double-bounce scatter would lead to an increase of σ0HH over σ0VV and a non-zero co-polarized phase difference (CPD). In the Lower Paraná River floodplain, a total of 11 full polarimetric RADARSAT-2 scenes from a wide range of incidence angles were acquired during a month. Flooded targets dominated by two herbaceous species were sampled: Schoenoplectus californicus (four sites, Bulrush marshes) and Ludwigia peruviana (three sites, Broadleaf marshes). As a general trend, σ0HH was higher than σ0VV, especially at the steeper incidence angles. By modeling CPD with maximum likelihood estimations, we found results consistent with double-bounce scatter in two Ludwigia plots, at certain scene incidence angles. Incidence angle accounted for most of the variation on σ0HH, whereas emergent green biomass was the main feature influencing σ0HV. Multivariate models explaining backscattering variation included the incidence angle and at least two of these variables: emergent plant height, stem diameter, number of green stems, and emergent green biomass. This study provides an example of using CPD to decide on the contribution of double-bounce scatter and highlights the influence of vegetation biomass on radar response. Even with the presence of water below vegetation, the contribution of double-bounce scatter to C-band backscattering depends on scene incidence angles and may be negligible in dense herbaceous targets.

Improved scatter correction model for high attenuation gamma-ray tomography measurements

In this study a state-of-the-art gamma-ray tomography (GRT) unit was used to measure the solids concentration distributions of high density, high attenuation clay/water/sand slurries in a 4 in. (100 mm) diameter recirculating pipe loop. The presence of neighbouring radiation sources on the GRT results in a scattered radiation contribution to the total intensity measured at each detector. The scattered radiation decreases the signal-to-noise ratio of the measured radiation intensity and introduces error in the measured tomography data and the reconstructed tomograms (Maad et al 2008 Meas. Sci. Technol. 19 6). Because of the high attenuation of the materials being measured, the scatter contribution was a significant fraction of the total radiation measured at each detector. To correct for the scattered radiation contribution in the measurements, a test campaign was undertaken to characterize and model the scattering behaviour of the GRT at the Saskatchewan Research Council. The scattered radiation was measured experimentally from empty pipe, water filled pipe and a number of flowing clay/water/sand slurries at densities ranging from 1206 kg m−3 to 1580 kg m−3. A semi-empirical scatter correction model has been developed which allows the scattered contribution at each detector to be calculated iteratively based on the measured uncorrected attenuated radiation intensity. This article is an extension of the work presented at the 9th World Congress for Industrial Process Tomography (Spelay et al 2018 Proc. 9th World Congress Industrial Process Tomography p 14).

STRUCTURE STATISTICS OF SCALE TO SCALE ENERGY TRANSFER OF WALL TURBULENCE BASED ON LBM

For the reason that the scale-to-scale energy transfer characteristics of wall bounded turbulence exhibit strong anisotropy in three dimensional physical space, understanding the spatial distribution of energy scale-to-scale transfer is of great importance for the further construction of high-fidelity anisotropic large eddy simulation models. Direct numerical simulation were performed at Reynolds number $Re_{\tau } =180$ by using lattice Boltzmann method (LBM). In comparison with the open source channel flow turbulence database, both the mean velocity and turbulent variables, such as Reynolds stress and velocity fluctuations, agree well with the results of Kim et al. (1987) and Moser et al. (2015). The ability and validity of LBM on simulating turbulent channel flow were verified. The filter-space technique is used to obtain the scale-to-scale transport of kinetic energy, and the statistical results show that the backward scatter and forward scatter contributions to the SGS dissipation were comparable. Combined with the novel structure identification method, the distribution and geometry properties of the energy flux structure is quantitatively investigated by the clustering methodology. The results show that small scale structures account for the majority in number, and the volume probability density distribution presents a -4/5 power law. The structures can further be divided into wall attached structures and wall detached structures according to their minimal distance to the wall. In spite of the relatively small number fraction, the attached structure accounts for most of volume fraction, which shows that most of the attached structures are large scale structures. Further statistics of the attached structures exhibit that the size of the structure has a certain power law relationship, indicating that the scale-to-scale energy transport structure also has the self similarity of the attached eddy hypothesis which proposed by Townsend. The forward and backward energy flux structure pairs tend to be arranged side by side along the span direction.

Cluster strong lensing with hierarchical inference

Context. Lensing by galaxy clusters is a versatile probe of cosmology and extragalactic astrophysics, but the accuracy of some of its predictions is limited by the simplified models adopted to reduce the (otherwise intractable) number of degrees of freedom. Aims. We aim to explore cluster lensing models in which the parameters of all cluster member galaxies are free to vary around some common scaling relations with non-zero scatter and deviate significantly from these relations if, and only if, the data require this. Methods. We devised a Bayesian hierarchical inference framework that enables the determination of all lensing parameters and the scaling relation hyperparameters, including intrinsic scatter, from lensing constraints and (if given) stellar kinematic measurements. We achieve this through BAYESLENS, a purpose-built wrapper around common parametric lensing codes that can sample the full posterior on parameters and hyperparameters; we release BAYESLENS with this paper. Results. We ran functional tests of our code against simple mock cluster lensing datasets with realistic uncertainties. The parameters and hyperparameters are recovered within their 68% credibility ranges and the positions of all the “observed” multiple images are accurately reproduced by the BAYELENS best-fit model, without over-fitting. Conclusions. We have shown that an accurate description of cluster member galaxies is attainable, despite a large number of degrees of freedom, through fast and tractable inference. This extends beyond the most updated cluster lensing models. The precise impact on studies of cosmography, galaxy evolution, and high-redshift galaxy populations can then be quantified on real galaxy clusters. While other sources of systematics exist and may be significant in real clusters, our results show that the contribution of intrinsic scatter in cluster member populations can now be controlled.

Improving linac integrated cone beam computed tomography image quality using tube current modulation.

Linac integrated cone beam CT (CBCT) scanners have become widespread tool for image guidance in radiotherapy. The current implementation uses constant imaging fluence across all the projection angles, which leads to anisotropic noise properties and suboptimal image quality for noncircular symmetric objects. Tube current modulation (TCM) is widely used in conventional CT. The purpose of this work was to implement TCM on a linac integrated CBCT scanner and evaluate its impact on image quality under varying scatter conditions and scatter correction strategies. We have implemented TCM on a nonclinical Elekta Versa HD linear accelerator with enhanced x-ray generator functionality including pulse width modulation. The pulse width was modulated using two Arduino programmable microcontrollers: one placed on the kV arm to measure the projection angle and the other connected to the kV generator control board to vary x-ray pulse width as function of gantry angle and precalculated transmission. An in-house developed phantom with a ratio of the left-right to anterior-posterior path length of 1.85:1 was scanned. Image quality was determined using the anisotropicity of the 2D noise power spectra (NPS) in the transverse plane and the contrast-to-noise ratio (CNR). In addition, to determine the impact of scatter on the applicability of the TCM method we have modified the generated scatter using three different collimators in the cranio-caudal direction as well as with and without an antiscatter grid (ASG). Application of the TCM led to 30-78% reduction of the angular anisotropicity of the NPS in the transverse plane. The amount of reduction depended on the scatter conditions, with lower values corresponding to higher scatter conditions. The same was true for the CNR: when scatter contribution was low (presence of an ASG or very aggressive collimation) the CNR was improved by about 30%, while in high scatter conditions the CNR was improved by about 12%. TCM has the potential to improve CBCT image quality, but this depends on the amount of detected x-ray scatter.

A semi-empirical model for scatter field reduction in digital mammography

X-ray mammography is the gold standard technique in breast cancer screening programmes. One of the main challenges that mammography is still facing is scattered radiation, which degrades the quality of the image and complicates the diagnosis process. Anti-scatter grids, the main standard physical scattering reduction technique, have some unresolved challenges as they increase the dose delivered to the patient, do not remove all the scattered radiation and increase the cost of the equipment. Alternative scattering reduction methods based on post-processing algorithms, have lately been under investigation. This study is concerned with the use of image post-processing to reduce the scatter contribution in the image, by convolving the primary plus scatter image with kernels obtained from simplified Monte Carlo (MC) simulations. The proposed semi-empirical approach uses up to five thickness-dependant symmetric kernels to accurately estimate the scatter contribution of different areas of the image. Single breast thickness-dependant kernels can over-estimate the scatter signal up to 60%, while kernels adapting to local variations have to be modified for each specific case adding high computational costs. The proposed method reduces the uncertainty to a 4%–10% range for a 35–70 mm breast thickness range, making it a very efficient, case-independent scatter modelling technique. To test the robustness of the method, the scattered corrected image has been successfully compared against full MC simulations for a range of breast thicknesses. In addition, clinical images of the 010A CIRS phantom were acquired with a mammography system with and without the presence of the anti-scatter grid. The grid-less images were post-processed and their quality was compared against the grid images, by evaluating the contrast-to-noise ratio and variance ratio using several test objects, which simulate calcifications and tumour masses. The results obtained show that the method reduces the scatter to similar levels than the anti-scatter grids.

NEUTRON AND GAMMA ALBEDO EVALUATION AT AN IRRADIATION FACILITY.

The irradiation facility of the University of Pisa (UNIPI) is a neutron and gamma irradiation facility of the University of Pisa used for calibration purposes and to evaluate detectors and dosemeters for mixed neutron and gamma fields. The facility consists of a fairly large room (5.0×7.8×2.5m3) with concrete walls and roof designed to minimise radiation scattering. Sealed neutron and gamma radionuclide sources are stored in the facility for calibration and test purposes. In order to perform accurate response measurements, this study applied a methodology of general validity to assess the neutron and gamma room scatter contributions (albedo) to the fluence and to the ambient dose equivalent rates. The assessment was done for the standard irradiation points of the facility, using detailed Monte Carlo simulations and considering several source-to-wall and source-to-detector distances.

Angle-resolved STEM using an iris aperture: Scattering contributions and sources of error for the quantitative analysis in Si

The angle-resolved electron scattering is investigated in scanning-transmission electron microscopy (STEM) using a motorised iris aperture placed above a conventional annular detector. The electron intensity scattered into various angle ranges is compared with simulations that were carried out in the frozen-lattice approximation. As figure of merit for the agreement of experiment and simulation we evaluate the specimen thickness which is compared with the thickness obtained from position-averaged convergent beam electron diffraction (PACBED). We find deviations whose strengths depend on the angular range of the detected electrons. As possible sources of error we investigate, for example, the influences of amorphous surface layers, inelastic scattering (plasmon excitation), phonon-correlation within the frozen-lattice approach, and distortions in the diffraction plane of the microscope. The evaluation is performed for four experimental thicknesses and two angle-resolved STEM series under different camera lengths. The results clearly show that especially for scattering angles below 50 mrad, it is mandatory that the simulations take scattering effects into account which are usually neglected for simulating high-angle scattering. Most influences predominantly affect the low-angle range, but also high scattering angles can be affected (e.g. by amorphous surface covering).