Anti-scatter Grid Research Articles

Tackling the commissioning of a digital radiography AEC system

AEC devices aim to maintain a consistent level of clinical image quality despite variations in patient body habitus and imaging method. The assessment of AEC performance has undergone significant revision with the advent of digital radiography and challenges still remain. Verification of manufacturers’ target detector air kerma (DAK) values with differing AEC testing protocols can be problematic, particularly when the system has been configured with integrated direct digital radiography (DDR) detectors. In addition, there is a need for comparative dose information from current DDR technology to be made available to the wider Medical Physics community. AEC testing was performed on 4 recently-installed Carestream DRX Evolution Plus digital radiographic systems as part of their commissioning tests. AEC calibration was assessed using 3 IPEM-recommended parameters; DAK, image pixel value and detector dose indicator (DDI). Testing was performed with both free and scatter-free configurations and with and without antiscatter grid. A mixture of exit dose and in-bucky measurements were performed with the assistance of a field service engineer (FSE) for both free and integrated detectors. Results from the AEC testing of the four Carestream digital radiographic systems are presented for different testing conditions along with hints and tips on assessing the AEC dose levels when commissioning the systems. DAK results are compared to the manufacturers target values and to Bowden et al.’s (2011) published results on other DDR systems. These results could be used as initial AEC reference values against which others can compare their results when commissioning comparable systems.

LIGA method of forming high-contrast collimators and anti-scatter grids with high aspect ratio

В статье рассматриваются вопросы создания и применения высококонтрастных антискаттерных рентгеновских сеток для рентгеновской визуализации в диапазоне 20-200 КЭВ из Микрофокусных рентгеновских трубок. Структура решетки никеля высоко-аспект-коэффициента прототипа использована для того чтобы рассматривать удар решетки анти -- разброса на дирекционных картинах трубки рентгеновского снимка ИМА-2-150Д. Показано подавление рассеянного излучения и сужение схемы направленного излучения. Контраст прототипа никеля, по-видимому, недостаточен. В статье представлены методы LIGA для изготовления антискаттерных рентгеновских сеток из золота в Сибирском центре Синхротронного и Терагерцового излучения. Описан метод формирования резистивных сетчатых структур с использованием глубинной рентгеновской литографии и показаны тестовые структуры. В настоящее время идет Разработка технологии изготовления рентгеновских сеток против рассеяния.

LIGA method of forming high-contrast collimators and anti-scatter grids with high aspect ratio

В статье рассматриваются вопросы создания и применения высококонтрастных антискаттерных рентгеновских сеток для рентгеновской визуализации в диапазоне 20-200 КЭВ из Микрофокусных рентгеновских трубок. Структура решетки никеля высоко-аспект-коэффициента прототипа использована для того чтобы рассматривать удар решетки анти -- разброса на дирекционных картинах трубки рентгеновского снимка ИМА-2-150Д. Показано подавление рассеянного излучения и сужение схемы направленного излучения. Контраст прототипа никеля, по-видимому, недостаточен. В статье представлены методы LIGA для изготовления антискаттерных рентгеновских сеток из золота в Сибирском центре Синхротронного и Терагерцового излучения. Описан метод формирования резистивных сетчатых структур с использованием глубинной рентгеновской литографии и показаны тестовые структуры. В настоящее время идет Разработка технологии изготовления рентгеновских сеток против рассеяния.

Comparison of screening performance metrics and patient dose of two mammographic image acquisition modes in the Danish National Breast Cancer Screening Programme

IntroductionIn this study, screening performance metrics and radiation dose were compared for two image acquisition modes for breast cancer screening with MAMMOMAT Inspiration (Siemens Healthcare GmbH, Forchheim, Germany). This mammography system can operate without an anti-scatter grid in place but using software scatter correction instead. This grid-less acquisition mode (PRIME) requires less patient dose due to the increase in primary radiation reaching the detector. This study retrospectively analyses data from the Region of Southern Denmark where the grid-less mode has been installed in November 2013 and replaced grid-based screening. Methods and materialsA total of 72,188 screening cases from the same geographical region in Denmark were included in the study. They were subdivided into two study populations: cases acquired before and after installation of the grid-less acquisition mode. Sensitivity and specificity of breast cancer screening were calculated for the two populations; thus representing the performance of grid-less and grid-based screening. To measure the entrance surface air kerma (ESAK) additional phantom tests were carried out. Polymethylmethacrylate (PMMA) attenuation plates with different thicknesses (20–70 mm in steps of 10 mm) simulated the compressed breast (21 mm–90 mm) and a solid-state dosimeter was used. ResultsStatistical testing of the results showed that screening with grid-less acquisition provides equivalent performance with respect to sensitivity and specificity compared to grid-based screening. The specificity was 98.11% (95% confidence interval (CI) from 97.93% to 98.29%) and 97.96% (95% CI from 97.84% to 98.09%) for screening with grid-less acquisition and grid-based acquisition, respectively. The cancer detection rate as a measure for sensitivity was equal (0.55%) for grid-less screening and grid-based screening. An average glandular dose saving between 13.5% and 36.4% depending on breast thickness in grid-less acquisition was obtained compared to grid-based acquisition. ConclusionStatistically significant equivalence was shown with an equivalence margin of 0.12% points for cancer detection rate and with an equivalence margin of 0.40% points for specificity. A marked patient dose savings in grid-less acquisition of up to 36% compared to grid-based acquisition was achieved. It can be concluded that grid-less acquisition with software scatter correction is an alternative to grid-based acquisition in mammography.

Focused anti-scatter grid for background reduction in x-ray fluorescence tomography.

X-ray fluorescence (XRF) tomography is an emerging imaging technology with the potential for high spatial resolution molecular imaging. One of the key limitations is the background noise due to Compton scattering since it degrades the signal and limits the sensitivity. In this Letter, we present a linear focused anti-scatter grid that reduces the Compton scattering background. An anti-scatter grid was manufactured and evaluated both experimentally and theoretically with Monte Carlo simulations. The measurements showed a 31% increase in signal-to-background ratio, and simulations of an improved grid showed that this can easily be extended up to >75%. Simulated tomographies using the improved grid show a large improvement in reconstruction quality. The anti-scatter grid will be important for in vivo XRF tomography since the background reduction allows for faster scan times, lower doses, and lower nanoparticle concentrations.

Improvement of radiographic visibility using an image restoration method based on a simple radiographic scattering model for x-ray nondestructive testing

In conventional radiography, image visibility is often limited mainly due to the superimposition of the object’s structure under investigation and scattered x-rays. Several methods, including the antiscatter grid technique, the air-gap technique, and scatter correction methods using measurements, mathematical-physical modeling, or a combination of both, have been extensively investigated in an attempt to overcome these difficulties. However, these methods require special equipment, geometry, and extra work to measure the scatter characteristics. In this study, we propose a new image restoration method based on a simple radiographic scattering model in which the intensity of the scattered x-rays and the direct transmission function of a given object are estimated from a single x-ray image by using the dark-channel prior. We implemented the proposed algorithm and performed a systematic experiment by using a 450-kV industrial x-ray inspection system to demonstrate its viability for nondestructive testing. Our results indicated that the structure of the examined object was much more clearly visible in the restored image, considerably improving the radiographic visibility.

A new measurement method of actual focal spot position of an x-ray tube using a high-precision carbon-interspaced grid

This study investigated the effectiveness of a new method for measuring the actual focal spot position of a diagnostic x-ray tube using a high-precision antiscatter grid and a digital x-ray detector in which grid magnification, which is directly related to the focal spot position, was determined from the Fourier spectrum of the acquired x-ray grid’s image. A systematic experiment was performed to demonstrate the viability of the proposed measurement method. The hardware system used in the experiment consisted of an x-ray tube run at 50 kVp and 1 mA, a flat-panel detector with a pixel size of 49.5 µm, and a high-precision carbon-interspaced grid with a strip density of 200 lines/inch. The results indicated that the focal spot of the x-ray tube (Jupiter 5000, Oxford Instruments) used in the experiment was located approximately 31.10 mm inside from the exit flange, well agreed with the nominal value of 31.05 mm, which demonstrates the viability of the proposed measurement method. Thus, the proposed method can be utilized for system’s performance optimization in many x-ray imaging applications.

Acuros CTS: A fast, linear Boltzmann transport equation solver for computed tomography scatter - Part I: Core algorithms and validation.

To describe Acuros® CTS, a new software tool for rapidly and accurately estimating scatter in x-ray projection images by deterministically solving the linear Boltzmann transport equation(LBTE). The LBTE describes the behavior of particles as they interact with an object across spatial, energy, and directional (propagation) domains. Acuros CTS deterministically solves the LBTE by modeling photon transport associated with an x-ray projection in three main steps: (a) Ray tracing photons from the x-ray source into the object where they experience their first scattering event and form scattering sources. (b) Propagating photons from their first scattering sources across the object in all directions to form second scattering sources, then repeating this process until all high-order scattering sources are computed using the source iteration method. (c) Ray-tracing photons from scattering sources within the object to the detector, accounting for the detector's energy and anti-scatter grid responses. To make this process computationally tractable, a combination of analytical and discrete methods is applied. The three domains are discretized using the Linear Discontinuous Finite Elements, Multigroup, and Discrete Ordinates methods, respectively, which confer the ability to maintain the accuracy of a continuous solution. Furthermore, through the implementation in CUDA, we sought to exploit the parallel computing capabilities of graphics processing units (GPUs) to achieve the speeds required for clinical utilization. Acuros CTS was validated against Geant4 Monte Carlo simulations using two digital phantoms: (a) a water phantom containing lung, air, and bone inserts (WLAB phantom) and (b) a pelvis phantom derived from a clinical CT dataset. For these studies, we modeled the TrueBeam® (Varian Medical Systems, Palo Alto, CA) kV imaging system with a source energy of 125kVp. The imager comprised a 600μm-thick Cesium Iodide (CsI) scintillator and a 10:1 one-dimensional anti-scatter grid. For the WLAB studies, the full-fan geometry without a bowtie filter was used (with and without the anti-scatter grid). For the pelvis phantom studies, a half-fan geometry with bowtie was used (with the anti-scatter grid). Scattered and primary photon fluences and energies deposited in the detector were recorded. The Acuros CTS and Monte Carlo results demonstrated excellent agreement. For the WLAB studies, the average percent difference between the Monte Carlo- and Acuros-generated scattered photon fluences at the face of the detector was -0.7%. After including the detector response, the average percent differences between the Monte Carlo- and Acuros-generated scatter fractions (SF) were -0.1% without the grid and 0.6% with the grid. For the digital pelvis simulation, the Monte Carlo- and Acuros-generated SFs agreed to within 0.1% on average, despite the scatter-to-primary ratios (SPRs) being as high as 5.5. The Acuros CTS computation time for each scatter image was ~1s using a single GPU. Acuros CTS enables a fast and accurate calculation of scatter images by deterministically solving the LBTE thus offering a computationally attractive alternative to Monte Carlo methods. Part II describes the application of Acuros CTS to scatter correction of CBCT scans on the TrueBeam system.

Acuros CTS: A fast, linear Boltzmann transport equation solver for computed tomography scatter - Part II: System modeling, scatter correction, and optimization.

To correct for scatter in kV cone-beam CT (CBCT) projection data on a clinical system using a new tool, Acuros® CTS, that estimates scatter images rapidly and accurately by deterministically solving the linear Boltzmann transport equation. Phantom and patient CBCT scans were acquired on TrueBeam® radiotherapy machines. A first-pass reconstruction was used to create water and bone density maps of the imaged object, which was updated to include a more accurate representation of the patient couch. The imaging system model accounted for the TrueBeam x-ray source (polychromatic spectrum, beam filtration, bowtie filter, and collimation hardware) and x-ray detection system (antiscatter grid, flat-panel imager). Acuros CTS then used the system and object models to estimate the scatter component of each projection image, which was subtracted from the measured projections. The corrected projections were then reconstructed to produce the final result. We examined the tradeoff between run time and accuracy using a Pareto optimization of key parameters, including the voxel size of the down-sampled object model, the number of pixels in the down-sampled detector, and the number of scatter images (angular down-sampling). All computations and reconstructions were performed on a research workstation containing two graphics processing units (GPUs). In addition, we established a method for selecting a subset of projections for which scatter images were calculated. The projections were selected to minimize interpolation errors in the remaining projections. Image quality improvement was assessed by measuring the accuracy of the reconstructed phantom and patient images. The Pareto optimization yielded a set of parameters with an average run time of 26 seconds for scatter correction while maintaining high accuracy of scatter estimation. This was achieved in part by means of optimizing the projection angles that were processed, thus favoring the use of more angles in the lateral (i.e., horizontal) direction and fewer angles in the AP direction. In a 40 cm solid water phantom reconstruction, nonuniformities were decreased from 217 HU without scatter correction to 51 HU with conventional (kernel-based) scatter correction to 17 HU with Acuros CTS-based scatter correction. In clinical pelvis scans, nonuniformities in the bladder were reduced from 85 HU with conventional scatter correction to 14 HU with Acuros CTS. Acuros CTS is a promising new tool for fast and accurate scatter correction for CBCT imaging. By carefully modeling the imaging chain and optimizing several parameters, we achieved high correction accuracies with computation times compatible with the clinical workflow. The improvement in image quality enables better soft-tissue visualization and potentially enables applications such as adaptive radiotherapy.

Anti-scatter grid artifact elimination for high-resolution x-ray imaging detectors without a prior scatter distribution profile.

Using anti-scatter grids with high-resolution imaging detectors could result in grid-line artifacts, with increasing severity as detector resolution improves. Grid-line mask subtraction can result in residual artifacts that are due to residual scatter penetrating the grid and not subtracted. By subtracting this residual scatter, the grid artifacts can be minimized. In the previous works, an initial residual-scatter estimate was derived by placing lead markers on a test object; however, any change in the object geometry requires a new scatter estimate. Such a method is impractical to implement during a clinical procedure. In this work, we present a new method to derive the initial scatter estimate to eliminate grid-line artifacts during a procedure. A standard stationary Smit-Roentgen x-ray grid (line density - 70 lines/cm, grid ratio - 13:1) was used with a high-resolution CMOS detector (Dexela Model 1207, pixel size - 75 μm) to image an anthropomorphic head phantom. The initial scatter estimate was derived from the image itself and the grid artifacts were eliminated using recursive correction estimation; this result was compared to that with the estimate derived from placing lead markers on the phantom. In both cases, the contrast-to-noise ratio (CNR) was improved compared to the original image with grid artifacts. Percentage differences in CNR's for three regions between the images corrected with the two estimates were less than 5%. With the new method no a priori scatter distribution profiles are needed, eliminating the need to have libraries of pre-calculated scatter profiles and making the implementation more clinically practical.

Validation of a Monte Carlo code system for grid evaluation with interference effect on Rayleigh scattering

Anti-scatter grids are commonly used in x-ray imaging systems to reduce scatter radiation reaching the image receptor. Anti-scatter grid performance and validation can be simulated through use of Monte Carlo (MC) methods. Our recently reported work has modified existing MC codes resulting in improved performance when simulating x-ray imaging.The aim of this work is to validate the transmission of x-ray photons in grids from the recently reported new MC codes against experimental results and results previously reported in other literature.The results of this work show that the scatter-to-primary ratio (SPR), the transmissions of primary (Tp), scatter (Ts), and total (Tt) radiation determined using this new MC code system have strong agreement with the experimental results and the results reported in the literature. Tp, Ts, Tt, and SPR determined in this new MC simulation code system are valid. These results also show that the interference effect on Rayleigh scattering should not be neglected in both mammographic and general grids’ evaluation. Our new MC simulation code system has been shown to be valid and can be used for analysing and evaluating the designs of grids.

Dependence of low contrast detail on exposure dose and tube voltage in digital flat-panel detector radiography—a pre-clinical phantom study

Purpose. Pre-clinical determination of reasonable simplified examination protocols before clinical application to assess the performance of a direct digital radiography (DDR) system. Material and methods. The experimental setup consists of a DDR system with fixed flat-panel detector (CsI). A phantom with low-contrast structures (LCS) was examined in two settings in order to simulate standard radiographs: (1) small peripheral joints like the wrist (no patient phantom, no anti-scatter grid) and (2) big proximal joints like the hip (20 cm water tank patient simulation and with an anti-scatter grid). Setup (1) was manually exposed at 55 kV/2 mAs, 60 k V/1.6 mAs and 65 k V/1.25 mAs. Setup (2) was automatically exposed at 1.6, 2.5, 4 and 6 μGy per kV-setting (80 ± 5 kV). Contrast-detail curves (CDC) were generated by analyzer software. Results. Exposures of setup (1) showed similar CDC (p = 0.99) with a decreasing curve with larger diameter of LCS. In all exposures of setup (2) CDC were similar for all kV-settings (p > 0.93) with a decreasing curve with larger LCS diameter. CDC insignificantly decrease with increasing exposure dose (p > 0.38). Conclusion. Low-contrast detectability was independent of kV and marginally dependent on the dose, both within a certain range. This could be used to further reduce radiation dose and simplify examination protocols to be tested in a further clinical study.

Two-dimensional antiscatter grid: A novel scatter rejection device for Cone-beam computed tomography.

Scattered radiation remains to be a major cause of image quality degradation in Flat Panel Detector (FPD)-based Cone-beam computed tomography (CBCT). We have been investigating a novel two-dimensional antiscatter grid (2D-ASG) concept to reduce scatter intensity, and hence improve CBCT image quality. We present the first CBCT imaging experiments performed with the 2D-ASG prototype, and demonstrate its efficacy in improving CBCT image quality. A 2D-ASG prototype with septa focused to x-ray source was additively manufactured from tungsten and mounted on a Varian TrueBeam CBCT system. CBCT projections of phantoms were acquired with an offset detector geometry using TrueBeam's "developer" mode. To minimize the effect of gantry flex, projections were gain corrected on angle-specific bases. CBCT images were reconstructed using a filtered backprojection algorithm and image quality improvement was quantified by measuring contrast-to-noise ratio (CNR) and CT number accuracy in images acquired with no antiscatter grid (NO-ASG), conventional one dimensional antiscatter grid (1D-ASG), and the 2D-ASG prototype. A significant improvement in contrast resolution was achieved using our 2D-ASG prototype compared to results of 1D-ASG and NO-ASG acquisitions. Compared to NO-ASG and 1D-ASG experiments, the CNR of material inserts improved by as much as 86% and 54% respectively. Using 2D-ASG, CT number underestimation in water equivalent material section of the phantom was reduced by up to 325 HU when compared to NO-ASG and up to 179 HU when compared to 1D-ASG. We successfully performed the first CBCT imaging experiments with a 2D-ASG prototype. 2D-ASG provided significantly higher CT number accuracy, higher CNR, and diminished scatter-induced image artifacts in qualitative evaluations. We strongly believe that utilization of a 2D-ASG may potentially lead to better soft tissue visualization in CBCT and may enable novel clinical applications that require high CT number accuracy.

Effect of Interspaces of Anti-scatter Grid on the Image Improvement Factor

The standard general purpose of anti-scatter girds: JIS Z 4910: 2015 (IEC 60627: 2013) has been revised, with the new addition of an image improvement factor (Q) to the physical characteristic. Using aluminum (Al) and fiber-interspaced anti-scatter grids; we studied the meaning of Q by calculating each of the physical characteristics and assessing the image. The experimental method was based on JIS Z 4910: 2015. The two anti-scatter grids had a grid ratio of 12: 1 and a strip frequency of 40 cm-1. Assessment items consisted of grid exposure factor (B), grid selectivity (Σ), contrast improvement ratio (K), and Q. In addition, the contrast to noise ratio (CNR) and contrast-detail curve (CD-curve) were determined from the contrast-detail phantom image, and the inverse image quality figure (IQF) was then calculated from the CD-curve. Compared to the Al-interspaced anti-scatter grid, the fiber-interspaced anti-scatter grid had B at 0.87, Σ at 0.95, K at 0.99, and Q at 1.14. In the assessment of the contrast-detail phantom image, the fiber-interspaced anti-scatter grid had an IQF of 1.02 times and a CNR of approximately 1.24 times when compared to the Al interspaced anti-scatter grid. The fiber-interspaced anti-scatter grid was superior with respect to the B and Q of the physical characteristics and to the CNR of image quality assessment.

Measurement of effective detective quantum efficiency for a photon counting scanning mammography system and comparison with two flat panel full-field digital mammography systems

Effective detective quantum efficiency (eDQE) describes the resolution and noise properties of an imaging system along with scatter and primary transmission, all measured under clinically appropriate conditions. Effective dose efficiency (eDE) is the eDQE normalised to mean glandular dose and has been proposed as a useful metric for the optimisation of clinical imaging systems. The aim of this study was to develop a methodology for measuring eDQE and eDE on a Philips microdose mammography (MDM) L30 photon counting scanning system, and to compare performance with two conventional flat panel systems.A custom made lead-blocker was manufactured to enable the accurate determination of dose measurements, and modulation transfer functions were determined free-in-air at heights of 2, 4 and 6 cm above the breast support platform. eDQE were calculated for a Philips MDM L30, Hologic Dimensions and Siemens Inspiration digital mammography system for 2, 4 and 6 cm thick poly(methyl methacrylate) (PMMA). The beam qualities (target/filter and kilovoltage) assessed were those selected by the automatic exposure control, and anti-scatter grids were used where available.Measurements of eDQE demonstrate significant differences in performance between the slit- and scan-directions for the photon counting imaging system. MTF has been shown to be the limiting factor in the scan-direction, which results in a rapid fall in eDQE at mid-to-high spatial frequencies. A comparison with two flat panel mammography systems demonstrates that this may limit image quality for small details, such as micro-calcifications, which correlates with a more conventional image quality assessment with the CDMAM phantom. eDE has shown the scanning photon counting system offers superior performance for low spatial frequencies, which will be important for the detection of large low contrast masses. Both eDQE and eDE are proposed as useful metrics that should enable optimisation of the Philips MDM L30.

Study of Monte Carlo Simulator for Estimation of Anti-Scatter Grid Physical Characteristics on IEC 60627:2013-Based

The anti-scatter grid (grid) is a major component of X-ray imaging devices, improving the quality of the image by removing X-rays scattered while passing through the subject. However, problems such as image distortions or increasing the dose of a patient unnecessarily may result if the grid is not suitable for a specific digital x-ray receptor (detector.) Selecting a suitable grid can take significant work and testing due to the wide range of specifications and physical characteristic of both grid and digital imaging x-ray detectors. In order to reduce the time cost and to improve the accuracy of selecting a suitable grid, this study implemented a Monte Carlo simulation for estimating the physical characteristics of the grid and verified the accuracy of the result by comparing with the physical characteristics of the actual grid. For the verification, this study compared the estimated physical characteristics with the measured physical characteristics for ten (10) grids with different specifications. The physical characteristics were measured at RQR (Radiation Qualities in Radiation Beams emerging from the X-ray Source Assembly) 4/6/8/9 of the Radiation conditions and analyzed Transmission of Primary radiation (Tp), Transmission of Total radiation (Tt) and Transmission of Scattered radiation (Ts) of the physical characteristics of the grid. As a result of the analysis, less than 1% average deviation between simulation and physical measurement was observed with all ten (10) grids. The changes of the physical characteristics as the specifications (line density and ratio) of the grid changed were also evaluated, and found to have a Pearson’s correlation coefficient of 0.998 between simulation and measurements. From the above results, the proposed program in this paper is judged reasonable as a grid physical characteristics prediction program.

Abstract ID: 176 Geant4 implementation of inter-atomic interference effect in small-angle coherent X-ray scattering for materials of medical interest

Advanced applications of digital mammography such as dual-energy and tomosynthesis require multiple exposures and thus deliver higher dose compared to standard mammograms. A straightforward manner to reduce patient dose without affecting image quality would be removal of the anti-scatter grid, provided that the involved reconstruction algorithms are able to take the scatter figure into account [1]. Monte Carlo simulations are very well suited for the calculation of X-ray scatter distribution and can be used to integrate such information within the reconstruction software. Geant4 is an open source C++ particle tracking code widely used in several physical fields, including medical physics [2,3]. However, the coherent scattering cross section used by the standard Geant4 code does not take into account the influence of molecular interference. According to the independent atomic scattering approximation (the so-called free-atom model), coherent radiation is indistinguishable from primary radiation because its angular distribution is peaked in the forward direction. Since interference effects occur between x-rays scattered by neighbouring atoms in matter, it was shown experimentally that the scatter distribution is affected by the molecular structure of the target, even in amorphous materials. The most important consequence is that the coherent scatter distribution is not peaked in the forward direction, and the position of the maximum is strongly material-dependent [4]. In this contribution, we present the implementation of a method to take into account inter-atomic interference in small-angle coherent scattering in Geant4, including a dedicated data set of suitable molecular form factor values for several materials of clinical interest. Furthermore, we present scatter images of simple geometric phantoms in which the Rayleigh contribution is rigorously evaluated.

Heavy Metal on a Tungsten Basis With a High Radiation Absorption Capacity Is an Ecological Alternative to Toxic Lead

EU Directive 2011/65/EU (Restriction of Hazardous Substances 2) regulates the use of certain hazardous substances in electrical and electronic equipment. Lead is ranked at the top of the list, even higher than mercury and cadmium. The heavy metal used as radiation shielding or for anti-scatter grids due to its high atomic mass is highly toxic and does not break down easily, which is why a weight proportion of only 0.1 percent is permitted in new electronic and electric equipment. It also has a low melting point of just 327°C, constituting an additional hazard in case of fire. The Bavarian company Wolfram Industrie offers an innovative, more environmentally friendly alternative with Triamet, a heavy metal alloy on a tungsten basis: thanks to its specific high density and the resulting radiation absorption capacity, the dimensionally stable product that is available in both an iron-nickel and a copper-nickel binder phase even shields against gamma radiation. While tungsten is normally difficult to process, it can be shaped into blanks using a powder-metallurgical process in this form. Subsequently these blanks can be processed further with little effort.

The Effect of Grid Ratio and Material of Anti-scatter Grid on the Scatter-to-primary Ratio and the Signalto-noise Ratio Improvement Factor in Container Scanner X-ray Imaging

Background: X-ray imaging detectors for the nondestructive cargo container inspection using MeV-energy X-rays should accurately portray the internal structure of the irradiated container. Internal and external factors can cause noise, affecting image quality, and scattered radiation is the greatest source of noise. To obtain a high-performance transmission image, the influence of scattered radiation must be minimized, and this can be accomplished through several methods. The scatter rejection method using an anti-scatter grid is the preferred method to reduce the impact of scattered radiation. In this paper, we present an evaluation the characteristics of the signal and noise according to physical and material changes in the anti-scatter grid of the imaging detector used in cargo container scanners. Materials and Methods: We evaluated the characteristics of the signal and noise according to changes in the grid ratio and the material of the anti-scatter grid in an X-ray image detector using MCNP6. The grid was composed of iron, lead, or tungsten, and the grid ratio was set to 2.5, 12.5, 25, or 37.5. X-ray spectrum sources for simulation were generated by 6- and 9-MeV electron impacts on the tungsten target using MCNP6. The object in the simulation was designed using metallic material of various thicknesses inside the steel container. Using the results of the computational simulation, we calculated the change in the scatter-to-primary ratio and the signal-to-noise ratio improvement factor according to the grid ratio and the grid material, respectively. Results and Discussion: Changing the grid ratios of the anti-scatter grid and the grid material decreased the scatter linearly, affecting the signal-to-noise ratio. Conclusion: The grid ratio and material of the anti-scatter grid affected the response characteristics of a container scanner using high-energy X-rays, but to a minimal extent; thus, it may not be practically effective to incorporate anti-scatter grids into container scanners.

Directional scatter imaging for the stereoscopic tracking of fiducial markers in a single kV exposure.

To demonstrate, via Monte Carlo simulation, that an image obtained from the patient-generated scattered radiation forced to impinge on the detector from a known direction by means of parallel-focused grids, can be used to complement the information conveyed by the primary image, such that accurate stereoscopic three-dimensional localization of fiducial markers can be achieved in a single kV x-ray exposure. A voxelized Zubal phantom was used to model the process of fiducial marker localization. The markers were represented as made of gold and cylindrical in shape with dimensions of 5mm in length and 1mm in diameter. Three such markers were placed in the Zubal phantom at the prostate level. Two gantry-mounted image acquisition geometries were modeled: a single kV imaging system and a dual kV-MV imaging system. The detector was modeled as a 30cm×40cm Gd2 O2 S screen with a thickness of 0.2cm and a resolution of 768×1024 pixels. The PENELOPE Monte Carlo code was used to calculate the absorbed dose in this detector imparted by the transmitted primary and directional scatter radiation. A grayscale conversion was then applied to obtain an image from which the positions of the markers were determined. Two parallel-focused grid geometries were modeled, one based on the standard lead-carbon fiber grids and a proposed modification using tungsten as the shielding material. Absorbed dose in the patient model was also calculated. It is shown that the combination of primary and directional scatter images provides the means for an accurate stereoscopic fiducial marker 3D localization in a single x-ray exposure, provided the antiscatter grids are made thick enough to allow radiation traveling only in a particular direction to reach the detector. For the proposed tungsten grid and the x-ray spectrum used in this work, grid ratios of 20 and thickness of 0.2cm, provide the necessary shielding while for the standard lead grids, a ratio of at least 166 and a thickness of 2cm are needed to obtain discernible directional scatter images. We have shown that it is in principle possible to determine the 3D position of fiducial markers in a single exposure by making use of the radiation scattered by the patient to form an image that complements the information obtained with the primary beam. The method here proposed requires minimal modification of existing clinical hardware.