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Emission of Characteristic L and K Radiation from Thick Tungsten Targets
Abstract
A method is proposed for calculating L and K photon emission from thick tungsten targets bombarded by electrons accelerated by potentials of 12–300 kV. Electron energy losses, electron backscatter losses, and photon attenuation in the target are included. Agreement with measured K emission is obtained using the Arthurs-Moiseiwitsch ionization cross sections and an expression of the form CZ (E<sub>0</sub>-k) 1- exp (-3k/E<sub>K</sub>) (k/E<sub>0</sub>)<sup>1/3</sup> 1- exp (-E<sub>0</sub>/E<sub>K</sub>) to describe the bremsstrahlung energy distribution. Satisfactory agreement with L-emission measurements is obtained using the Mott-Massey cross-section formula with constants B<sub>L<sub>i</sub></sub>=4 E<sub>L<sub>i</sub></sub>, b<sub>L<sub>I</sub></sub>=0.25×1.6, b<sub>L<sub>II</sub></sub>=0.25×2.75, and b<sub>L<sub>III</sub></sub>=0.25×4.2. Indirect radiation contributes 54–82% of the total K emission and 5–8% of the total L emission. The Webster-Clark empirical relation I<sub>i</sub>=C<sub>i</sub>(U<sub>0</sub>-1)<sup>n</sup>i agrees with calculation for U<sub>0</sub>≪3, with C<sub>K</sub>=5.1×10<sup>11</sup> photon/sec mA sr, n<sub>K</sub>=1.67, C<sub>L</sub>=2.6×10<sup>11</sup> photon/sec mA sr, and n<sub>L</sub>=1.5.
... The study was conducted using xSpect simulation code developed at Henry Ford Health Systems ͑Detroit, MI͒ which is based on a semi-empirical x-ray generation model. [25][26][27] We simulated x-ray spectra from a tungsten ͑W͒ target with tube potentials ranging from 40 to 100 kVp in various increments ͑2-3 kVp increments between 40 and 55 kVp, 5 kVp increments thereafter͒, using a 14°anode angle, and with filter thicknesses ranging from 10th to 1000th value layer. The complete simulation flow is as follows ͑Fig. ...
... Scattered radiation and electronic noise were not explicitly included in our simulations. The semi-empirical spectral model used was based on the work of Storm 25,26 and is comparable to similar spectral models used by other researchers. Other approaches include spectral generation using interpolating polynomials and published constant potential x-ray spectra. ...
The purpose of this simulation study was to evaluate the feasibility, benefits, and potential operating parameters of a quasi-monochromatic beam from a tungsten-target x-ray source yielding projection images. The application is intended for newly developed cone beam computed mammotomography (CmT) of an uncompressed breast. The value of a near monochromatic x-ray source for a fully 3D CmT application is the expected improved ability to separate tissues with very small differences in attenuation coefficients. The quasi-monochromatic beam is expected to yield enhanced tomographic image quality along with a low dose, equal to or less than that of dual view x-ray mammography. X-ray spectra were generated with a validated projection x-ray simulation tool (XSpect) for a range of tungsten tube potentials (40-100 kVp), filter materials (Z=51-65), and filter thicknesses (10th to 1000th value layer determined at 60 kVp). The breast was modeled from ICRU-44 breast tissue specifications, and a breast lesion was modeled as a 0.5 cm thick mass. The detector was modeled as a digital flat-panel detector with a 0.06 cm thick CsI x-ray absorption layer. Computed figures of merit (FOMs) included the ratio of mean beam energy post-breast to pre-breast and the ratio of lesion contrasts for edge-located and center-located lesions as indices of breast beam hardening, and SNR2/exposure and SNR2/dose as indices of exposure and dose efficiencies. The impact of optimization of these FOMs on lesion contrast is also examined. For all simulated filter materials at each given attenuation thickness [10th, 100th, 500th, 1000th value layers (VLs)], the mean and standard deviation of the pre-breast spectral full-width at tenth-maximum (FWTM) were 16.1 +/- 2.4, 10.3 +/- 2.2, 7.3 +/- 1.4, and 6.5 +/- 1.5 keV, respectively. The change in beam width at the tenth maximum from pre-breast to post-breast spectra ranged from 4.7 to 1.1 keV, for the thinnest and thickest filters, respectively. The higher Z filters (Z=57-63) produced a quasi-monochromatic beam that allowed the widest tube potential operating range (50-70 kVp) while maintaining minimal beam hardening and maximal SNR2/exposure and SNR2/dose, and providing a contrast greater than that obtained in the unfiltered case. Figures of merit improved with increasing filter thickness, with diminishing returns beyond the 500th value layer attenuation level. Operating parameters required to produce optimal spectra, while keeping exposures equal to that of dual view mammography, are within the capability of the commercial x-ray tube proposed for our experimental study, indicating that use of these highly attenuating filters is viable. Additional simulations comparing Mo/Mo, Mo/Rh, and W/Rh target/filter combinations indicate that they exhibit significantly lower SNR2/exposure than the present approach, precluding them from being used for computed mammotomography, while maintaining dose limitations and obtaining sufficient SNR. Beam hardening was also much higher in the existing techniques (17%-42%) than for our technique (2%). Simulations demonstrate that this quasi-monochromatic x-ray technique may enhance tissue separation for a newly developed cone beam computed mammotomography application for an uncompressed breast.
... The energy spectrum of the x-rays was modeled to resemble an x-ray tube with a tungsten-rhenium anode at 125 kVp-in order to match the spectrum of the x-rays in the experimentusing the XSPECT software. 25,26 The x-ray beam fluence in the simulation was made to match the fluence used in the experiment based on x-ray tube mAs: the x-ray tube mAs used in the experiment was recorded, and the x-ray tube mAs used in the simulation can be calculated based on recording the number of x-ray histories that are simulated and converting that number into the corresponding mAs from the fluence per unit mAs values calculated by XSPECT (which for the 125 kVp, tungstenrhenium anode x-ray tube used in this work was 17 × 10 6 x-rays per mAs in the 0.75 mm-diameter collimated pencil beam). The energy spectrum from XSPECT was validated in two ways: by measuring the directly transmitted x-ray beam spectrum in the experiment using an energy-sensitive photon-counting CdTe detector-array with 6.5 keV full width at half maximum (FWHM) energy resolution that we used for the coherent scatter imaging measurements in this work, and by measuring the Compton scatter off of a known object with a single energysensitive pixel (Amptek model XR100T) that has 1 to 2 keV FWHM energy resolution. ...
A scatter imaging technique for the differentiation of cancerous and healthy breast tissue in a heterogeneous sample is introduced in this work. Such a technique has potential utility in intraoperative margin assessment during lumpectomy procedures. In this work, we investigate the feasibility of the imaging method for tumor classification using Monte Carlo simulations and physical experiments. The coded aperture coherent scatter spectral imaging technique was used to reconstruct three-dimensional (3-D) images of breast tissue samples acquired through a single-position snapshot acquisition, without rotation as is required in coherent scatter computed tomography. We perform a quantitative assessment of the accuracy of the cancerous voxel classification using Monte Carlo simulations of the imaging system; describe our experimental implementation of coded aperture scatter imaging; show the reconstructed images of the breast tissue samples; and present segmentations of the 3-D images in order to identify the cancerous and healthy tissue in the samples. From the Monte Carlo simulations, we find that coded aperture scatter imaging is able to reconstruct images of the samples and identify the distribution of cancerous and healthy tissues (i.e., fibroglandular, adipose, or a mix of the two) inside them with a cancerous voxel identification sensitivity, specificity, and accuracy of 92.4%, 91.9%, and 92.0%, respectively. From the experimental results, we find that the technique is able to identify cancerous and healthy tissue samples and reconstruct differential coherent scatter cross sections that are highly correlated with those measured by other groups using x-ray diffraction. Coded aperture scatter imaging has the potential to provide scatter images that automatically differentiate cancerous and healthy tissue inside samples within a time on the order of a minute per slice.
... where l en /q is the mass energy-absorption coefficient of the phosphor; qx, the mass loading of the phosphor; I 0 (E), the incident X-ray photon spectrum; and E, the photon energy. We calculated I 0 (E) for the tube voltage range from 40 to 140 kV by using the Birch-Marshall formula [11] for the continuous part of the spectrum and the Webster-Clark formula [12] for the characteristic lines, with an added filtration of 20 cm acrylic resin to simulate abdominal radiography. The linear attenuation coefficient l of acrylic resin is almost equivalent to muscle for the diagnostic Xray energy range. ...
Our purpose in this study was to compare the energy absorbed in a computed radiography (CR) plate with that absorbed by a film-screen system over the diagnostic X-ray tube voltage range. A Fuji ST-II and Fuji HGM/UR2 were selected as a CR plate and film-screen system, respectively. The X-ray energy absorbed by the phosphor layer (per mA per unit area) was calculated theoretically as an index of the radiographic sensitivity by use of the incident X-ray photon spectrum with the Birch-Marshall formula and the sensitivity spectrum for the range of 40-140 kV. The radiosensitive media were treated as layers involving mass loading. The relative radiographic sensitivity of the ST-II CR plate best approached that of the HGM/UR2 film-screen at 60 kV; it was considerably lower for the ST-II CR plate on both sides of 60 kV. The relative sensitivities at 40 and 140 kV of the ST-II CR plate to the HGM/UR2 film-screen decreased by 16 and 30%, respectively. This result implies that, in the CR system, the mAs values must be increased by 16 and 30% at 40 and 140 kV, respectively. These results were explained by the relative positions of the K-absorption edges of the phosphors. The theoretically calculated result was in good agreement with the experimental result obtained with an acrylic resin phantom. These results would be useful in preventing under- or overexposure in a CR system and thus controlling the dose administered to the patient.
... Characteristic X-rays from the Target In addition to the X-ray continuum, characteristic fluorescence X-rays are produced by the tungsten target. This relationship is discussed elsewhere (16,17). Birch [8] where Nk= number of characteristic K-X-rays of tungsten produced by an incident electron; AR= mass number of target; C= constant= 1; R = correction of backscatter of election 1; Uo = Ti/Ek; and Ek= K-absorption energy of target. ...
This paper describes establishment by computational and experimental methods of the American National Standard Institute (ANSI) N13.11 X-ray radiation fields by the Korea Atomic Energy Research Institute (KAERI). These fields were used in the standard irradiations of various personal dosimeters for the personal dosimetry performance test program performed by the Ministry of Science and Technology of Korea in the autumn of 1995. Theoretical X-ray spectra produced from two KAERI X-ray generators were estimated using a modified Kramers' theory with target attenuation and backscatter correction and their spectral distributions experimentally measured by a high-purity germanium semiconductor detector through proper corrections for measured pulse height distributions with photopeak efficiency, Compton fraction, and K-escape fraction. The average energies and conversion coefficients obtained from the computation and experimental methods, when compared with ANSI N13.11 and the recently published National Institute of Standards and Technology X-ray beams, appeared to be in good agreement--(+/-)3% between corresponding values--and thus, could be satisfactorily applied in the performance test of personal dosimeters.
... In this work, we used a 250-m-thick lead edge device. Using a semiempirical x-ray spectra model that accounts for both characteristic x-ray and bremsstrahlung radiation, 44,45 the transmission through 250 m lead was predicted to be within 0.05 to 0.15 for a 90-115 kVp x-ray beam. ...
The modulation transfer function (MTF) of radiographic systems is frequently evaluated by measuring the system's line spread function (LSF) using narrow slits. The slit method requires precise fabrication and alignment of a slit and high radiation exposure. An alternative method for determining the MTF uses a sharp, attenuating edge device. We have constructed an edge device from a 250-microm-thick lead foil laminated between two thin slabs of acrylic. The device is placed near the detector and aligned with the aid of a laser beam and a holder such that a polished edge is parallel to the x-ray beam. A digital image of the edge is processed to obtain the presampled MTF. The image processing includes automated determination of the edge angle, reprojection, sub-binning, smoothing of the edge spread function (ESF), and spectral estimation. This edge method has been compared to the slit method using measurements on standard and high-resolution imaging plates of a digital storage phosphor (DSP) radiography system. The experimental results for both methods agree with a mean MTF difference of 0.008. The edge method provides a convenient measurement of the presampled MTF for digital radiographic systems with good response at low frequencies.
It is known that conventional x-ray imaging provides a maximum contrast between cancerous and healthy fibroglandular breast tissues of 3% based on their linear x-ray attenuation coefficients at 17.5 keV, whereas coherent scatter signal provides a maximum contrast of 19% based on their differential coherent scatter cross sections. Therefore in order to exploit this potential contrast, we seek to evaluate the performance of a coded- aperture coherent scatter imaging system for breast cancer detection and investigate its accuracy using Monte Carlo simulations. In the simulations we modeled our experimental system, which consists of a raster-scanned pencil beam of x-rays, a bismuth-tin coded aperture mask comprised of a repeating slit pattern with 2-mm periodicity, and a linear-array of 128 detector pixels with 6.5-keV energy resolution. The breast tissue that was scanned comprised a 3-cm sample taken from a patient-based XCAT breast phantom containing a tomosynthesis- based realistic simulated lesion. The differential coherent scatter cross section was reconstructed at each pixel in the image using an iterative reconstruction algorithm. Each pixel in the reconstructed image was then classified as being either air or the type of breast tissue with which its normalized reconstructed differential coherent scatter cross section had the highest correlation coefficient. Comparison of the final tissue classification results with the ground truth image showed that the coded aperture imaging technique has a cancerous pixel detection sensitivity (correct identification of cancerous pixels), specificity (correctly ruling out healthy pixels as not being cancer) and accuracy of 92.4%, 91.9% and 92.0%, respectively. Our Monte Carlo evaluation of our experimental coded aperture coherent scatter imaging system shows that it is able to exploit the greater contrast available from coherently scattered x-rays to increase the accuracy of detecting cancerous regions within the breast.
The analysis of X-ray scatter patterns has been demonstrated as an effective method of identifying specific materials in mixed object environments, for both biological and non-biological applications. Here we describe an X-ray scatter imaging system for material identification in cluttered objects and investigate its performance using a large-scale Monte Carlo simulation study of one-thousand objects containing a broad array of materials. The Geant4 Monte Carlo source code for Rayleigh scatter physics was modified to model coherent scatter diffraction in bulk materials based on experimentally measured form factors for 33 materials. The simulation was then used to model coherent scatter signals from a variety of targets and clutter (background) materials in one thousand randomized objects.
The resulting scatter images were used to characterize four parameters of the imaging system that affected its ability to identify target materials: (a) the arrangement of materials in the object, (b) clutter attenuation, (c) type of target material, and (d) the X-ray tube current. We found that the positioning of target materials within the object did not significantly affect their detectability; however, a strong negative correlation was observed between the target detectability and the clutter attenuation of the object. The imaging signal was also found to be relatively invariant to increases in X-ray tube current above 1 mAs for most materials considered in the study.
This work is the first Monte Carlo study to our knowledge of a large population of cluttered object of an X-ray scatter imaging system for material identification and lays the foundation for large-scale studies of the effectiveness of X-ray scatter imaging systems for material identification in complex samples.
The purpose of this study was to determine the optimum beam quality in
terms of kVp and filtration for a tungsten-target x-ray source for a
newly developed cone beam computed mammotomography application. The
optimized beam is expected to yield enhanced image quality along with a
low dose, equal to or less than that of dual view x-ray mammography.
X-ray spectra were computer generated for a range of tube potentials,
filter materials, and filter and breast thicknesses. The uncompressed
breast was modeled from 8, 12, and 16 cm thick tissue, and breast
lesions were modeled as a 0.5 cm thick striated muscle mass and a 0.02
cm thick microcalcification. The detector was modeled as a digital
flat-panel detector with a 0.06 cm thick CsI x-ray absorption layer.
Figures of merit computed included the ratio of the mean beam energy
post-breast to pre-breast as an index of beam hardening, ratio of lesion
contrasts with and without filtering, and SNR/exposure. Tube potentials
between 50 and 70 kVp provided continuous spectra that, when filtered
with Z-filters between 56 and 62 yielded quasi-monochromatic x-ray
spectra with optimal SNR/exposure and contrast while providing minimum
beam hardening. Figures of merit improve with increasing filter
thickness, but diminishing returns are seen beyond the 500th value
attenuation layer. While uncompressed breast thickness affected absolute
values of the measurement criteria, they had little effect on beam
hardening and contrast ratio and did not alter the optimal operating
range. Filter thickness near the 500th value layer is expected to be
well within the operating range for an intended commercial x-ray tube,
indicating that these highly attenuating filters can provide superior
performance for mammotomography.
Traditional film/screen mammograms are obtained using Molybdenum or Rhodium target x-ray tubes. The energy spectrum from these sources matches the limited latitude of film/screen systems. For digital imaging systems, the latitude is linear over a wide range of exposures and arbitrary H&D curves can be obtained with image processing. This allows the recorded contrast to noise ratio (CNR) to be optimized by considering a wide range of radiographic techniques. For this work, we modeled the radiographic process for a digital (amorphous selenium) mammography system. The optimal CNR relative to dose was determined for several target/filter combinations, for a wide range of kVp values, and for varying breast thickness. The target/filter combinations included: Mo/Mo, Mo/Rh, Rh/Rh, W/Al, W/Mo, W/Ag, and W/Sn. As breast thickness increased, the use of a tungsten target with a tin filter resulted in a 34% improvement in CNR for the same dose to the breast when compared to the use of a Molybdenum target with a Molybdenum filter. Notably, the W/Sn target/filter combination resulted in a significantly lower mA-s for the same breast dose (2/3 to 1/5 lower for a breast thickness from 4 to 8cm). In mammography applications, use of a Tungsten tube rather than the traditional Molybdenum tube should lead to significant reductions in exposure time and tube heat while maintaining similar image quality and dose.
Simulation results from previous studies indicate that a quasi-monochromatic x-ray beam can be produced using a newly developed beam filtration technique. This technique utilizes heavy filtration with novel high Z filter materials having k-edges just above those of CsI, producing a near monochromatic beam with mean energy optimized for detection. The value of a near monochromatic x-ray source for a fully 3D tomography application is the expected improved ability to separate tissues with very small differences in attenuation coefficients for a range of uncompressed breast sizes while maintaining dose levels at or below existing dual view mammography. In this study, we experimentally investigate a set of filter materials (Al, Cu, Ag, Ce, W, and Pb), filter thicknesses (10th, 100th, and 200th VL), and tube potentials (40-80 kVp) using a newly constructed test apparatus. Initial experimental results corroborate simulations and indicate that this approach can improve image quality (SNR) at constant dose. Al, Cu, W, and Pb provide optimal exposure efficiency results at 60 kVp and above. Decreasing relative improvements are observed above 100th VL filter thickness at 78 cm SID. Results are obtained without significant tube heating (except at 40 kVp). In addition, simulations indicate significant reductions in beam hardening. This optimized beam will be incorporated into a novel cone-beam x-ray computed mammotomography sub-system together with an emission tomograph in a dual modality CT/SPECT application specific emission and transmission tomography system for fully 3D uncompressed breast imaging.
The purpose of this study was to critically assess and compare the intrinsic resolution, noise, and signal-to-noise transfer characteristics of two modern digital photostimulable phosphor (PSP) radiographic systems. Two commercial PSP systems were evaluated by identical methodologies. Measurements were made at three beam qualities using standard-resolution and high-resolution screens. The presampled modulation transfer functions (MTF) of the systems were measured using an edge method. The noise power spectra (NPS) were determined by 2D Fourier analysis of uniformly exposed radiographs. The frequency-dependent detective quantum efficiencies (DQE) were obtained from the MTF and NPS measures and the input signal-to-noise ratio, determined by a computational model for x-ray spectrum which was verified against exposure and half-value-layer measurements. The physical performance of the systems were very similar; a DQE of 0.24 and a spatial frequency of 2.5 cycles/mm at 0.2 MTF were estimated for both systems at 115 kVp using a pixel size of 0.1 mm and standard-resolution screens. The HD screen provided improved MTF with an adverse effect on noise as compared to the GP-25 screen using the same pixel size and beam quality. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
A Monte Carlo method was used to calculate the emission of bremsstrahlung and characteristic K X-rays, and the backscattering of electrons, from thick tungsten irradiated by electrons with energies between 500 and 50 keV, incident on the target surface at many angles, from perpendicular to almost grazing. An extensive database was produced consisting of the spectra of emitted X-rays, and of reflected electrons, emerging in many directions. This database is available on a compact disk. Auxiliary calculations were developed to take into account the modification of the X-ray spectra by filters, and to determine quantities which characterize the X-ray output: (1) the fraction of the incident electron energy that is converted to emitted X-ray energy; (2) the exposure rate (in roentgen per second) per unit power input, and per unit rate of energy deposition in the target.
We evaluate the feasibility, benefits, and operating parameters of a quasimonochromatic beam for a newly developed X-ray cone beam computed mammotomography application. The value of a near monochromatic X-ray source for a fully 3D tomography application is the expected improved ability to separate tissues with very small differences in attenuation coefficients while maintaining dose level at or below existing dual view mammography. X-ray spectra were simulated for a range of tungsten tube potentials, K-edge filter materials, filter thicknesses, and a 12 cm uncompressed breast. The detector was modeled as a digital flat-panel CsI(Tl) detector. Figures of merit computed included: ratio of mean beam energy postbreast to prebreast; ratio of measured lesion contrasts with and without filtering; and exposure efficiency (SNR2/exposure). Initial experiments were performed with a commercially available X-ray tube/generator system, Ce foil filters, and plastic breast and lesion tissue-equivalent slabs. Simulation results showed that tube potentials of 50-70 kVp with filters of Z=57-63 yielded quasimonochromatic X-ray spectra with optimal FOMs. Initial experimental measurements with Ce foil filters corroborate simulation results. Simulations and initial experiments with one type of filter show that this approach can improve image quality while reducing exposure, and implemented in our multimodality X-ray CT/SPECT computed mammotomography system.
X-ray photoionization of elements and the detection of their characteristic fluorescent radiation has long been a basic research tool in atomic physics (see Chapter 4). It is also a widely applied analytical technique for the determination of elemental composition in support of both research and technology. The analytical application is the subject of this chapter. Synchrotron radiation must offer some important advances over present analytical techniques or little justification can be found for the use of a facility remote to most users. The analytical capabilities of the limited synchrotron facilities available will not supply all the routine analytical services required. However, increasing demands on analytical services caused by advanced technologies and growing concern for environmental monitoring is exceeding the performance capabilities of standard analytical methods. Justifications for applying synchrotron radiation to measurements of chemical composition include lowering of the detection limits, reducing heat or damage to the sample, improving the spatial resolution and contrast of microprobe analysis, reducing the time for analysis, improving the means of chemical identification by measuring absorption edge shifts or exafs, providing more accurate quantitative analysis, and extending analytical measurements to samples, configurations, and environments that are impractical, if not impossible, to analyze with present techniques.
The Kαh hypersatellite spectra of Mg, V, Cr, Mn, and Fe are measured with a plane crystal Bragg spectrometer for the first time in photon excitation. The experimental energies of the Kα2h lines are obtained and the energies of the Kα1,2 and Kα1,2h lines are calculated using a multiconfiguration Dirac-Fock program of Desclaux. The K-shell double photoionization cross sections are estimated from the experimental hypersatellite intensities. Results agree qualitatively with existing theoretical calculations based on the shake theory and correlated wave functions.
Bremsstrahlung radiation has been used to excite nearly monoenergetic x rays in secondary targets which can be utilized to study the energy response of radiation detectors if the intensity and purity are known. A method is suggested of calculating the spectral intensity of the secondary‐target radiation, including the K‐fluorescent x rays, and the bremsstrahlung plus characteristic‐line radiation scattered from the target. Both coherent and incoherent scatter are included in the calculation. To test the theory, bremsstrahlung radiation from an x‐ray unit operating in the 100–300‐kV potential range was used to excite K‐fluoresecent radiation in secondary targets ranging in atomic number from 29 to 90. Both the primary and secondary spectra were measured with NaI and germanium detectors. The measured primary spectral intensities were used as input to the secondary spectral intensity calculation. Calculated secondary spectra were within 20% agreement with measurement. Optimization of the secondary‐target intensity and purity is discussed as a function of target thickness, potential, and selective filtration.
Objectives: A dual-mode Single Photon Emission Computed Tomography (SPECT)/X-ray CT (XCT) system for full-field mammotomography is proposed for evaluation of breast lesions, where dual-view X-ray mammography has proven to be ineffective or inconclusive, particularly in radiographically dense breasts. Investigated design parameters include combined SPECT/XCT system geometry, X-ray focal spot size, tube loading, kVp, filtration, and detector type to determine operating parameters for a novel quasi-monochromatic X-ray beam for use in X-ray computed mammotomography that would use a dose equal to that from dual view mammography and that could be combined in a dual-mode system with SPECT. Methods: The relatively compact design of the hybrid SPECT/XCT will allow both systems to rotate on the same gantry below a patient. The hybrid system will be composed of a versatile, goniometer-based SPECT gantry, and an XCT system with a fixed position tungsten target, cone-beam geometry and digital flat panel detector frame nearly orthogonal to the SPECT system. The X-ray beam was modeled by xSpect, a semi-empirical simulation code for X-ray production, attenuation, and detection. The idealized operating parameters for the quasi-monochromatic beam were determined by evaluating a beam hardening (minimum) figure of merit (FOM) along with quantitative spectral characteristics, and an exposure efficiency (maximum) FOM. These FOMs were evaluated in 8–16 cm thick uncompressed breasts. Filters with atomic numbers (Z) from 51–65 with 10th to 1000th value attenuating layer thicknesses were modeled, while considering tube loading limitations. Results: Using a tungsten target, increased tube potential and filtration can yield minimal beam hardening with improved exposure efficiency and contrast for various thicknesses of uncompressed breasts. Optimal tube operating range between 50–70 kVp with filter thickness of between 100th and 500th value layer for Z filters 56–62 result in optimized FOMs while remaining at or below expected dose of two-view X-ray mammography for the entire breast. Mean quasi-monochromatic beam energies were near 40 keV, with beam widths of <15% FWHM. Diminishing improvements are seen for filters greater than Z=65 and for thicknesses greater than the 500th value layer. Heavy filtration also demonstrated near-independence of FOM outcomes based on breast size. Conclusion: Simulated data show that a quasi-monochromatic beam with minimal beam hardening can readily be generated. Given the small expected source-image distance, it is geometrically feasible for the XCT system to be mounted on the SPECT gantry so that simultaneous SPECT/XCT images of a pendant, uncompressed breast can be obtained with identical imaging geometries.
This paper proposes a new technique for reducing the patient dose when employing medical radiographs prepared by using screen-film systems. In this technique the patient dose can be reduced by employing scattered X-rays in order to obtain the same film density as that realized without the use of scattered X-rays. The minimum perceptible thickness difference ΔX(min), which can be recognized by liminal vision, was psychophysically calculated by considering the energy spectrum of incident X-ray, sensitivity spectrum of the screen layer, and the perception capability of human vision. From the calculated ΔX(mins) in various conditions, the permissible upper limit of scatter fraction for obtaining the same ΔX(min) for three kinds of luminances, and the fraction of reduction in the primary X-rays were determined. As an example of the results, when the object size required for perception is 1.3 mm, a scatter fraction up to 42% can be permitted at a density D of 1.0 for a luminance of 2548 cd m(-2). When we increase the luminance of the viewer from 478 cd m(-2) to 2548 cd m(-2), the upper limit of the permitted scatter fraction varies from 30% to 42% at a D of 1.0, i.e., the patient dose can be reduced by 17% under the same perceptibility of ΔX(min) by utilizing scattered X-rays. This reduction can be successfully achieved by changing the lead content of the grid from 0.45 to 0.38 g cm(-2).
Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.
The accuracy of dose distributions calculated in radiation therapy can be improved significantly if the effects of anatomical inhomogeneities such as air cavities, lung and bone are considered. Data about such inhomogeneities in the form of attenuation coefficients can be derived from computed tomography (CT) scans and used as input for the dose calculation. In those instances where there is little bone in the treatment field, CT data can be used without modification. If there is a large amount of bone in the treatment field, the attenuation coefficients measured at the lower photon energy of the diagnostic procedure must be transformed to the higher photon energy of the therapy treatment. Sufficient data are available from a single scan to make this transformation. An alternate method of using two scans at different kVp settings does not lead to an improvement in accuracy that is significant for radiation therapy. For a photon beam generated at 140 kVp and filtered by 7 mm Al, both methods contribute less than 1.5% to the error in the calculated radiation dose.
A study was undertaken to evaluate the effect of combining irradiation with X rays of various energies and an iodine-based contrast agent on the induction of chromosome aberrations in the peripheral lymphocytes of blood samples taken from healthy young donors. Although no enhancement of the effect of radiation was induced when blood samples with the iodine-based contrast agent were given 35 kV X irradiation, an 80 kV X-ray exposure induced an enhanced level of chromosome aberrations, and at 250 kV X irradiation, an enhancement of the frequencies of chromosome aberrations was seen in blood samples with the iodine-based contrast agent, especially when a Lucite phantom was employed in studies to increase the scattered rays. It was thus shown by microdosimetric analysis that X irradiation combined with an iodine-based contrast agent causes an enhancement of the absorbed radiation dose, which is dependent on the X-ray energies employed. This phenomenon may have clinical use in the radiotherapeutic management of tumors, although further extensive studies of tumor vascularity must be pursued before this can be applied clinically.
To measure the radiologic characteristics of subtle lung nodules and develop nodule phantoms that simulate these characteristics.
From a database of chest radiographs, 16 radiographs that showed confirmed subtle lung nodules were selected. The optical density values surrounding each nodule were obtained, and the radiologic characteristic of the nodules were evaluated. A set of Teflon nodule phantoms with similar x-ray transmission and variability was designed and fabricated. The contrast characteristics of the images of the nodule phantoms were measured.
Most of the subtle nodules appeared to have diffuse edges with Gaussian-like contrast profiles. The contrast and size characteristics of the nodule phantom images fulfilled the design criteria within a 4% deviation.
The radiologic pattern of subtle lung nodules is poorly simulated by nodule phantoms with simple spherical or disk shapes. The nodule phantoms in this study produce realistic radiographic patterns with an energy-dependent attenuation equivalent to that of tissue. The nodule phantoms are intended for use in experiments involving human subjects.
Many kinds of x-ray films having various characteristic curves have been developed for chest radiographs. In general, a phototiming device for determination of a mAs value which gives a proper exposure has been used for a chest radiography. For each film, however, the x-ray tube voltage has been determined by the subjective evaluation of radiologists or radiological technologists. In this paper, we propose a new method for determining the optimum tube voltage for chest radiographs using psychophysical analysis. The optimum density and the optimum density range of a screen/film system are obtained from the gradient curve of film and the minimum perceptible contrast delta Dmin [Acta Radiol. Diagnos. 4, 463-476 (1966)]. The optimum tube voltage, by which the lowest density of a mediastinum and the highest density of a lung field just cover the optimum density range, is obtained using the x-ray photon spectrum and sensitivity spectrum of the screen. This objective method does not depend on personal subjective evaluation, therefore it is available for the determination of optimum tube voltage for chest radiographs to be observed by many doctors of various departments.
The purpose of this study was to compare the image quality for a digital storage phosphor system using 1760 x 2140 (2k) and 3520 x 4280 (4k) image arrays. Measurements were made on a chest radiography system (Fuji FCR-9501) with special provisions to be operated in both 2k (standard) and 4k (HQ) modes. Presampled modulation transfer functions (MTF) were measured using an edge method. Noise power spectra (NPS) were determined for different input exposures by two-dimensional Fourier analysis. These measures along with exposure measurements and an x-ray spectral model were used to determine the frequency-dependent detective quantum efficiency DQE (f) of the system for the 4k and the 2k modes. The magnitude of the NPS for the 4k mode was about 1/2 that of the 2k mode. A MTF value of 0.5 was found at 1.25 cycles/mm for the 4k system and 1.50 cycles/mm for the 2k system. The 4k images had an extended MTF of 0.1 at 4.5 cycles/mm in the plate-scan direction. Overall, the DQE (f) of the 4k mode was slightly better than that for the 2k mode by about 0.02 due primarily to its better noise characteristics.
The intrinsic resolution, noise, and signal-to-noise transfer characteristics of five commercial digital computed radiography (CR) systems were compared using identical experimental methods. The reader/screen combinations evaluated were Agfa ADC-Compact/MD-10, Agfa ADC-Compact/MD-30, Agfa ADC-Solo/MD-10, Agfa ADC-Solo/MD-30, Lumisys CR-2000/MD-10, Fuji FCR-9501 (HQ)/ST-Va, Kodak CR-400/GP-25, and Kodak CR-400/HR. Measurements were made at 70 and 115 kVp with 19 mm added aluminum filtration. The presampled modulation transfer functions (MTFs) of the systems were measured using an edge method. The noise power spectra (NPS) were determined by 2D Fourier analysis of uniformly exposed radiographs. The frequency-dependent detective quantum efficiencies (DQEs) were computed from the MTF, NPS, exposure measurements, and computational estimates of the ideal signal-to-noise ratios. Using 70 kVp and 0.1-0.12 mm pixel sizes, spatial frequencies of 2.1, 2.0, 2.2, 1.9, 2.0, 2.0, 2.3, 2.3, and 3.5 cycles/mm were measured at 0.2 MTF for the eight reader/screen combinations, respectively. Using 70 kVp, 7.74 x 10(-8) C/kg (0.3 mR), and 0.1-0.12 mm pixel sizes, DQE(0.15) values of 20.3%, 22.9%, 24.6%, 28.6%, 22.2%, 30.0%, 29.5%, and 17.3% were obtained for the eight combinations, respectively. The corresponding values at 115 kVp were 15.9%, 18.5%, 21.5%, 21.8%, 15.3%, 23.1%, 22.3%, and 13.8%, respectively. The findings of the study demonstrate the pixel size, orientation, beam quality, screen, and reader dependencies of image quality in CR systems. The physical performance of the systems having standard-resolution screens demonstrated similar resolution performance but more notable variations in DQE. The one high-resolution screen tested had reduced DQE and increased MTF at high frequencies.
The increasing use of small animals in basic research has spurred interest in new imaging methodologies. Digital subtraction angiography (DSA) offers a particularly appealing approach to functional imaging in the small animal. This study examines the optimal x-ray, molybdenum (Mo) or tungsten (W) target sources, and technique to produce the highest quality small animal functional subtraction angiograms in terms of contrast and signal-difference-to-noise ratio squared (SdNR2). Two limiting conditions were considered-normalization with respect to dose and normalization against tube loading. Image contrast and SdNR2 were simulated using an established x-ray model. DSA images of live rats were taken at two representative tube potentials for the W and Mo sources. Results show that for small animal DSA, the Mo source provides better contrast. However, with digital detectors, SdNR2 is the more relevant figure of merit. The W source operated at kVps >60 achieved a higher SdNR2. The highest SdNR2 was obtained at voltages above 90 kVp. However, operation at the higher potential results in significantly greater dose and tube load and reduced contrast quantization. A reasonable tradeoff can be achieved at tube potentials at the beginning of the performance plateau, around 70 kVp, where the relative gain in SdNR2 is the greatest.
An experimental set-up for x-ray fluorescence analysis with synchrotron radiation was built and installed at the Stanford Synchrotron Radiation Project. X-ray spectra were taken from numerous and varied samples in order to assess the potential of synchrotron radiation as an excitation source for multielement x-ray fluorescence analysis. For many applications, the synchrotron radiation technique is shown to be superi- or to other x-ray fluorescence methods, especially those employing electrons and protons as excitation sources.
We evaluate the feasibility, benefits, and operating parameters of a quasimonochromatic beam for a newly developed x-ray cone beam computed mammotomography application. The value of a near monochromatic x-ray source for fully 3D dedicated mammotomography is the expected improved ability to separate tissues with very small differences in attenuation coefficients while maintaining dose levels at or below that of existing dual view mammography. In previous studies, simulations for a range of tungsten tube potentials, K-edge filter materials, filter thicknesses, and a 12 cm uncompressed breast, with a digital flat-panel CsI(Tl) detector model, indicated that thick, rare earth filter materials may provide optimized image quality. Figures of merit computed included: lesion contrast under different filtering conditions; ratio of measured lesion contrast with and without filtering; and exposure efficiency (SNR<sup>2</sup>/exposure). Initial experiments are performed with a custom built x-ray mammotomography system, cerium foil filters, and plastic breast and lesion tissue-equivalent slabs. Simulation results showed that tube potentials of 50-70 kVp with filters of Z=57-63 yielded quasimonochromatic x-ray spectra with improved FOMs. Initial experimental measurements corroborate simulation results in that, relative trends and rank order of contrast ratios and exposure efficiency were in agreement. These studies show that this approach can be implemented practically with simple hardware and yield improved exposure efficiency versus the unfiltered or minimally filtered case.
In this article we shall discuss the methods available for the calculation of the rates of atomic collision processes. We shall exclude from consideration any methods which are of specific application to nuclear or high energy collisions as these are discussed in Vols. XXXIX, XL, XLI and XLIII of this Encyclopedia though some of the methods which will be discussed are applicable to such collisions. No mention will be made of the rates of chemical reactions as these are adequately dealt with in chemical textbooks.
An expression for the cross-section for K-shell ionization of atoms by electrons is obtained by using Moller's relativistic modification of the Born approximation. Results are presented for the elements with atomic numbers less than 30. For nickel the calculated cross-sections are in good agreement with those measured by Kirkpatrick and his collaborators, a marked improvement due to the allowance for relativistic effects being found at high energies of impact. The normalization of the wave function of the ejected electron is discussed in the appendix.
The total number and energy distribution of backscattered electrons at 0.2-4 kev incident energy (V) have been measured for six elements using electrostatic retarding potential techniques. For atomic number Z~30, backscattering decreases with decreasing V below 2-3 kev to values less than those for elements of Z~=30, and it no longer is a simple function of Z. The ratio of the mean energy of the backscattered electrons to that of the primaries is found to be close to one-half for Z=6 and to increase only slightly for the heavier elements. These results are shown to indicate a dominant role of inelastic processes in the scattering of intermediate energy electrons, in contrast to scattering at very high energies, where elastic interactions control the phenomenon.
The mechanisms of the reorganization of atoms with inner shell vacancies are reviewed. All available experimental values of K-, L-, and M-shell fluorescence yields, together with L-shell Coster-Kronig yields are summarized and compared with available theoretical calculations. Rather good agreement exists in general between theory and experiment for the K shell, but experimental values of the L2 and L3 subshell fluorescence yields disagree substantially with the semitheoretical results of Listengarten. Related phenomena in mu-mesic atoms are included. Experimental methods of measurement are summarized and discussed, together with suggestions for future studies with high resolution techniques. Results from the literature have been included up to 10 May 1966.
The numberN_{L_alpha }^{dir} (produced) of L alpha -photons produced by electron-bombardment in a thick target of tungsten per incident electron has been measured absolutely with the Ross-filter method and relatively with the crystal-spectrometer method in the energyregion up to the 3.6 times the L III-ionization energyE_{L_{III} } . The result can be presented in the following empirical form:N_{L_alpha }^{dir} (produced)=4 pi·t·( U 0-1) n with t=0.52·10-4±5% and n=1.44±0.02(U_0 = E_0 /E_{L_{III} }< 3.6). Out of this the numbern_{L_{III} } of L III-ionizations per electron which is slowed down to the energyE_{L_{III} } within the target, has been evaluated. The computation ofn_{L_{III} } out of the elementary process by using Bethe's non-relativistic formulae for total L III-ionization cross section Q L and energy loss- dE/ds is in full agreement with experiment in the region 2< U 0
Cross sections for K-shell ionization have been measured at electron bombarding energies of 0.10, 0.15, 0.20, 0.25, 0.30, and 1.00 MeV for Ag, and of 0.20, 0.60, 0.80, 1.00, 1.20, 1.40, 1.70, and 2.00 MeV for Sn and Au. Below 0.50 MeV, the results for Ag are in excellent agreement with the relativistic calculations of Arthurs and Moiseiwitsch. For Sn at 0.20 and 0.60 MeV, where the theoretical values are available, the experimental cross sections are also in very close agreement. Although the experimental cross section for Au at 0.20 MeV is very nearly the same as the calculated result of Arthurs and Moiseiwitsch at this energy, at 0.60 MeV and higher electron energies the experimental results for Au are, on the average, about 15% lower than the values of Arthurs and Moiseiwitsch.
The agreement with experiment is reasonably good for the K-shell ionization, but only fair in the case of the rather meagre experimental data available for the L-shell. The values obtained for the relative ionization in the K- and L-shells are in good agreement with those to be expected from experiment.(Received August 06 1939)
Mit einer elektrischen Gegenfeldmethode wurde die Energieverteilung der in einen Halbraum rckdiffundierten Elektronen an den Elementen Al, Cu, Ag und Pt, bei Primrenergien von 20, 30 und 40 keV und bei Auftreffwinkeln der Primrelektronen von 90 und 45 untersucht. Die erhaltenen Gegenspannungskurven sind im angegebenen Bereich unabhngig von der Primrenergie, ebenso die Rckdiffusionskoeffizienten, fr die sich relative Werte ergeben, die etwas schwcher mit der OrdnungszahlZ anwachsen als frhere Werte vonSchonland. Durch graphische Differentiation der Gegenspannungskurven ergeben sich die Kurven der Energieverteilung, mit einem Maximum bei geringen Energieverlusten und einem anfangs steileren, schlielich sehr flachen Abfall gegen grere Energieverluste. Das Maximum rckt bei steigender Ordnungszahl nher an die Primrenergie, ebenso beim bergang zum kleineren Auftreffwinkel.
The total cross section for ionization of K‐electrons by electrons has been derived using the method of virtual photons combined with the photoelectric cross section for distant collisions and the Møller cross section for close collisions. Good agreement with experiments is obtained for the elements silver (Z = 47) and tin (Z = 50) when the electron energy is much higher than the ionization energy.
Distributions of energy fluence with respect to energy were measured by three methods for x-rays produced in a tungsten target tube with a 1 mm beryllium window. The tube was energized by a constant potential generator operated at 15, 20, 25 and 30 kv. The methods used were gas proportional counting, scintillation counting and attenuation analysis. The percentage of the total energy fluence due to tungsten L-characteristic radiation was measured by each of the above techniques and also by using balanced (Ross) filters. It reached a maximum of about 33%. The methods gave results which were in reasonable agreement with one another and with theory.
An attempt is made to apply some theoretical data regarding continuous X-ray spectra, due to H. A. Kramer, to the conditions obtaining in the use of low kilovoltage beryllium window tubes. Thus, after introducing the appropriate absorption coefficients, transmitted spectral dose rate distributions are computed for selected absorbers (beryllium, aluminium and air thicknesses). Thence, relative outputs and qualities relating to constant potential generation are derived by graphical integration. These results are then extended to pulsating tension units by applying the data to the actual kV and tube current waveforms to which the experimental equipment used by the writer is subject. By this means dose rate waveforms are obtained and their relative areas provide the relative outputs under pulsating tension which can be compared with the corresponding experimental data previously published. From a consideration of relative quality data, a correction for the self-hardening effect of the cathode beam target-penetration is made in terms of an equivalent filtration. The dependence of output on tube current, and air absorption losses are also analysed.
The ranges considered are (1) 10 to 50 kVp; (2) 0 to 2 mm Al external filtration (1 mm Be. inherent filtration); (3) 2·5 to 50 mA tube current; (4) 10 to 50 cm F.S.D.
X‐Ray Optics and Microanalysis, III (Academic
- M Green
- Bothe W.