X-ray Focal Spot Size Research Articles

Monte Carlo simulation of phase-contrast X-ray imaging with a single-grid setup

Phase-contrast X-ray imaging is widely used to visualize the internal structures of samples that cannot be seen with conventional X-ray imaging. Among various phase-sensitive imaging techniques available—propagation-based, grating-interferometry, edge-illumination, and speckle-based—single-grid-based imaging, a variant of the speckle-based technique, provides benefits like single exposure and a simple setup. In this study, we developed a Monte Carlo (MC) simulator based on Geant4 for phase-contrast imaging with a single-grid setup. We then validated the MC simulations of single-grid imaging against theoretical predictions. Our results demonstrate that the phase-contrast images obtained using the simulator align well with the theoretical predictions. Moreover, we quantitatively investigated the effects of imaging parameters, such as X-ray focal spot size and grid period, on image quality using the MC simulator. This confirmed the viability of the simulator. We expect that this simulator will be useful in designing an optimal single-grid-based imaging setup.

Assessing the influence of CT acquisition parameters on flaw detectability through simulation

X-Ray Computed Tomography (XCT) is a unique tool to fully visualize and understand the nature and size of flaws in industrial parts, with a growing application in different fields such as aeronautics and more recently metal additive manufacturing. The inevitable questions underlying any XCT inspection concern the detectability limit of the measure. What size of defect will be detected with my current configuration? How can I optimize my acquisition material or parameters to improve the detectability limit? Naturally, the known characteristics of the XCT systems (detector pixel size, X-ray tube voltage and focal spot size, magnification) give a first answer to these questions, at least in terms of spatial resolution, but it is more difficult to estimate a priori the visibility of a flaw in terms of contrast. The cost of XCT inspection, the difficulty to design specimen with narrow internal defects and the influence of the geometry of the part on the XCT image quality make experimental analysis of the detectability limits difficult. The simulation brings therefore a promising alternative, provided that it gives a thorough representation of a real inspection.

A Geant4 tool for edge-illumination X-ray phase-contrast imaging

The PEPI project is developing a new experimental facility integrating a chromatic photon-counting detector within an edge-illumination (EI) phase-contrast setup. In this context, a novel Geant4-based simulation tool has been introduced with the aim of defining the optimal design of the experimental setup. The code includes a custom X-ray refraction process and allows simulating the whole EI system, comprising a polychromatic and extended source, absorbing masks, substrates, their movement during acquisition, and X-ray detection. In this paper, a realistic spectral detector model is introduced and its energy response validated against experimental data acquired with synchrotron radiation at energies between 26 and 50 keV. Moreover, refraction and transmission images of a plastic phantom are reconstructed from simulation data and successfully compared with theoretical predictions. Finally, an optimization study aiming at finding the effect of the X-ray focal spot size (i.e. spatial coherence) on image quality is presented; the results suggest that, in the considered configuration, the system can tolerate source sizes up to 30 μm, while, for a fixed exposure time, the best signal-to-noise ratio in refraction images is found for source sizes in the order of 10 to 15 μm.

Fast microfocus x-ray tube based on carbon nanotube array

A full vacuum-sealed macrofocus x-ray tube with a vertically-aligned ring-shaped carbon nanotube (CNT) emitter grown by microwave plasma enhanced chemical vapor deposition is presented in this paper. The external grid allowed the CNT-based x-ray tube to exhibit transient switching on and off. The total emission current was 200 μA, which corresponds to a maximum emission current density of 10.1 A/cm2 from the ring-shaped CNT emitter when the grid voltage was 2.4 kV. The optimized focus electrode controlled the beam convergence on the target to produce a very small x-ray focal spot size less than 5 μm. Consequently, this microfocus x-ray tube could produce x-ray images with very high spatial resolution. X-ray fluoroscopy images of a multilayer printed circuit board (PCB) and field programmable gate array show distinct gold PCB traces with approximately 20 μm width.

Measuring the average slope error of a single-bounce ellipsoidal glass monocapillary X-ray condenser based on an X-ray source with an adjustable source size

Abstract An equation for the relation between the X-ray source size and focal spot size of a single-bounce ellipsoidal glass monocapillary X-ray condenser (SBEGMXC) was constructed. According to this equation, a method by measuring the focal spot size of an SBEGMXC was used to determine the average slope error of the SBEGMXC, and this method based on an X-ray source with an adjustable source size. Compared with the existing methods for measuring the slope error of an SBEGMXC, this method could be used to obtain the average of slope errors originating from every point on the inner surface of the SBEGMXC.

Physical Modeling and Performance of Spatial-Spectral Filters for CT Material Decomposition.

Material decomposition for imaging multiple contrast agents in a single acquisition has been made possible by spectral CT: a modality which incorporates multiple photon energy spectral sensitivities into a single data collection. This work presents an investigation of a new approach to spectral CT which does not rely on energy-discriminating detectors or multiple x-ray sources. Instead, a tiled pattern of K-edge filters are placed in front of the x-ray to create spatially encoded spectra data. For improved sampling, the spatial-spectral filter is moved continuously with respect to the source. A model-based material decomposition algorithm is adopted to directly reconstruct multiple material densities from projection data that is sparse in each spectral channel. Physical effects associated with the x-ray focal spot size and motion blur for the moving filter are expected to impact overall performance. In this work, those physical effects are modeled and a performance analysis is conducted. Specifically, experiments are presented with simulated focal spot widths between 0.2 mm and 4.0 mm. Additionally, filter motion blur is simulated for a linear translation speeds between 50 mm/s and 450 mm/s. The performance differential between a 0.2 mm and a 1.0 mm focal spot is less than 15% suggesting feasibility of the approach with realistic x-ray tubes. Moreover, for reasonable filter actuation speeds, higher speeds are shown to decrease error (due to improved sampling) despite motion-based spectral blur.

Simulation of hard x-ray focusing using an array of cylindrical micro-holes in a diamond film

The focusing of hard x-rays was numerically simulated using a beam propagation method implemented in the BeamPROP software package and a Rayleigh-Sommerfeld integral. It was shown that when focusing hard X-rays of wavelength 1.34 A with sequentially arranged circular cylindrical microlenses (shaped as 10-µm micro-holes) found in a polycrystalline diamond film, a nearly 10 times shorter focal length of the microlens array can be achieved, reducing from 14.4 cm to 15 mm, via increasing the number of microlenses from 1 to 20. Meanwhile, the x-ray focal spot size was found to decrease 7 times along the minor axis from 1.5 μm to 200 nm, while the maximum intensity in the focus experienced a 40-times increase.

Study of the Microfocus X-Ray Tube Based on a Point-Like Target Used for Micro-Computed Tomography.

For a micro-Computed Tomography (Micro-CT) system, the microfocus X-ray tube is an essential component because the spatial resolution of CT images, in theory, is mainly determined by the size and stability of the X-ray focal spot of the microfocus X-ray tube. However, many factors, including voltage fluctuations, mechanical vibrations, and temperature changes, can cause the size and the stability of the X-ray focal spot to degrade. A new microfocus X-ray tube based on a point-like micro-target in which the X-ray target is irradiated with an unfocused electron beam was investigated. EGS4 Monte Carlo simulation code was employed for the calculation of the X-ray intensity produced from the point-like micro-target and the substrate. The effects of several arrangements of the target material, target and beam size were studied. The simulation results demonstrated that if the intensity of X-rays generated at the point-like target is greater than half of the X-ray intensity produced on the substrate, the X-ray focal spot is determined in part by the point-like target rather than by the electron beam in the conventional X-ray tube. In theory, since it is able to reduce those unfavorable effects such as the electron beam trajectory swinging and the beam size changing for the microfocus X-ray tube, it could alleviate CT image artifacts caused by the X-ray focal spot shift and size change.

Laser wakefield generated X-ray probe for femtosecond time-resolved measurements of ionization states of warm dense aluminum

We have developed a laser wakefield generated X-ray probe to directly measure the temporal evolution of the ionization states in warm dense aluminum by means of absorption spectroscopy. As a promising alternative to the free electron excited X-ray sources, Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure. The X-ray probe system consists of an adjustable Kirkpatrick-Baez (KB) microscope for focusing the Betatron emission to a small probe spot on the sample being measured, and a flat Potassium Acid Phthalate Bragg crystal spectrometer to measure the transmitted X-ray spectrum in the region of the aluminum K-edge absorption lines. An X-ray focal spot size of around 50 μm was achieved after reflection from the platinum-coated 10-cm-long KB microscope mirrors. Shot to shot positioning stability of the Betatron radiation was measured resulting in an rms shot to shot variation in spatial pointing on the sample of 16 μm. The entire probe setup had a spectral resolution of ~1.5 eV, a detection bandwidth of ~24 eV, and an overall photon throughput efficiency of the order of 10(-5). Approximately 10 photons were detected by the X-ray CCD per laser shot within the spectrally resolved detection band. Thus, it is expected that hundreds of shots will be required per absorption spectrum to clearly observe the K-shell absorption features expected from the ionization states of the warm dense aluminum.

Relationship between x‐ray illumination field size and flat field intensity and its impacts on x‐ray imaging

X-ray cone-beam CT (CBCT) is being increasingly used for various clinical applications, while its performance is still hindered by image artifacts. This work investigates a new source of reconstruction error, which is often overlooked in the current CBCT imaging. The authors find that the x-ray flat field intensity (I(0)) varies significantly as the illumination volume size changes at different collimator settings. A wrong I(0) value leads to inaccurate CT numbers of reconstructed images as well as wrong scatter measurements in the CBCT research. The authors argue that the finite size of x-ray focal spot together with the detector glare effect cause the I(0) variation at different illumination sizes. Although the focal spot of commercial x-ray tubes typically has a nominal size of less than 1 mm, the off-focal-spot radiation covers an area of several millimeters on the tungsten target. Due to the large magnification factor from the field collimator to the detector, the penumbra effects of the collimator blades result in different I(0) values for different illumination field sizes. Detector glare further increases the variation, since one pencil beam of incident x-ray is scattered into an area of several centimeters on the detector. In this paper, the authors study these two effects by measuring the focal spot distribution with a pinhole assembly and the detector point spread function (PSF) with an edge-spread function method. The authors then derive a formula to estimate the I(0) value for different illumination field sizes, using the measured focal spot distribution and the detector PSF. Phantom studies are carried out to investigate the accuracy of scatter measurements and CT images with and without considering the I(0) variation effects. On our tabletop system with a Varian Paxscan 4030CB flat-panel detector and a Varian RAD-94 x-ray tube as used on a clinical CBCT system, the focal spot distribution has a measured full-width-at-half-maximum (FWHM) of around 0.4 mm, while non-negligible off-focal-spot radiation is observed at a distance of over 2 mm from the center. The measured detector PSF has an FWHM of 0.510 mm, with a shape close to Gaussian. From these two distributions, the author calculate the estimated I(0) values at different collimator settings. The I(0) variation mainly comes from the focal spot effect. The estimation matches well with the measurements at different collimator widths in both horizontal and vertical directions, with an average error of less than 3%. Our method improves the accuracy of conventional scatter measurements, where the scatter is measured as the difference between fan-beam and cone-beam projections. On a uniform water cylinder phantom, more accurate I(0) suppresses the unfaithful high-frequency signals at the object boundaries of the measured scatter, and the SPR estimation error is reduced from 0.158 to 0.014. The proposed I(0) estimation also reduces the reconstruction error from about 20 HU on the Catphan©600 phantom in the selected regions of interest to less than 4 HU. The I(0) variation is identified as one additional error source in x-ray imaging. By measuring the focal-spot distribution and detector PSF, the authors propose an accurate method of estimating the I(0) value for different illumination field sizes. The method obtains more accurate scatter measurements and therefore facilitates scatter correction algorithm designs. As correction methods for other CBCT artifacts become more successful, our research is significant in further improving the CBCT imaging accuracy.

Effect of X-Ray Computed Tomography Scanning Parameters on the Estimated Porosity of Foam Specimens

X-Ray Computed Tomography (CT) scanning is an effective method for estimating the porosity of various engineering materials and biomedical specimens such as tissue scaffolds and bones. However, the scanning and analysis parameters play a significant role in the accuracy of the porosity value determined from CT scan. This paper presents details of an investigation carried out to understand the effect of system parameters, namely the voxel size, X-ray focal spot size and segmentation threshold, on the estimated porosity by taking an example of safety-critical foam used for impact protection applications. Different voxel resolutions and focal spot sizes are selected in a total of 12 scanning tests and the effect of segmentation threshold is analyzed on each of these tests. The study indicates that the obtained porosity value is greatly influenced by the choice of voxel size at larger spot sizes and less influenced at smaller spot sizes. The threshold also has significant effect on the porosity value, especially at larger voxel sizes.

Line-Source Based X-Ray Tomography

Current computed tomography (CT) scanners, including micro-CT scanners, utilize a point x-ray source. As we target higher and higher spatial resolutions, the reduced x-ray focal spot size limits the temporal and contrast resolutions achievable. To overcome this limitation, in this paper we propose to use a line-shaped x-ray source so that many more photons can be generated, given a data acquisition interval. In reference to the simultaneous algebraic reconstruction technique (SART) algorithm for image reconstruction from projection data generated by an x-ray point source, here we develop a generalized SART algorithm for image reconstruction from projection data generated by an x-ray line source. Our numerical simulation results demonstrate the feasibility of our novel line-source based x-ray CT approach and the proposed generalized SART algorithm.

Optimization of X-ray target parameters for a high-brightness microfocus X-ray tube

Parameters of a transmission-type X-ray target have been optimized for a microfocus X-ray tube with an emphasis on maximizing the intensity of X-rays generated from the target while reducing the X-ray focal spot size. Monte Carlo simulation code MCNPX was used for the calculation of X-ray intensity generated from the X-ray tube. The calculation shows that the target thickness plays a dominant role in the enhancement of X-ray intensity. The optimum thickness to get maximum X-ray intensity for a given beam current was derived from the calculation for a broad range of electron energies from 30 keV to 150 keV. In addition, stability of a transmission-type target against localized thermal loading, which is attributed to the electron beam impinging on a micron-sized target region, was also investigated by the calculation of the temperature distribution within the target. From the analysis, we present the maximum allowable electron-beam power loading for a stable operation of the transmission-type target at different incident electron beam energies.

Conceiving a high resolution and fast X-ray CT system for imaging fine multi-phase mineral particles and retrieving mineral liberation spectra

Mineral liberation studies on fine particulate systems require high resolution and fast examination of a (statistically) significant number of particles. Commercial X-ray computed tomography systems allow high resolution on very small samples (say few micrometers on single particles) and only limited resolution on large particulate samples: thus they are unsuitable for fine particle liberation studies. A computed tomography system was conceived that is capable of high resolution (say micrometric or sub-micrometric) and allows imaging at once a large number of particles. It uses a detector design with a fiber optic bundle that allows enlarging the Field of View of a standard high resolution CCD in one direction. The thin particle multi-layer in a straw offers low X-ray attenuation and permits scanning with low current and voltages that in turn allows keeping small the size of X-ray focal spot and the induced geometrical un-sharpness of the projected particles. At low magnification values (i.e., with the sample close to the detector fiber optic input end) the overall resolution is controlled by CCD pixel size and the detector allows to scan more particles. At higher magnification values the detector design allows also improvements in resolutions. The system allows studying directly in 3D those liberation problems that require micrometer-size resolution.

Development of a carbon nanotube based microfocus x-ray tube with single focusing electrode

We report a detailed study on the design of carbon nanotube based microfocus x-ray tube with one electrostatic focusing electrode. Based on the electron optics simulations, such parameters as geometrical distances and applied voltages among all the electrodes are considered, respectively, in relation to the size of x-ray focal spot. The stability of the x-ray focal spot size is also examined with respect to the variation of gate and anode voltages. Experimental results that agree well with the simulated data are also provided to corroborate the design method. We also discuss the operating stability and limitations when designing a carbon nanotube based microfocus x-ray tube with only one electrostatic focusing electrode. The designed x-ray tube with an isotropic focal spot sees wide applications in in vivo medical imaging studies.

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Development of an optimal X-ray beam for dual-mode emission and transmission mammotomography

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 8–16cm 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 50–70kVp 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 40keV, 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.

Integration of a hard x-ray microprobe with a diffractometer for microdiffraction

A hard x-ray Fresnel zone-plate-based microprobe has been designed and integrated into a Newport kappa diffractometer for microdiffraction studies at the 2ID-D beamline at the Advance Photon Source. The microprobe employs 10 and 40 cm (focal length at 8 keV) zone plates to provide high and moderate focusing power, respectively. Each zone-plate assembly has two identical zone plates stacked together to provide higher focusing efficiency for higher energy (30 keV) applications. The mounting base of the microprobe is supported by the side of the base of the diffractometer at one end and the central table of the diffractometer at the other end, thus minimizing the instability of relative positions between the focal spot and the specimen. A x-ray focal spot size smaller than 360 nm and angular repeatability of the sample circles smaller than 0.001° have been demonstrated with this setup.

Focal Spot Measurements for Quality Control Purposes Using a Random Object Distribution

The use of a random object distribution for the characterization of x-ray focal spots for quality control purposes is described. This technique, utilizing a coherent optical processor, allows for the direct generation of the two-dimensional modulation transfer function (MTF). The distance betwep the first zeroes of the MTF, in the orientation of interest, are then measured to provide an estimation of the x-ray focal spot size. This technique is compared to the conventional pinhole image measurement technique and the failure of resolution (star measurement) technique and is found to produce similar variability and sensitivity to changes in the focal spot size.

Microradiography. II. Projection microradiography.

The simplest method of microradiography uses a normal X-ray tube and a fine-grained photographic plate in close contact with the specimen. The negative is enlarged optically up to ×500 depending on the grain size and density distribution in the photographic emulsion and also on the thickness of specimen, X-ray focal spot size and tube distance. This contact method has had a long and varied history, starting less than two years after Rontgen's discovery of X rays. One of the earliest references is the programme of the Inaugural Meeting of the Rontgen Society, November 5, 1897, where Mr. F. H. Neville (from this Laboratory) showed Microskiagrams of sections of sodium—gold alloys (Robertson, 1954). Since that time other workers have used contact microradiography in biology, medicine, metallurgy and other fields with a gradual realisation of the value of the method. More recently, Engstrom (1950) has placed microradiography on a quantitative basis for determining the mass and element distribution in a specime...