Line focal X-ray source imaging

AS ANGIOGRAPHIC diagnosis becomes more and more refined there is a demand for radiographic demonstration of smaller and smaller vessels. The various factors restricting resolution of x-ray images are well known, and it is proposed here to make some comments on the problem of geometric unsharpness and the use of primary or direct magnification radiography in vivo. Optimal resolution of vascular detail is obtained by low kilovoltage radiography of thin specimens in contact with fine-grain emulsion film, followed by photographic enlargement. In order to apply secondary or photographic magnification to in vivo angiography, the organ to be studied must be relatively thin and placed in direct contact with the film-holder. Margulis used this method in small experimental animals and on the human bowel at surgery (5). Recently Bosniak et al. applied this method to the surgically exposed kidney and bowel of dogs, using industrial-type film (3). In both studies, film grain limited enlargement to tenfold at most. Because of limitations on the rapidity of exposure and excessive radiation dosage with fine-grain film, as well as the inconvenience of the photographic step, secondary magnification has few clinical applications. Primary or direct magnification radiography does not require fine-grain film or special screens so that the overall quantity of radiation need not be excessive. The limiting factor is geometrical blurring due to the size of the focal spot relative to the object studied. Focal spot size is limited on the small side by the concentration of heat on the anode during the x-ray exposure. Experimental tubes with ultrafine focal spots have been described but find little clinical application because of the severely restricted heat capacity (9). Recent interest in direct magnification has been stimulated by the development of x-ray tubes with improved heat dissipation and relatively small focal spots, made possible by such features as decreased anode surface angulation and increased rotor speed and surface emissivity. As a result, studies can now be performed on the human body, through even its thick parts, with acceptably short exposures, at two- to fourfold magnification (4, 8, 10). On the basis of simple geometric considerations' it is possible to estimate the size of the smallest object, relative to the focal spot, that could be demonstrated with a given primary magnification, assuming that only the umbra image is useful. Bellman noted that an image composed only of penumbra would have poor contrast (1). Milne suggested that the apparent size of an object smaller than the focal spot would actually diminish with increasing magnification, referring, of course, to the umbra component only (6). Takahashi described the disappearance of objects smaller than the focal spot as a “filter effect” (9). The validity of these theories depends on whether an image could indeed be usefully detected if it were made up entirely of penumbra.

Following publication of the Forrest Report (Department of Health and Social Security (DHSS), 1986) and with the implementation of a breast cancer screening programme in the UK, there is considerable interest in the technical aspects of mammographic systems. The equipment requirements for mammographic X-ray screening units have been identified (DHSS, 1987) and guidance on the commissioning and routine testing of mammographic X-ray equipment is also expected (Institute of Physical Sciences in Medicine (IPSM), 1989). These documents relate to all aspects of the mammographic X-ray set including the specification and measurement of the size of the X-ray tube focal spot. It is recommended (DHSS, 1987) that effective focalspot sizes are measured using a pair of slit radiograms in the manner described in BS 6530 (British Standards Institution (BSI), 1985). This standard provides details on the construction of the slit diaphragm and the geometry to be employed when performing focal-spot measurements. In particular, the method requires that the centre of the slit diaphragm and its axis of symmetry are aligned with the reference axis, which is defined as the line through the centre of the radiation source and parallel to the reference direction or that specified direction to which the characteristics such as target angle are referenced. The manufacturer should specify the reference axis when providing data on anode target angle and size of focal spot.

PurposeThe purpose of this work was to develop and make available x-ray spectra for some of the most widely used digital mammography (DM), breast tomosynthesis (BT), and breast CT (bCT) systems in North America.MethodsThe Monte Carlo code MCNP6 was used to simulate minimally-filtered (only beryllium) x-ray spectra at 8 tube potentials from 20 to 49 kV for DM/BT, and 9 tube potentials from 35 to 70 kV for bCT. Vendor-specific anode compositions, effective anode angles, focal spot sizes, source-to-detector distances, and beryllium filtration were simulated. For each 0.5 keV energy bin in all simulated spectra, the fluence was interpolated using cubic splines across the range of simulated tube potentials to produce spectra in 1 kV increments from 20 to 49 kV for DM/BT and from 35 to 70 kV for bCT. The HVL of simulated spectra with conventional filtration (at 35 kV for DM/BT and 49 kV for bCT) was used to assess spectral differences resulting from variations in: (1) focal spot size (0.1 and 0.3 mm IEC), (2) solid angle at the detector (i.e. small and large FOV size), and (3) geometrical specifications for vendors that employ the same anode composition.ResultsAveraged across all DM/BT vendors, variations in focal spot and FOV size resulted in HVL differences of 2.2% and 0.9%, respectively. Comparing anode compositions separately, the HVL differences for Mo (GE, Siemens) and W (Hologic, Philips, and Siemens) spectra were 0.3% and 0.6%, respectively. Both the commercial Koning and prototype “Doheny” (UC Davis) bCT systems utilize W anodes with a 0.3 mm focal spot. Averaged across both bCT systems, variations in FOV size resulted in a 2.2% difference in HVL. In addition, the Koning spectrum was slightly harder than Doheny with a 4.2% difference in HVL. Therefore to reduce redundancy, a generic DM/BT system and a generic bCT system were used to generate the new spectra reported herein. The spectral models for application to DM/BT were dubbed the Molybdenum, Rhodium, and Tungsten Anode Spectral Models using Interpolating Cubic Splines (MASMICSM-T, RASMICSM-T, and TASMICSM-T ; subscript “M-T” indicating mammography and tomosynthesis). When compared against reference models (MASMIPM, RASMIPM, and TASMIPM; subscript “M” indicating mammography), the new spectral models were in close agreement with mean differences of 1.3%, −1.3%, and −3.3%, respectively, across tube potential comparisons of 20, 30, and 40 kV with conventional filtration. TASMICSbCT-generated bCT spectra were also in close agreement with the reference TASMIP model with a mean difference of −0.8%, across tube potential comparisons of 35, 49, and 70 kV with 1.5 mm Al filtration.ConclusionsThe Mo, Rh, and W anode spectra for application in DM and BT (MASMICSM-T, RASMICSM-T, and TASMICSM-T) and the W anode spectra for bCT (TASMICSbCT) as described in this study should be useful for individuals interested in modeling the performance of modern breast x-ray imaging systems including dual energy mammography which extends to 49 kV. These new spectra are tabulated in spreadsheet form and are made available to any interested party.

The goal of this study is to evaluate a new method based on a coded aperture mask combined with a digital x-ray imaging detector for measurements of the focal spot sizes of diagnostic x-ray tubes. Common techniques for focal spot size measurements employ a pinhole camera, a slit camera, or a star resolution pattern. The coded aperture mask is a radiation collimator consisting of a large number of apertures disposed on a predetermined grid in an array, through which the radiation source is imaged onto a digital x-ray detector. The method of the coded mask camera allows one to obtain a one-shot accurate and direct measurement of the two dimensions of the focal spot (like that for a pinhole camera) but at a low tube loading (like that for a slit camera). A large number of small apertures in the coded mask operate as a "multipinhole" with greater efficiency than a single pinhole, but keeping the resolution of a single pinhole. X-ray images result from the multiplexed output on the detector image plane of such a multiple aperture array, and the image of the source is digitally reconstructed with a deconvolution algorithm. Images of the focal spot of a laboratory x-ray tube (W anode: 35-80 kVp; focal spot size of 0.04 mm) were acquired at different geometrical magnifications with two different types of digital detector (a photon counting hybrid silicon pixel detector with 0.055 mm pitch and a flat panel CMOS digital detector with 0.05 mm pitch) using a high resolution coded mask (type no-two-holes-touching modified uniformly redundant array) with 480 0.07 mm apertures, designed for imaging at energies below 35 keV. Measurements with a slit camera were performed for comparison. A test with a pinhole camera and with the coded mask on a computed radiography mammography unit with 0.3 mm focal spot was also carried out. The full width at half maximum focal spot sizes were obtained from the line profiles of the decoded images, showing a focal spot of 0.120 mm x 0.105 mm at 35 kVp and M = 6.1, with a detector entrance exposure as low as 1.82 mR (0.125 mA s tube load). The slit camera indicated a focal spot of 0.112 mm x 0.104 mm at 35 kVp and M = 3.15, with an exposure at the detector of 72 mR. Focal spot measurements with the coded mask could be performed up to 80 kVp. Tolerance to angular misalignment with the reference beam up to 7 degrees in in-plane rotations and 1 degrees deg in out-of-plane rotations was observed. The axial distance of the focal spot from the coded mask could also be determined. It is possible to determine the beam intensity via measurement of the intensity of the decoded image of the focal spot and via a calibration procedure. Coded aperture masks coupled to a digital area detector produce precise determinations of the focal spot of an x-ray tube with reduced tube loading and measurement time, coupled to a large tolerance in the alignment of the mask.

The characterization of X-ray focal spots is of great significance for the diagnosis and performance optimization of focusing systems. X-ray free-electron lasers (XFELs) are the latest generation of X-ray sources with ultrahigh brilliance, ultrashort pulse duration and nearly full transverse coherence. Because each XFEL pulse is unique and has an ultrahigh peak intensity, it is difficult to characterize its focal spot size individually with full power. Herein, a method for characterizing the spot size at the focus position is proposed based on coherent diffraction imaging. A numerical simulation was conducted to verify the feasibility of the proposed method. The focal spot size of the Coherent Scattering and Imaging endstation at the Shanghai Soft X-ray Free Electron Laser Facility was characterized using the method. The full width at half-maxima of the focal spot intensity and spot size in the horizontal and vertical directions were calculated to be 2.10 ± 0.24 µm and 2.00 ± 0.20 µm, respectively. An ablation imprint on the silicon frame was used to validate the results of the proposed method.

Focusing polycapillary optics yield high gains in intensity and increased spatial resolution for a variety of clinical, lab-based, synchrotron, or in situ analysis applications. In this dissertation we investigate the extension of two applications of focusing polycapillary optics. The first is the application of polycapillary optics in radiation therapy. This discussion includes measurements and calculation of dose for focused beam orthovoltage therapy. A system has been designed to investigate whether the polycapillary optics can produce an X-ray beam which can give more accurate dose painting due to the higher dose concentration at the focal spot. X-ray exposures were measured with radio sensitive film. The measured spectrum of the X-ray beam was used to calculate the dose and compared with the measured dose on the films. The second application is medical imaging and quantitative phase retrieval. X-ray phase imaging can display subtle differences in low-density materials more readily than conventional X-ray imaging. However, due to spatial coherence requirements on the X-ray source, producing X-ray phase images has traditionally required either highly specialized sources, such as synchrotrons or small and low power microfocus sources, or multiple exposures with several carefully stepped precision gratings. To find appropriate approaches for enabling X-ray phase imaging in a clinically meaningful way, two techniques were tested. First, focusing polycapillary optics were applied in this work with conventional X-ray sources to enable propagation-based X-ray phase imaging. Quantitative phase retrieval was compared for two simple computational algorithms. Second, mesh-based X-ray phase imaging was demonstrated. In this technique, a simple, low-cost, coarse wire mesh and simple processing relaxes the spatial coherence constraint and allows quantitative phase retrieval. The mesh-based system combined with polycapillary optics significantly improved the accuracy of quantitative phase retrieval.

The spatial resolution of new, photon counting detector (PCD) CT scanners is limited by the size of the focal spot. Smaller, brighter focal spots would melt the tungsten focal track of a conventional X-ray source. To propose focal spot multiplexing (FSM), an architecture to improve the power of small focal spots and thereby enable higher resolution clinical PCD CT. In FSM, the source rapidly alternates between multiple focal spot locations. The dwell time at each focal spot location is much shorter than the readout interval of the PCD, and each location is visited many times during the readout interval so that heat can be effectively distributed over a larger surface area. The PCD accumulates recorded events on the detector (prior to slip ring transmission) into multiple spatial bins, as a second dimension to conventional energy bins. We estimated the maximum power permissible for a square, 0.2 mm focal spot assuming a maximum allowed surface temperature of 2900 K and a maximum transient temperature increase at the focal spot of 1400 K. This was performed using two separate thermal simulation codes: first, a commercial finite element method (FEM) was used to estimate bulk heating; second, a custom solver linearizing the heat equation was used to estimate track heating. FSM was simulated assuming that focal spot locations lay on parallel focal tracks, that instantaneous switching between tracks was possible, and that the space charge limit was never reached. We assumed a focal track velocity of 100 m/s and a 1 mm tungsten focal track layer with TZM alloy backing. The tube power was constant over a 4 s acquisition period. Without FSM, the 0.2 mm focal spot could attain a maximum power output of 17 kW before reaching the thermal limit. FSM required a minimum switching frequency of 0.5 MHz to produce benefit. With two focal tracks and a switching frequency of 4 MHz, the power could be increased to 28 kW, and with eight focal tracks and a switching frequency of 16 MHz, the power could be increased to 98 kW. In all cases, the 1400 K transient increase thermal limit was reached before the 2900 K maximum surface temperature constraint. The relationship between maximum power, switching frequency, and number of focal tracks is discussed in relation to the underlying physics of heat transport. Focal spot multiplexing is an architecture that exploits the digital nature of photon counting to improve the power of high-resolution CT X-ray sources. With fast switching and several parallel focal tracks, it may be possible to halve the size of the focal spot while maintaining source power. This work motivates the development of new source technologies that can achieve fast electronic switching.

A Monte Carlo study of the influence of focal spot size, intensity distribution, breast thickness and magnification on spatial resolution of an a-Se digital mammography system using the generalized MTF

Mammography x-ray tube focal spot sizes are routinely measured during acceptance testing and annual performance audits. The National Electrical Manufactures Association (NEMA) recommends the slit camera for this purpose. Investigated were the effect of slit rotational misalignment, tilt misalignment, image film density, film and screen-film image receptors, microscope magnification and reticule accuracy, and observer variation on slit camera focal spot measurements. Our results indicate that small rotational misalignment (< 5 degrees) and tilt misalignment (< 3 degrees) introduce insignificant error. Measured focal spot size increased slightly with image optical density, indicating that for consistent results the image optical density variations should be minimized. Also desirable for accurate field measurements is a high power microscope (25-50x) and a reticule with divisions of < or = 0.02 mm. Screen-film imaging consistently resulted in a slightly smaller measured focal spot size than direct film. The greatest source of error was due to observer variation. Of interest is that reader variability showed a consistent pattern and variation between two measurements by the same observer was much smaller than between observer variation, suggesting that standardized criteria should be established and a method of reader training developed. The length of the focal spot is defined at a reference axis angle specified by the mammography unit manufacturer. Presented is a tabulation of the focal spot geometry and reference axis angles for the majority of mammography units currently and recently marketed in North America.

This study has the following objectives: To measure the geometric efficiency (GE) of a multidetector CT (MDCT) system and investigate its dependence on several exposure parameters and to correlate GE with radiation dose burden of patients undergoing MDCT imaging. Dose profiles for all available beam collimations, tube voltage values, focal spot sizes, and modes of operation were determined for a modern MDCT scanner using an array of thermoluminescent chip dosimeters positioned side by side. Dose profiles measured free-in-air and at the center and periphery of the standard polymethyl methacrylate (PMMA) phantoms were used to estimate GE. The standard free-in-air and a new measure of GE determined using the standard PMMA phantoms were correlated with volume computed tomography dose index normalized to the z-axis coverage per rotation which is directly related to patient radiation dose burden. GE was found to be from 30% to 88% as beam collimation was changed from 1.2 to 24 mm, with thin beam collimations corresponding to higher "wasted" dose. For the same tube voltage and beam collimation, the use of small focal spot was associated with higher GE compared to the large focal spot. Besides, beam quality was found to have a much weaker effect on the GE value. In comparison to free-in-air, a weighted GE determined using the standard PMMA phantoms was found to have significantly better correlation with patient radiation burden (p < 0.05). Overbeaming strongly depends on the beam collimation and focal spot size, while the impact of beam quality on GE is less pronounced. Thin beam collimations are associated with a GE of as low as 30%.

Off-the-shelf beam analysis systems work very well in investigating and measuring collimated laser beam spatial profiles, focus spot spatial profiles, and the waist caustic. These standard systems have some shortcomings, however, related to viewing very small focus spot sizes at any wavelength, but especially at 10.6um. Another problem that these beam analysis systems can solve is alignment of optics and other components that can be damaged by high power beams if improperly set-up. This presentation will investigate the use of novel but simple imaging systems to allow in-depth analysis and real-time imaging of focus spot sizes from ∼10um to 100um at any laser wavelength, including 10.6um, allowing resolution of features as small as 0.25um. It will also investigate the use of low power pointing laser beams and artificial incoherent light to produce fine alignment and registration of optics and opto-mechanical components to high power laser beams. Examples will also employ ray-tracing information to validate the optical systems.Off-the-shelf beam analysis systems work very well in investigating and measuring collimated laser beam spatial profiles, focus spot spatial profiles, and the waist caustic. These standard systems have some shortcomings, however, related to viewing very small focus spot sizes at any wavelength, but especially at 10.6um. Another problem that these beam analysis systems can solve is alignment of optics and other components that can be damaged by high power beams if improperly set-up. This presentation will investigate the use of novel but simple imaging systems to allow in-depth analysis and real-time imaging of focus spot sizes from ∼10um to 100um at any laser wavelength, including 10.6um, allowing resolution of features as small as 0.25um. It will also investigate the use of low power pointing laser beams and artificial incoherent light to produce fine alignment and registration of optics and opto-mechanical components to high power laser beams. Examples will also employ ray-tracing information to validat...

This article discusses the simulation, design and performance of an 80 kW strip electron gun. Electrons from an electrically heated, multisegmented tantalum filament are electrostatically focused to provide a sheet of electrons in the postanode region, that when bent through 270° along a circle of radius 120 mm by a uniform magnetic field, focuses on the target. This gun has been tested up to 80 kW (45 kV×1.8 A), and delivers power densities on the target in the range of 20 kW cm−2. The linear current density is uniform up to 20%, over a focal spot of dimension 80×5 mm2. Good correspondence between filament geometry and focal spot image of the electron beam on the target has been obtained, which is desirable in applications involving metal vapor coatings. This design is scaleable along the length of the focal spot without sacrificing uniformity, which is not possible in scanning or multiple pencil-type of electron guns. Experimental measurement of gun parameters such as position of filament in relation to the cathode opening, distribution of current along the length of the beam, focal spot size, extraction efficiency, etc., along with data concerning electron beam quality as a function of various parameters, are presented. The performance of this gun under different experimental conditions, as well as the simulation and modeling using an in-house three-dimensional beam dynamics code, is discussed.

Most X-ray tubes comprise a rotating anode that is bombarded with electrons to produce X-rays. A substantial amount of heat is generated, and to increase the area of the anode exposed to the electrons, without increasing the apparent size of the focal spot, the focal track of the anode is generally beveled with a very shallow angle (typically 5 degrees-7 degrees in a computed tomography (CT) tube). Due to the line focus principle, this allows a fairly large area of the focal track to be exposed to electrons while retaining a fairly small effective projected focal spot. One side effect of anode angulation is that the focal spot appears different from different positions in the detector array; the effective focal spot size at a constant distance from the tube will be larger for a peripheral detector channel than for a central one. These differences in the effective size of the focal spot across the field-of-view lead to worse resolution in the periphery than in the center of reconstructed images. In this work we describe a method for achieving more uniform resolution in fanbeam CT images by correcting for these focal spot angulation effects. We do so by modeling the effects as a series of local blurrings in the space of transmitted CT intensities and determining the effective coefficients of the corresponding discrete convolutions. The effect of these blurrings can then be compensated for in the sinogram domain through the use of a penalized-likelihood sinogram restoration model we have recently developed.

We report speckle-free digital holographic phase microscopy (DHPM) and tomography using partially spatially coherent monochromatic light with improved image sharpness, resolution and phase noise. Effects of high spatial and temporal coherence of laser light in DHPM, such as speckle effect, spurious fringe generation are discussed and these effects lead to poor phase reconstruction. We present our recent work on the speckle-free phase microscopy and spatial coherence gated tomography with improved phase reconstruction and resolution.

Microsphere has been demonstrated for imaging resolution improvement based on optical microscope, and applied in many engineering and biological fields successfully, while its mechanism is still inconclusive due to the complex physics of both near-field and far-field optics involved. In this paper, a comprehensive simulation model based on point spread function is built to develop a physical insight into microsphere optical nanoscope by numerical simulation. With this model, the Fresnel-region focusing of microsphere lens is studied, which shows the contradiction between focal spot size and focal length. Furthermore, the gap between microsphere and the sample surface is investigated by numerical simulation and experimental verification. As increasing of this gap, the image resolution declines rapidly, and the image becomes blur more and more. At last, a customized sample with a flower pattern etched on silicon substrate is demonstrated with microsphere optical nanoscope, and its feature sizes of 51 nm line width are imaged clearly. While it is generally accepted that microsphere optical nanoscope can be implemented in nano-imaging, these findings quantified the limit on image resolution of this setup, which serves as a general guideline in system design of the other microsphere or micro-particle assisted microscope/nanoscope.