Slit camera focal spot measurement errors in mammography

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.

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.

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.

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.

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.

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

High spatial coherent and bright X-ray beam is necessary condition for acquiring high quality radiography image. However, traditional X-ray tube can only provide high flux X-ray light or high spatial coherent light. In general, X-ray photons are generated by using energetic electrons with several tens or even hundreds keV to hit a target. Unfortunately, over 99% electron energy are converted into heat rather than the energy of X-ray photons. Thus, the heat dissipation of the target restricts the emission power and radiation flux. Increasing the emission area of X-ray can relieve the heat dissipation, but it would bring another serious problemlow spatial coherence that is in inverse proportion to emission area or focal spot. In order to solve the conflict between brightness and spatial coherence, an X-ray source with one-dimensional coherence is proposed in this work. The new X-ray source has a special focal spot where one side is small enough to ensure the spatial coherence and the perpendicular side is big enough to provide sufficient X-ray flux. In the direction of long side, the long size of focal spot will result in losing the image details. Consequently, an algorithm of superposition and rotation, in which many images with different rotation angles are added together, is proposed to retrieve the lost information. On the other hand, the spatial transfer function of superposition is analyzed in the frequency domain, and the result shows that the method of superposition can transfer more components of frequency than single image. Based on a traditional X-ray tube, a line focal spot source is designed and fabricated. Two series of experiments are performed for different destinations. After 17 images of a chip with different rotation angles and the line focal spot are collected, those images are rotated in the reverse direction and added together. The image of superposition clearly presents some details which are invisible in one of 17 images. At the same tube voltage and for the same object, some comparative experiments with micro-focus source, line focal spot source and normal focal spot source are presented. Compared with traditional X-ray tube, line focal spot source can provide high-resolution image. In the aspect of image visibility, the new source has definite advantages compared with micro-focus source, despite no improvement in imaging resolution. Finally, the reason for the difference in imaging resolution is discussed from the aspects of spatial coherence and light flux.

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...

It is not generally appreciated that there is a gradation of the focal spot size from the anode to cathode end of an x-ray field that can dramatically affect radiographic detail from one end of a radiograph to the other. The authors name this gradation the "focal effect." Gradations in radiation intensity and focal spot size were measured from anode end to cathode end of a 14 x 17-inch field and were visually demonstrated with lymphangiograms and line-pair patterns. The degradation in spatial resolution along the anode-cathode axis was found to be as much as 75%, depending on the orientation of the patient with respect to the x-ray tube. Radiographic detail is, therefore, significantly improved (even when the large focal spot is used) by exploiting the focal effect and placing the body part requiring the best radiographic detail at the anode end of the table.

The physical assessment of the spatial resolution produced by broad and fine focal spot sizes has been well established. There is however an evident lack of study into the impact of focal spot selection on clinical image quality. While the excessive use of the fine focus has an impact on tube life, the benefit of its use in radiological imaging should be investigated. Cadaver images were produced in order to compare the 0.8mm and 1.8mm focal spot sizes. The range of radiographic projections assessed included the medio-lateral ankle, antero-posterior (AP) knee, AP thoracic spine and horizontal beam lateral (HBL) lumbar spine. Five clinicians analysed the images using a 1 - 4 visual grading analysis score against a reference image to assess the visibility of specific anatomical criteria. A Mann- Whitney U statistical test was employed to assess the results. No significant statistical differences between the scores for the broad and fine focus images were found, although a non-significant higher score in image quality was shown for the fine compared with the broad focus images with large object to detector distance. No difference in image quality was shown for examinations traditionally produced with a fine focus. The study results questions the wide spread usage of fine foci for specific examinations, particularly for extremity examinations. Current practice based on international guidelines can lead to a reduced life and increased cost with little clinical benefit.

The performance characteristics of a new dedicated mammography unit (a Mammo Diagnost U-M, Philips Medical Systems, Inc., Shelton, Conn.) are presented. The unit has capabilities for contact imaging with or without a grid and magnification imaging without a grid, using a molybdenum target and either of two different beam filtrations (molybdenum or aluminum). Characteristics evaluated include the performance of the beam restriction system, accuracy of technique parameter selector indication of kilovoltage, tube current and exposure time, focal spot size, exposure output and beam quality characteristics as a function of kilovoltage and filtration, compression device attenuation and automatic exposure control system performance. The resultant data have been used to calculate breast entrance exposure and average glandular dose for each imaging modality and to compare the different imaging modalities in terms of equivalent bandwidth, fe, using established models for the modulation transfer functions of the focal spot and image receptor. Modifications to the system to enhance clinical utility and to allow for routine calculations of patient exposure are also described.

Purpose: To find the optimum geometric magnification (Mopt) for a specific range of spatial frequencies in the object plane by taking the focal spot blur into account for three different detectors. Methods: The generalized modulation transfer functions (GMTFs) (which gives spatial resolution referenced to the object plane) were calculated for a range of magnifications for two focal spots (small [0.3 mm] and medium [0.5 mm]) for three different detectors, namely the micro‐angiographic fluoroscope (MAF), the solid‐state x‐ray image intensifier (SSXII) and a flat‐panel detector (FPD). The magnification factor (Mopt) corresponding to the maximum GMTF value for a particular frequency was determined as a function of frequency in the range of (1–2.5) cycles/mm for the FPD and (1– 6) cycles/mm for the other two detectors. Results: The Mopt decreases from 2 to 1.6 for the FPD and from 2 to 1.2 for both the MAF and the SSXII for the small focal spot in the mid‐frequency range of (1–2.6) cycles/mm while it decreases from 1.2 to 1.1 for both the FPD and the SSXII and from 1.2 to 1.0 for the MAF for the medium focal spot for the same frequency range. For high frequencies (above 2.6 cycles/mm), the Mopt decreases for both the MAF and the SSXII from 1.2 to 1.1 and from 1.1 to 1.0 in the case of small and medium focal spot, respectively. Conclusion: The value of optimum magnification is different in a particular range of frequencies for a given combination of detector and focal spot. For the small focal spot and mid‐frequencies, the Mopt drops sharply for high resolution detectors (MAF and SSXII) compared to the low resolution FPD, while it did not change much for the medium focal spot or for high spatial frequencies for the high resolution detectors. NIH Grant 2R01EB002873

Many medical studies have investigated the safety threshold of ocular media at various wavelengths. Understanding and determining the damage threshold of laser exposure to the retina is important in order to set safety guidelines. A phenomenon known as thermal lensing where a media's index of refraction changes as the laser energy is absorbed causes the beam profile and subsequently the position of the focal point of the beam to change as the temperature changes within the media. These changes in ocular media such as vitreous and aqueous humor, the cornea, crystalline lens, and retina all result in a slight change in the beam profile making the prediction of the focal point and laser spot size at the retina difficult to accurately predict. This paper describes how the effects of thermal lensing in the eye can be investigated to aid in determining the damage threshold for pulse durations of a few nanoseconds and continuous wave, CW, laser irradiation delivered to the retina and the relationship of how the damage is related to multiple pulse exposures. We present a first-order modeling effort for the thermal lensing effect in the eye along with initial estimates of impact on damage thresholds as predicted through computational biophysics thermal damage models.