Measurement of focal spot in X-ray tube using imaging plate

Comparisons of focal-spot size measurements with a direct-exposure X-ray film, screen-film (S/F) system, and imaging plate (IP) were carried out for slit and star resolution pattern camera techniques. The focal-spot measurements of X-ray units with the nominal size of 0.1-1 mm were performed, and measurement using the computed radiography (CR) system was investigated. Compared with the direct-exposure X-ray film method, the measurements of focal-spot size by the S/F system were slightly small for all focal spots. The measurements obtained with IP were slightly large for large and small focal spots, although the measurement of micro focal-spot size was overestimated, extending the tolerance level owing to the sampling. With the star pattern camera technique, differences between the three kinds of detectors were minimal. Therefore, it was thought that measurement of focal-spot size using the CR system would be practical for both large and small focal spots if this method were used for quality control in medical facilities.     

Measurement of the focal spot size of diagnostic x-ray tubes--a comparison of pinhole and resolution methods.

The pinhole camera technique has been compared with resolution and other methods for measuring the focal spot dimensions fo diagnostic X-ray tubes. These comparisons show that the resolution method offers an accurate estimation of the focal spot dimensions without the difficulties normally encountered in pinhole camera techniques. Accuracy and reproducibility of the "star-test" resolution method is examined. The current standard specifications for pinhole camera measurements are also discussed, and in the light of results obtained, changes in these specifications are suggested, particulary with regard to correction factors for non-uniformity of the intensity distribution and manufacturing tolerances.     

The evaluation of the performance of a new-concept screen-film system]

Aim of this work is to show the results of the evaluation of the sensitometric properties of a new Du Pont anti cross-over radiographic film system called Ultra Vision, based on a new film and different YTaO4 undoped screens whose emission is centered at about 335 nm. Fog, contrast, latitude, sensitivity, noise and resolution were tested: the MTFs were obtained by exposing a standard resolution pattern and using the "zero"-frequency step as a knife edge. The image of this knife edge was scanned with a microdensitometer with a 20 microns slit. The results (at 70 kVp with 2.5 mm Al and 2.5 mm Cu total filtration), compared with those of Du Pont conventional radiographic screen-film systems Cronex 10 S/Quanta, show that the new system significantly increases contrast, sensitivity and resolution and will consequently improve, in the clinical use, radiographic image quality.     

Recent advances in screen-film mammography

Today there are many dedicated mammographic x-ray units available that are capable of providing high-quality screen-film mammograms. Likewise, screen-film combinations designed for mammography are capable of providing images with appropriate contrast, resolution, and noise levels. Proper film processing is most important in order to obtain the appropriate film speed and contrast. A higher-speed screen-film combination designed for mammography can provide mammograms with significantly lower radiation dose, especially for grid and magnification techniques. Designing x-ray units and techniques as well as screen-film combinations with the singular goal of reducing radiation dose will always involve compromises and trade-offs. The key is to always consider optimizing all of the factors that affect image quality: (1) appropriate beam quality, (2) breast compression, (3) consideration of the use of grids, (4) good geometry, (5) selection of an appropriate screen-film combination, and (6) proper film processing. Optimization of all appropriate imaging factors will produce high-quality mammograms at the lowest radiation dose to the patient.     

Absolute speeds of screen-film systems and their absorbed-energy constants

Absolute speeds (mR-1) of screen-film systems were determined accurately and precisely from ten independent measurements over a six month period at several beam qualities that included both scatter and scatter-free conditions. The absorbed x-ray energies in the screens that yielded a net film density of 1.0 were obtained by means of Monte Carlo calculations and a semianalytical method. The standard deviations of the calculated absorbed x-ray energies at different beam qualities were less than 10% for all of the screen-film systems studied. Therefore, the absorbed x-ray energy that produces a given film density for these systems can be regarded as constant at the beam qualities used. Quantum absorption and fractional energy absorption of the screens were also calculated. The predicted Wiener spectral values based on the number of photons detected were compared with the measured Wiener spectral values of the screen-film systems.     

Practicality of NEMA performance specification measurements for user-based acceptance testing and routine quality assurance

National Electrical Manufactures Association ( NEMA ) performance specifications provide the only standardized and traceable measurements of scintillation-camera performance that are widely accepted by manufacturers. The NEMA publication describing the performance specifications suggests that elaborate equipment beyond a standard imaging computer is required for the measurements. For this reason the tests are currently unsuitable for both user-based acceptance testing and daily quality assurance. We have implemented five of the eight NEMA performance measurements as routine quality-assurance procedures on our computerized scintillation cameras. In addition, we have shown that seven of the eight NEMA measurements can be performed in a manner traceable to NEMA , with energy resolution as the single exception. With a standard imaging computer, NEMA phantom, and minor modification to NEMA collection and analysis constraints, we have analyzed images for intrinsic uniformity, resolution, linearity, and multiple-window spatial registration as well as for system spatial resolution both with and without scatter.     

Mammography equipment: principles, features, selection

1. Three-phase and constant potential generators may offer advantages over single-phase generators. These include shorter exposure times with less chance for motion unsharpenss, more adequate exposure and possible lower dose, and longer tube life. 2. Control of tube voltage in increments of 1 kVp are desirable for screen-film mammography and 2 kVp for xeromammography. 3. Tube loading limitations should be capable of allowing an adequate mA and mAs output. A mammography unit with a lower mA output may require longer exposure times with consequent motion unsharpness and/or higher dose. The maximum mA output is limited by the power rating of the tube and not by the power rating of the generator, which is usually much higher. Thus, tube rating charts rather than generator power rating should be used to compare the mA output of different mammography units. 4. Phototimer capability can reduce exam time, facilitate proper exposure, and reduce radiation dose. Units with a greater number of density steps and three or more photocell locations, one of which is near the edge of the film holder, provide better phototimer results. 5. A molybdenum target, beryllium window tube with 0.03 mm molybdenum added filtration is presently preferred for screen-film mammography. A tungsten target tube with 1 to 2 mm added aluminum filtration is advised for xeromammography. 6. Effective local spot size for contact mammography should be in the range of 0.2 to 0.5 mm. However, high resolution also requires sufficiently long source-to-image-receptor distances. Using measured focal spot size and source-to-image-receptor distance, it has been shown that some mammography units do not fully utilize the resolution capabilities of some available screen-film systems. On the other hand, a focal spot that is too small may limit the field size and lead to uneven energy distribution in the x-ray field. 7. In order to perform standard and supplementary mammographic projections, the C arm assembly must be capable of being located at multiple positions within a 180 degrees rotation. Greater degrees of rotation are not required. 8. The heat dissipation capability should be adequate to accommodate the anticipated work-load in terms of number of patients and patient scheduling. A mammographic unit with an anode heat capacity of 90,000 heat units or more should be adequate for most practices. A mammography unit with a heat capacity of 20,000 heat units or less may require waiting time between exposures or studies in a high-volume mammography practice. 9. The compression plate should be made of plastic, usually 1 to 4 mm thick.(ABSTRACT TRUNCATED AT 400 WORDS)     

PET performance measurements for an LSO-based combined PET/CT scanner using the National Electrical Manufacturers Association NU 2-2001 standard.

Results of performance measurements for a lutetium oxyorthosilicate (LSO)-based PET/CT scanner using new National Electrical Manufacturers Association (NEMA) NU 2-2001 standards are reported. METHODS: Performance measurements following the NU 2-2001 standards were performed on an LSO-based PET/CT scanner. In addition, issues associated with the application of the NEMA standard to LSO-based tomographs in the presence of intrinsic radiation are discussed. RESULTS: We report on some difficulties experienced in following the suggested NEMA measurement techniques and describe alternative approaches. Measurements with the new standard (as compared with NU-1994) incorporate the effects of activity outside the scanner and facilitate measurements of the entire axial field of view. Realistic clinical conditions are also simulated in image quality measurements of a torso phantom. CONCLUSION: We find that, with appropriate modifications, NU 2-2001 can be successfully applied to LSO-based scanners.     

Digital subtraction cerebral angiography by intraarterial injection: comparison with conventional angiography

For 4 months, a prototype digital subtraction system was used to obtain images of the cerebral vasculature after intraarterial contrast injections. In 12 instances, the intraarterial injections were recorded with both a digital subtraction unit and conventional direct magnification film-screen system. The digital subtraction and conventional film subtraction images were compared and graded for quality and information content by three skilled observers. In addition, quantitative measurements of contrast-detail performance and spatial resolution were obtained on both the digital system and the screen-film imaging chain. In a clinical setting, both the digital subtraction and conventional film-screen systems provided similar quality images and angiographic information. Contrast-detail curves demonstrated that digital subtraction angiography outperformed conventional film technique for low-contrast objects. Digital subtraction angiography also reduced the time required to obtain the angiogram, markedly reduced film cost, and lowered the contrast agent burden.     

Reduction of doses in teleradiography of the spine. Criteria of radiation protection and studies of dosimetry

To reduce the radiation exposure during full spine X-ray examinations, the following techniques were instituted: 72" tube to film distance; X-ray beam collimation; fast screen-film combination (rare earth gradual screens with high speed films); additional compensation filters; shielding of the most radiation-sensitive organs. Dosimetric measurements are reported. A very high reduction in exposure, above all of breasts and gonads, was obtained, without significant loss in the quality of radiographs.     

Image quality in mammography with special reference to anti-scatter grids and the magnification technique

Some of the parameters determining image quality in mammography are analyzed: the effects of primary photon spectra, focal spot size and screen-film systems on spatial resolution are discussed as are scattered radiation, development temperature and absorbed dose. The parameters limiting spatial resolution and contrast are evaluated for the standard and magnification techniques. Methods of reducing scattered radiation to improve contrast are evaluated. Scatter to primary ratios for different scatter reducing methods are compared, using the physical quantity energy imparted. For the standard technique the spatial resolution has been found to be limited by the fluorescent screen. With magnification technique the focal spot is the weakest link for the spatial resolution. The contrast is mainly set by the amount of scatter using the standard technique considering the use of a low tube potential (approximately 25 kVp). Using the magnification technique the amount of scatter is so small, that the tube potential is the limiting factor. We have found the optimized standard mammographic technique to be achieved under the following conditions: 25 kVp, 0.3 to 0.6 mm focal spot, film-focus distance 500 mm, anti-scatter grid, developing temperature 36 to 38 degrees C and 4 minutes total processing time with the screen-film system we have used. In magnification technique an air gap of at least 20 mm is desired. With an FFD of about 500 mm this will give a magnification ratio of 1.8 to 2.0 and a 0.1 mm X 0.1 mm focus spot is mandatory. With this technique, it is necessary to use a faster screen-film system than that used in standard mammography.     

Evaluation of radiographs developed by a new ultrarapid film processing system

The image quality of radiographs developed by a new ultrarapid processor was evaluated to determine if faster processing causes degradation in the image. The processor used was the Konica Super-Rapid SRX-501 model. Two films designed for this processor (Konica MGH-SR and MGL-SR) were processed in 45 sec and were compared with standard rapid processing in 90 sec of corresponding conventional films (Kodak TMG and OC). Rare-earth screens (Kodak Lanex Regular and Lanex Medium) used with the new and conventional films interleaved during angiographic studies or for phantom images were assessed for image quality. The basic imaging properties of the screen-film systems were examined by measuring (1) Hurter and Driffield curves, (2) modulation transfer functions by using the slit method, and (3) noise Wiener spectra. Subjective clinical assessment showed that the images obtained with ultrarapid processing were acceptable, with increased contrast and graininess. Hurter and Driffield curve measurements confirmed higher gradients. Modulation transfer function measurements were the same as for the conventional films. Noise Wiener spectrum measurements showed a 10% increase in noise for MGH-SR vs TMG film and a 30% increase for MGL-SR vs OC film. We conclude that acceptable image quality can be obtained using ultrarapid processing, with processing time approximately 60% that of conventional rapid processing. Potential applications include all areas in which rapid availability of the radiograph for interpretation is important. Although the processor studied was the first of its kind available, our evaluation indicates that the technology is available for a new class of ultrarapid processors.     

Collimation for imaging the myocardium.

Line-source response functions and modulation transfer functions (MTFs) were used to compare the resolution obtained with four myocardial-imaging agents: 129Cs, 43K, 13N-ammonia, and 81Rb contamination. When an Anger camera with pinhole collimator was used, the order of decreasing resolution was 129Cs, 43K, 13N, and 81Rb. Two techniques were employed to determine the extent to which spatial resolution could be improved. The first, involving the addition of lateral shielding, improved the MTFs for all the agents studied. The second, which utilized a subtraction mode, gave the best overall results. With the second method the MTF curves for 129Cs and 43K became very similar and were superior to both 81Rb and 13N. Both techniques were found useful in improving the spatial resolution of certain myocardial-imaging agents by reducing or eliminating the effects of penetration of the pinhole collimator walls by high-energy photons.     

Intrinsic uniformity requirements for pinhole SPECT

Pinhole SPECT is a fully 3-dimensional tomography technique. Uniformity requirements for gamma-cameras have been studied for 2-dimensional SPECT performed with parallel-hole collimators. This study investigated reconstruction artifacts in pinhole SPECT arising from intrinsic uniformity defects of the gamma-camera in the case of a pinhole aperture (5 mm) and a rotation radius (10 cm) suitable for human studies. METHODS: A cylindric phantom was filled with water and 99mTc. The count density in the pinhole SPECT projections largely exceeded the density that would be expected in human studies. Two-dimensional gaussian-shaped positive uniformity defects of various heights and various full widths at half maximum were simulated at 5 locations on the 64 projections. All these sets of projections were reconstructed using an iterative 3-dimensional ordered-subset expectation maximization (OSEM) algorithm tailored to the pinhole geometry. The influence of the number of OSEM iterations (2, 5, or 8 for 8 subsets) was also investigated. The height of the ring artifacts generated by the uniformity defects was measured on the reconstructed transverse slices and was compared with the noise in the noncorrupted slices. The uniformity defects were also generated on a 30-megacount flood image from the camera. These images were inspected visually, and the National Electrical Manufacturers Association (NEMA) differential and integral uniformities were calculated. RESULTS: Defects centered on the line corresponding to the orthogonal projection of the camera axis of rotation onto the detector plane generated artifacts whose magnitude-to-noise ratio exceeded 1% when the defect height was larger than 5%-10%, depending on the defect full width at half maximum. These defects were clearly visible on the slices and the flood images. Defects located elsewhere on the camera detector generated ring artifacts of magnitude-to-noise ratio smaller than 1%. They were visually observable both on the reconstructed slices and on the flood images for defect heights larger than 5%. The increase in the number of OSEM iterations conducted to a decrease in the artifact magnitude-to-noise ratio. CONCLUSION: Although hardly visible on the flood images and only slightly increasing the NEMA differential or integral uniformity, a detector uniformity defect of 3% height is able to generate a visible artifact on the reconstructed transverse slices. The study was conducted on a relatively high-count pinhole SPECT acquisition. Considering that far fewer counts would accumulate in clinical practice, any camera that fulfils these uniformity requirements should not lead to the presence of visible uniformity artifacts in the reconstructed slices.     

Evaluation of radiation dose, focal spot, and automatic exposure of newer film-screen mammography units

Fifteen recently introduced mammographic units representing eight different models and manufacturers were evaluated to see whether the equipment performance equaled claims made by the manufacturers. Focal-spot size was measured by pinhole and slit methods. Eight automatic exposure control systems on five models were tested by measuring the optical density produced by imaging at 28 and 30 kVp, with and without a grid and with test objects of varying thicknesses of BR12 epoxy material. Comparable mean glandular dose for a 4-cm-thick BR12 test object was measured for 13 of the units under manual or automatic exposures. Nominal focal-spot size was larger than stated for six of the 15 large focal spots and for seven of the 12 microfocal spots. Comparable doses under automatic exposure control varied more than 100% between units from different manufacturers, while the doses of units from the same manufacturer varied less than 20%. Automatic exposure control systems were operational in eight of the 15 units, but four of these eight were found to be out of calibration. There are considerable performance variations that need to be evaluated before acceptance and use of new mammographic equipment.     

Digital chest radiography with a solid-state flat-panel x-ray detector: contrast-detail evaluation with processed images printed on film hard copy

PURPOSE: To evaluate and compare human observer performance in a contrast-detail test by using postprocessed hard-copy images from a digital chest radiography system and conventional screen-film radiographs. MATERIALS AND METHODS: The digital radiography system is based on a large-area flat-panel x-ray detector with a structured cesium iodide scintillator layer and an amorphous silicon thin-film transistor array for image readout. Images of a contrast-detail phantom were acquired at two exposure levels by using two standard thoracic screen-film systems and the digital system at matched dose. By using images of the phantom processed with standard chest image postprocessing techniques, a four-alternative forced-choice observer perception study was performed, and the number of detectable test signals (disk-shaped objects 0.3-4.0 mm in diameter) was determined for each image type. RESULTS: On average, observers detected more test signals on digital images than on screen-film radiographs at all diameters up to 2.0 mm and an equivalent number at larger diameters. Test signals with lower inherent subject contrast were detected more readily on digital images than on screen-film images, even when x-ray exposure levels for the digital system were reduced by 20%. CONCLUSION: Observer performance in a contrast-detail detection task can be improved by using images acquired with the flat-panel digital chest radiography system as compared with those acquired with state-of-the-art screen-film combinations.     

A crystal camera for ultra-small-angle x-ray scattering using synchrotron radiation

We describe a novel USAXS camera that combines the use of synchrotron radiation with collimation by perfect-crystal optics. The outstanding result is that high measuring intensities and extreme angular resolution are achieved even with a point-focusing geometry. Along the principles of the original design (U. Bonse and M. Hart, Z. Phys. 189, 151 (1966)) which had to be operated at an x-ray tube, we employ two sets of pairs of multiply reflecting channel-cut crystals diffracting in the horizontal and vertical planes. The collimation characteristics thus obtained are equivalent to the point-focusing geometry of conventional SAXS cameras based on slit collimation. We present results from samples of polystyrene spheres which were used for test measurements performed with synchrotron radiation of DORIS at HASYLAB/DESY in Hamburg. Taking into account the number of reflections within the channel-cut crystals, the theoretical resolution was calculated and found to agree well with that derived from measured scattering patterns. Structures as large as about 1.3 μm could easily be identified from the scattering curves. As expected with point-focusing geometry, desmearing of raw data was unnecessary.     

Digital radiography of the chest: design features and considerations for a prototype unit

The general features of a prototype digital chest unit are described along with the rationale for the choice of design factors employed. It is shown that the scanning-slit, linear-detector-array approach employed can, with available x-ray tube technology, achieve a spatial resolution of 1 cy/mm and detector radiation levels comparable with those obtained with conventional screen-film systems. Also discussed are the unit's exposure latitude and its ability virtually to eliminate scatter.     

Three-pinhole collimator to improve axial spatial resolution and sensitivity in pinhole SPECT

Purpose The aim of this study was to improve the uniformity of the axial spatial resolution and sensitivity in pinhole single-photon emission computed tomography (SPECT) and to extend the axial field of view, by using a dedicated three-pinhole collimator. Methods A rectangular tungsten plate with three pinhole apertures of 1.5 mm diameter was designed to image a cylindrical field of view of 55 mm diameter and 160 mm axial length using a clinical gamma camera. To evaluate the non-uniformity of spatial resolution and noise, a multiple-disk phantom was built. The phantom was filled with Tc-99m, and data were acquired using a circular orbit and reconstructed with a dedicated iterative reconstruction algorithm. The axial spatial resolution together with the noise was measured in each disk. These measurements were compared to a single-pinhole system using an identical acquisition geometry and reconstruction. Results At the central slice, a spatial resolution of 2.7 mm was observed for both the three-pinhole and single-pinhole geometries. At 17.5 mm from the central slice, the axial spatial resolution deteriorated to 10.3 mm when using a single pinhole, while the spatial resolution remained 2.7 mm for the three-pinhole system. In the central slice, 19% noise was observed for both geometries. At 31.5 mm from this central slice, the noise remained 19% for the three-pinhole geometry, while it increased to 32% using a single pinhole. Conclusion The presented three-pinhole collimator improves the uniformity of the axial spatial resolution and sensitivity in pinhole SPECT and consequently extends the axial field of view, a requirement for whole-body small-animal imaging.     

Prospects for x-ray exposure reduction using rare earth intensifying screens.

From measurements of screen-film sensitometry, Wiener spectra, and x-ray spectra, the speed, mottle, and quantum efficiency of several rare earth screen-film systems have been characterized. These systems offer exposure reductions of from two- to tenfold over conventional calcium tungstate systems. At the slower end, the noise (mottle) and resolution are comparable to what is available with conventional systems. Moreover, films designed to be used in such systems can be made with significantly lower silver content. As the speed advantage is increased from two- to genfold, however, the mottle also increases.