Physical and clinical evaluation of new high-strip-density radiographic grids

The imaging performance of new high-strip-density (HSD) grids having 57 lines/cm was compared with that of conventional low-strip-density (LSD) grids having 33 or 40 lines/cm. The unique advantage of HSD grids is that, under most standard radiographic conditions, the grid lines are not noticeable on the final image, even if the grid is stationary. This is due to the combined effect of the high fundamental spatial frequency of HSD grids, the modulation transfer function of screen-film systems and of the human visual system, and scattered radiation. Monte Carlo simulation studies, phantom images, and clinical evaluation indicate that HSD grids can provide contrast improvement factors and Bucky factors that are comparable to or slightly better than those obtained with LSD grids. Therefore, it may now be possible to eliminate moving Bucky trays from radiographic tables and fluoroscopic devices.     

Mammography using stationary grids. Qualitative and dosimetric considerations

The authors report their experience with ultra-high-strip-density (UHSD) stationary grids in mammography, based upon dosimetric measurements carried out in a phantom and mammographic examination of 62 patients. Utilization of UHSD grids made it necessary an increase in radiation exposure by a factor 3.5 in a breast 5 cm thick 50% water and 50% fat by weight. Improvement in image-quality is apparent with voltages not exceeding 30 kV, particularly in cases of questionable masses within areas of locally increased breast density. The authors believe that UHSD grids may be useful in selected cases, such as in the performance of adjunctive whole-breast or locally aimed radiograms and in the radiological examination of surgical specimens.     

Scatter dose calculation for anti-scatter linear grids in mammography

Monte Carlo simulations were used to optimize the geometry of a mammography anti-scatter linear grid to achieve minimum scatter-to-primary ratio (SPR) for different X-ray tube voltages. A single optimum design of the grid with 0.9 mm septa height, 12 μm septa thickness and 100 μm interspace thickness was found for breast phantom thicknesses between 30 and 80 mm. The optimal grid has 0.153–0.330 scatter-to-primary ratio, a Bucky factor (BF) less than 2.5 and a contrast improvement factor (CIF) of 1.3.     

Grid and slot scan scatter reduction in mammography: comparison by using Monte Carlo techniques

PURPOSE: To evaluate a comprehensive array of scatter cleanup techniques in mammography by using a consistent methodology. MATERIALS AND METHODS: Monte Carlo techniques were used to evaluate the Bucky factor (BF) and the contrast improvement factor (CIF) for linear and cellular grids and for slot scan and scanning multiple-slot assembly (SMSA) systems. RESULTS: For a 28-kVp molybdenum anode-molybdenum filter spectrum with a standard detector and a 6-cm-thick 50% adipose-50% glandular breast phantom, slot scan techniques delivered an ideal BF. For slot widths greater than 4 mm, however, the CIF was lower than that achieved by the high-transmission cellular grid with a grid ratio of 3.8:1. A tungsten-septa air-interspaced cellular grid with a 4:1 grid ratio outperformed the high-transmission cellular grid in both BF and CIF. The SMSA was shown to be efficacious when 4-mm-wide slots were separated by at least 20 mm. In comparison with the literature, 3.6% agreement was achieved with other Monte Carlo studies, 3.3% with an experimental study that used a digital detector, and 13%-29% agreement was demonstrated in comparison to film-based experimental studies. CONCLUSION: With use of consistent methods for comparison, cellular grids were shown to substantially outperform linear grids but have slightly higher BFs compared with that of slot scan geometries at the same CIF.     

A comparison of image quality and radiation dose between a cellular grid and a linear grid using a mammography phantom

The aim of this study is to establish whether a cellular grid can offer any improvement over a linear grid in terms image quality and radiation dose for a single analogue mammography system.Using a prospective experimental design, a range of exposure factors and Perspex thicknesses were used to simulate conditions found in clinical practice. A Contrast Detail Mammography (CDMAM) test phantom was utilized to produce Image Quality Factor (IQF) and Mean Glandular Dose (MGD) data sets.An identical range of exposure factors/phantom thicknesses were selected for both grids and a direct comparison was made of the calculated IQF and the MGD for each pair of factors.The cellular grid produced a higher IQF for every exposure technique tested, when compared to the linear grid (increase: 1.2% to 35.8%, mean 15.8%, p < 0.005). MGD values with the cellular grid were also minimally increased for the majority of exposure techniques, (increase: ?2.4% to 7.9%, mean 3.3%, p < 0.01).In this phantom study, use of the cellular grid resulted in increased IQF with an acceptable increase in MGD under all exposure conditions tested with the largest improvement in IQF seen in the extremes of phantom thickness. This suggests that use of a cellular grid in clinical practice might help improve cancer detection rates, without breaching current radiation dose guidelines for accepted dose levels and be particularly beneficial when examining both very small and very large breasts.     

Mammographic equipment, technique, and quality control

The most important improvements in mammographic technique were the introduction of single- or double-emulsion high-contrast film-screen combinations for mammography, the use of a specially designed low-kilovoltage Bucky grid to reduce scattered radiation, and the introduction of smaller focal spots to improve imaging geometry. Magnification techniques, especially the spot-film technique, yields clearer delineation of high-contrast microcalcifications. Dedicated mammographic equipment with specially designed x-ray tubes is necessary for modern high-quality mammography. However, in many modern mammographic units, the automatic exposure controller still fails to provide appropriate and constant optical film density over a wide range of tissue thickness and absorption. Extended-cycle processing of single-emulsion mammographic films can yield better image contrast and reduce exposure by up to 30%. Exposure times of less than 1 second are recommended to avoid the unnecessary higher doses caused by longer exposure times and reciprocity law failure. The wide dynamic range in mammography can be reduced by a beam equalization filter, and thus be better adapted to the decreased latitude of modern high-contrast mammographic screen-film systems. Mammographic film reading (detection of subtle microcalcifications) can be facilitated by modern computer evaluation of previously digitized mammograms. Standardization and assurance of image quality have been major challenges in the technical development of mammography. Different technical and anthropomorphic phantoms have been designed to measure and compare practical image quality. Detailed quality control measures have been developed. The benefit of a single or annual screening mammography, calculated in gained life expectancy, by far outweighs the relative risk for radiation-induced breast cancer.     

Evaluating the use of anti-scatter grids in adult knee radiography

IntroductionAnti-scatter grids efficiently reduce scatter radiation from reaching the imaging receptor, enhancing image quality; however, the patient radiation dose increases in the process. There is disagreement regarding the thickness thresholds for which anti-scatter grids are beneficial. This study aims to establish a thickness threshold for the use of anti-scatter grids to optimise adult knee radiography.MethodsThe study consisted of two phases. In Phase 1 phantom knee radiographs were acquired at varying thicknesses (10–16 cm) and tube voltages (60–80 kV). For each thickness and tube voltage, images with and without an anti-scatter grid were obtained. In Phase 2, two radiologists and three radiographers, evaluated the image quality of these images. Visual Grading Analysis (VGA) scores were analysed using Visual Grading Characteristics (VGC) based on the visualisation of five anatomic criteria.ResultsThe average DAP decreased by 72.1% and mAs by 73.1% when removing the anti-scatter grid. The VGC revealed that overall images taken with an anti-scatter grid have better image quality (AUC ≥0.5 for all comparisons). However, the anti-scatter grids could be removed for thicknesses 10, 12 and 14 cm in conjunction with using 80 kVp,.ConclusionAnti-scatter grids can be removed when imaging adult knees between 10 and 12 cm using any kVp setting since the radiation dose is reduced without significantly affecting image quality. For thicknesses >12 cm, the use of anti-scatter grids significantly improves image quality; however, the radiation dose to the patient is increased. The exception is at 14 cm used with 80 kVp, where changes in image quality were insignificant.Implications for practiceOptimisation by removing anti-scatter grids in adult knee radiography seems beneficial below 12 cm thickness with any kVp value. Since the average knee thickness ranges between 10 and 13 cm, anti-scatter grid can be removed for most patients. Nevertheless, further studies are recommended to test if this phantom-based threshold applies to human subjects.     

High-contrast mammography with a moving grid: assessment of clinical utility

Mammography techniques using moving grids produce superior breast images in many patients but result in increased radiation dose. This prospective controlled study of 1000 unselected screen-film mammography patients identifies a subset of women who are most likely to benefit from higher-dose grid-assisted techniques. In approximately 60% of the patients, the increased contrast of grid films produced a noticeable improvement in overall image quality. In only 20% of cases did this translate into clinically useful information, however, usually resulting in an increased level of confidence in interpretation. In virtually all the cases in which grid images aided mammographic diagnosis, the patients were women having more than 50% dense fibroglandular tissue or compressed breast thickness greater than 6 cm (only 37% of the study population). We suggest that the use of grid techniques be restricted to patients with such dense or thick breasts, because only in these women can the increase in radiation dose be justified.     

Investigating the use of an antiscatter grid in chest radiography for average adults with a computed radiography imaging system

Objective:

The aim of this study was to investigate via simulation a proposed change to clinical practice for chest radiography. The validity of using a scatter rejection grid across the diagnostic energy range (60–125 kVp), in conjunction with appropriate tube current–time product (mAs) for imaging with a computed radiography (CR) system was investigated.

Methods:

A digitally reconstructed radiograph algorithm was used, which was capable of simulating CR chest radiographs with various tube voltages, receptor doses and scatter rejection methods. Four experienced image evaluators graded images with a grid (n = 80) at tube voltages across the diagnostic energy range and varying detector air kermas. These were scored against corresponding images reconstructed without a grid, as per current clinical protocol.

Results:

For all patients, diagnostic image quality improved with the use of a grid, without the need to increase tube mAs (and therefore patient dose), irrespective of the tube voltage used. Increasing tube mAs by an amount determined by the Bucky factor made little difference to image quality.

Conclusion:

A virtual clinical trial has been performed with simulated chest CR images. Results indicate that the use of a grid improves diagnostic image quality for average adults, without the need to increase tube mAs, even at low tube voltages.

Advances in knowledge:

Validated with images containing realistic anatomical noise, it is possible to improve image quality by utilizing grids for chest radiography with CR systems without increasing patient exposure. Increasing tube mAs by an amount determined by the Bucky factor is not justified.Radiography of the chest is one of the most frequently performed diagnostic radiographic examinations in the UK. In 2010, the Health Protection Agency (now Public Health England) reported¹ that chest radiographs represented 19.6% of all radiographic examinations in 2008 (although the contribution to collective dose was small at about 0.5%), so optimization of radiation dose (i.e. ensuring dose is as low as reasonably practicable) and image quality (i.e. ensuring all required clinical structures are visible to the reporting healthcare professional so that an acceptable diagnosis is possible) in chest radiography is an important research area, especially since digital imaging has all but replaced its film-screen counterpart. It is also a legal requirement in the UK under the Ionising Radiation (Medical Exposure) Regulations 2000² to optimize all medical exposures, consistent with the intended purpose.One such technique to optimize image quality in chest radiography is to use a scatter rejection grid. They work by preferentially removing radiation scattered by the body prior to reaching the detector, and their improvement of image quality in film-screen imaging has been recognized for decades.^3–6 This improvement was described by the contrast improvement factor⁷ but came with a cost. Film requires a given level of incident exposure to ensure adequate optical density (OD), and because a grid attenuates most of the scattered radiation (as well as some primary radiation), this necessitates an increase in tube current–time product (mAs). The Bucky factor describes the necessary multiplication by which exposure parameters must be increased, and for film-screen can be anything between 2–6 times the “non-gridded” exposure.^8,9 Regardless of this, there are clear guidelines recommending the use of scatter rejection grids with film-screen systems for adult chest radiography,¹⁰ but none for digital imaging modalities, although Fritz and Jones¹¹ have recently published guidelines for scatter rejection techniques in paediatric digital radiology.Digital image detectors, such as computed radiography (CR) photostimulable powder phosphors, have a larger dynamic range than does film,¹² and grey levels in the resulting image are usually adjusted, irrespective of incident detector dose, to match the output of the display monitor. Therefore, unlike film, digital imaging is not contrast (OD) limited. At doses used clinically (e.g. air kerma of approximately 2–15 µGy at the receptor), digital images are dominated by quantum noise (i.e. other noise sources such as electronic and structural are typically ≤2% of the total noise as consistently demonstrated through in-house routine quality assurance testing of this CR system), which depends on the level of air kerma incident on the detector (signal) and the detector''s detective quantum efficiency. Therefore, when a scatter rejection grid is used, appropriate exposure factors (tube''s peak kilo-voltage and/or mAs) are required to maintain a level of image signal-to-noise ratio (SNR)^13,14 acceptable to the image evaluator, although there is no agreement as to what these appropriate exposure factors should be. A common school of thought, described in a considerable resource by Carlton and Adler,¹⁵ suggests that the lower limit on the increase in mAs should be the reciprocal of the primary transmission (T_p) of the grid and the upper limit, the Bucky factor. The reciprocal of T_p for modern grids is typically 1.2–1.4, which suggests that an increase in mAs of at least 20% is required. However, Tanaka et al¹⁶ have recently demonstrated that the use of grids (grid ratios 5 : 1 to 14 : 1) without increasing exposure factors (compared with “non-gridded” exposures), actually improved the effective noise equivalent quanta (eNEQ) when acquiring images of 20 cm of polymethylmethacrylate (PMMA). They concluded that the improvement to image quality owing to removal of scatter outweighs the increase in quantum noise when a grid is used, although they acknowledged that their work did not use any images with anatomical structure. Similarly, Fetterly and Schueler¹⁷ studied numerous grids with different thicknesses of uniform solid water and suggested that a scatter rejection grid can provide improvement in SNR without increasing exposure for large patients.Given the scarcity of evidence/guidelines in the literature, the aim of this study was two-fold: firstly, to investigate the validity of using a scatter rejection grid for chest radiography of average adults with an Agfa CR imaging system (Agfa, Peissenberg, Germany) across the diagnostic energy range (60–125 kVp); and secondly, to investigate appropriate tube mAs to identify what increase in patient dose (if any) there needs to be. Many recent studies have demonstrated that anatomical noise (the influence of projected anatomy on the image evaluator’s ability to detect potential abnormalities and provide an accurate diagnosis) is the limiting factor in chest radiography,^18–27 so it was felt that the inclusion of realistic anatomy in the images used in this study was of particular importance, rather than using uniform PMMA or solid water only. Therefore, computer-simulated chest radiographs (each containing realistic projected anatomical noise and lung abnormalities/nodules), reconstructed with a scatter rejection grid, were compared by expert image evaluators with images reconstructed without a grid (as per current clinical protocol in our radiology department).     

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.     

Quality assurance in mammography: artifact analysis.

Evaluation of mammograms for artifacts is essential for mammographic quality assurance. A variety of mammographic artifacts (i.e., variations in mammographic density not caused by true attenuation differences) can occur and can create pseudolesions or mask true abnormalities. Many artifacts are readily identified, whereas others present a true diagnostic challenge. Factors that create artifacts may be related to the processor (eg, static, dirt or excessive developer buildup on the rollers, excessive roller pressure, damp film, scrapes and scratches, incomplete fixing, power failure, contaminated developer), the technologist (eg, improper film handling and loading, improper use of the mammography unit and related equipment, positioning and darkroom errors), the mammography unit (eg, failure of the collimation mirror to rotate, grid inhomogeneity, failure of the reciprocating grid to move, material in the tube housing, compression failure, improper alignment of the compression paddle with the Bucky tray, defective compression paddle), or the patient (e.g., motion, superimposed objects or substances [jewelry, body parts, clothing, hair, implanted medical devices, foreign bodies, substances on the skin]). Familiarity with the broad range of artifacts and the measures required to eliminate them is vital. Careful attention to darkroom cleanliness, care in film handling, regularly scheduled processor maintenance and chemical replenishment, daily quality assurance activities, and careful attention to detail during patient positioning and mammography can reduce or eliminate most mammographic artifacts.     

Mammography in the eighties

Mammography has experienced the greatest change of any existing radiologic examination in recent years. In 1985, as a part of the Nationwide Evaluation of X-Ray Trends (NEXT) program, a national survey was conducted of a statistically selected sample (n = 232) of facilities performing mammography examinations in the United States. By 1988, the number of mammography facilities in the United States had increased to over 6,400, an increase of over 60% from the 1985 level. To assess the consequence of this expansion as well as the impact of recent technological and other significant developments on mammography, a NEXT survey of mammography facilities was repeated in 1988 (n = 226). Screen-film mammography accounted for 83% of the facilities surveyed in 1988, and dedicated equipment dominated screen-film systems (99%). There was a 26% increase in the overall mean phantom image score, over 45% increase in the use of grids, and 10% increase in mean glandular dose for systems using grids.     

Evaluation of scatter rejection system using a metal filter

Grids are used for the purpose of reducing scattered radiation. In portable radiography, however, accurate positioning of the grid is difficult. Errors in alignment may cut off the primary beam, which can result in misdiagnosis. We devised a metal filter with scatter rejection properties and evaluated its performance. The filter, which has no intrinsic alignment mechanism, is placed in front of the IP. We evaluated aluminum (Al), copper (Cu), and tantalum (Ta) filters by comparing them with low-ratio grids of 3:1 and 5:1. In the total evaluation, Ta (0.03mm) demonstrated high visualization of light and minute vascular shadow and visceral pleura, and the Bucky factor was lower than that of the 3:1 grid, while the clinical target was very clear. Because of its high atomic number, Ta can absorb a low energy component effectively even if it is very thin. Because of the K absorption edge, Ta also decreases the high-energy components that cause photographic contrast to decline. Therefore, Ta proved to be a highly suitable material for this research. An air gap within 4cm was not effective for the purpose of supporting the reduction in scattered radiation of the metal filter. The method of placing a metal filter in contact with the patient is more practical and makes the system very thin. This system is thought to be effective for portable radiography because it is light, easy to use, and flexible in structure and does not cause misalignment.     

The effect of x-ray spectra from molybdenum and tungsten target tubes on image quality in mammography.

The measured x-ray spectra from a molybdenum anode Senographe mammography unit and a tungsten anode unit were used to calculate the x-ray energy spectra transmitted through various thicknesses of fat and water (breast-equivalent materials). The dependence of subject contrast and patient exposure on (a) x-ray spectra, (b) attenuation properties of two breast-equivalent materials, and (c) thickness of breast-equivalent material was predicted. Radiographs of resected breast tissue confirmed these predictions and demonstrated the general relative effects of x-ray beam quality on image contrast in mammography.     

Evaluation of image quality of breast phantom radiographs by fuzzy measure theory: cross grid and single grid

Our study was carried out to compare and evaluate two types of grids (single and cross types) for the removal of scattered X-rays exclusive to mammography in an ACR-specified 156 model phantom using fuzzy measure theory (fuzzy measure and its fuzzy integration). When three simulated shadows of the breast phantom (fibrous, calcified, and tumor) were integrated, the cross grid (Gc) showed a slightly higher evaluation value than the single grid (Gs). In addition, when two shadows were combined or each shadow was alone, the Gc was evaluated as 2% or 3% higher. It is difficult to determine the physical properties of a grid for removal of scattered X-rays exclusive to the breast considering the structure of equipment, but if a visual and subjective evaluation is quantitatively conducted by applying fuzzy kinetics, comparison and evaluation can be carried out.     

Investigation of the effect of anode/filter materials on the dose and image quality of a digital mammography system based on an amorphous selenium flat panel detector

A comparison, in terms of image quality and glandular breast dose, was carried out between two similar digital mammography systems using amorphous selenium flat panel detectors. The two digital mammography systems currently available from Lorad-Hologic were compared. The original system utilises Mo/Mo and Mo/Rh as target/filter combinations, while the new system uses W/Rh and W/Ag. Images of multiple mammography phantoms with simulated compressed breast thicknesses of 4 cm, 5 cm and 6 cm and various glandular tissue equivalency were acquired under different spectral conditions. The contrast of five details, corresponding to five glandular compositions, was calculated and the ratio of the square of the contrast-to-noise ratio to the average glandular dose was used as a figure-of-merit (FOM) to compare results. For each phantom thickness and target/filter combination, there is an optimum voltage that maximises the FOM. Results show that the W/Rh combination is the best choice for all the detection tasks studied, but for thicknesses greater than 6 cm the W/Ag combination would probably be the best choice. In addition, the new system with W filter presents a better optimisation of the automatic exposure control in comparison with the original system with Mo filter.Over the past decade, several digital mammography systems based on different detector technologies have become available and the process of optimisation of digital systems has been developing in parallel with the adoption of those systems. On the one hand, this process involves the optimisation of digital detectors, but, on the other hand, it involves the optimisation of the X-ray sources and exposure factors with regards to the minimum radiation dose required to maintain the highest possible image quality. In conventional mammography this optimisation has mostly been achieved using the anode/filter combination Mo/Mo, but it may be questioned whether it is also optimal for digital mammography.In the past few years, several studies and simulation works have been carried out in order to investigate the factors that affect choice of X-ray spectra for mammography.Dance et al [1] in their Monte Carlo study concluded that in digital mammography with a gadolinium oxysulphide detector, the standard Mo/Mo combination is superior only for 2 cm compressed breasts. For all other compressed breast thicknesses and glandularities, each of the alternative anode/filter combinations (Mo/Rh, W/Rh, Rh/Rh and Rh/Al) can offer a lower dose for the same contrast-to-noise ratio (CNR). In particular, for compressed breast thicknesses of 4 cm to 6 cm, W/Rh is recommended.In a previous work, Andre and Spivey [2] developed a parametric model for digital mammography to evaluate optimisation of X-ray spectra for a particular sensor. The model computes spectra and average glandular doses (AGD) for combinations of W target, beam filters (Al, Sn, Rh, Mo and Ag), kVp, breast type and thickness. On the basis of their results, the authors suggest the use of Mo filter for 30 mm breast, Ag filter for 45 mm, Sn filter for 60 mm and Al filter for 75 mm breast thicknesses.Fahrig and Yaffe [3, 4], in their simulation study, demonstrated that using a digital detector based on Gd₂O₂S scintillator, a W target is preferable to a Mo target for the detection of infiltrating ductal carcinoma and calcifications.In the work of Flynn and colleagues [5], the radiographic process for a digital amorphous selenium (aSe) mammography system was modelled. The optimal CNR relative to dose was determined for several target/filter combinations, for a wide range of kVp values, and for varying breast thickness. The target/filter combinations included Mo/Mo, Mo/Rh, Rh/Rh, W/Al, W/Mo, W/Ag and W/Sn. Results show that when breast thickness increased, the use of a W target with a Sn filter resulted in a 34% improvement in CNR for the same dose to the breast compared with the use of a Mo target with a Mo filter.In their recent work, Bernhardt et al [6] related the image quality and patient risk through simulations and phantom studies regarding the detection of microcalcifications and tumours for different breast thickness and breast compositions, anode/filter combinations, different filter thicknesses and different tube voltages. They found that, for an aSe detector, the W/Rh target/filter combination is the best choice for all breast thicknesses and composition and for the detection of both microcalcifications and tumours.Similar results were found by Toroi et al [7], who measured the CNR between aluminium sheets and homogeneous background for various radiation qualities and breast thicknesses to determine the optimal radiation quality when using an aSe detector based system. By using a W anode in combination with an Rh filter, they achieved the same CNR with a significantly lower dose than when using Mo anode with an Mo or Rh filter. This result is valid for all breast thicknesses, but is most significant for the thickest breasts.In a recent paper Williams et al [8] compared, for each of the major commercially available full-field digital mammography (FFDM) systems, the impact of the selection of exposure factors on image quality and dose for a range of breast thickness and tissue types. They used the figure-of-merit (FOM) (signal-to noise ratio²/mean glandular dose (MGD)) to compare the technique factors of all available target/filter combinations. A key finding of their study was the better performance of W/Rh combinations, as compared with Mo/Mo and Mo/Rh combinations, using the same aSe detector.Given these and other similar published results, manufacturers have introduced mammography units with different anode/filter combinations, such as Mo/Rh, Rh/Rh, W/Rh and W/Ag, and the process of optimisation is on-going.In particular, Lorad-Hologic (Hologic Inc., Danbury, CT) has recently introduced to the market a new Selenia digital mammography system. The new system is very similar to the previous system and differs primarily in respect of the target/filter combinations available – it consists of an X-ray tube with W anode material combined with Rh and Ag filter materials.The aims of this study were twofold. One goal was to establish optimum spectra for breasts of different thicknesses and compositions imaged with both the original and the new Lorad system. In addition, we aimed to compare the two generations of Lorad system in terms of X-ray spectra to the optimum.     

Contrast and scatter in x-ray imaging

The effect of scattered radiation on x-ray image contrast is reviewed. Without scatter control, the information content of x-ray images is severely compromised. Typical grid performance in general radiography and mammography and grid selection considerations are presented. Contrast can be significantly improved and patient dose reduced if more efficient methods of scatter control can be developed. This can be accomplished in chest radiography with scanning slit and improved grid techniques, in mammography with multiple scanning slit techniques, and in abdominal radiography with scanning grid techniques.     

Influence of anode/filter material and tube potential on contrast, signal-to-noise ratio and average absorbed dose in mammography: a Monte Carlo study

The comparative performance of mammographic X-ray systems that use different anode/filter combinations has been assessed for screen-film and digital imaging. Monte Carlo techniques have been used to calculate average glandular dose as well as contrast and signal-to-noise ratio for imaging two test details. Five anode/filter combinations have been studied to establish the potential for dose saving or image quality improvement. For screen-film mammography, it was found that little benefit is gained by changing from a standard 28 kV molybdenum/molybdenum spectrum for breasts up to 6 cm thick. For thicker breasts, where the tube potential for the standard technique might be increased, 20% improvement in contrast can be achieved without dose penalty using molybdenum/rhodium or rhodium/rhodium spectra, whereas dose savings of more than 50% can be attained whilst maintaining contrast using tungsten/rhodium or rhodium/aluminium spectra. In digital mammography, a molybdenum/molybdenum spectrum delivers the lowest dose for a 2 cm breast, but gives the highest dose for thicker breasts. Tungsten/rhodium or rhodium/aluminium spectra provide the lowest doses at greater thicknesses. It is concluded that for screen-film mammography, molybdenum/molybdenum is the spectrum of choice for all but the thickest or most glandular breasts. In digital mammography, an alternative spectrum is preferable for breasts thicker than 2 cm.     

The intensity of scattered radiation in mammography

The ratio of scattered-to-primary radiation has been measured for a range of x-ray tube voltages, field sizes and phantom thicknesses that typify clinical mammographic situations. The relative intensity of scattered radiation measured was essentially independent of kVp but increased as the phantom thickness and radiation field size increased. For the range of field sizes and phantom thicknesses that typify clinical situations the intensity of scattered radiation varied from about 40 to 85% of the primary beam intensity indicating that only from about 54 to 71% of the primary beam contrast is imaged in mammography.     

Detection of simulated microcalcifications in a phantom with digital mammography: effect of pixel size

PURPOSE: To evaluate the effect of pixel size on the detection of simulated microcalcifications in a phantom with digital mammography. MATERIALS AND METHODS: A high-spatial-resolution prototype imager that yields variable pixel size (39 and 78 microm) and a clinical full-field digital mammography (FFDM) system that yields a 100-microm pixel size were used. Radiographic images of a contrast-detail (CD) phantom were obtained to perform four-alternative forced-choice observer experiments. Polymethylmethacrylate was added to obtain phantom thicknesses of 45 and 58 mm, which are typical breast thicknesses encountered in mammography. Phantom images were acquired with both systems under nearly identical exposure conditions by using an antiscatter grid. Twelve images were acquired for each phantom thickness and pixel size (for a total of 72 images), and six observers participated in this study. Observer responses were used to compute the fraction of correctly detected disks. A signal detection model was used to fit the recorded data from which CD characteristics were obtained. Repeated-measures analyses with mixed-effects linear models were performed for each of the six observers. All statistical tests were two sided and unadjusted for multiple comparisons. A P value of .05 or less was considered to indicate a significant difference. RESULTS: Statistical analysis revealed significantly better CD characteristics with 39- and 78-microm pixel sizes compared with 100-microm pixel size for all disk diameters and phantom thicknesses (P<.001). Increase in phantom thickness degraded CD characteristics regardless of pixel size (P<.001). CONCLUSION: On the basis of the conditions of this study, reducing pixel size below 100 mum with low imaging system noise enhances the visual perception of small objects that correspond to typical microcalcifications.