A comparison of the performance of digital mammography systems

An objective analysis of image quality parameters was performed for six digital mammography systems. The presampled modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE) for the systems were determined at different doses, for 28 kVp with a Mo/Mo or W/Al target/filter combination and 2 mm of additional aluminium filtration. The flat-panel units have higher MTF and DQE in the mid to high frequency range than standard CR systems. The highest DQE, over the whole dose range, is for the slit-scanning direct photon counting system. Dual-side read CR can overcome the inherent x-ray absorption and signal collection limitations of standard CR mammography, improving the low-frequency DQE by 40%, to the same level as full-field systems, but it does not improve the poor spatial resolution of phosphor.     

Maximum likelihood fitting of FROC curves under an initial-detection-and-candidate-analysis model

We have developed a model for FROC curve fitting that relates the observer's FROC performance not to the ROC performance that would be obtained if the observer's responses were scored on a per image basis, but rather to a hypothesized ROC performance that the observer would obtain in the task of classifying a set of "candidate detections" as positive or negative. We adopt the assumptions of the Bunch FROC model, namely that the observer's detections are all mutually independent, as well as assumptions qualitatively similar to, but different in nature from, those made by Chakraborty in his AFROC scoring methodology. Under the assumptions of our model, we show that the observer's FROC performance is a linearly scaled version of the candidate analysis ROC curve, where the scaling factors are just given by the FROC operating point coordinates for detecting initial candidates. Further, we show that the likelihood function of the model parameters given observational data takes on a simple form, and we develop a maximum likelihood method for fitting a FROC curve to this data. FROC and AFROC curves are produced for computer vision observer datasets and compared with the results of the AFROC scoring method. Although developed primarily with computer vision schemes in mind, we hope that the methodology presented here will prove worthy of further study in other applications as well.     

A comparison of the performance of modern screen-film and digital mammography systems

This work compares the detector performance and image quality of the new Kodak Min-R EV mammography screen-film system with the Fuji CR Profect detector and with other current mammography screen-film systems from Agfa, Fuji and Kodak. Basic image quality parameters (MTF, NPS, NEQ and DQE) were evaluated for a 28 kV Mo/Mo (HVL = 0.646 mm Al) beam using different mAs exposure settings. Compared with other screen-film systems, the new Kodak Min-R EV detector has the highest contrast and a low intrinsic noise level, giving better NEQ and DQE results, especially at high optical density. Thus, the properties of the new mammography film approach those of a fine mammography detector, especially at low frequency range. Screen-film systems provide the best resolution. The presampling MTF of the digital detector has a value of 15% at the Nyquist frequency and, due to the spread size of the laser beam, the use of a smaller pixel size would not permit a significant improvement of the detector resolution. The dual collection reading technology increases significantly the low frequency DQE of the Fuji CR system that can at present compete with the most efficient mammography screen-film systems.     

Performance comparison of full-field digital mammography to screen-film mammography in clinical practice

Results of acceptance testing 18 full-field digital mammography systems for clinical use and of conducting annual physics surveys of 38 screen-film mammography systems were compared in terms of exposure times, mean glandular breast doses, and image quality. These evaluations were made using the same test tools on all systems, with emphasis on assessing automatic exposure control performance and image quality on both digital and screen-film systems using clinical techniques. Survey results indicated that digital mammography systems performed similarly to screen-film systems in terms of exposure times and mean glandular doses for thin to intermediate breasts, but that digital mammography systems selected shorter exposure times and lower mean glandular doses for thicker breasts. For all breast thicknesses, digital mammography systems yielded mean contrast-detail scores higher than those for screen-film systems. For all breast thicknesses, the 18 digital mammography systems demonstrated less variance in terms of exposure times, mean glandular doses, and contrast-detail scores than did the 38 screen-film systems tested. These results indicate that the clinical use of digital mammography may generally improve image quality for equal or lower breast doses, while providing tighter control on exposures and image quality than screen-film mammography.     

Film-screen mammography: comparison of views

The authors performed oblique, mediolateral, and cephalocaudal film-screen mammographic views for all 9,662 patients examined at the UCLA Medical Center from January 1, 1980 to December 31, 1985. In these patients, biopsies yielded 172 cancers; 87 were nonpalpable. There was a mammographic mass in 113, only calcifications in 38, and distortion or asymmetry of breast parenchyma in 12. We retrospectively determined how each view contributed to depiction of tumors: 125 cancers were seen on all views, 10 on none, 11 on the oblique only, 4 on the mediolateral only, and 3 on the cephalocaudal only. The remaining cancers were detected on various combinations of views. Cancers were missed in individual views because of overlying dense tissue or because the tumor was outside the area depicted in the film. Breast cancer screening is performed with two views of each breast. Oblique-cephalocaudal two-view mammograms showed 158 cancers; mediolateral-cephalocaudal two-view mammograms showed 151 cancers.     

The comparison of femoral curves and curves of contemporary intramedullary nails

The aim of this study was to evaluate both the cortical and the medullary anterior bowing of the femur, and to compare these measurements with current intramedullary nails to assess the adequacy of their design. Methods: Lateral digital radiographic views of left femurs of 104 normal subjects (18–68 years old) were obtained. Radii of cortical and medullary curvatures of femurs were calculated using these images. The values obtained were compared to the radius of curvatures of ten different intramedullary nails. Results: Medullary bowing was between 114 and 1,389 mm (mean: 722 mm, SD: 230 mm) and the cortical bowing was between 109 and 1,666 mm (mean: 770 mm, SD: 267 mm). For males, these values were 114–1,389 mm (mean: 722 mm, SD: 230 mm) and 109–1,666 mm (mean: 770 mm, SD: 267 mm), respectively. For females, they were 114–1,389 mm (mean: 722 mm, SD: 230 mm) and 109–1,666 mm (mean: 770 mm, SD: 267 mm), respectively. The differences between genders were not significant. Cortical and medullar bowing was strongly correlated with age (r=−0.269, p<0.006 and r=−0.234, p<0.017, respectively). These significances were produced by females only. Radii of curvatures of intramedullary nails ranged between 150 and 300 cm and were higher than the mean cortical (77 cm) and medullary (72.2 cm) bowings. Conclusion: The difference between the curves of femur and the contemporary femoral nails implicates the inadequacy of the design of such nails for the Caucasian race living in Anatolia. Therefore, such nails should be revised accordingly to prevent the above-mentioned complications.     

Evaluation of detector dynamic range in the x-ray exposure domain in mammography: a comparison between film-screen and flat panel detector systems

Digital detectors in mammography have wide dynamic range in addition to the benefit of decoupled acquisition and display. How wide the dynamic range is and how it compares to film-screen systems in the clinical x-ray exposure domain are unclear. In this work, we compare the effective dynamic ranges of film-screen and flat panel mammography systems, along with the dynamic ranges of their component image receptors in the clinical x-ray exposure domain. An ACR mammography phantom was imaged using variable mAs (exposure) values for both systems. The dynamic range of the contrast-limited film-screen system was defined as that ratio of mAs (exposure) values for a 26 kVp Mo/Mo (HVL=0.34 mm Al) beam that yielded passing phantom scores. The same approach was done for the noise-limited digital system. Data from three independent observers delineated a useful phantom background optical density range of 1.27 to 2.63, which corresponded to a dynamic range of 2.3 +/- 0.53. The digital system had a dynamic range of 9.9 +/- 1.8, which was wider than the film-screen system (p<0.02). The dynamic range of the film-screen system was limited by the dynamic range of the film. The digital detector, on the other hand, had an estimated dynamic range of 42, which was wider than the dynamic range of the digital system in its entirety by a factor of 4. The generator/tube combination was the limiting factor in determining the digital system's dynamic range.     

On the measurement of half-value layer in film-screen mammography

Sources of variations in the measurement of half-value layer (HVL) in film-screen mammography are evaluated. Two principal sources are the energy responses of ionization chambers and the elemental compositions of the 1100 aluminum-alloy attenuation materials. Methods in measuring the thickness of attenuation materials are also an important potential source of error. These factors as well as the influence of geometry of measurement, position of measurement in the field, and precision of radiation output are examined. Recommendations for consistency in measurement are provided.     

Characterization of the reciprocity law failure in three mammography screen-film systems.

The purpose of this project was to quantify reciprocity law failure (RLF) for mammography screen-film systems. Three widely used screen-film systems were evaluated: the Kodak MinR 2000 system. Fuji UM Mammo Fine screen and Fuji UM MA HC film, and Agfa MR Detail screen and Agfa Mammoray MR5 film. The logit algorithm that linearizes logistic curve shapes was utilized to characterize film sensitometric response. Different values of mammographic phantom thickness, tube current, and kVp were used to vary screen-film exposure rates. RLF was quantified by examining the dependence of logit parameters (maximum and minimum film density, curve shift, and slope) on exposure rate. The shift of the logit curve was found to be a good indicator of the screen-film system speed, while the slope of the logit curve is affected by the RLF. RLF leads to changes in film contrast as well as speed. For the range of exposure rates measured (50-fold), screen-film contrast and speed varied by factors of 2 and 3.5, respectively. Film contrast decreased as exposure rate increased. The greatest changes were observed with the Kodak MinR 2000 screen-film system.     

Information-theoretic CAD system in mammography: entropy-based indexing for computational efficiency and robust performance

We have previously presented a knowledge-based computer-assisted detection (KB-CADe) system for the detection of mammographic masses. The system is designed to compare a query mammographic region with mammographic templates of known ground truth. The templates are stored in an adaptive knowledge database. Image similarity is assessed with information theoretic measures (e.g., mutual information) derived directly from the image histograms. A previous study suggested that the diagnostic performance of the system steadily improves as the knowledge database is initially enriched with more templates. However, as the database increases in size, an exhaustive comparison of the query case with each stored template becomes computationally burdensome. Furthermore, blind storing of new templates may result in redundancies that do not necessarily improve diagnostic performance. To address these concerns we investigated an entropy-based indexing scheme for improving the speed of analysis and for satisfying database storage restrictions without compromising the overall diagnostic performance of our KB-CADe system. The indexing scheme was evaluated on two different datasets as (i) a search mechanism to sort through the knowledge database, and (ii) a selection mechanism to build a smaller, concise knowledge database that is easier to maintain but still effective. There were two important findings in the study. First, entropy-based indexing is an effective strategy to identify fast a subset of templates that are most relevant to a given query. Only this subset could be analyzed in more detail using mutual information for optimized decision making regarding the query. Second, a selective entropy-based deposit strategy may be preferable where only high entropy cases are maintained in the knowledge database. Overall, the proposed entropy-based indexing scheme was shown to reduce the computational cost of our KB-CADe system by 55% to 80% while maintaining the system's diagnostic performance.     

A comparison of reconstruction algorithms for C-arm mammography tomosynthesis

Digital tomosynthesis is an imaging technique to produce a tomographic image from a series of angular digital images in a manner similar to conventional focal plane tomography. Unlike film focal plane tomography, the acquisition of the data in a C-arm geometry causes the image receptor to be positioned at various angles to the reconstruction tomogram. The digital nature of the data allows for input images to be combined into the desired plane with the flexibility of generating tomograms of many separate planes from a single set of input data. Angular datasets were obtained of a low contrast detectability (LCD) phantom and cadaver breast utilizing a Lorad stereotactic biopsy unit with a coupled source and digital detector in a C-arm configuration. Datasets of 9 and 41 low-dose projections were collected over a 30 degrees angular range. Tomographic images were reconstructed using a Backprojection (BP) algorithm, an Iterative Subtraction (IS) algorithm that allows the partial subtraction of out-of-focus planes, and an Algebraic Reconstruction (AR) algorithm. These were compared with single view digital radiographs. The methods' effectiveness at enhancing visibility of an obscured LCD phantom was quantified in terms of the Signal to Noise Ratio (SNR), and Signal to Background Ratio (SBR), all normalized to the metric value for the single projection image. The methods' effectiveness at removing ghosting artifacts in a cadaver breast was quantified in terms of the Artifact Spread Function (ASF). The technology proved effective at partially removing out of focus structures and enhancing SNR and SBR. The normalized SNR was highest at 4.85 for the obscured LCD phantom, using nine projections and IS algorithm. The normalized SBR was highest at 23.2 for the obscured LCD phantom, using 41 projections and an AR algorithm. The highest normalized metric values occurred with the obscured phantom. This supports the assertion that the greatest value of tomosynthesis is in imaging fibroglandular breasts. The ASF performance was best with the AR technique and nine projections.     

On the noise variance of a digital mammography system

A recent paper by Cooper et al. [Med. Phys. 30, 2614-2621 (2003)] contains some apparently anomalous results concerning the relationship between pixel variance and x-ray exposure for a digital mammography system. They found an unexpected peak in a display domain pixel variance plot as a function of 1/mAs (their Fig. 5) with a decrease in the range corresponding to high display data values, corresponding to low x-ray exposures. As they pointed out, if the detector response is linear in exposure and the transformation from raw to display data scales is logarithmic, then pixel variance should be a monotonically increasing function in the figure. They concluded that the total system transfer curve, between input exposure and display image data values, is not logarithmic over the full exposure range. They separated data analysis into two regions and plotted the logarithm of display image pixel variance as a function of the logarithm of the mAs used to produce the phantom images. They found a slope of minus one for high mAs values and concluded that the transfer function is logarithmic in this region. They found a slope of 0.6 for the low mAs region and concluded that the transfer curve was neither linear nor logarithmic for low exposure values. It is known that the digital mammography system investigated by Cooper et al. has a linear relationship between exposure and raw data values [Vedantham et al., Med. Phys. 27, 558-567 (2000)]. The purpose of this paper is to show that the variance effect found by Cooper et al. (their Fig. 5) arises because the transformation from the raw data scale (14 bits) to the display scale (12 bits), for the digital mammography system they investigated, is not logarithmic for raw data values less than about 300 (display data values greater than about 3300). At low raw data values the transformation is linear and prevents over-ranging of the display data scale. Parametric models for the two transformations will be presented. Results of pixel variance measurements made on raw data images will be presented. The experimental data are in good agreement with those of Cooper et al. It will be shown that the slope of 0.6 found by Cooper et al. for the log-log plot at low exposure is not due to transfer function nonlinearity, it occurs because of an additive variance term-possibly due to electronic noise. It will also be shown, using population statistics from clinical images, that raw data values below 300 are rare in tissue areas. Those tissue areas with very low raw data values are within about a millimeter of the chest wall or in very dense muscle at comers of images.     

Physical characteristics of five clinical systems for digital mammography

The purpose of this study was to evaluate and compare the physical characteristics of five clinical systems for digital mammography (GE Senographe 2000D, Lorad Selenia M-IV, Fischer Senoscan, Agfa DM 1000, and IMS Giotto) currently in clinical use. The basic performances of the mammography systems tested were assessed on the basis of response curve, modulation transfer function (MTF), noise power spectrum, noise equivalent quanta (NEQ), and detective quantum efficiency (DQE) in an experimental setting closely resembling the clinical one. As expected, all the full field digital mammography systems show a linear response curve over a dynamic range from 3.5 to 500 microGy (0.998相似文献   

A comparison of dose-response curves in cognitive-behavioral and psychodynamic psychotherapies

Changes in patient- and therapist-rated process items, and patient-rated symptom severity assessed over a maximum of 30 sessions of psychodynamic or cognitive-behavioral psychotherapy were compared in an outpatient mental-health clinic. Patients' ratings in psychodynamic psychotherapy on two of the process items were superior to ratings by patients in cognitive-behavioral therapy, and interactions with time were discovered for an interpersonal item. Comparisons between patients who reported their initial symptom severities as most severe and those with less severe symptoms also were made, with results indicating differential effects of treatment according to perceptions of symptom severity. Interactions between length of stay in treatment and number of sessions also were investigated. Findings indicated that patients benefit from psychotherapy over time and support the usefulness of a contextual model for psychotherapy funding.     

Computer-aided detection of masses in digital tomosynthesis mammography: comparison of three approaches

The authors are developing a computer-aided detection (CAD) system for masses on digital breast tomosynthesis mammograms (DBT). Three approaches were evaluated in this study. In the first approach, mass candidate identification and feature analysis are performed in the reconstructed three-dimensional (3D) DBT volume. A mass likelihood score is estimated for each mass candidate using a linear discriminant analysis (LDA) classifier. Mass detection is determined by a decision threshold applied to the mass likelihood score. A free response receiver operating characteristic (FROC) curve that describes the detection sensitivity as a function of the number of false positives (FPs) per breast is generated by varying the decision threshold over a range. In the second approach, prescreening of mass candidate and feature analysis are first performed on the individual two-dimensional (2D) projection view (PV) images. A mass likelihood score is estimated for each mass candidate using an LDA classifier trained for the 2D features. The mass likelihood images derived from the PVs are backprojected to the breast volume to estimate the 3D spatial distribution of the mass likelihood scores. The FROC curve for mass detection can again be generated by varying the decision threshold on the 3D mass likelihood scores merged by backprojection. In the third approach, the mass likelihood scores estimated by the 3D and 2D approaches, described above, at the corresponding 3D location are combined and evaluated using FROC analysis. A data set of 100 DBT cases acquired with a GE prototype system at the Breast Imaging Laboratory in the Massachusetts General Hospital was used for comparison of the three approaches. The LDA classifiers with stepwise feature selection were designed with leave-one-case-out resampling. In FROC analysis, the CAD system for detection in the DBT volume alone achieved test sensitivities of 80% and 90% at average FP rates of 1.94 and 3.40 per breast, respectively. With the 2D detection approach, the FP rates were 2.86 and 4.05 per breast, respectively, at the corresponding sensitivities. In comparison, the average FP rates of the system combining the 3D and 2D information were 1.23 and 2.04 per breast, respectively, at 80% and 90% sensitivities. The difference in the detection performances between the 2D and the 3D approach, and that between the 3D and the combined approach were both statistically significant (p = 0.02 and 0.01, respectively) as estimated by alternative FROC analysis. The combined system is a promising approach to improving automated mass detection on DBTs.     

Image quality assessment in digital mammography: part I. Technical characterization of the systems

In many European countries, image quality for digital x-ray systems used in screening mammography is currently specified using a threshold-detail detectability method. This is a two-part study that proposes an alternative method based on calculated detectability for a model observer: the first part of the work presents a characterization of the systems. Eleven digital mammography systems were included in the study; four computed radiography (CR) systems, and a group of seven digital radiography (DR) detectors, composed of three amorphous selenium-based detectors, three caesium iodide scintillator systems and a silicon wafer-based photon counting system. The technical parameters assessed included the system response curve, detector uniformity error, pre-sampling modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE). Approximate quantum noise limited exposure range was examined using a separation of noise sources based upon standard deviation. Noise separation showed that electronic noise was the dominant noise at low detector air kerma for three systems; the remaining systems showed quantum noise limited behaviour between 12.5 and 380 μGy. Greater variation in detector MTF was found for the DR group compared to the CR systems; MTF at 5 mm(-1) varied from 0.08 to 0.23 for the CR detectors against a range of 0.16-0.64 for the DR units. The needle CR detector had a higher MTF, lower NNPS and higher DQE at 5 mm(-1) than the powder CR phosphors. DQE at 5 mm(-1) ranged from 0.02 to 0.20 for the CR systems, while DQE at 5 mm(-1) for the DR group ranged from 0.04 to 0.41, indicating higher DQE for the DR detectors and needle CR system than for the powder CR phosphor systems. The technical evaluation section of the study showed that the digital mammography systems were well set up and exhibiting typical performance for the detector technology employed in the respective systems.     

A comparison of statistical techniques for analysis of growth curves

Frequently, experiments are conducted in order to investigate the effects of various treatments on an animal's growth rate. The data from these investigations usually consist of each animal's body weight or accumulative weight gain at specific times during the experiment. The most common statistical techniques for analysis of growth rates (increments in body weight over time) consider only terminal body weights or final accumulative weight gain. In this study, we compare growth rates over the duration of the experiment and use standard simultaneous testing procedures in order to accommodate more than two treatment groups. Results obtained by comparison of regression lines randomization analysis of variance, and repeated measures analysis are presented.     

A comparison of the imaging properties of CCD-based devices used for small field digital mammography

The objective imaging characteristics of three systems that use charge coupled devices (CCD) for small-field digital mammography (SFDM) have been compared in terms of spatial resolution and signal to noise ratio. The results indicate that although they are designed for nominally the same tasks of stereotactic localization and spot imaging these detectors have significantly differing physical imaging properties. Imaging system design parameters such as the phosphor screen type and thickness, screen configuration and method of optically coupling the phosphor to the CCD have significant effects on the imaging performance of the detectors.     

A comparison of the stimulus-response curves of aortic and carotid sinus baroreceptors in decerebrated cats

Recordings of total nerve activity suggested differences in the sensitivities and working ranges between aortic and carotid sinus baroreceptors. This result however, conflicts with single fibre studies from isolated receptor zones. Thus it appeared of some interest to compare the function curves of aortic and carotid sinus baroreceptors in the intact animal.This was achieved by comparing the response characteristics of two groups of aortic and carotid sinus baroreceptors in decerebrated cats. One smaller group consisted of 11 receptor pairs, each member of the pair being studied simultaneneously in the same cat, and a larger group consisting of 98 aortic and 49 carotid sinus baroreceptors studied independently of each other.The response of each receptor to wide pressure variations was recorded by inflating and deflating an intraaortic catheter tip balloon. Function curves were derived by plotting receptor discharge in terms of spikes per second against mean aortic pressure. No significant differences were found either in the slope of the function curves or their mean pressures at minimum activity, the latter appearing to be set by the working blood pressure level.Thus it was concluded that aortic and carotid sinus baroreceptors differ neither in their sensitivities nor in their working ranges when in their physiological environment.The support of the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 30, Kardiologie Düsseldorf is gratefully acknowledged