Digital detectors for mammography: the technical challenges

Phantom-based analysis of variations in automatic exposure control across three mammography systems: implications for radiation dose and image quality in mammography, DBT, and CEM

BACKGROUND

Automatic exposure control (AEC) plays a crucial role in mammography by determining the exposure conditions needed to achieve specific image quality based on the absorption characteristics of compressed breasts. This study aimed to characterize the behavior of AEC for digital mammography (DM), digital breast tomosynthesis (DBT), and low-energy (LE) and high-energy (HE) acquisitions used in contrast-enhanced mammography (CEM) for three mammography systems from two manufacturers.

METHODS

Using phantoms simulating various breast thicknesses, 363 studies were acquired using all available AEC modes 165 DM, 132 DBT, and 66 LE-CEM and HE-CEM. AEC behaviors were compared across systems and modalities to assess the impact of different technical components and manufacturers' strategies on the resulting mean glandular doses (MGDs) and image quality metrics such as contrast-to-noise ratio (CNR).

RESULTS

For all systems and modalities, AEC increased MGD for increasing phantom thicknesses and decreased CNR. The median MGD values (interquartile ranges) were 1.135 mGy (0.772-1.668) for DM, 1.257 mGy (0.971-1.863) for DBT, 1.280 mGy (0.937-1.878) for LE-CEM, and 0.630 mGy (0.397-0.713) for HE-CEM. Medians CNRs were 14.2 (7.8-20.2) for DM, 4.91 (2.58-7.20) for a single projection in DBT, 11.9 (8.0-18.2) for LE-CEM, and 5.2 (3.6-9.2) for HE-CEM. AECs showed high repeatability, with variations lower than 5% for all modes in DM, DBT, and CEM.

CONCLUSIONS

The study revealed substantial differences in AEC behavior between systems, modalities, and AEC modes, influenced by technical components and manufacturers' strategies, with potential implications in radiation dose and image quality in clinical settings.

RELEVANCE STATEMENT

The study emphasized the central role of automatic exposure control in DM, DBT, and CEM acquisitions and the great variability in dose and image quality among manufacturers and between modalities. Caution is needed when generalizing conclusions about differences across mammography modalities.

KEY POINTS

• AEC plays a crucial role in DM, DBT, and CEM. • AEC determines the "optimal" exposure conditions needed to achieve specific image quality. • The study revealed substantial differences in AEC behavior, influenced by differences in technical components and strategies.

Investigation of CdTe, GaAs, Se and Si as Sensor Materials for Mammography

Despite the benefits of mammography investigations, some studies have shown that X-ray exposure from the mammography screening itself can statistically cause breast cancer in a small fraction of women. Therefore, a dose reduction in mammography is desirable. At the same time, there is a demand for a higher spatial resolution in mammographic imaging. The most promising way to achieve these goals is the use of advanced photon-processing semiconductor X-ray detectors with optimum sensor materials. This study addresses the investigation of the optimum semiconductor sensor material for mammography in combination with the photon-processing detector Medipix3RX. The influence of K-shell fluorescence from the sensor material on the achievable contrast-to-noise ratio is investigated, as well as the attenuation efficiency. The three different sensor materials, CdTe, GaAs, and Si are studied, showing advances of CdTe-sensors for mammography. Furthermore, a comparison of the contrast-to-noise ratio between a clinical Se-detector and Medipix3RX detectors with Si- and CdTe-sensors is shown using a self-produced mammography phantom that is based on real human tissue.

A new generation of direct X-ray detectors for medical and synchrotron imaging applications

Large-area X-ray imaging is one of the most widely used imaging modalities that spans several scientific and technological fields. Currently, the direct X-ray conversion materials that are being commercially used for large-area (> 8 cm × 4 cm without tiling) flat panel applications, such as amorphous selenium (a-Se), have usable sensitivities of up to only 30 keV. Although there have been many promising candidates (such as polycrystalline HgI2 and CdTe), none of the semiconductors were able to assuage the requirement for high energy (> 40 keV) large-area X-ray imaging applications due to inadequate cost, manufacturability, and long-term performance metrics. In this study, we successfully demonstrate the potential of the hybrid Methylammonium lead iodide (MAPbI3) perovskite-based semiconductor detectors in satisfying all the requirements for its successful commercialization in synchrotron and medical imaging. This new generation of hybrid detectors demonstrates low dark current under electric fields needed for high sensitivity X-ray imaging applications. The detectors have a linear response to X-ray energy and applied bias, no polarization effects at a moderate bias, and signal stability over long usage durations. Also, these detectors have demonstrated a stable detection response under BNL's National Synchrotron Light Source II (NSLS-II) 70 keV monochromatic synchrotron beamline.

Digital zoom of the full-field digital mammogram versus magnification mammography: a systematic review

OBJECTIVES

To summarise and compare the performance of magnification mammography and digital zoom utilising a full-field digital mammography (FFDM) system in the detection and diagnosis of microcalcifications.

METHODS

We ran an extended search in MEDLINE, EMBASE, CINAHL, Engineering Village and Web of Science. Diagnostic test studies, experimental breast phantom studies and a Monte Carlo phantom study were included. A narrative approach was selected to summarise and compare findings regarding the detection of microcalcifications, while a hierarchical model with bivariate analysis was used for the meta-analysis of sensitivity and specificity for diagnosing microcalcifications.

RESULTS

Nine studies were included. Phantom studies suggested that the size of microcalcifications, magnification or zoom factor, exposure factors and detector technology determine whether digital zoom is equivalent to magnification mammography in the detection of microcalcifications. Pooled sensitivity for magnification and zoom calculated from the diagnostic test studies was 0.93 (95% CI 0.84-0.97) and 0.85 (95% CI 0.70-0.94), respectively. Pooled specificity was 0.55 (95% CI 0.51-0.58) and 0.56 (95% CI 0.50-0.62), respectively. The differences between the sensitivities and specificities were not statistically significant.

CONCLUSIONS

Digital zoom may be equivalent to magnification mammography. Diagnostic test studies and phantom studies using newer detector technology would contribute additional knowledge on this topic.

KEY POINTS

• The performance of digital zoom is comparable to magnification for detecting microcalcifications when newer detector technology and optimised imaging procedures are utilised. • The accuracy of digital zoom appears equivalent to geometric magnification in diagnosing microcalcifications.

Negative threshold voltage shift in an a-IGZO thin film transistor under X-ray irradiation

We investigated the effects of X-ray irradiation on the electrical characteristics of an amorphous In–Ga–Zn–O (a-IGZO) thin film transistor (TFT). The a-IGZO TFT showed a negative threshold voltage (V_TH) shift of −6.2 V after 100 Gy X-ray irradiation. Based on spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) analysis, we found that the Fermi energy (E_F) changes from 2.73 eV to 3.01 eV and that the sub-gap state of D1 and D2 changes near the conduction band minimum (CBM) of the a-IGZO film after X-ray irradiation. These results imply that the negative V_TH shift after X-ray irradiation is related to the increase in electron concentration of the a-IGZO TFT active layer. We confirmed that the sources for electron generation during X-ray irradiation are hydrogen incorporation from the adjacent layer or from ambient air to the active layer in the TFT, and the oxygen vacancy dependent persistent photocurrent (PPC) effect. Since both causes are reversible processes involving an activation energy, we demonstrate the V_TH shift recovery by thermal annealing.

We studied the effect of X-ray irradiation on the negative threshold voltage shift of bottom-gate a-IGZO TFT. Based on spectroscopic analyses, we found that this behavior was caused by hydrogen incorporation and oxygen vacancy ionization.

Investigation of viewing procedures for interpretation of breast tomosynthesis image volumes: a detection-task study with eye tracking

Objectives

To evaluate the efficiency of different methods of reading breast tomosynthesis (BT) image volumes.

Methods

All viewing procedures consisted of free scroll volume browsing and three were combined with initial cine loops at three different frame rates (9, 14 and 25 fps). The presentation modes consisted of vertically and horizontally orientated BT image volumes. Fifty-five normal BT image volumes in mediolateral oblique view were collected. In these, simulated lesions were inserted, creating four unique image sets, one for each viewing procedure. Four observers interpreted the cases in a free-response task. Time efficiency, visual attention and search were investigated using eye tracking.

Results

Horizontally orientated BT image volumes were read faster than vertically when using free scroll browsing only and when combined with fast cine loop. Cine loops at slow frame rates were ruled out as inefficient.

Conclusions

In general, horizontally oriented BT image volumes were read more efficiently. All viewing procedures except for slow frame rates were promising when assuming equivalent detection performance.

Key Points

• Breast tomosynthesis is increasingly used for breast cancer detection

• There is a benefit in reading breast tomosynthesis image volumes presented horizontally

• Align image content to visual field, especially for dynamic 3D images

• Reading at slow frame rates was considered inefficient

Computation of realistic virtual phantom images for an objective lesion detectability assessment in digital mammography

Image quality assessment is required for an optimal use of mammographic units. On the one hand, there are objective image quality assessment methods based on the measurement of technical parameters such as modulation transfer function (MTF), noise power spectrum (NPS) or detection quantum efficiency (DQE) describing performances of digital detectors. These parameters are, however, without direct relationship with lesion detectability in clinical practice. On the other hand, there are image quality assessment methods involving time consuming procedures, but presenting a direct relationship with lesion detectability. This contribution describes an X-ray source/digital detector model leading to the simulation of virtual contrast-detail phantom (CDMAM) images. The virtual image computation method requires the acquisition of only few real images and allows for an objective image quality assessment presenting a direct relationship with lesion detectability. The transfer function of the proposed model takes as input physical parameters (MTF* and noise) measured under clinical conditions on mammographic units. As presented in this contribution, MTF* is a modified MTF taking into account the effects due to X-ray scatter in the breast and magnification. Results obtained with the structural similarity index prove that the simulated images are quite realistic in terms of contrast and noise. Tests using contrast detail curves highlight the fact that the simulated and real images lead to very similar data quality in terms of lesion detectability. Finally, various statistical tests show that quality factors computed for both the simulated images and the real images are very close for the two data sets.

Amorphous and polycrystalline photoconductors for direct conversion flat panel x-ray image sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI₂ and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

The basics and implementation of digital mammography

Current day digital mammography acquisition units have already been shown to be equal or better than screen film systems for the detection and classification of breast lesions. The optimal multimodality breast imaging diagnostic workstations and connectivity to existing picture and archiving communication systems and information systems is still a work in progress, but with more and more facilities transitioning to digital imaging it is only a matter of time until these hurdles are overcome.

[Comparison between electronic zoom and geometric magnification of clusters of microcalcifications on digital mammography]

PURPOSE

With the advent of digital mammography with electronic zoom capabilities, we have sought to determine the need for geometric magnification for the evaluation of clusters of microcalcifications.

PATIENTS AND METHODS

Eighty-eight clusters of microcalcifications were reviewed by two experienced radiologists using electronic zoom (ZOOM) and geometric magnification (MAG). The following criteria were evaluated: image quality, shape and number of microcalcifications, size and shape of the clusters. The clusters were classified based on malignancy risk using the BI-RADS criteria. Histological results from macrobiopsy or surgery as well as 2 year follow-up were used as reference for statistical analysis.

RESULTS

Sensitivity (100% for MAG and 90% for ZOOM), specificity (52% versus 39%), positive predictive value (51% versus 44%) and negative predictive value (100% versus 88%) were superior for geometric magnification compared to electronic zoom irrespective of the reviewer but without reaching statistical significance. However, image quality was significantly superior with geometric magnification (p<<0.05). In addition, reviewers were more confident in their interpretation of geometric magnification images.

CONCLUSION

Geometric magnification remains necessary in routine clinical practice for the characterization of microcalcifications and BI-RADS classification.

Comparison of standard mammography with digital mammography and digital infrared thermal imaging for breast cancer screening

Breast cancer is the most common malignancy in women. Screen-film mammography (SFM) has been considered the gold standard for breast cancer screening and detection. Despite its recognized value in detecting and characterizing breast disease, mammography has important limitations and its false-negative rate ranges from 4% to 34%. Given these limitations, development of imaging modalities that would enhance, complement, or replace mammography has been a priority. Digital mammography (FFDM) and digital infrared thermal imaging (DITI) are some of these alternative modalities.

Effect of dose reduction on the ability of digital mammography to detect simulated microcalcifications

The purpose of this article was to report the relationship between radiation dose and the ability of sentence digital mammography to detect microcalcifications. All images were acquired by computed radiography and an anthropomorphic breast phantom. The tube voltage and anode/filter combination used were 28 kVp and Mo/Mo. Simulated microcalcifications with an approximate diameter of 250-350 μm were positioned on the phantom. Groups of six microcalcifications were arranged in one of two patterns, a line cluster 1 cm long or a hexagonal cluster 4 mm wide. One of the six microcalcifications was removed to create a negative control. Each cluster was placed on 25 different points. Four levels of milliampere-second (mAs) values were applied: 100%, 50%, 25%, and 12.5%. Five staff radiologists participated in an observer performance test. All observers used a workstation with a 3-megapixel monochrome LCD monitor. The areas under the receiver-operating characteristics curves (AUC) were used to compare diagnostic performance among the four doses. The overall AUC scores were 0.97 with 100% mAs, 0.93 (n.s.) with 50%, 0.90 (p < 0.05) with 25%, and 0.81 (p < 0.01) with 12.5% mAs. Among the negative series, the percentage of images on which observers were able to identify the removed microcalcification point decreased from 88.8% with 100% mAs to 83.6% (n.s.) with 50%, 74.8% (p < 0.001) with 25%, and 67.2% (p < 0.001) with 12.5% mAs. A certain level of dose reduction in digital mammography may be an option.

Physical characterization and performance comparison of active- and passive-pixel CMOS detectors for mammography

We investigated the physical characteristics of two complementary metal oxide semiconductor (CMOS) mammography detectors. The detectors featured 14-bit image acquisition, 50 microm detector element (del) size and an active area of 5 cm x 5 cm. One detector was a passive-pixel sensor (PPS) with signal amplification performed by an array of amplifiers connected to dels via data lines. The other detector was an active-pixel sensor (APS) with signal amplification performed at each del. Passive-pixel designs have higher read noise due to data line capacitance, and the APS represents an attempt to improve the noise performance of this technology. We evaluated the detectors' resolution by measuring the modulation transfer function (MTF) using a tilted edge. We measured the noise power spectra (NPS) and detective quantum efficiencies (DQE) using mammographic beam conditions specified by the IEC 62220-1-2 standard. Our measurements showed the APS to have much higher gain, slightly higher MTF, and higher NPS. The MTF of both sensors approached 10% near the Nyquist limit. DQE values near dc frequency were in the range of 55-67%, with the APS sensor DQE lower than the PPS DQE for all frequencies. Our results show that lower read noise specifications in this case do not translate into gains in the imaging performance of the sensor. We postulate that the lower fill factor of the APS is a possible cause for this result.

Image Quality, lesion detection, and diagnostic efficacy in digital mammography: Full-field digital mammography versus computed radiography-based mammography using digital storage phosphor plates

OBJECTIVE

To compare image quality, the lesion detection, and the diagnostic efficacy of full-field digital mammography (FFDM) and computed radiography-based mammography using digital storage phosphor plates (DSPM) in the evaluation of breast lesions.

MATERIALS AND METHODS

In this prospective study, 150 patients with suspicious breast lesions underwent FFDM and DSPM. Nine aspects of image quality (brightness, contrast, sharpness, noise, artifacts, and the detection of anatomic structures, i.e., skin, retromamillary space, glandular tissue, and calcifications) were evaluated by five radiologists. In addition, the detection of breast lesions and the diagnostic efficacy, based on the BI-RADS classification, were evaluated with histologic and follow-up correlation.

RESULTS

For contrast, sharpness, and the detection of all anatomic structures, FFDM was rated significantly better (p<0.05). Mass lesions were equally detected, whereas FFDM detected more lesions consisting of calcifications (85 versus 75). DSPM yielded two false-negative results. Both lesions were rated BI-RADS 4 with FFDM, but BI-RADS 2 with DSPM. Both were invasive carcinoma at histology. The sensitivity, specificity, PPV, NPV, and accuracy of FFDM were 1.0, 0.397, 0.636, 1.0, and 0.707, compared to 0.974, 0.397, 0.630, 0.935, and 0.693 of DSPM.

CONCLUSION

Based on image quality parameters, FFDM is, in part, significantly better than DSPM. Furthermore, the detection of breast lesions with calcifications is favorable with FFDM. However, the diagnostic efficacy of FFDM and DSPM was equal. The interpretation of the false-negative results suggests that the perception and characterization of breast lesions is not defined solely by the digital mammography system but is strongly influenced by the radiologist, who is one of the determinants in the interpretation of breast imaging.

Solid-state, flat-panel, digital radiography detectors and their physical imaging characteristics

Solid-state, digital radiography (DR) detectors, designed specifically for standard projection radiography, emerged just before the turn of the millennium. This new generation of digital image detector comprises a thin layer of x-ray absorptive material combined with an electronic active matrix array fabricated in a thin film of hydrogenated amorphous silicon (a-Si:H). DR detectors can offer both efficient (low-dose) x-ray image acquisition plus on-line readout of the latent image as electronic data. To date, solid-state, flat-panel, DR detectors have come in two principal designs, the indirect-conversion (x-ray scintillator-based) and the direct-conversion (x-ray photoconductor-based) types. This review describes the underlying principles and enabling technologies exploited by these designs of detector, and evaluates their physical imaging characteristics, comparing performance both against each other and computed radiography (CR). In standard projection radiography indirect conversion DR detectors currently offer superior physical image quality and dose efficiency compared with direct conversion DR and modern point-scan CR. These conclusions have been confirmed in the findings of clinical evaluations of DR detectors. Future trends in solid-state DR detector technologies are also briefly considered. Salient innovations include WiFi-enabled, portable DR detectors, improvements in x-ray absorber layers and developments in alternative electronic media to a-Si:H.

Digital mammography: what do we and what don't we know?

High-quality full-field digital mammography has been available now for several years and is increasingly used for both diagnostic and screening mammography. A number of different detector technologies exist, which all have their specific advantages and disadvantages. Diagnostic accuracy of digital mammography has been shown to be at least equivalent to film-screen mammography in a general screening population. Digital mammography is superior to screen-film mammography in younger women with dense breasts due to its ability to selectively optimize contrast in areas of dense parenchyma. This advantage is especially important in women with a genetic predisposition for breast cancer, where intensified early detection programs may have to start from 25 to 30 years of age. Tailored image processing and computer-aided diagnosis hold the potential to further improve the early detection of breast cancer. However, at present no consensus exists among radiologists on which processing is optimal for digital mammograms. Image processing may also vary significantly among vendors with so far limited interoperability. This review aims to summarize the available information regarding the impact of digital mammography on workflow and breast cancer diagnosis.

[Digital mammography: performance evaluation parameters and available systems in France]

The purpose of this article is to review the features of digital mammography systems available in France. This article includes three parts: definition of the different components of a digital mammography system, description of different performance evaluation parameters, and review of systems currently available in France. Data from the literature as well as questionnaires from different manufacturers were used. The following vendors agreed to participate: Agfa, Fujifilm, GE Healthcare, Hologic-Lorad, IMS, Kodak, Konica Minolta, Philips, Planmed, Sectra, Siemens. For each vendor, the type of detector, type of available mammographic units and workstation, optional features (CAD), archival and printing possibilities (when vendors specifically offer dedicated solutions).

Revisión de la evidencia científica sobre la aplicación clínica de la mamografía digital

OBJECTIVE

To review the scientific evidence with respect to the use of digital mammography and compare it with analogical mammography in the clinical context.

MATERIAL AND METHOD

We searched Medline and EMbase for studies published between 1989 and 2005 that compared the results of digital and analogical mammography in the same group of patients or in two different groups of patients to evaluate their respective diagnostic accuracy (sensitivity, specificity, area under the ROC curve), recall rate, biopsy rate, and exposure to radiation. We reviewed and compared the different methodologies of the studies published.

RESULTS

Eight articles and eight presentations at congresses were found. No statistically significant differences were observed between digital and analogical mammography for the detection of breast cancer. Until the publication of the Oslo II (2004) and Digital Mammographic Imaging Screening Trial (2005) studies, the series presented were small and used a variety of methodologies. This made it impossible to appreciate small differences in diagnostic accuracy between the two techniques and to group the, sometimes, contradictory results. Furthermore, these first series did not include follow-up.

CONCLUSION

There are no statistically significant differences between the two techniques for diagnostic accuracy, except in women with dense or heterogeneously dense breasts, in those under 50 years of age, and in peri- or pre-menopausal women, in which cases digital mammography is significantly better. These data should be confirmed in longer term studies to enable the effects on the breast cancer mortality rate to be specifically evaluated. Cost-effectiveness studies are important when considering changing techniques.

Advances in Digital Radiography: Physical Principles and System Overview

During the past two decades, digital radiography has supplanted screen-film radiography in many radiology departments. Today, manufacturers provide a variety of digital imaging solutions based on various detector and readout technologies. Digital detectors allow implementation of a fully digital picture archiving and communication system, in which images are stored digitally and are available anytime. Image distribution in hospitals can now be achieved electronically by means of web-based technology with no risk of losing images. Other advantages of digital radiography include higher patient throughput, increased dose efficiency, and the greater dynamic range of digital detectors with possible reduction of radiation exposure to the patient. The future of radiography will be digital, and it behooves radiologists to be familiar with the technical principles, image quality criteria, and radiation exposure issues associated with the various digital radiography systems that are currently available.

Physical characterization of a high-resolution CCD detector for mammography

The physical characteristics of charge-coupled device (CCD) mammography detector with 16-bit dynamic range and 27 microm detector element size were investigated. The detector, with an active area of 1 cm x 20 cm is suitable for slot-scanning systems. We evaluated the detector resolution by measuring the modulation transfer function (MTF) using a tilted edge. We also measured the noise power spectra (NPS) and detective quantum efficiency (DQE) using tungsten spectra filtered with 3 mm Al. We carried out measurements in two modes of operation: the frame mode where the detector is stationary and the scan mode where the detector operates in a slot-scanning configuration. The specific beam qualities and exposure ranges employed were 30 kVp, HVL 1.4 mm Al, 1.24 microC kg(-1) to 12.44 microC kg(-1), and 40 kVp, HVL 2.1 mm Al and 3.26 microC kg(-1) to 16.64 microC kg(-1). The product of the normalized noise power spectrum and exposure was also computed to evaluate the quantum limited characteristic of the detector. The detector MTF was 12% at 15 lp mm(-1). The product of the noise power spectra and exposure was independent of exposure level, indicating a quantum limited detector. The DQE in the scan and frame modes near zero frequency was 40% and 60%, respectively. Our results show that the slot-scanning configuration was less efficient than the performance capabilities of the detector. This detector is comparable to other digital mammography sensors evaluated in the literature.

Automated analysis of phantom images for the evaluation of long-term reproducibility in digital mammography

The performance of an automatic software package was evaluated with phantom images acquired by a full-field digital mammography unit. After the validation, the software was used, together with a Leeds TORMAS test object, to model the image acquisition process. Process modelling results were used to evaluate the sensitivity of the method in detecting changes of exposure parameters from routine image quality measurements in digital mammography, which is the ultimate purpose of long-term reproducibility tests. Image quality indices measured by the software included the mean pixel value and standard deviation of circular details and surrounding background, contrast-to-noise ratio and relative contrast; detail counts were also collected. The validation procedure demonstrated that the software localizes the phantom details correctly and the difference between automatic and manual measurements was within few grey levels. Quantitative analysis showed sufficient sensitivity to relate fluctuations in exposure parameters (kV(p) or mAs) to variations in image quality indices. In comparison, detail counts were found less sensitive in detecting image quality changes, even when limitations due to observer subjectivity were overcome by automatic analysis. In conclusion, long-term reproducibility tests provided by the Leeds TORMAS phantom with quantitative analysis of multiple IQ indices have been demonstrated to be effective in predicting causes of deviation from standard operating conditions and can be used to monitor stability in full-field digital mammography.

How do lesion size and random noise affect detection performance in digital mammography?

RATIONALE AND OBJECTIVES

We investigated the effect of random noise and lesion size on detection performance in mammography.

MATERIALS AND METHODS

Digital mammograms were obtained of an anthropomorphic breast phantom with and without simulated mass lesions. Digital versions of the mass lesions, ranging in size from 0.8 to 12 mm, were added back to the breast phantom image. Four alternate forced choice experiments were performed to determine the lesion contrast required to achieve a 92% correct lesion detection rate, denoted I92. Experiments were performed using identical phantom images and different versions of phantom images obtained using the same techniques but with different random noise patterns.

RESULTS

For lesions larger than 1 mm, the slope of the contrast detail curves was always positive. This behavior contrasts with conventional contrast-detail curves in uniform backgrounds in which the slope is approximately -0.5. There was no difference between twinned experiments and those obtained using different patterns of random noise for lesions greater than 1 mm. When the lesion size was reduced below 1 mm, the detection threshold increased indicating a deterioration of lesion detectability, and detection performance was significantly lower when random noise patterns were used.

CONCLUSION

Our results suggest that lesion detection is dominated by anatomical structure for lesions with a size >1 mm, but by random noise for submillimeter sized lesions.

Current status of digital mammography for screening and diagnosis of breast cancer

PURPOSE OF REVIEW

Digital mammography is becoming the preferred technique for investigation of the breast. It is important, therefore, to analyze not only the accuracy of this technique in the screening and the diagnostic environment but also to evaluate its strengths and limits. In addition, communication with other specialists inside and outside the hospital is essential.

RECENT FINDINGS

Recent publications of large clinical trials have shown that digital mammography is as accurate as film-screen mammography in a screening setting. Technical protocols for acceptance testing of these modalities are emerging but they are not yet complete. The literature shows that reading on soft copy may be preferred to hard copy. The high cost is still an important limiting factor in the easy introduction of full-field digital mammography in a hospital.

SUMMARY

Digital mammography is becoming the method of choice in the detection and characterization of breast cancer. Today, physical and technical protocols as well as large clinical trials are assessing the performance of this technology. A lot of work remains in the optimization of the different parts of the imaging chain: exposure setting, acceptability of detectors, dedicated post processing, viewing conditions and computer-aided detection. In parallel with these developments, newer digital technologies are being explored (tomosynthesis).

Theoretical analysis of high-resolution digital mammography

The performance of a high-resolution charge coupled device-based full-field digital mammography imager was analysed using a mathematical framework based on an adaptation of cascaded linear systems theory described by other investigators. This work has been conducted in order to understand the impact of various design parameters on the physical performance characteristics of the imager. Specifically, the effect of pixel size, scintillator thickness and packing density, x-ray spectra, air kerma, dark current, charge integration time, and pixel fill-factor on the frequency dependent detective quantum efficiency was studied using a charge-coupled device as a reference platform. The imaging system was modelled as a series of physical processes with gain and spatial spreading. For each stage, the signal and noise power spectra were computed and propagated through the imaging chain as inputs to subsequent stages. Good agreement between experimental and theoretical predictions was obtained for various x-ray spectral conditions that were investigated. The modulation transfer function, MTF(f) and detective quantum efficiency DQE(f) characteristics obtained in this study are encouraging and comparable to other digital mammography systems. The results of this study strongly suggest the feasibility of large area scintillator-based digital mammography imagers with pixel sizes below 100 microm.

Dose comparison between screen/film and full-field digital mammography

The study purpose was the comparison between doses delivered by a full-field digital mammography system and a screen/film mammography unit, both using the same type of X-ray tube. Exposure parameters and breast thickness were collected for 300 screen/film (GE Senographe DMR) and 296 digital mammograms (GE Senographe 2000D). The entrance surface air kerma (ESAK) was calculated from anode/filter combination, kV(p) and mAs values and breast thickness, by simulating spectra through a program based on a catalogue of experimental X-ray spectra. The average glandular dose (AGD) was also computed. Results showed an overall reduction of average glandular dose by 27% of digital over screen/film mammography. The dose saving was about 15% for thin and thick breasts, while it was between 30% and 40% for intermediate thicknesses. Full-field digital mammography dose reduction is allowed by wider dynamic range and higher efficiency of digital detector, which can be exposed at higher energy spectra than screen/film mammography, and by the separation between acquisition and displaying processes.

Intracranial 2D and 3D DSA with flat panel detector of the direct conversion type: initial experience

The purpose of this study was to compare the image quality of two-dimensional (2D) digital subtraction angiography (DSA) between a flat panel detector (FPD) of the direct conversion type with low radiation dose and a conventional image intensifier (I.I.)-TV system, and to assess 3D DSA with the FPD system in the depiction of intracranial vessels. Fifteen consecutive patients (five men, ten women; age range: 18-82 years; mean age: 55.5 years) were prospectively included in this study. All patients underwent 2D DSA with both the FPD and I.I.-TV system in one projection. The radiation doses during angiography were evaluated using a phantom. The 3D DSA images were created from the rotational DSA data with the FPD system. Two blinded radiologists independently evaluated 2D DSA with the FPD system and I.I.-TV system using a 5-point assessment scale (excellent to not visible) to assess the depiction of intracranial vessels. MIP and volume rendering (VR) images of 3D DSA with the FPD system were also evaluated using a 5-point scale (excellent to not visible). DSA and fluoroscopy dose measurements with the phantom showed a dose reduction of approximately 85% and 9% with the FPD system compared with the I.I.-TV system, respectively. For 2D DSA, the FPD system was significantly superior to the I.I.-TV system with respect to the visibility of the peripheral and perforating vessels (p<0.05). The peripheral and perforating vessels were also sufficiently visualized on MIP images of 3D DSA in all 15 cases. Our FPD system was found to be superior to the I.I.-TV system in visualizing small intracranial vessels combined with a significant reduction of radiation dose, and was able to create high-quality 3D DSA images on which high spatial resolution allowed precise visualization of small vessels such as perforating ones.

Evaluation of the diagnostic value of a computed radiography system by comparison of digital hard copy images with screen–film mammography: results of a prospective clinical trial

The purpose of the study was to determine prospectively the diagnostic value of a computed radiography (CR) system by comparing mammographic hard copy images with screen-film mammography (SFM). A series of 100 patients, who came for diagnostic investigation, underwent two-view SFM (Lorad M-IV Platinum) and digital mammography with a CR system (AGFA CR system). The images were obtained by double exposure, i.e. same view without removing compression of the corresponding breast. The CR images were processed with dedicated processing for mammography. Six radiologists read sets of SFM and CR images. The primary efficacy parameter was the overall diagnostic value. The secondary efficacy parameters were lesion conspicuity and lesion details (for masses and micro-calcifications), tissue visibility at chest wall and at skin line, axillary details, overall density and sharpness impression and the overall noise impression. These parameters were scored by a 7-point scoring system. "CR non-inferior to SFM" was concluded if the lower confidence interval bound exceeded 80%. The confidence interval for the overall diagnostic value was between 96.4% and 100%. Pooled analysis of the ten features for image quality comparison demonstrated for all but one feature (lesion details of the calcifications) CR non-inferiority to SFM.

Comparison of full-field digital mammography to screen-film mammography with respect to contrast and spatial resolution in tissue equivalent breast phantoms

To determine if the improved contrast resolution of full-field digital mammography (FFDM) with reduced spatial resolution allows for superior or equal phantom object detection compared with screen-film mammography (SFM). Tissue equivalent breast phantoms simulating an adipose to glandular ratio of 50/50,30/70, and 20/80 were imaged according to each manufacturers' recommendation with four full-field digital mammography units (Fuji, Sectra, Fischer, and General Electric) and a screen-film mammography unit (MammoMatII 2000, Siemens, Munich, Germany). A total of 20 images were obtained in both hard- and soft-copy formats. For the purpose of soft-copy display, the screen-film hard-copy images were digitized with a 50 microm micron scanner. Six radiologists, experts in breast imaging, and three physicists, experts in scoring mammography phantoms, participated in a reader study where each reader scored each phantom for visibility of line-pairs and for 24 objects (fibers, clusters of specks, and masses). The data were recorded, entered into a database, and analyzed by a mixed-effect model. The limiting spatial resolution in line-pairs per millimeter visible with the digital units was less, regardless of display modality used, than that provided by the screen-film unit. The difference was statistically significant for the General Electric (p < 0.01) and Fuji digital mammography units (p = 0.03). With respect to the number of visible objects, a statistically significant higher number could be detected with the screen-film unit as compared to the Fischer (p < 0.01) and Sectra (p < 0.01) digital mammography units, but there was no significant difference between the other digital units and screen film. Overall, there was significantly better performance on the 50/50 phantom than with the 30/70 and 20/80 phantoms (p = 0.01, p < 0.01) for object visibility. For the digital mammography units, soft-copy display performed better than hard-copy display for the Fischer and Sectra images, but worse for Fuji and General Electric. In addition, soft-copy display of digitized screen-film images was significantly better than hard-copy display (p =0.02) of the original screen films for object visibility, but worse for spatial resolution. The higher contrast resolution of the FFDM units tested did not result in improved detection of line-pair resolution or objects in the phantoms tested versus screen-film mammography. The phantom performance of a digital mammography unit seems to be influenced by the type of detection task (line-pair resolution versus object visibility), the display modality (soft-copy versus hard-copy) chosen to score the phantoms, and the parenchymal pattern composition of the phantom.

Effect of soft-copy display supported by CAD on mammography screening performance

Diagnostic performance and reading speed for conventional mammography film reading is compared to reading digitized mammograms on a dedicated workstation. A series of mammograms judged negative at screening and corresponding priors were collected. Half were diagnosed as cancer at the next screening, or earlier for interval cancers. The others were normal. Original films were read by fifteen experienced screening radiologists. The readers annotated potential abnormalities and estimated their likelihood of malignancy. More than 1 year later, five radiologists reread a subset of 271 cases (88 cancer cases having visible signs in retrospect and 183 normals) on a mammography workstation after film digitization. Markers from a computer-aided detection (CAD) system for microcalcifications were available to the readers. Performance was evaluated by comparison of A(z)-scores based on ROC and multiple-Reader multiple-case (MRMC) analysis, and localized receiver operating characteristic (LROC) analysis for the 271 cases. Reading speed was also determined. No significant difference in diagnostic performance was observed between conventional and soft-copy reading. Average A(z)-scores were 0.83 and 0.84 respectively. Soft-copy reading was only slightly slower than conventional reading. Using a mammography workstation including CAD for detection of microcalcifications, soft-copy reading is possible without loss of quality or efficiency.

Digital mammography: current state and future aspects

The introduction of digital technique in mammography has been the last step in completing the process of digitalization in diagnostic imaging. Meanwhile, some different digital techniques as well as a couple of different digital mammography systems were developed and have already been available for some years. In this review article, the relevant data of key studies are reported, the current status is defined, and perspectives of digital mammography are described.

Are phantoms useful for predicting the potential of dose reduction in full-field digital mammography?

A phantom study was performed in full-field digital mammography to investigate the opportunity and the magnitude of a possible dose reduction that would leave the image quality above the accepted thresholds associated with some classical phantoms. This preliminary work is intended to lay the groundwork for a future clinical study on the impact of dose reduction on clinical results. Three different mammography phantoms (ACR RMI 156, CIRS 11A and CDMAM 3.4) were imaged by a full-field digital mammography unit (GE Senographe 2000D) at different dose levels. Images were rated by three observers with softcopy reading and scoring methods specific to each phantom. Different types of data analysis were applied to the ACR (American College of Radiology) and the other two phantoms, respectively. With reference to the minimum acceptance score in screen/film accreditation programmes, the ACR phantom showed that about 45% dose reduction could be applied, while keeping the phantom scores above that threshold. A relative comparison was done for CIRS and CDMAM, for which no threshold is defined. CIRS scoring remained close to the reference level down to 40% dose reduction, the inter- and intra-observer variability being the main source of uncertainty. Contrast-detail curves provided by CDMAM overlapped down to 50% dose reduction, at least for object contrast values ranging between 30% and 3%. This multi-phantom study shows the potential of further reducing the dose in full-field digital mammography beyond the current values. A common dose reduction factor around 50% seems acceptable for all phantoms. However, caution is required before extrapolating the results for clinical use, given the limitations of these widely used phantoms, mainly related to their limited dynamic range and uniform background.

Flat detectors and their clinical applications

Diagnostic and interventional flat detector X-ray systems are penetrating the market in all application segments. First introduced in radiography and mammography, they have conquered cardiac and general angiography and are getting increasing attention in fluoroscopy. Two flat detector technologies prevail. The dominating method is based on an indirect X-ray conversion process, using cesium iodide scintillators. It offers considerable advantages in radiography, angiography and fluoroscopy. The other method employs a direct converter such as selenium which is particularly suitable for mammography. Both flat detector technologies are based on amorphous silicon active pixel matrices. Flat detectors facilitate the clinical workflow in radiographic rooms, foster improved image quality and provide the potential to reduce dose. This added value is based on their large dynamic range, their high sensitivity to X-rays and the instant availability of the image. Advanced image processing is instrumental in these improvements and expand the range of conventional diagnostic methods. In angiography and fluoroscopy the transition from image intensifiers to flat detectors is facilitated by ample advantages they offer, such as distortion-free images, excellent coarse contrast, large dynamic range and high X-ray sensitivity. These characteristics and their compatibility with strong magnetic fields are the basis for improved diagnostic methods and innovative interventional applications.