Digital mammography: what do we and what don’t we know?
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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.
KeywordsDigital mammography Breast cancer screening Image processing Workflow Quality assurance
Digital mammography systems
CCD array(3 × 4 mosaic)
18.6 × 24.8 cm
6,400 × 4,800
Digital Breast Imager
Trex (Lorad, Bennett)b
No longer available
CCD slot scanning
1.4-cm wide array of 4 CCDs
22.1 × 30.4 cm
4,096 × 5,625
No grid necessary
Phosphor flat panel
Array of photo diodes/TFT
19.2 × 23 cm
1,914 × 2,294
Images can be obtained in rapid sequence
24 × 30.7 cm
2,394 × 3,062
Selenium flat panel
Array of electrode pads/TFT
25 × 29 cm
3,584 × 4,096
Direct conversion of X-ray photons to electric charge
17.4 × 23.9 cm or 23.9 × 30.5 cm
2,016 × 2,816 or 2,816 × 3,584
Array of X-ray photon counters
24 × 26 cm
4,800 × 5,200
Very high DQE, currently no AEC
Computed radiography (CR)
18 × 24 cm or 24 × 30 cm
Approx. 1,800 × 2,400 or 2,400 × 3,000
FCR Profect CSFCR 5000 MA
CR 75.0 CR 85-X
REGIUS 190 Mammo
Measuring image quality in digital mammography
Image quality of conventional film-screen systems can be fairly accurately described by three parameters: (1) the characteristic curve of a film-screen system, (2) the sensitivity or “speed” and (3) the high-contrast line-pair resolution. All three concepts do not apply to digital imaging.
Digital mammography detectors have a linear relationship between detector dose and signal intensity, and no fixed characteristic curve as in film-screen mammography exists. Translation of detector signal intensities into monitor brightness is achieved by specific window settings and non-linear look-up tables, which can be modified to optimize the contrast in a certain image area of interest [8, 9].
While the sensitivity of a film-screen system defines the amount of dose required to reach a certain optical density of the developed film, there is no single optimal detector dose in digital mammography. With decreasing detector dose, image noise increases in the digital image and vice versa. By changing the X-ray beam energy spectrum, image noise can be exchanged against image contrast while keeping the parenchymal dose to the patient constant. In digital mammography, it is often beneficial to move to a higher energy spectrum than with film-screen mammography, since image noise is lower and the resulting loss in image contrast can be compensated for by adjusting the window setting [10, 11].
Digital mammography systems
There are now several different types of digital mammography systems available, which all are capable of producing high-quality digital mammograms, but all have specific advantages and disadvantages. Digital mammography systems can be grouped according to detector material, whether they are integrated or cassette-based systems, or whether they use an area detector or slot scanning technique (Table 1). Integrated systems usually allow for a higher throughput than cassette-based CR systems, but are more expensive. Slot scanning systems often can operate at a lower dose, since the slot collimation is effective in reducing scatter radiation, thus obliviating the need for an additional anti-scatter grid. Disadvantages of the slot scanning systems include longer scan times, high tube strain and the need for exact mechanical registration of the moving collimation slot and the detector. There are two different basic types of integrated area detectors, one on the bases of a phospor scintillator combined with an array of photo diodes capturing the light generated by the phosphor layer and the second type using an amorphous selenium layer with direct conversion of the X-ray photons to an electric charge. Both systems use a TFT array mounted on a amorphous silicon base for signal read-out. Detector elements in the phosphor flat-panel systems with a pixel size of 100 μm are usually slightly larger than in the selenium-based systems (70–85 μm). Reduction in detector element size in phosphor flat-panel systems is difficult, since with a decreasing size of the detector elements, the relative portion of the active detector area would decrease rapidly compared to the relatively fixed inactive portion of the detector related to the signal read-out, resulting in a lower DQE and higher parenchymal dose for the patient. An advantage of phosphor flat-panel systems is that images can be obtained in relatively short sequence, which is useful for patient throughput and advanced applications such as tomosynthesis and contrast-enhanced mammography [27, 28]. A disadvantage of selenium-based systems compared to phosphor flat-panel systems is the higher amount of image lag (image signal carried over from a previous to a subsequent exposure) and ghosting (temporary change in sensitivity base on prior exposure history). However, detector development in this area is ongoing, and both lag and ghosting have been reduced significantly in newer clinical selenium-based systems .
The value of CR mammography systems compared to integrated full-field systems has recently been under intense discussion. One major selling point of CR systems is the lower investment cost, especially if existing mammography equipment can be used for acquiring the mammographic images. This cost advantage, however, is significantly smaller when considering a new installation. One argument brought forward against CR mammography is that the dose necessary to operate CR systems at acceptable image quality levels is higher than that of integrated full-field systems. Although there is no doubt that CR systems have a slightly lower DQE than integrated full-field systems, part of this dose disadvantage may be explained by other factors. Integrated systems usually optimize the entire imaging chain including choice of the exposure parameters such as kVp and the anode/filter combination. It has been shown that the major dose savings with digital mammography systems are achieved in patients with larger breasts by switching to a higher energy beam spectrum earlier than with conventional film-screen systems [16, 30]. Since CR mammography systems are used with standard mammography equipment traditionally designed for film-screen mammography, this optimization of exposure parameters often does not occur. Another common problem with CR mammography is that imaging processing algorithms developed for other radiographic exams (e.g., chest films) are used. Image noise with digital images is higher in areas of lower detector dose, e.g., the mediastinum. Since this noise may be perceived as disturbing, special processing algorithms have been developed for CR images to suppress noise in bright (underexposed) image areas . In mammography, such algorithms will lead to impaired visibility of microcalcifications in areas of dense parenchyma and should therefore not be used.
Clinical comparison of digital and film-screen mammography
Prospective clinical screening trials comparing film-screen and digital mammography
Number of sites
Number of exams
Cancer detection rate
GE phosphorflat panel prototypea
Paired, double-reading with consensus
[GE Senographe 2000D]
OSLO II 
Randomized,double-reading with consensus
GE Senographe 2000D
GE Senographe 2000 D (45%)Fischer Senoscan (23%)Fuji FCR (22%)Lorad Digital Breast Imager and Hologic Selenia(together around 10%)b
Impact of double examination on cancer detection in paired screening trials
Number of exams
Number of cancersa detected by mammography
Gain by adding the second modalityb
OSLO I 
Digital mammography with the possibility to locally optimize image contrast has, however, a clear advantage in younger patients with dense breasts, as was impressively demonstrated by the Digital Mammographic Screening Trial (DMIST) . Interestingly, the rapid decline in sensitivity as typically seen with film-screen mammography in denser breasts  was not observed with digital mammography in the DMIST trial, where the sensitivity of digital mammography in the subgroup of women with dense breasts was identical to the sensitivity in the entire group . This advantage of digital mammography in women with dense breasts will be especially valuable in patients with a genetic predisposition for breast cancer, in whom intensified early detection measures including mammography may have to start as early as 25 to 30 years of age [41, 42]. However, it is uncertain whether the DMIST results can be translated into the European situation, where screening mammography exams are usually double-read and recall rates are much lower. Per Skaane in the Oslo II study at a recall rate of 3.7% for digital and 3.0% for film-screen mammography (compared to around 10% for women <50 years of age in the DMIST trial) found a much smaller, statistically not significant advantage for digital mammography in women below the age of 50 (Table 2). To be able to take full advantage of digital mammography in women with dense breasts, it may therefore be necessary to aggressively recall even subtle findings, so-called “minimal signs” as defined in the Dutch screening program . In European population-based screening programs, however, there is a tendency to initially ignore these minimal signs in order to keep the recall rate at an acceptable low level .
For a long time, the question whether digital mammography with a spatial resolution lower than film-screen mammography systems can adequately visualize small microcalcifications has been at the center of an intense debate. In theory, digitization with a limited spatial resolution may impair visualization of small details in two ways. Objects smaller than the pixel size of a digital detector will be shown larger and with lower contrast. In addition, the shape information of small objects may be lost, since objects slightly larger than the pixel size will be depicted by a few square pixels [4, 44]. However, both experimental studies [45, 46] as well as clinical trials [47, 48, 49] have shown this to be irrelevant both for detection and characterization of microcalcifications. This is due to the fact that in overview (unmagnified) mammographic images, only microcalcifications larger than approximately 130 μm can be detected [2, 50]. With these small microcalcifications just at the detection threshold, also with film-screen mammography no real shape information is discernible due to screen unsharpness, scatter radiation and geometric blur associated with the larger focus. Although on average there may be no differences in the depiction of microcalcifications between film-screen and digital mammography, both systems may have advantages and disadvantages in certain patient populations. Integrated digital systems with a high DQE imaged at sufficient dose may be superior to film-screen mammography in depicting microcalcifications in dense parenchyma due to higher contrast. This is not true for CR systems, which at clinically acceptable dose levels have a relatively high noise level in areas of dense parenchyma, limiting the visualization of subtle microcalcifications. While in general the lower spatial resolution of digital mammography will not play a role in clinical practice, digital mammography may be at a slight disadvantage in older patients with small and transparent breasts, in whom film-screen mammography may depict details smaller than the usual visibility threshold of around 130 μm. Although not analyzed separately, there may have been a slight advantage for film-screen mammography in the DMIST trial in patients ≥50 years of age and with transparent breasts [51, 52], which could support this hypothesis.
Impact on workflow
Introduction of digital imaging into mammography has significant workflow implications. Most of the advantages of digital mammography are related to getting rid of film. With integrated digital systems, the lack of film cassette handling allows for a higher patient throughput  and lets the technologist concentrate more on the patient. Especially interventional procedures such as preoperative wire localizations are much faster with integrated digital systems without the need for films to be developed between each step of the procedure. Digital images can automatically be transferred, stored and retrieved without the need for human interaction. There are no lost or misplaced films and digital images can be viewed by several different people at the same time. Film library space and personnel are freed up, and the higher investment costs for digital mammography are at least in part compensated for by these savings . When considering the impact of digital mammography on the reading of mammographic studies, the picture is less clear-cut. There is no doubt that images acquired digitally should best be read as soft-copy on a monitor. Only in this way can the main advantages of digital mammography such as tailored image processing and contrast optimization be harvested. However, depending on detector area and pixel size, digital mammograms may have an image matrix of up to 4,800 × 6,000 pixels with a file size of more than 50 MByte. These images are too large to be displayed at full 1:1 resolution on a high-resolution 5-megapixel monitor, the current standard for mammography review workstations. Both hard- and software of mammography review stations has improved significantly over the last few years. Dedicated mammography review stations now allow to switch almost instantaneously between different image layouts including a sequential magnified quadrant zoom, with softcopy reading speed approaching or even surpassing that of batch film reading [56, 57]. Viewing the entire image piece by piece in full resolution may be tedious, but necessary to ensure that no microcalcifications are overlooked. There has been some discussion recently about the use of computer-aided diagnosis (CAD) techniques, which have a near perfect sensitivity for microcalcifications of more than 98%, as a preprocessing tool, only showing those areas of the image in full resolution to the radiologist, where the CAD system has detected possible microcalcifications . This concept has the potential for significantly speeding up softcopy reading of digital mammograms.
Different opinions exist on how to handle prior mammographic films when reading digital mammogram soft-copy. Digitization of prior films is expensive, and image characteristics of digitized films are different from primary digital mammograms, making direct comparison difficult. Keeping a film viewer next to the computer workstation is not only cumbersome, but light from the view box may interfere with the image display on the monitor. Some groups have therefore decided with softcopy reading not to offer prior mammographic films during the primary reading session, but only in the consensus conference in case abnormal findings exist on the current exam requiring comparison with older exams .
Positive predictive value of CAD marks in a screening setting
Average number of CAD marks per normal case
Positive-predictive value (ppv)a
Number of positive CAD marks/abnormal readings to detect one cancer
Digital mammography has established itself as a true alternative to film-screen mammography offering significant workflow improvements, and there is no doubt that in the long run digital mammography will replace film-screen mammography. Although in general the diagnostic accuracy of digital mammography is similar to that of film-screen mammography, digital mammography may have specific advantages in younger women due to the possibility to selectively enhance image contrast in areas of dense parenchyma. Digital mammography enables an array of advanced applications such as contrast-enhanced mammography, tomosynthesis and computer-aided diagnosis, although the value of these new techniques in clinical practice has yet to be shown. Future efforts should aim to further optimize image acquisition parameters in digital mammography resulting in the lowest possible radiation exposure to the breast as well as to improve and standardize image processing techniques.
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