Chantal Van Ongeval MD
André Van Steen
Department of Radiology
University Hospitals Leuven Belgium
Full-field digital mammography (FFDM) has definitive (practical) advantages over screen-film mammography (SFM). For some systems, the workflow can be markedly increased. Images are digital; thus, they can be archived electronically and sent to a remote site for second reading. Computer-aided detection (CAD) can be applied on the full depth of the data, and dedicated image processing can improve the perception of the lesions. The performance of a digital system, however, should ultimately be evaluated by clinical patient studies. The role of the new modality for screening should be tested in a screening environment. The first such studies were run on a Senographe 2000D (GE, Milwaukee, WI, USA). These studies concluded that there was no difference in detection rate between FFDM and SFM.(1–3) In the ACRIN (American College of Radiology Imaging Network) trial, multiple systems of different vendors were included. This trial also concluded that the overall diagnostic accuracy of digital and film mammography in a screening environment is similar for the entire population, but that FFDM is more accurate in women under the age of 50 years.(4)
Technical acceptance tests aim to guarantee an acceptable level in terms of clinical performance. A first protocol was recently published by the European Commission.(5) The main system test discussed in this protocol is the contrast-detail analysis. This test relies on a test object with inserts of different size and depth and limiting values on the minimal thickness (or contrasts) that should be visible for specified diameters. This test is the subject of a lot of debate in the literature. There is an important degree of subjectivity related to this test, as images have to be scored manually.
The main parameters that characterise the performance of the detector are: the spatial resolution (or modulation transfer function: MTF), noise and detective quantum efficiency (DQE).(6) Currently, the digital detector is still considered to have a dominant impact on image quality, while processing protocols and viewing conditions are often not evaluated.(7) The link between these parameters and clinical performance remains a challenge today. We are in need of more information regarding their impact on clinical performance. One next challenge is concerned with the use of these modalities in screening. How should all digital advantages be used efficiently?
Clinically and physically, the overall conclusion is that FFDM should become the method of choice in the detection of breast cancer. But before the method can be considered a mature technology, the challenges, as formulated above, should be addressed.
The first group of challenges is related to the specific technical characteristics of FFDM
The requirements on spatial resolution, contrast resolution and noise of a detector system depend on the radiological task. Breast cancer can be present as microcalcifications or masses. Detection of calcifications requires a very high-resolution imaging technique.(8) Masses induce only small changes in contrast, and their detection necessitates high-contrast images. Compared with general radiology, mammography is a very demanding technique. The spatial resolution of digital mammography is limited by the pixel size. FFDM has intrinsically a lower spatial resolution than SFM. This could cause problems in detecting subtle microcalcifications. However, a better performance at lower spatial frequencies seems to compensate for the relatively large pixel sizes. The apparent contrast resolution is largely dependent upon the energy of the X-rays and the image processing that takes place in the computer system after signal acquisition. Differences in X-ray densities can be more accentuated by post- processing (see Figure 1). Image processing may react in very different ways on these anatomical backgrounds. Processing is often driven by the X-ray distribution in the detector and the texture of the breast. The influence of the X-ray distribution on the result of the applied processing is only rarely investigated. Small studies on the evaluation of image quality showed that processing was not optimal in all patients and for all exposure settings. With inappropriate postprocessing tools, differentiation between noise, calcifications and small densities can be difficult.
In practice, aspects of spatial resolution, contrast resolution, noise and processing are interrelated. It is difficult to predict the outcome of combinations of characteristics on image quality. Image quality criteria are necessary for the quantification of the global image quality. The experience with such criteria remains limited today and, as far as we know, even criteria for conventional mammography have not been applied on a large scale, and limiting values or acceptance levels have not been proposed.(9)
In digital mammography, there is a shift from hardcopy to softcopy display and reading. Many different display stations are on the market, but until now there has been no investigation of the effect of a monitor on the reading results of the radiologist. Practical tools are being developed to control the stability of the monitors.(10)
The second group of challenges is related to screening mammography
In the screening population, FFDM offers specific advantages. Integrating FFDM into an existing screening programme, however, creates a lot of practical problems, in terms of both informatics and image quality. Different types of imaging systems are on the market. All of them have other physical charateristics. In addition, most manufacturers have developed image-processing algorithms along with their acquisition system. This results in an always different look to images in the different systems. Images of subsequent runs in a screening programme can therefore appear very variable because of upgrading of the system, changes in the applied processing, etc. Image processing will be vendor-specific, and image quality differences, induced by different quality of processing, will be obvious.
Radiologists involved in second reading (ie, reading images from other mammographic sites) have to deal with all these different kinds of images (see Figure 2). Can they compare conventional and digital images of subsequent rounds of the same patient?
For all these reasons, radiologists face an important learning stage. Dose reduction while maintaining sufficient image quality is of high importance in a screening population, especially when screening starts at the age of 40 years.(4,11) For a lot of systems, it remains to be explored at which dose level images should be acquired. The cost-benefit of a screening programme should not be worse than with conventional systems. This figure of merit of the screening programme should be reassessed. As CAD might be used as a second reader and as this tool has already been shown to improve cancer detection, application of CAD to all the images of a screening population is of great interest. The improvement, however, is dependent on the experience of the radiologist in mammographic reading and on the type of lesion (better for calcifications than for masses).(11)
FFDM is already a solution to many of the inherent limitations of screen-film mammography. A series of items are not fully understood today. The influence of the type of detectors, processing and display possibilities on the image and on cancer detection is not sufficiently investigated. More studies are also needed to explore the optimal dose levels. Finally, there is only limited experience with turning an existing screening unit into a fully digital environment that takes full advantage of the digital nature of these systems.
- Skaane P, Young K, Skjennald A. Population-based mammography screening: comparison of screen-film and full-field digital mammography with soft-copy reading – the Oslo I study. Radiology 2003;229:877-84.
- Skaane P, Skjennald A. Screen-film mammography versus full-field digital mammography with soft-copy reading:randomized trial in a population-based screening program – The Oslo II Study. Radiology 2004;232:197-204.
- Lewin FM, Hendrick RE, D’Orsi CJ, Isaacs MF. Comparison of full-field digital mammography with screen-film mammography for cancer detection: results of 4.945 paired examinations. Radiology 2001;218:873-80.
- Pisano ED, Gatsonis C, Hendrick E. Diagnostic performance of digital versus film mammography for breast-cancer screening.N Engl Med 2005;353.
- European Commission. European Guidelines for Quality Assurance in Mammography Screening Part B: Digital mammography. In the Fourth Edition of the European Guidelines for Breast Cancer Screening. In print by the European Commission. Downloadable from:www.euref.org
- Noel A, Thibault F. Digital detectors for mammography: the technical challenges.Eur Radiol 2004;14:1990-8.
- Bosman s H, Carton AK, Rogge F, Zanca F, Jacobs J, Van Ongeval C et al. Image quality measurements and metrics in full-field digital mammography: an overview. Rad Prot Dosimetry 2005;117(1-3):120-30.
- Fischer U, Baum F, Obenauer S, et al.Comparative Study in patients with microcalcifications: full-field digital mammography vs screen-film mammography.Eur Radiol 2002;12:2679-83.
- Van Ongeval C, Bosmans H, Van Steen A. Current challenges of full field digital mammography.Rad Prot Dosimetry 2005;117(1-3):148-53.
- Jacobs J, Deprez T, Rogge F, Marchal G, Bosmans H. Validation of a new dynamic pattern for daily quality control of medical screen devices. 91st Scientific Assembly and Annual Meeting of the RSNA, McCormick Place, Chicago, November 2005.
- Fischer U, Hermann KP, Baum F. Digital mammography: current state and future aspects. Eur Radiol 2006;16:38-44.