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Breast imaging: digital mammography.


Technological advances have resulted in a significant improvement in imaging methods. The art of breast imaging has traditionally relied on analogue screen-film mammography (SFM), although ultrasound scan and magnetic resonance imaging (MRI) scans are now routinely used as adjuncts to mammography in the evaluation of patients. Further advances have now led to the introduction of new technologies in breast imaging, such as digital mammography. In this article, we will review the current advances in imaging techniques in breast radiology with particular reference to digital mammography.

Digital mammography

The National Health Service Breast Screening Programme (NHSBSP) began in 1998, screening women 3-yearly from the ages of 50 to 65 years with two-view SFM at the initial screening session followed by single-view mammography at the following screening sessions. This was subsequently extended to two-view mammography in women up to the age of 70 years by 2005. The next phase in the evolution of the NHSBSP, along with the proposal to extend the screening programme to women from ages 47 to 73 years [1], is the conversion from analogue SFM to full-field digital mammography (FFDM).

The evidence

There have been numerous studies on digital mammography and its utilisation in the screening population. Despite the technology being available since the early 1990s, the uptake and implementation of digital mammography has been slow with the high cost being one of the major reasons. This is compounded by a lack of data conferring an advantage of digital mammography over SFM. The initial results were disappointing as many studies have shown that there is no difference in cancer detection rate observed between SFM and FFDM [2-5]. Other studies [6,7] have suggested that there is no improvement in the positive predictive value for the diagnosis of breast cancer with FFDM compared with SFM. However, two studies [2,4] have shown that FFDM resulted in lower recall rates than SFM. This is an important point to consider as it has been shown that women with benign disease who were recalled for further investigation suffered adverse psychological consequences [8,9] with 15% of these women not returning for their subsequent routine 3-year screening [8].

Other more recently published studies on digital mammography have shown more encouraging results.

The Oslo II study [10], which involved 23,929 women aged 45 to 69 years who were randomly assigned to FFDM or SFM, showed that FFDM resulted in a statistically significant higher cancer detection rate than SFM. In total, 16,985 of these women underwent SFM and 6944 underwent FFDM. The overall cancer detection rate was 0.59% for FFDM and 0.38% for SFM. The positive predictive values, however, were comparable between the two imaging modalities.

Further studies have also shown a better cancer detection rate with digital mammography, especially lesions seen as microcalcifications [11,12], and allowed correct classification of more breast cancers than SFM [13]. It has also been shown that FFDM is superior to SFM in detecting simulated small masses and microcalcifications even at a lower radiation dose [14].

Other recent large trials have produced mixed results. The Digital Mammographic Imaging Screening Trial (DMIST) was a study performed in North America involving 49,528 women who underwent both digital and film mammography. Although this has shown no significant difference in diagnostic accuracy between FFDM and SFM [15], the DMIST did show that digital mammography is more accurate in women under the age of 50 years, women with radiographically dense breasts, and premenopausal and perimenopausal women [15,16]. The increased accuracy in women of younger age and in those with dense breasts is particularly relevant in patients with familial breast cancer in view of the publication from the National Institute for Health and Clinical Excellence (NICE) recommending annual mammographic surveillance in this group of women [17]. The extension of the NHSBSP to include women from the age of 47 years [1] would also involve screening of younger women with dense breasts and digital mammography may prove to be beneficial in this group of women.

Advantages of a digital system

Apart from the benefits for certain groups of patients discussed above, the main advantage of the digital system is the separation of the processes of image acquisition, processing, display and storage. This enables the optimisation of each step in the hope of further improving the quality of the images. The process of image acquisition is faster with digital mammography which therefore leads to greater throughput of patients. The images can be checked immediately to ascertain whether the film is adequate and repeated if necessary without having to bring the patient back into the mammography room. The images are processed and typically reviewed on display monitors with high spatial resolution. This method of soft-copy reporting enables radiologists to manipulate the images by altering the window and magnifying the images as necessary. FFDM has a wide dynamic range and post-processing capabilities which may lead to a reduction in recall rates and improvement in the interpretation of dense breasts. FFDM has been shown to produce better image quality and lesion characterisation although the lesion detection rate was similar to that of SFM [18]. Other advantages of digital mammography include the ease of storage, retrieval and transmission of images. This would be in line with the government plan for a filmless radiology department with Picture Archiving and Communications System (PACS) [19]. Radiologists would have the ability to obtain second opinions from other radiologists, possibly from other remote sites.

The retrieval of images would also be much easier for presentation at multidisciplinary team meetings and conferences, and for teaching.

Future applications

The conversion to a digital system allows the option of development and application of other technologies to mammography. One of these technologies is digital tomosynthesis. Early indications have suggested that digital tomosynthesis may reduce the selection of patients for biopsy, improve cancer detection rates [20] and reduce recall rates [20,21]. Digital tomosynthesis is similar to conventional X-ray tomography, involving a moving X-ray source and digital detector to enable the acquisition of a three-dimensional volume of data. The X-ray source moves along an arc and acquires data through a series of 11 sections using a low dose of radiation during the image acquisition [20]. The images are then reconstructed using algorithms as in computed tomography. Digital tomosynthesis eliminates the problem of tissue overlap and would be particularly helpful in women with dense breasts. This technique is currently not in clinical use but further studies are being performed to evaluate its potential for use in clinical practice.

Contrast-enhanced mammography relies on tumour angiogenesis for identification and characterisation of malignant lesions. Similar to contrast-enhanced MRI, this technique relies on the uptake and washout of contrast material. An advantage of digital mammography over MRI is the high spatial resolution, and the application of contrast enhancement improves the mammographic visibility of lesions while retaining the ability to analyse the lesions in detail [22]. A pilot study has shown that contrast mammography may be useful in detecting lesions, especially in dense breast, although further research is required [23].

Computer-aided detection (CAD) relies on computer software to assist the reader to detect abnormalities by placing prompts over possible abnormal areas on mammograms which may otherwise be overlooked. CAD has been shown in many published trials to increase the cancer detection rates in screening mammograms [24-26]. A study [24] involving 21,349 screening mammograms showed an increase in the cancer detection rate by 7.6%. The detection rate of microcalcification was 100% in another study [26], although this study only included 93 patients in total, all of them with breast cancer.

The concern with CAD is the high false-positive rates of up to 0.29 per image [26]. The expansion of digital mammography makes the application of CAD easier compared to the analogue films which had to be digitised first, which is a time-consuming and labour-intensive process. With regard to the NHSBSP, a study involving 10,267 mammograms was performed to compare the performance of a single reader with CAD versus double reading [27]. This study, the Computer-Aided Detection Evaluation Trial (CADET) I study, has shown that the performance of a single reader with CAD is equivalent to the performance of double reading. The cancer detection rate was 6.5% higher for single reading with CAD but the recall rate was also higher at 8.6%, compared to 6.5% for double reading. The application of CAD may assist in the workforce problems in breast radiology.

The CADET II study, which is a prospective multi-centre study evaluating the use of CAD in the NHSBSP, is due to be published in the near future.


As discussed above, it has been shown that FFDM is at the very least comparable to SFM in lesion detection and is advantageous in premenopausal and perimenopausal women and in those with dense breasts; this would be particularly relevant in the screening of the patients under the age of 50 years and those with familial breast cancer. The use of a digital system allows optimisation of image acquisition, processing, display and storage which could lead to further improvement in image quality. It also enables the application of other techniques that may improve lesion detection and characterisation. The above advantages would have to be balanced against the high cost.

Future development in this technology should bring further improvement in the performance of the digital system. This should translate into improvement in clinical practice and lead to reduction in mortality in these patients.


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Yit Lim and Caroline Boggis

Department of Radiology, Nightingale Centre, University Hospital of South Manchester, UK

Correspondence to: Yit Lim, Department of Radiology, Nightingale Centre, University Hospital of South Manchester, Southmoor Road, Wythenshawe, Manchester M23 9LT, UK (email:
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Title Annotation:Feature Article
Author:Lim, Yit; Boggis, Caroline
Publication:Advances in Breast Cancer
Article Type:Report
Geographic Code:4EUUK
Date:Jun 1, 2008
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Next Article:Targeted therapies in breast cancer.

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