Digital Mammography

Digital Mammography with Computed Radiography

Background

Mammography is a branch of diagnostic radiology that could benefit greatly from the assimilation of digital imaging technologies. This would lead to more efficient and reproducible acquisition of high quality mammographic images. In addition it should ensure superior imaging of patients with radiographically dense breasts and potentially significant reductions in patient dose in mammography. Beyond this digital imaging technologies will support a range of novel digital imaging facilities to enhance mammographic services. For example the combination of digital image detectors with high resolution softcopy workstations and digital image archival will make film-free mammography possible. Other digital facilities such as tele-mammography and computer aided diagnosis are now becoming a technical reality in clinical service. Obviously an essential pre-cursor to all these advances is the availability of devices to capture high quality breast X-ray image data at the outset.

CR Based Mammography

The X-ray Imaging Group at Leeds University has been interested in developments in digital mammography for over a decade. In 1990 in co-operation with Dr Geoff Parkin, (radiologist responsible for mammography services at the General Infirmary at Leeds), this group participated in a major research project to investigate the technical and clinical feasibility of photo-stimulable phosphor computed radiography based mammography. This three year R&D project was funded by the UK Department of Health and Philips Medical Systems.

The project investigated the technical and operational requirements for an effective clinical mammography service based upon CR. Notable among the project's findings were the needs to develop digital image enhancement algorithms matched to the characteristics of breast X-ray images, plus the need for high physical performance standards to ensure optimum image acquisition and display.

The study also confirmed that arguments regarding minimum spatial resolution requirements for digital mammography was of much less clinical significance than had previously been stated. Specifically the Leeds project showed that a digital imaging matrix based upon 100 micron pixel size provided by CR mammography represented a practical minimum spatial resolution specification for viable clinical mammography. For example a psychophysical study showed that the minimum size of (micro-calcification like) detail visible on both modern screen-film mammography and CR mammography is ~ 130 microns, for currently accepted levels of dose per image. The superior detective quantum efficiency, contrast rendition and exposure dynamic range of CR mammography plus lower repeat rate, lead overall to a superior imaging specification which in practice more than compensated for the modest spatial resolution.

The project was originally established as a laboratory based investigation using a Philips 7000 CR reader in the radiology department but all image processing and printing facilities off line in the Medical Physics Department. However early technical successes meant that by 1992 the focus of the project could then shift to the evaluation of CR mammography in the clinical department. A programme of studies to compare the diagnostic image quality of conventional and CR mammography was set up during 1993 which confirmed the viability of the latter in clinical use. As a result Dr Parkin reported that "CR mammography offered superior rendition of the skin edge plus sub-cutaneous tissue and dense parenchyma while ensuring adequate detectability of micro-calcification clusters".

Clinical Implementation

During 1994 a Philips CR system incorporating the imaging specification outlined above was installed in the clinical Mammography Suite at the General Infirmary. The clinical CR installation comprised a Philips AC-3 compact computed radiography acquisition system using HR-V high resolution image plates, plus an EasyVision RAD workstation. A conventional design of mammography X-ray unit (Philips UBC) was used to expose the patient and CR image plates.

Layout of CR installation in 1994

Digital image enhancement was performed not by the default image processing software, but via a special image enhancement option known as "Dynamic Range Regulation or DRR", which was developed specifically for the clinical phase of the project. Clinical mammograms were presented to the radiologist as laser hardcopy images.

4 on 1 filmImages were printed on an Imation HQ 969 hardcopy unit installed in an adjacent room in the clinical department. Images have been printed in a four (-breast) images on one (-film) format with corresponding pairs of MLO and CC images printed side-by-side. This format provided a cost-effective, efficient and elegant presentation of each clinical examination. The EasyVision RAD workstation supported all image enhancement and printer image composition functions automatically with minimum intervention by radiographic staff.

Since 1994 the clinical mammography service provided by the Infirmary has been supported exclusively by computed radiography and no screen film mammography has been performed. In the intervening period Dr Parkin's team has successfully produced approximately 20,000 digital mammography images using CR.

Future Directions

The clinical CR mammography service described above is based upon a modern but entirely conventional design of CR acquisition device. The associated EasyVision RAD software however has been upgraded to support specialised image enhancement and print format facilities. More recently the Fuji Company in the USA have launched a new CR system designed specifically for mammography with a significantly higher specification than that available at Leeds. This system features finer matrix resolution (50 microns pixel size), higher DQE image readout, a new suite of digital image enhancement software as well as digital support facilities for digital image archival, soft-copy reporting and computer aided diagnosis.

Beyond this a new range of solid-state digital X-ray image detectors (usually of flat-panel design) designed specifically for mammography have been developed. Among these is the Senographe 2000D Full-field digital mammography. Again this system boasts a wide range of digital support facilities. One potential benefit of this technology may be greater dose efficiency than either conventional or CR mammography. No doubt further exciting designs of solid-state digital mammography system will continue to emerge.

Mammography is entering a new era when film-based imaging will come under increasing pressure from competing digital products as the imaging limitations of the former come under increasing scrutiny, and the wider benefits of the latter become firmly established. We encourage those responsible for managing and operating mammography equipment to explore these benefits for themselves.

LXi's Mammography publications

Parkin GJS and Cowen AR "Digital Mammography with CR: Experiences with 20,000 patients," Medica Mundi, Volume 45(2), pages 11-15, 2001. [detail]

Coleman NJ, Cowen AR and Parkin GJS "The effect of the anti-scatter grid on computed mammography image quality," Radiography, Volume 6(3), pages 199-204, 2000. [detail]

Cowen AR "A tutorial on digital mammography imaging equipment Part 1: Advances in image acquisition & display," Radiography, Volume 4, pages 159-171, 1998. [detail]

Cowen AR "A tutorial on digital mammography imaging equipment Part 2: Development in imaging support technologies," Radiography, Volume 4, pages 239-249, 1998. [detail]

Conmy PM, Parkin GJS, Cowen AR, Davies AG and Kengyelics SM "Effect of lossy wavelet compression on the threshold detectability of low contrast features in digital mammography," In: , Eds. Piqueras J and Carreno JC, pages 99-102, 1998, Publisher: (). [detail]

Cowen AR, Parkin GJS and Hawkridge P "Direct digital mammography image acquisition," European Radiology, Volume 7(6), pages 918-930, 1997. [detail]

Cowen AR, Launders JH, Jadev M and Brettle D "Visibility of micro-calcifications in computed mammography and screen film mammography," Physics in Medicine and Biology, Volume 42(8), pages 1533-1548, 1997. [detail]

McLeod G, Parkin GJS and Cowen AR "Automatic detection of clustered micro-calcifications using Wavelets," In: , Eds. Doi K, pages 311-316, 1996, Publisher: Elsevier Science BV (Amsterdam). [detail]

Brettle D, Thompson J, Parkin GJS and Cowen AR "Technical note: Dual compression mammography using computed radiography," British Journal of Radiology, Volume 68, pages 761-763, 1995. [detail]

Brettle D, Workman A, Parkin GJS and Cowen AR "Computed Radiography as a direct digital mammography image acquisition system," In: , Eds. Gale AG and Astley S, pages 163-172, 1994, Publisher: Elsevier Science BV (Amsterdam). [detail]

Brettle D and Cowen AR "Dual-energy digital mammography utilising storage phosphor computed radiography," Physics in Medicine and Biology, Volume 39, pages 1989-2004, 1994. [detail]

Brettle D and McLeod G "Automatic micro-calcification localisation using matched Fourier filtering," In: , Eds. Gale AG and Astley S, pages 21-30, 1994, Publisher: Elsevier Science BV (Amsterdam). [detail]

Brettle D, Ward SC, Parkin GJS, Sumsion H and Cowen AR "Clinical (ROC) comparison between conventional and digital mammography utilising computed radiography," British Journal of Radiology, Volume 67, pages 464-468, 1994. [detail]

Sumsion H, Davies AG, Parkin GJS and Cowen AR "Application of a computed radiography database to a mammography reporting environment," In: , Eds. Gale AG and Astley S, pages 379-386, 1994, Publisher: Elsevier Science BV (Amsterdam). [detail]

Sumsion H, Parkin GJS and Cowen AR "A training and assessment package for digital mammography," In: , Eds. Gale AG and Astley S, pages 367-386, 1994, Publisher: Elsevier Science BV (Amsterdam). [detail]

Giles A, Cowen AR and Parkin GJS "Clinical workstation for digital mammography," In: Medical Imaging 93: Biomedical Image Processing and Computer Vision, Eds. Goldgof DB, pages 806-817, 1993, Publisher: Society of Photo-Optical Intrumentation Engineers (Bellingham, WA). [detail]

Cowen AR and Brettle D "Technical note: compensation for field non-uniformity on a mammographic X-ray unit," British Journal of Radiology, Volume 66(782), pages 150-154, 1993. [detail]

Cowen AR, Brettle D and Giles A "An investigation into digital mammography using photo-stimulable phosphor computed radiography," In: Applications of Image Processing in Mass Health Screening Digest No. 1992/056, Eds. , pages 5/1-5/6, 1992, Publisher: Institute of Electrical Engineers (London). [detail]

Cowen AR, Brettle D, Coleman NJ and Parkin GJS "A preliminary investigation of the imaging characteristics of photo-stimulable phosphor computed radiography using a new design of mammography quality control test object," British Journal of Radiology, Volume 65(774), pages 528-535, 1992. [detail]

Parkin GJS, Cowen AR, Brettle D and Giles A "Direct digital mammography utilising a computed radiography. system : clinical evaluation and potential improvements," In: Workshop on QA and Radiation Protection, Eds. Busch HP and Georgi M, pages 68-69, 1992, Publisher: Schnetzor-Verlag GmbH (Konstanz). [detail]