Unified Procedures for Quality Controls in Analogue and Digital Mammography

Breast cancer is the most commonly diagnosed cancer in women [1]. Current attempts to control breast cancer concentrate on early detection by means of massive screening campaign, via periodic mammography and physical examination, because ample evidence indicates that such screening indeed can be effective in lowering the death rate [2]. Early diagnosis of breast cancer plays a leading role in reducing the mortality and improving the prognosis of this disease [3]. Mammography consists in imaging the female breast using X-rays with low contrast (to keep the delivered dose low), but at the same time high resolution (especially used for early detection). The goal of mammography is to achieve the image quality required for a given detection task, while ensuring that the patient-absorbed dose is kept as low as reasonably achievable [4]. As practised now, it normally requires a dedicated X-ray tube with special anode materials such as molybdenum or rhodium, small focal spots, operating at a tube voltage around 25 to 32 kV, and carefully chosen films and screens in dedicated cassettes. Stationary or moving grids are used as in other branches of plain film radiography. Present-day mammography can be described as a low-dose procedure [5]. In recent years, advances in screen-film technology and film-processing techniques have contributed to major improvements in the quality of mammographic images. At present, two distinct mammographic techniques exist: oAnalogue mammography in which the image is recorded on a film. oDigital mammography in which the image is digitalised. The production of analogue or digital mammography images is based on two distinct concepts of image formation [6]. The analogue image is a continuous representation of spatial and intensity variations of the X-ray pattern transmitted by the tissue under analysis. Traditionally, the mammographic image is analogue, obtained using conventional screen-film image receptors as the standard detector [7]. The advantages of screen-film mammography are: high spatial resolution and low contrast sensitivity achieved through improvements in X-ray tube design, screen-film combinations, grids, and film processing [8]. Thus, analogue mammography permits high image quality, low patient dose, and most importantly, the ability to detect small, nonpalpable breast cancers. In digital systems, image acquisition and display are two independent processes [4]. In such systems images are captured as a digital signal, making electronic transfer and storage of images possible. Digital systems offer a large dynamic range of operation, improving visualization of all areas of the breast and increasing exposure latitude. Also, the digital format allows grayscale adjustment to optimize contrast for any imaging task. In addition, with the digitalization of the diagnostic image, new medical applications have now emerged, such as Computer-Aided Diagnosis (CAD), stereo mammography, tomosynthesis, contrast medium imaging and dual energy imaging [7]. For a successful screening function the mammograms should contain sufficient diagnostic information to be able to detect breast cancer, using a radiation dose as low as reasonably achievable (ALARA principle). In this context, it is necessary to establish and actively maintain regular and adequate Quality Assurance (QA) procedures that take into account medical, organisational and technical aspects. The QA procedure should include periodic tests to ensure accurate target and critical structure localization. Such tests are referred to as Quality Controls (QC). They are fundamental for the QA procedure because they help ascertain that the equipment performs consistently at a high quality level. However, whilst the requirement for standardisation is imp