**Image Quality Requirements for Digital Mammography in Breast Cancer Screening**

Margarita Chevalier1, Fernando Leyton2,3, Maria Nogueira Tavares3, Marcio Oliveira3, Teogenes A. da Silva3 and João Emilio Peixoto4 *1Complutense University of Madrid 2Diego Portales University 3Centro de Desenvolvimento da Tecnologia Nuclear 4Instituto Nacional do Cáncer 1Spain 2Chile 3,4Brazil* 

#### **1. Introduction**

114 Imaging of the Breast – Technical Aspects and Clinical Implication

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screen film mammography in the prevalent round of a population-based screening

mammography: comparison within the UK breasts screening program and

Mammography is currently considered to be the best tool for early detection of breast cancer. The target groups of most of the population-based screening programmes are women aged between 50 and 65 years. A recent study has also shown mortality benefit in the age group 40-49 (Hellquist, et al, 2010). Screen-film mammography has been to date the traditional test for breast screening having been shown its efficacy in reducing breast cancer mortality in large randomised trials (Duffy, et al, 2006). The potential advantages of digital mammography over screen-film techniques have been the subject of several investigations which provides an improved diagnosis in dense breasts and an increase in breast cancer detection rate (Pisano et al, 2006; Hendrick et al, 2010).

Breast screening using X-ray mammography only confers a benefit on the screened population if it is able to detect breast cancer at an early stage, whereby the prognosis is improved. This can only be achieved by having high quality breast images to assure as much as possible the detection of small and subtle lesions in the breast (Muller, 1997;Karellas, 2004; Lewin, 2004; ICRU, 2009). High quality mammography must be achieved and maintained by applying rigorous and comprehensive quality assurance and control programmes.

The quality of the breast images depends critically on the design and performance of the radiographic unit, the image receptor, and on how that equipment is used to acquire and process the mammogram. The type of display and the conditions under which the image is viewed have an important effect on the ability of the radiologist to extract the information recorded in the mammogram. The diagnostic information is integrally related to the quality of the image and higher image quality will result in more accurate diagnosis (Nishikawa, 2004). The systematic monitoring of both image quality and radiation dose is needed to

Image Quality Requirements for Digital Mammography in Breast Cancer Screening 117

Fig. 1. Indirect method of image acquisition with CsI(Tl)/a-Si. The CsI scintillators hold needle structures and work as channels which guide the light perpendicularly to the surface

Fig. 2. Method of direct acquisition of an image with a-Se (Peixoto, 2009).

(PSP) which is introduced into a cassette of similar characteristics than the one used with the

Inside the cassette, the photostimulable phosphorus plate is used to absorb and store the energy of the X-ray transmitted through the breast, thus producing a 'latent image'. The energy stored in the phosphorous plate is associated to the electrons raised to excited levels of energy in which they hold trapped ("F-centre"). This is the non-observable electronic latent image, where the number of electrons trapped is proportional to the number of incident X-ray photons (Marcelino V.A. Dantas., 2010). As follows, the cassette is inserted into the reading unit (Fig. 3). Inside this unit, the plate is scanned with a low energy intense laser light (~ 2 eV) which is highly focused. The electrons trapped in the phosphorus photostimulable matrix (PSP) are stimulated by the laser energy, and a significant fraction

of the photodiodes (Peixoto, 2009).

film.

guarantee a constant high quality of the mammography examination (ICRU, 2009; Ng, 2005). Conventional film/screen mammography is being gradually substituted by digital technology in most countries. Consequently, there is an important activity related with developing quality control protocols adapted to this new digital technologies (CEC,2006; SEFM, 2008;NHSBSP, 2009; IAEA, 2011).

Data retrieved from programmes in the Netherlands (Beckers, 2003), Sweden (Leitz, 2001), Norway (Pedersen, 2000) and the UK (NHSBSP, 2003) show that the levels of DG in screenfilm mammography range between 0.8 and 2.5 mGy for a 5.3 cm compressed breast thickness. Thus, several national and international protocols have established an accepted DG limit of 2.5 mGy for a 5.3 cm standard breast thickness. Data from a European survey (Report EUR 14821, 2001) on radiation doses developed in 56 mammography institutions showed DG values ranging from 1.0 to 3.0 mGy for 6.0 cm thick breasts. This value was established from measurements using an acrylic simulator.

This chapter is devoted to describe the relevant parameters and procedures for the quality control of digital mammography systems making the necessary distinctions among the two technologies (computed radiography (CR) and flat panel detectors (DR)).
