La imagen dentro de un contexto situacional y cultural

2.5.1 Mammography

The production of low contrast, fine detail, two-dimensional breast images using X-ray is known as mammography (Law, 2006). Unlike screening mammograms, which are undertaken on healthy asymptomatic women, diagnostic mammograms are utilised to investigate breast tissue abnormalities in women with either breast symptoms or abnormal findings resulting from a screening mammogram (ACS, 2015a). To date, there are two technologies for mammography - conventional film-screen mammography or digital mammography, which has progressively replaced film-screen techniques (Schueller et al., 2008). The main advantages of digital mammography over film-screen are that it is more sensitive in cancer detection within dense breasts and has a lower associated radiation dose (Pagliari et al., 2012). However, some film-screen systems can produce images with spatial resolution three times better than those of digital detectors (Obenauer, Hermann, & Grabbe, 2003). Overall, studies such as the Digital Mammographic Imaging Screening Trial (DMIST) have illustrated that for a screening population (50-69 years old) both film-screen and digital mammography are equal in diagnostic accuracy. Nevertheless, the use of digital mammography improves diagnostic accuracy for younger population with dense heterogeneous breast (Pagliari et al., 2012; Pisano et al., 2005; Thierens et al., 2009).

Despite the reported superiority of digital mammography in breast cancer detection, it has been found that 20% - 30% of breast cancers cannot be detected by two-dimensional mammograms due to the superimposition of dense breast tissue with cancers (Rafferty et al., 2013). To overcome the mammographic ‗anatomical noise‘, Digital Breast Tomosynthesis (DBT) is used to produce three-dimensional images by reconstructing low dose, two- dimensional, thin slice images of breast volume (Svahn et al., 2012). This is achieved, as in conventional tomography, by the movement of the X-ray tube across an arc or linearly above the breast in order to expose the breast at different small angles (Williams, Judy, Gunn, & Majewski, 2010). Consequently, each plane of the breast can be clearly seen with less tissue overlap, thereby improving the lesion detectability (Young, 2006).

16 2.5.2 Breast Computed Tomography (BCT)

Compared to DBT, dedicated BCT allows the acquisition of high resolution volumetric breast image data (Sechopoulos, Bliznakova, Qin, Fei, & Feng, 2012). Accordingly, BCT can overcome the problem of tissue overlap resulting in accurate detection of breast cancers whilst at the same time obtaining more detailed information about the shape, location, and size of any lesion (Shen et al., 2014). Dedicated BCT consists of a gantry, which encloses the X-ray tube and detector assembly, that rotates around the breast during imaging. To avoid unnecessary radiation to the woman‘s chest, the breast is protruded downward through an opening in the patients‘ table while the woman is lying in prone position; see Figure (2-3) (Shaw & Whitman, 2013). Breast compression is not required during BCT making the technique more comfortable than the conventional mammographic procedure (Shen et al., 2014). The potential cost of BCT comes from the complex processes associated with data acquisition, analysis, visualisation, and interpretation (Russo, Coppola, Mettivier, Montesi, & Lauria, 2009). Although the reduced dose in dedicated BCT when compared to conventional chest CT, dedicated BCT exposes breast tissue to a higher radiation dose than mammography (Sechopoulos et al., 2012).

(a) (b)

Figure (2-3) Shows a diagram for dedicated breast CT (a) Woman positioning and (b) Gantry design with examined breast (Shaw & Whitman, 2013).

2.5.3 Breast Ultrasound

Breast ultrasound is one of the most common tools used for breast cancer detection. Since ultrasonography has lower specificity for breast cancer detection than mammography, it is primarily used as a diagnostic tool within a triple assessment process (Jan, Mattoo, Salroo, & Ahangar, 2010; Silverstein et al., 2009). The lower cost of ultrasound compared to

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mammography is the main advantage of breast ultrasonography over mammography. Also, more detailed information about lesion size, boundary and blood circulation can be obtained by ultrasound (Wang et al., 2013). However, the high false-positive rate and the time required for ultrasonic examinations make breast ultrasonography unlikely to be a cost- effective screening technique. Therefore, it is used as a supplementary screening tool for intermediate cancer risk women and for those with dense breasts (Kopans, 2007).

Breast ultrasound depends on the production of longitudinal mechanical waves with frequencies higher than the audible range (20 - 20000 Hz). For medical imaging 1 - 15 MHz frequencies are used. In general, higher frequencies result in better resolution within the ultrasonic image so that more than 10 MHz frequency is often required for mammography. The scan head of an ultrasound system contains a number of piezoelectric crystals (transducers) which emit the ultrasound wave; they then receive the reflected back waves from the tissue. The reflected portion is used to determine the [acoustic] properties of the tissue. Then, the reflected ultrasonic waves are recorded over time to produce a two- dimensional image called a B-mode image. Colour and power Doppler techniques are additional tools of diagnostic ultrasound. Both of them are of great importance in breast cancer detection because they give information about a lesion‘s vascularity. The principle of work for both is similar and depends on ultrasound frequency shift due to blood motion. Colour Doppler ultrasound gives information about the speed and direction of motion. Power Doppler ultrasound is more flow sensitive than colour Doppler but it does not give information about flow direction (Kopans, 2007; Whitman, Khisty, & Stafford, 2013). 2.5.4 Breast Magnetic Resonance Imaging (MRI)

Clinical MRI utilises a strong magnetic field to polarize the magnetic moment of water molecule protons in the body. Next, an oscillating magnetic field, within the radiofrequency range, is applied to rotate the magnetisation vector into a transverse plane to the magnetic field. This oscillating magnetisation is measured by a radiofrequency coil which gives the primary MRI signal (Lane, Stafford, & Whitman, 2013). The MRI signal produces high contrast cross-sectional images of breast tissue (Saslow et al., 2007). Similar to breast CT, in MRI procedures women should lie prone with their breast protruding in a specially designed platform. No breast compression is required. However, the MRI scan time is long, often up to

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an hour, as in most cases contrast enhancement is required (ACS, 2015a). Breast MRI is widely used for screening high risk women and for pre/post-operative evaluation of breast cancers (Lane et al., 2013).