In mammograms, the breast tissue pattern is primarily created by the shadow of fatty components as well as the functional element (glandular tissue) of the breast (Kopans, 2007). The differential radiographic contrast of these elements enables X-ray breast imaging to occur (Wentz & Parsons, 1997). Breast composition differs among women, through the relative proportions of fatty and glandular tissue, the higher the proportion of glandular tissue, the more dense the breast. Since the glandular tissue attenuates X-ray more than adipose tissue, the radiolucent fat arises as dark areas, whereas dense glandular tissue appears as light areas, see Figure (3-6) (Boyd et al., 2007; Boyd et al., 2010). However, these attenuation differences are small, therefore high contrast mammographic imaging systems are required to make breast anatomy visible (Wentz & Parsons, 1997).
As was noted in section 3.5.2 (page 46), mammography examinations may require supplementary views in addition to the two basic CC / MLO views (Yankaskas & Gill, 2005). The basic mammographic views were the CC and lateral, used until the late 1970s wherein the MLO view was first described by Lundgren (1977). The MLO view depended on the anatomical fact that when the arm is raised the breast will appear continuous with the pectoral muscle in caudo-medial direction (Lundgren, 1977). It has been found that the use of single-view mammography results in a higher recall rate and leads to reduction in mammographic breast cancer detectability, wherein 11% - 25% of breast cancers can be missed (Kopans, 2007). The radiation dose limit for standard mammographic views was determined by the European Commission, IPEM and NHSBSP to be less than 2.5 mGy per image (European Commission, 2006; IPEM, 2005; Strudley et al., 2014). Since the pectoral muscle is displayed in MLO view, the breast MGD is more than in CC view. This MGD difference between CC and MLO may be up to 30% (Gomes et al., 2011).
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Figure (3-6) Breast MLO image illustrates the radiographic appearance of different breast tissues (Darlington, 2015).
Breast imaging is performed while the breast is compressed onto the image receptor. As mentioned in section 3.4.2 (page 37), breast compression helps to improve image quality and reduces the breast radiation dose (Kita, Highnam, & Brady, 1998). The compression should be sufficient to prevent the slip of breast tissue during exposure but not too much as to cause patient discomfort. Sometimes, pain may result from skin pinch during compression; in this case the compression should be stopped and released (Kopans, 2007).
3.6.1 Cranio-caudal (CC) Projection
CC projection is one of the routinely obtained projections when using conventional, two- dimensional breast X-ray imaging (Kopans, 2007). In CC projection, the X-ray beam is directed to enter the superior aspect of the breast and leave through the inferior aspect. The image receptor is positioned parallel in the horizontal orientation and perpendicular to the central ray (Magnus, 1995). The breast should be positioned on the AEC device. CC projection is used to visualise the beast tissue as a whole, except the most lateral and axillary portion (European Commission, 2006). The ideal CC projection should visualise several important anatomical structures including: the sharp shadow of the pectoral muscle on the border of the image; the shadow of retro-mammary fat tissue; the medial and lateral glandular tissue without folds; a symmetrical view of the right and left breasts; and the nipple
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seen in profile, Figure (3-7) (Gomes et al., 2011; European Commission, 2006). Another positioning consideration when producing CC images is to set the height of the breast support correctly in relation to the infra-mammary angle (Smith, Szczepura, Mercer, Maxwell, & Hogg, 2015; European Commission, 2006). The typical compressed breast shape in CC projection is semi-circular with approximately 10 cm between the nipple and the chest wall and an approximate breast base length of 20 cm for an ‗average‘ woman (Feng , Patel, & Sechopoulos, 2013). The average compressed breast area in the CC projection, which was assessed in 880 British women by Diffey (2012), was 157.6 cm2. A similar result (157.3 cm2) was reported by Boone, Lindfors, Cooper, and Seibert (2000).
1. Represent the shadow of pectoral muscle. 2. The shadow of retro-mammary fat line. 3. Shadow of medial breast tissue.
4. Lateral glandular tissue shadow. 5. Nipple shadow in profile.
Figure (3-7) Shows a diagram of the breast anatomical criteria that should be seen in typical CC projection (IAEA, 2005).
3.6.2 Medio-lateral Oblique (MLO) Projection
The MLO projection visualises the whole breast tissue in one view; especially the supero- lateral part of the breast which is more commonly affected by cancer than other breast parts (Magnus, 1995). For this reason the MLO projection is chosen for DBT imaging. The typical MLO projection should image the breast from the axilla down to include the infra-mammary angle. Unlike other radiography areas, the word oblique in mammography refers to the breast compression plane (Kopans, 2007). Much more care is required to produce MLO images than CC images because an imperfect MLO projection affects the image quality criteria more than that in CC (IAEA, 2005). In MLO projection, the X-ray beam enters the breast through supero-medial aspect and leaves it through the infero-lateral aspect (Magnus, 1995). The
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image receptor is set at an angle of approximately 45o from horizontal, but can range from 40o to 60o depending on the woman‘s body habitus (Mercer, Hill, Kelly, & Smith, 2015). According to the European Commission (2006), the breast anatomical criteria that should be seen in MLO images are: the infra-mammary angle, although the visualisation of this angle as a whole is affected by breast size , it is strongly related to breast positioning; supero-lateral glandular tissue, this criteria can be easily achieved; the shadow of nipple and retro-glandular adipose tissue; the symmetrical image of both breasts with no recognised skinfold; the reproduction of the pectoral muscle in the image angle, which is one of the most important anatomical criteria to indicate the correct positioning in MLO projection, see Figure (3-8) (Gomes et al., 2011; Bentley, Poulos, & Rickard, 2008; European Commission, 2006). In the upper posterior margin of the image, the pectoral muscle should be visualised as a triangular shadow with a mean value of its length and the width at approximately 140 mm and 46 mm (Spuur, Poulos, Currie, & Rickard, 2010). The shape and size of the pectoral muscle in the mammogram image generally depends on individual body variations, thorax length and muscle development, and positioning angulation (Bentley, Poulos, & Rickard, 2008). The breast area demonstrated in the MLO mammogram is greater than that in CC by approximately 17.7 cm2 (Diffey, 2012). Kunosic (2012) stated that in MLO projection the mean compressed breast thickness is 20-23% higher than that in CC projection, but other researchers found that this increment to be around 5mm (Dellie, Rao, Admassie, & Meshesha, 2013; Helvie, Chan, Adler, & Boyd, 1994; IAEA, 2005).
1. Shadow of pectoral muscle. 2. Infra-mammary angle.
3. Superio-lateral glandular tissue. 4. Shadow of retro-glandular fat tissue. 5. Shadow of the nipple in full profile.
Figure (3-8) Shows a diagram of the breast anatomical criteria that should be seen in typical MLO projection (IAEA, 2005).
53 3.6.3 Supplementary Mammographic Projections
Diagnostic mammography can include extra projections in addition to the standard projections (CC and MLO). These supplementary mammographic projections are usually performed for either symptomatic women with palpable breast abnormality, breast discharge, painful breasts, or abnormal skin changes, or for women with suspected or positive screening results (Yankaskas & Gill, 2005). Due to chest geometry, the extreme medial and extreme lateral tissues of the breast cannot be included in the CC/MLO standard mammographic projections, and supplementary projections are usually utilised to investigate the lesions in these aspects of the breast (Kelly, 2015; Kopans 2007). However, these increase patient radiation dose by 3.9 - 5.2 mGy depending on the number of projections (Destounis & Gruttadauria, 2015).