Chapter 15 Mammography
Within general radiography the differences in density of varying parts of the subject (the subject or patient contrast) is usually visible at 45 kV. This is due to the attenuation of the X-ray beam by various tissues being dependent on the cube of the proton number of the structure (Z3):
Soft tissue radiography is utilised when imaging structures where the proton numbers of the tissues are very similar to amplify the differences in density to allow the different tissues to be identified.
The relative radiolucency of breast tissue also demands an X-ray beam of penetrating ability (beam quality) of 20–35 kVp. In mammography the lowest kVp should be selected so as to give the highest subject contrast, as the absorption coefficient of soft tissue falls rapidly with an increasing kV at low energies. However, if the energy is too low, there will not be enough intensity to penetrate the breast to produce an image. This will just increase the radiation dose and time of the exposure. So there will be a compromise between good image quality and keeping radiation doses as low as reasonably practicable. Therefore, to optimise the image the breast must be compressed; this helps reduce the overall thickness and, consequently, the radiation dose (as there is improved beam penetration), reduce the risk of movement unsharpness, and improve the contrast resolution, by reducing scatter. The compressed breast will be closer to the image receptor, which improves the spatial resolution.
The adult breasts are two hemispherical organs situated on the anterior and lateral aspects of the chest wall, on the surface of the pectoralis major muscle (Fig. 15.1). The breast extends from the second rib superiorly (and clavicle) to the sixth rib inferiorly, and laterally from the mid-axillary line to the sternal edge medially. The glandular tissue in the upper outer quadrant of the breast, which extends into the axilla, is known as ‘axillary tail’. The nipple usually lies at the level of the fourth/fifth rib space; however, the breast is a mobile structure, which varies considerably in size and shape, so this should not be seen as a reliable surface marking.
The adult breast is composed of glandular tissue and fatty tissue, which accounts for the variation in size and shape. The glandular tissue is formed from 15–20 lobes, each consisting of several small lobules. A lobule has a number of secretory alveoli, which open into lactiferous ducts. The lobule and the duct form the basic unit of histopathology as the majority of benign and malignant lesions arise in the terminal ductal lobular unit (TDLU). Each lobe of the breast is drained by one lactiferous duct, which passes upwards to form dilated sinuses (ampulla) behind the areola. The areola is the area of pigmented skin surrounding the nipple. The lactiferous sinuses exit by individual orifices onto the nipple surface. The fatty tissue lies under the skin, separating it from the irregular boundary of the anterior surface of the glandular tissue known as the superficial fascia. Fibrous strands, which extend from the skin to the superficial fascia, form the suspensory ligaments (Cooper’s ligaments).
The lymph drains from the central portion of the breast, the circumareolar region and the skin surface, into a plexus of vessels on the surface of the pectoral muscle (Fig. 15.4). Approximately 75% of lymph drains into the axillary glands with only 25% passing from the medial half of the breast into the internal mammary nodes. There is also a limited amount of crossover between breasts and into the abdominal lymphatics. The relatively high occurrence of cancers in the upper outer quadrant and central areas of the breast and the fact that the majority of the lymphatic drainage passes into the axillary region have a direct importance in the selection of the mediolateral oblique as a screening position for breast cancer.1
The adolescent female breast develops from rudimentary tissues that start to form during fetal development. There are varying periods of inactivity; however, the process speeds up during pre-puberty and puberty. The adolescent breast has a greater amount of developing glandular tissue but the shaping is usually caused by the accumulation of adipose tissue. The breast is a radiosensitive organ that is at greatest risk from radiation between 10 and 19 years of age, but there is also a risk between the ages of 20 and 39.
The adult breast is less dense than that of the adolescent and has a greater fat content. The exception to this occurs during pregnancy and lactation when the TDLU increase in size and the epithelial cells prepare for the secretion of milk. After lactation ceases the TDLU reduce in size and number.
During the menstrual cycle changes occur in the breast which relate to the hormonal effects on the epithelial cells in the TDLU. Although the changes do not appear on the image the discomfort experienced during the pre-menstrual period may produce difficulties in positioning and compression of the tender breast.
The density of the breast tissues on a mammogram image depends on the relative amounts of glandular, fibrous and adipose tissue present (degree of obesity of the patient). There is a gradual decrease in the glandular tissue of the breast, which is termed as involution. This can begin to occur as early as 35 years and is associated with the decline and cessation of ovarian function. In the menopausal and postmenopausal period (45–75 years) there is a more noticeable increase in fat deposition and glandular tissue reduction. Screening mammography is carried out between the ages of 50 and 64+; this correlates to the menopausal/postmenopausal phase of the breast. On a mammogram this is indicated by the majority of the dense tissue occupying the upper outer quadrant, hence the indication for the mediolateral oblique. This also becomes important in the positioning of the automatic exposure control (AEC) chamber (sometimes also referred to as an automatic exposure device, AED), as the exposure will be insufficient to penetrate the dense tissue if the AEC is under an area of adipose tissue.
A constant potential generator supplies the X-ray tube. The source–object distance (SID) is shorter than normal diagnostic X-ray examinations (45–60 cm) and the use of a fine focal spot of between 0.15 and 0.4 mm reduces geometric unsharpness. Image quality needs to be of an absolute high standard (i.e. micro calcifications, fine details).
With mammography equipment the tube housing is more compact than a standard X-ray tube (Fig. 15.6). The lower kVp employed is a favourable factor in permitting a smaller tube housing. Both high-tension cables enter the tube housing via the anode end to avoid patient contact. One particular type of unit has a system whereby the tube head moves out of the way during procedures such as biopsy or stereotactic localisation. As well as being able to move the X-ray tube head there is also a Perspex face guard to prevent the head from moving into the X-ray field. The tube is orientated with the anode further away from and the cathode closer to the patient, thus utilising the ‘anode heel effect’. The increased output at the cathode side of the X-ray tube is directed at the thicker chest wall edge of the breast whilst the ‘heel effect’ is over the thinner nipple edge.
The main difference to a normal diagnostic tube is the use of molybdenum (Mb) as the target material. Traditionally, the only way to reduce the beam quality was to use a lower kVp; however, varying the target material and filtration can improve the penetrating power whilst reducing the glandular tissue dose in dense breasts. Tungsten (W) does not have any characteristic radiation at low voltages, unlike molybdenum (Fig. 15.7). Despite the lower intensity of brehmsstrahlung from molybdenum, this reduction in intensity is offset by the characteristic X-ray emission of molybdenum. Rhodium (Rh) is now also used as a target material and filter, its characteristic radiation is slightly higher than molybdenum,which makes it useful for patients with dense breast tissue or who have undergone radiation treatment or hormone therapy.
The filtration of the beam is essential to remove the low energy photons and minimise skin dose. By utilising multiple target and filtration combinations it is possible to tailor the beam quality to the specific tissue-type requirements. The beam penetration can be improved by employing the same material as a filter and target material. The material used for the exit port is also important in ‘hardening’ the beam in general X-ray equipment; however, in mammography a beryllium (Be) window permits the softer radiation to pass through and enhance the subject contrast.
Overall, the equipment should be non-threatening in appearance and have a quiet operation level. This is important to reassure the woman and thus encourage the uptake rate and re-attendance. Mammography, by virtue of the technique, may be uncomfortable due to the compression of the breast; however, there are various adaptations, which should enhance the experience.