Mammography: A Special Imaging Method in Diagnostic Radiology

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Mammography:
A special imaging method in diagnostic radiology

Adam Purvis 1154987
Medical Physics 1E03
TA: Sarah McNeil
Professor: Mic Farquharson

Over the past two decades, mammography has become the central tool used to detect cancerous regions within the breast. This evolution has been enabled by advancements in the technology itself and its implementation into healthcare regulations of numerous countries around the world (Pisano et al., 2004). In its beginning stages, breast cancer is a relatively treatable disease, and increased awareness for both mammograms and self-breast examinations have led to earlier diagnoses. Studies suggest that increases in mammography screenings have been one of the main reasons that breast cancer mortality rates in Canada have decreased by over 35% since peaking in 1986 (Chappell et al., 2011).

Breast cancer is most common among females over 40 years of age and comparatively rare among males. For females, it originates in one of two places: either the ducts which carry the milk to the nipple, or the lobules (Chen & Zieve, 2010). Although current breast cancer mortality rates are the lowest they have been since 1950, approximately one in twenty-nine Canadian females will die of breast cancer during their lifetime (Chappell et al., 2011). Continued research and advancements within the field of mammography will be very important in coming years in order to further reduce this rate. Figure [ 1 ]: Linear Attenuation Coefficients of Fat, Fibroglandular Tissue, and Tumour versus Energy of Beam (Johns & Yaffe, 1987) Figure [ 1 ]: Linear Attenuation Coefficients of Fat, Fibroglandular Tissue, and Tumour versus Energy of Beam (Johns & Yaffe, 1987)

Much of this research should begin with diagnosis. The key problem with mammograms is that they can be very inaccurate in detecting cancerous areas due to the difficulty in observing tumours directly off of the image. Problems with viewing the image stem from similarities between atomic numbers of elements in soft tissues which comprise the breast. The focal two soft tissues contained inside of the breast are fat and fibroglandular tissue (Yaffe, 2010). When a beam of x-rays come into contact with the breast that is being irradiated, a certain amount of the incident radiation is attenuated depending on the material. This attenuation of the beam is proportional to the atomic number of the material and inversely proportional to the energy of the beam (Farquharson, 2012). The x-ray beam is attenuated more with a higher atomic number due to the increased amount of protons which allow for x-rays to have a greater chance of being absorbed (Farquharson, 2012). The attenuation, energy of the beam and atomic number of the material are further linked through a complex function referred to as the linear attenuation coefficient, which describes the fraction of photons which interact per unit thickness of a given material (Yaffe, 2010). Consequently, soft tissues have similar linear attenuation coefficients because of similarities in their atomic number, and therefore absorb and scatter x-rays at approximately the same probabilities (Farquharson, 2012).

Soft tissues have relatively low linear attenuation coefficients and are therefore ineffective at absorbing incident radiation. The result of this is a higher degree of blackening (opacity) on the x-ray film, which is usually Figure 3: Mammograms of normal, dense breasts (Kaufmann, 1994) Figure 3: Mammograms of normal, dense breasts (Kaufmann, 1994) Figure 2: Characteristic curve of a mammographic screen-film system (Pisano et al., 2004) Figure 2: Characteristic curve of a mammographic screen-film system (Pisano et al., 2004) transposed as a faint white hue on the film (Farquharson, 2012). Figure 2 shows that denser tissues will have a higher physical thickness, therefore absorbing more of the beam, resulting in a lower opacity. Figure 2 shows that fibroglandular tissue has a...
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