Mammography - A Special Imaging Method in Medical Radiology

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Feb 20, 2024

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Mammography: A Special Imaging Method in Medical Radiology Dhruvil Shah McMaster University LIFESCI 1D03: Medical Imaging Physics Michael Farquharson Saturday, February 18 th , 2023

Introduction In 1965, a man by the name of Charles Gross produced the first functioning mammography machine. It used not a tungsten x ray tube but a molybdenum x ray tube to produce low intensity x rays which were perfect to examine breast tissue (Kalaf, 2014). Mammography was invented to screen for early signs of breast cancer affecting approximately 1 in 8 american females according to the american cancer society (howlader 2018). Due to advancements in the field of mammography and early screening of breast cancer, mortality of the disease has dropped 43% since 1989. Mammography Mammography is a medical imaging technique that utilizes low-energy x-ray beams to produce high-quality images of breast tissue. Its most notable use is in the early diagnosis of asymptomatic breast cancer. The procedure begins by placing the breast on a flat surface with a clear plastic paddle. It then compresses it to flatten the breast tissue, increasing surface area and producing a more informative image on one plane. X Rays are then passed through the breast tissue from multiple angles producing high-quality images that can be screened for most commonly breast tumors and signs of other diseases (Reeves & Kaufman, 2022). What is the problem with breast imaging? Compared to conventional x-ray imaging, where different tissues have largely variable linear attenuation coefficients, in breast tissue, tumors often look very similar to the surrounding tissue due to similar attenuation making it challenging to differentiate between what is and is not a tumor. Furthermore, the density and volume of breast tissue largely depend on weight, age, and family history, making each mammogram very different from the last. In fatty breast tissue

(adipose tissue), the Linear attenuation coefficient is lower, creating a dark image, whereas in connective and epithelial tissue (including milk ducts and glands) is dense, creating light spots on the image. The problem arises when tumor tissue is present. Similar to connective and epithelial tissue, tumors also have high linear attenuation coefficients creating a very abstract image that is difficult to interpret. Due to this, women with dense breast tissue, more often than not, are susceptible to misdiagnosis. Fig 1--- This figure displays different breast tissue densities among women of different ages and genetic compositions. (Baker, 2016) Moreover, during a mammography procedure, the breast is compressed using a compression table, often creating discomfort to the patient, but it is necessary as this reduces the patient radiation dose. When the breast compresses, the x-rays have less distance to travel, reducing the time the x-rays spend in the breast, thus reducing the radiation dose. Compression of the breast also reduces image blur, often caused by patient movement. In addition, it allows

the breast to be imaged on a single plane, making it easier to differentiate between tumor and breast tissue (NIBIB, n.d). What is the solution? The differences between the breast and cancerous tissue are minor due to their similar compositions. The one key identifier which assists in diagnosing abnormalities is a slight increase in the linear attenuation coefficient of cancerous breast tissue, changing the contrast between the two tissues. This makes it crucial to produce high-contrast images to reduce misdiagnosis and increase early detection of breast diseases. Fig 2--- This figure displays the varying linear attenuation coefficients of fat, glandular tissue (including milk ducts and glands), and Infiltrating ductal carcinoma (a type of cancer) which occurs in the breast. As can be seen, attenuation has the most significant difference at low energies, with attenuation differences decreasing at higher energies (Bushberg et al., 2020) Compared to conventional x-ray tubes, which utilize tungsten (Z - 74) as its anode target with binding energies of 69.5 keV for K-shell and ~12.1 keV for L-shell (Deslattes et al., 2021),

which produce high-intensity x rays required to image bone, to image soft tissue as present in breasts, low-intensity x rays are best as they create excellent contrast between fatty tissue, dense tissue, and abnormalities. To achieve this, either molybdenum (Z- 42) or rhodium (Z - 45) anode targets are used in mammography machines as they produce low keV characteristic x-rays (10-15 keV) due to their low binding energies at 20.18keV and 2.31keV for molybdenum and 23.21keV and 2.952keV for K-shell and L-shell respectively (Deslattes et al., 2021). This produces x-rays on a different spectrum than a tungsten anode perfect for imaging breasts. As the intensity of x-rays increases out of this range, contrast decreases, decreasing image quality as dense tissue, adipose tissue, and breast abnormalities begin to blend. Focal Spots Compared to focal spots on conventional x-rays (around 0.6 to 1.2mm), which use tungsten as its anode, Mammography machines have a much smaller focal spot at around (0.1mm - 0.3mm). This change in focal size increases the image's sharpness and spatial resolution. Furthermore, mammography uses a technique known as geometric magnification in which the breast is brought closer to the x-ray beam than the detector to magnify a specific breast region. This allows detailed structures to be seen and allows minor abnormalities to be visible in images. Blurring is mitigated with a small focal spot, improving complete resolution. (Bushberg et al., 2020) Filters Conventional x-ray tubes often utilize varying thicknesses of aluminum, lead, and copper as filters to remove low-energy x-ray beams, which have low to no possibility of penetrating the patient into the detector. Instead of the patient absorbing these low-energy x-rays increasing the

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