Mammography - A Special Imaging Method in Medical Radiology

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

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

to be flattened, causing the thickness of the breast to be reduced. This allows x-rays to penetrate more effectively. This is important as it reduces tissue overlap and blurring, which can conceal minor abnormalities or microcalcifications (small white patterns of tissue), which are often non-cancerous but, in rare cases, lead to larger tumors (Holland et al., 2017). 2. Reducing Motion Artifacts: If the breast were being imaged without compression, movement in the body would translate to movement in the mammogram causing motion artifacts (Kennedy, 2020). Remembering that mammograms are exposure images and not instantaneous captures is essential. These lead to blurry images, making deciphering the image challenging. Compression of the breast holds it in place, completely mitigating this issue. 3. Improved Image Quality: When x rays pass through an object, some x rays always scatter, as seen in Coherent and Compton scattering. These x-rays introduce noise into the final image (Holland et al., 2017). When the breast is compressed, it produces a more uniform shape making it easier for x-rays to pass, reducing total scattered radiation and improving image quality. 4. Reduced Patient Radiation Dose: Breast compression allows the same high-quality images to be taken with a lower radiation dose. The concept of HVL (Half value layer) is required to understand this best. HVL is the thickness of a material that reduces the intensity of the original x-ray beam by 50%. For example, If the breast were 10 cm thick

uncompressed with an HVL of 10 cm. 50% of the incident x-rays would be absorbed by the breasts leading to an increased risk of radiation-induced cancer. If the same breasts were compressed to 5 cm, the patient radiation dose would be reduced by 50%. Image Recording using Digital Imaging Over the years, digital imaging has gained popularity due to its many advantages over traditional film-based techniques. Digital Imaging is the process of using electronic devices which produce and store radiography images compared to using traditional photographic film. A significant advantage of digital imaging is its ability to produce high-resolution images with incredible speed and efficiency compared to traditional film imaging. To produce a film image, first, the image has to be captured and processed, all of which takes time, as these films must be treated with chemical solutions, developed, washed, and dried several times to produce one image. Compared to digital imaging, it allows almost instantaneous processing and quicker analysis and diagnosis (Sermon, 2000). Furthermore, digital images do not need to be physically stored, allowing quick sharing of images. In addition, digital imaging can produce images with optimal levels of spatial and contrast resolutions. Spatial resolution is the ability of an imaging system to produce images where it is easy to distinguish between closely spaced objects. In contrast, contrast resolution is the ability of a system to distinguish between different shades of gray in the images (corresponding to different tissue types) (Huda & Abrahams, 2015). In medical imaging, achieving optimal levels of both to provide accurate diagnosis is crucial due to minor differences accounting for abnormalities.

attenuation of tissues found in breasts, including adipose tissue, glandular tissue, small tumors, and microcalcifications. During testing, phantom images are compared to a control to ensure that consistent, accurate images are produced every time (Yaffe, 2011). Fig. 5 ---This image displays what a phantom image from the TISSUE EQUIVALENT PHANTOM FOR MAMMOGRAPHY sold by Sun Nuclear Medical Company looks like. The different squares seen in the middle of the image showcase the different contrasts produced by different tissues. On the left being glandular and cancerous tissues and on the right being fatty adipose tissue (Sun Nuclear, 2021). Likewise, spatial and contrast resolution is tested to ensure mammography machines are in perfect working order. In addition, the Patient dose is also monitored to ensure that patients are only receiving the required dose of radiation to produce the image no more. If a mammography machine is to fail a quality control test, they are either repaired or decommissioned (Yaffe, 2011).

What alternatives are there to X-Ray Imaging? Although mammography is the most common method of imaging the breasts, ultrasound is also a prominent imaging technique to provide additional diagnostic information. Unlike mammography, ultrasound uses sound waves to image internal structures (Mayo Clinic Staff, 2022). Often physical lumps in the breast are too small to show up on a mammogram, and ultrasound provides additional information. According to a study by Dr. Wendie A. Berg, 28% more cancers were found using mammography and ultrasound compared to mammography alone, although four times as many false positives were recorded, leading to unnecessary biopsies. (Berg et al., 2008). Another imaging technique that is used is Magnetic resonance imaging (MRI). MRI machines use high-power magnets and radio waves to align protons producing internal images of the body. MRIs are most commonly taken when a patient is known to have breast cancer to produce a treatment plan. However, if a patient is at high risk for breast cancer with inconclusive mammograms, MRIs can be used as a highly accurate imaging technique to diagnose breast abnormalities (American Cancer Society, 2014).

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