Attenuation describes the gradual reduction in the intensity of an energy beam as it passes through a material. In mammography, this refers to the weakening of the X-ray beam as it travels through the breast tissue before reaching the detector. The degree of this signal weakening creates the image, as different tissues attenuate the X-rays differently.
How Energy Interacts with Breast Tissue
When an X-ray beam is directed at the breast, the photons interact with the tissue in two primary ways. The first is photoelectric absorption, where an X-ray photon transfers all its energy to an electron in a tissue atom, causing the photon to disappear completely. This total energy transfer is the source of the contrast seen in the final image, as denser materials cause a higher rate of absorption. The second process is scattering, predominantly the Compton effect, where the X-ray photon deflects off an electron and continues traveling in a different direction with reduced energy.
The photons that successfully pass through the tissue without interacting—known as primary photons—carry the diagnostic information to the detector. Scattered photons strike the detector at random locations, introducing a background signal that degrades image clarity.
The Role of Breast Density and Composition
The primary factor determining the degree of X-ray attenuation is the composition of the breast tissue itself. The breast is composed of two tissue types: adipose (fatty) tissue and fibroglandular tissue, which includes ducts, lobules, and connective fibers. Fibroglandular tissue is significantly denser than fatty tissue, containing more atoms per unit volume, particularly water and collagen.
This difference means that fibroglandular tissue causes a much higher degree of X-ray attenuation than fatty tissue. A breast with a high proportion of fibroglandular tissue is considered radiographically dense, appearing brighter white on a mammogram due to greater X-ray absorption. Conversely, a breast composed mostly of adipose tissue is less attenuating, appearing darker on the image.
The relative proportions of these tissues often change with age and hormonal status. The attenuation coefficient of the fibroglandular tissue is much closer to that of potential cancer lesions than it is to fatty tissue. This narrow difference between the attenuation of normal dense tissue and a tumor creates a major challenge for accurate diagnosis.
Impact on Diagnostic Imaging Accuracy
The varying levels of breast attenuation have consequences for diagnostic imaging, particularly in women with dense breasts. High attenuation due to dense fibroglandular tissue presents a “masking effect” on the mammogram. Because both the normal dense tissue and a potential cancerous lesion attenuate X-rays strongly, the lesion can be visually obscured by the surrounding bright white tissue.
Attenuation also directly affects image quality parameters, specifically contrast and image noise. The small difference in attenuation between glandular tissue and a tumor means there is inherently low subject contrast. Heavy attenuation in dense tissue reduces the number of primary X-ray photons reaching the detector, which lowers the signal-to-noise ratio (SNR) and increases quantum noise. Increased noise makes subtle features harder to resolve.
Attenuation also plays a role in determining the radiation dose delivered to the patient. To ensure enough X-ray photons penetrate a highly attenuating breast to produce a high-quality image, technical factors such as exposure time or energy must be increased. Imaging systems must compromise between maximizing the contrast needed for diagnosis and minimizing the mean absorbed glandular dose. Scattered radiation further degrades image contrast by adding a uniform background signal.
Techniques Used to Compensate for Attenuation
Several strategies are employed to mitigate the challenges posed by breast attenuation.
Specialized Filtration
One method is the use of specialized filtration within the X-ray tube to tailor the beam’s energy spectrum to the specific tissue composition. Filters made of elements like rhodium or molybdenum select X-ray energies that maximize the difference in attenuation between fatty and glandular tissue, thereby increasing image contrast.
Automatic Exposure Control (AEC)
Modern mammography units rely on AEC systems, which measure the total X-ray attenuation through the breast during a brief pre-exposure. This system automatically adjusts the exposure parameters, such as the X-ray tube current or time, to ensure the optimal number of photons reach the detector for the specific thickness and composition of the compressed breast.
Breast Compression
Firm breast compression is a technique that directly manages the effects of attenuation. Compression reduces the physical thickness of the breast, which significantly decreases the overall amount of tissue the X-ray beam must pass through. This thinning reduces the amount of scattered radiation generated, thereby improving image contrast and lowering the necessary radiation dose.
Alternative Modalities
For women with highly attenuating, dense breast tissue, alternative imaging modalities often supplement mammography. Digital Breast Tomosynthesis (DBT) acquires multiple low-dose X-ray projections from different angles. This allows a three-dimensional reconstruction that effectively separates overlapping tissue structures, overcoming the masking effect caused by high attenuation.