Why does the beam attenuate quickly in mammography?

• A. High kVp
• B. Low kVp, so it may only be about 100 mR (15% of ESE)
• C. Use of contrast media
• D. Long exposure time

Answer: (B)

Low energy X-ray photons are absorbed more readily by breast tissue, so the beam loses intensity quickly as it passes through. In mammography, the energy is kept low on purpose because absorption differences between soft tissues (fat vs glandular tissue) become more pronounced at these energies, mainly due to the photoelectric effect. This makes the transmitted signal drop off rapidly with tissue thickness, giving high contrast images of the structures inside the breast. If we used higher energy (a high kVp), photons would penetrate more easily and the attenuation would be less pronounced, reducing image contrast. Contrast media isn’t used to cause this rapid attenuation in standard mammography, and simply increasing exposure time would raise dose without changing the fundamental attenuation behavior.