A mammogram uses low-energy X-rays to create detailed images of breast tissue, revealing lumps, calcium deposits, and other changes that can’t be felt during a physical exam. The machine sends X-rays through the breast while a digital detector on the other side captures what comes through, producing an image based on how different tissues absorb the radiation. The entire process takes about 20 minutes, with each individual X-ray exposure lasting only a few seconds.
How X-Rays Reveal Breast Tissue
Mammography relies on one key principle: different types of tissue absorb X-rays at different rates. Fat absorbs very little radiation and appears dark on the image. Glandular tissue and connective tissue absorb more and appear lighter. Tumors and calcium deposits also absorb more radiation, showing up as bright white spots or areas against the surrounding tissue.
The challenge is that the differences in absorption between normal glandular tissue and cancerous tissue are extremely small. To make those tiny differences visible, mammography equipment uses X-ray beams at very specific, low energy levels, typically around 15 to 18 keV. At these low energies, the contrast between tissue types is at its highest. At higher energies, everything starts to look the same on the image, and subtle abnormalities disappear. The X-ray tube also uses a very small focal spot, roughly 0.3 millimeters, to produce the sharpest image possible.
Why the Machine Compresses Your Breast
The compression plates are the least comfortable part of a mammogram, but they serve a real purpose. Flattening the breast holds the tissue still so the image doesn’t blur from movement. It also spreads the tissue out so overlapping structures are less likely to hide an abnormality. Equally important, compression reduces the thickness the X-rays need to travel through, which means a lower radiation dose can produce a clearer image. Without compression, the resulting images would be significantly harder to read.
2D Versus 3D Mammography
A standard 2D mammogram captures two flat images of each breast, one from top to bottom and one at an angle. This gives radiologists two views to work with, but all the tissue layers are stacked on top of each other in a single image, which can make overlapping tissue look like a suspicious area or hide a real problem behind normal tissue.
3D mammography, called tomosynthesis, solves this by taking a series of images as the X-ray tube rotates in an arc over the compressed breast. The tube stops at multiple positions along that arc (which spans roughly 11 to 50 degrees depending on the system) and captures a low-dose image at each angle. A computer algorithm then combines all those projections to reconstruct thin slices through the breast, similar to flipping through pages of a book. This lets the radiologist examine one layer of tissue at a time, reducing the chance that overlapping structures create a false alarm or mask a real finding.
What Shows Up on the Images
Radiologists look for two main types of findings: masses and calcifications. A mass is any area where tissue appears denser than the surrounding breast. It might turn out to be a cyst, a benign growth called a fibroadenoma, or in some cases a tumor. The shape and edges of the mass help the radiologist gauge how suspicious it is. Smooth, well-defined borders are more reassuring; irregular, spiky edges raise more concern.
Calcifications are tiny deposits of calcium that show up as bright white specks. Many are completely harmless. Benign calcifications tend to be larger, more regular in shape, and easy to identify. They include things like vascular calcifications in blood vessel walls or coarse, popcorn-shaped deposits. Suspicious calcifications are typically smaller, more irregular, and may appear in patterns that follow the path of a milk duct. The combination of a calcification’s shape and how it’s distributed in the breast guides the radiologist’s assessment.
How Results Are Scored
Every mammogram receives a standardized score from 0 to 6, using a system called BI-RADS. This scoring system ensures that results mean the same thing regardless of where you get your mammogram.
- Category 0: The images are incomplete. The radiologist needs additional views, an ultrasound, or a comparison with older mammograms before making an assessment.
- Category 1: Negative. Nothing abnormal was found.
- Category 2: A benign finding was identified, such as a simple cyst or a fibroadenoma. No further workup is needed.
- Category 3: A finding that is probably benign but not proven. You’ll typically be asked to come back for follow-up imaging in six months to make sure it isn’t changing.
- Category 4: A suspicious abnormality. Findings in this category have anywhere from a 2% to 95% chance of being cancer, and a biopsy is recommended.
- Category 5: Highly suggestive of cancer, with at least a 95% likelihood. A biopsy is strongly recommended.
- Category 6: A known, biopsy-confirmed cancer. This score is used when imaging is being done to monitor treatment response or assess the extent of a previously diagnosed cancer.
The Dense Breast Challenge
Breast density plays a major role in how well a mammogram works. Dense breasts contain a higher proportion of glandular and connective tissue relative to fat. On a mammogram, both dense tissue and tumors appear white. This creates a masking effect: a cancer can hide in plain sight because it looks almost identical to the surrounding normal tissue. Mammography is simply less sensitive in women with dense breasts, meaning it is more likely to miss a cancer that is present.
About half of women who get mammograms have dense breasts. Many states now require that facilities notify you if your mammogram shows dense tissue, because you may benefit from supplemental screening with breast ultrasound or MRI. Density is not something you can feel or predict based on breast size. It can only be determined from imaging.
Radiation Exposure
A standard screening mammogram with two views of each breast delivers about 0.4 millisieverts of radiation. That’s roughly equivalent to seven weeks of natural background radiation, the kind you absorb just from living on Earth. For context, a chest X-ray delivers about 0.1 millisieverts, and a cross-country flight exposes you to around 0.03 millisieverts. The dose from a mammogram is low enough that the benefit of detecting cancer early far outweighs the minimal radiation risk for women at screening age.
Quality Standards for Facilities
In the United States, every mammography facility must meet requirements set by the Mammography Quality Standards Act (MQSA), enforced by the FDA. Facilities must be accredited by a federally approved organization, undergo annual equipment inspections by a medical physicist, and have their clinical image quality reviewed on a regular basis. The law also sets standards for the qualifications of the radiologists who read the images, the technologists who operate the equipment, and the physicists who maintain it. These requirements exist because the tissue differences mammography is trying to detect are so subtle that even small drops in image quality can mean a missed diagnosis.