A mammogram is the most well-known tool for catching breast cancer early. Starting at age 40, all women should plan to get a screening mammogram every single year, even if they don't have concerning symptoms.

But if your doctor is also recommending a breast ultrasound, you might wonder why it's necessary and how it differs from mammography.

"A mammogram uses X-rays to capture images of the breast tissue by compressing the breasts from various angles — top to bottom and side to side," explains Dr. Kelli Ha, a breast radiologist at Houston Methodist. "It gives us the big picture, like a map of the breast tissue."

Each year, your breast radiologist re-assesses your breast tissue map, looking for changes that may signal breast cancer.

"The vast majority of breast cancers are found this way, which is why mammogram is the gold standard," adds Dr. Ha. "But ultrasound can also play an important role in breast cancer screening and breast cancer detection."

Here's what you need to know if your radiologist is recommending an ultrasound after a mammogram.

What is a breast ultrasound?

If a mammogram is a map of your breast tissue, a breast ultrasound is somewhat like a magnifying glass — it's used to explore areas that are hard to see on a mammogram or to take a closer look at something detected on a mammogram.

"Instead of X-rays, a breast ultrasound uses sonic waves to visualize breast tissue," says Dr. Ha. "And there is no need to compress the breast for this test."

Reasons a breast ultrasound may be recommended include:

• If you have dense breasts
• If you have a high risk of breast cancer (for example, strong family history)
• To evaluate an abnormal finding seen on a screening mammogram
• To guide a breast biopsy
• To evaluate a breast infection

One of the most common reasons to need a breast ultrasound after a mammogram is if you have dense breast tissue.

Is ultrasound better for dense breasts?

Dense breast tissue can make detecting cancer via mammogram more difficult — though, as Dr. Ha points out, not impossible. (Related: What Does It Mean to Have Dense Breast Tissue?)

"Fatty tissue is easier to see through, whereas cancers can be obscured in women with dense breast tissue," explains Dr. Ha. "Women with very dense, busy breasts can be a challenge to read with just the mammogram alone."

With an ultrasound, masses and dense tissue can often be distinguished more clearly. It's why Dr. Ha adds that, "Women who have very dense breast tissue may benefit from having a breast ultrasound as a supplement for screening."

She emphasizes, however, that the recommendation for all women — even those with dense breasts — is still to get a screening mammogram every year starting at age 40. (Related: 4 Reasons to Make Time for Your Annual Mammogram)

"Ultrasound is just a supplemental screening that gives us another way to make sure we are not missing any subtle findings obscured by dense breast tissue," Dr. Ha adds.

Can a breast ultrasound replace a mammogram?

Breast ultrasound has many advantages, but it is not an alternative to mammogram. Data from the American College of Radiology clearly shows that the best way to catch breast cancer early — when it's easiest to treat — is to get a mammogram every year starting at age 40.

"The two can complement each other well, but they are not interchangeable," says Dr. Ha.

The only situations in which an ultrasound is used in place of a mammogram include:

• If you've previously had a mastectomy and notice an area of concern within the reconstructed breast or chest wall
• If you are pregnant and have a problem

"It is still important to investigate breast problems during pregnancy, but we prefer to omit the radiation and use ultrasound instead," says Dr. Ha. "And we recommend coming in for your screening mammogram after you deliver."

Why do I need a breast ultrasound after a mammogram?

Again, there are several reasons to have a breast ultrasound. One of those reasons is as a supplement to mammogram for dense breast tissue.

"When we see something abnormal on a screening mammogram or if a woman is experiencing a breast problem, we use a diagnostic mammogram to more closely investigate the area of concern," explains Dr. Ha. "At that same appointment, we also typically perform an ultrasound to be sure we are not missing anything."

(Related: 5 Signs of Breast Cancer Beyond Feeling a Lump)

Ultrasound is also used for needle guidance during a biopsy.

How long does a breast ultrasound take?

A breast ultrasound is quick and painless. "The duration depends on the size of the breasts, but it usually only takes a few minutes," says Dr. Ha.