The imaging technique can be particularly useful for women with dense breasts or those under 30 who find a suspicious lump. Here's why.

If you’ve ever felt a suspicious lump or something painful in your breast, chances are it’s nothing. But if you’re under the age of 30, chances are also good that your doctor will suggest an ultrasound to make absolutely sure.

Ultrasound is the imaging choice for younger women who need to have potentially problematic areas in their breasts checked out. It uses sound waves to get a picture of the inside of your breasts. In older age groups, ultrasound is usually combined with another imaging modalities—like screening mammography—to suss out suspicious areas.

“In very young women without any risk factors [for breast cancer], we’ll start with ultrasound if there are symptoms to limit radiation exposure,” says Sabala Mandava, MD, director of the Division of Breast Imaging at Henry Ford Health System in Detroit. “The risk of radiation outweighs the benefit of obtaining a mammogram. If we do see something suspicious on ultrasound, then a mammogram may be performed.”

A mammogram is a low-dose X-ray, so it does emit some radiation. Ultrasound does not, which also makes it safer for women who are pregnant.

At any age, ultrasound is better than mammography at finding problems in dense breasts. Having dense breasts may slightly up your risk of developing breast cancer.

Once it’s time for a woman to start regular breast cancer screening—as long as she has no symptoms and average risk—the imaging test of choice is mammography. An ultrasound might come into the picture if you get a dreaded call-back after the mammogram (suggesting something’s amiss), or if you can feel something in your breast that doesn’t show up on a mammogram.

“Ultrasound is not a replacement for a screening mammogram. It’s kind of an adjunct to mammography,” says Dr. Mandava. “The biggest area that ultrasound is useful for is to further evaluate lesions we see on a mammogram. Is it a solid mass? Is it cystic? Does it have any vascular flow? Then we can figure out if it’s leaning to malignant or benign.”

A radiologist should be able to tell right away if it’s a cyst or something else benign or something that might be cancer and needs further testing, such as a biopsy.

About 90% of screening mammograms are negative, Dr. Mandava says. The other 10 to 12% end up with a woman getting called back for more views or possibly an ultrasound. Of those cases, up to 70% will be normal and 30% will still show problems. Many of those women will get a biopsy. The good news? “Eighty percent of biopsies tend to come back benign,” she adds.

And that something “suspicious” on the original mammogram? It could be as harmless as “overlapping tissue that appears mass-like depending on a certain angle or less compression,” she adds.

There are other reasons to use ultrasound. One is to look for complications with breast implants, though magnetic resonance imaging or MRI is more commonly used for this. Others are to guide biopsies (to make sure the needle takes a sample from the right location) and to assess response to therapy if the person already has cancer.

MRI is also sometimes combined with a mammogram. “It can be used as an adjunct screening for women with dense breast tissue or women who are high risk because they have a family history or because they have had cancer before,” says Dr. Mandava. “It’s also used for women who are newly diagnosed with breast cancer to see the extent of the disease. MRI is much more sensitive than ultrasound screening.”

If you have a choice between the two, Dr. Mandava points out ultrasound is cheaper and easier to do. MRI requires injecting a contrast agent, which some people are allergic to. It also involves being shuttled into a closed tube—often an unpleasant experience if you’re claustrophobic. You also typically can’t do an MRI if you have a metal implant, like a pacemaker.

Mammography guidelines vary a little from organization to organization. The American Cancer Society recommends that women ages 45 to 54 get a mammogram every year. Women 40 to 44 should have the choice to do so, while women 55 and over can skip every other year if they choose. The U.S. Preventive Services Task Force recommends screening every other year for women ages 50 to 74. Talk to your doctor about the best schedule for you, especially if you have any risk factors.

“Everyone should definitely get a mammogram,” says Dr. Mandava. “[When] is really going to depend on risk factors—and that your doctor will know.”