Screening for breast cancer isn't like looking for a needle in a haystack. It's harder. It's like looking for needles in a big field of haystacks, where some of the haystacks have needles, while most don't, but you don't know which are which, so you have to look in all of them.

Mammography is the best technique available right now to look for breast cancers in women who don't have any symptoms. On average, screening mammograms correctly identify 80 to 85 percent of women who have cancer and about 90 percent of women who don't.

Even as debate continues about who should get mammograms — and when, and how often — researchers are working on a blizzard of new approaches to breast imaging in hopes of reducing the number of cancers that are missed and the number of false alarms that lead to unnecessary biopsies. Here's a look at some of the approaches under study.

Digital mammography

The screening mammography we get today takes X-ray images — usually two of each breast — looking for abnormalities that can't be felt in a physical exam. These include small tumors and tiny deposits of calcium, called microcalcifications, which, in clusters, may be a sign of cancer.

It's harder for mammograms to find cancers in the dense breast tissue that is normal in young, premenopausal women, because dense tissue and cancerous tissue each look white on a mammogram. Fatty breast tissue — more common in older women — looks dark on a mammogram, so any white cancerous tissue stands out.

In digital mammography, which was approved by the Food and Drug Administration in 2000, images are stored in a computer instead of on film. The technique may provide an edge in some cases.

A large clinical trial — the Digital Mammographic Imaging Screening Trial (DMIST) — compared the accuracy of conventional and digital mammography on nearly 50,000 women. Results, published in 2005, showed that across the entire sample, accuracy of the two kinds of mammography was similar, but digital was more accurate in premenopausal women, women younger than 50 and women with dense breasts.

Another study published last year found that breast cancer detection rates nearly doubled — from 4.1-4.5 per 1,000 women to 7.9 per 1,000 — at a diagnostic center in San Luis Obispo, Calif., after the center switched to digital mammography.

But digital mammography is not available everywhere. "And it's more important simply to get a mammogram than to wait to get a digital one," says Dr. Pulin Sheth, chief of breast MRI and assistant professor of radiology at the University of Southern California's Keck School of Medicine.

Stochastic resonance

Scientists have discovered that by adding the right kind of noise (interference) to a mammogram image, they can make the image clearer. In a study of 75 images published last year, researchers found they could detect cancers as well or better than mammography alone, while reducing the number of false alarms by as much as 36 percent.

Digital tomosynthesis

First reported in 2007, digital tomosynthesis takes at least 11 X-ray images of the breast at different angles, which a computer combines into three-dimensional images.

While mammography pulls the breast away from the body and squeezes it between two glass plates, tomosynthesis uses just enough pressure to hold the breast still — so it doesn't hurt the way mammography can. And because it doesn't squeeze the breast, tomosynthesis doesn't create overlaps in tissue that hide cancers.

It also may result in fewer false alarms. In a study reported last year, radiologists interpreted images from 125 women, 35 who had cancer and 90 who did not. False-alarm recall rates were reduced by 30 percent by using both tomosynthesis and digital mammography instead of digital mammography alone.

Stereoscopic mammography

In this emerging technology, radiologists take two digital X-ray images of the breast about eight degrees apart and then fuse them into one stereoscopic view. At a 2007 meeting, researchers reported interim results for 1,093 patients with above-average breast cancer risk. Of 109 cancers, standard mammography missed 40, while stereoscopic missed 24 — 40 percent fewer.

Ultrasound (sonography)