You're probably referring to an experimental imaging technique called digital tomosynthesis. It works by taking multiple X-rays of each breast from many angles. These scans are used to construct a three-dimensional image of the breast.

Conventional mammography typically involves taking two X-rays of each breast. Tumors, which are denser than most healthy tissue, appear as shadows. One limitation of X-rays is that overlapping of normal breast tissue can hide cancers. It can also produce shadows that may be wrongly interpreted as cancer.

Tomosynthesis allows doctors to look at cross-sections of the breast, which should reduce the problem of overlapping tissues. This technology may improve detection of early-stage breast cancer and reduce the number of unnecessary biopsies. But it's currently available only for research purposes.