Specially trained technologists and physicians perform the procedure. The images are obtained not with x-ray-exposed film, as in conventional mammography, but using computerized or digital imaging in place of a film cassette. This reduces x-ray exposure to the breast, and also permits the images to be viewed on a computer monitor seconds after exposure—compared with the several minutes needed to develop x-ray film. The principle of stereotactic biopsy is that a lesion can be located precisely in three dimensions by calculating its apparent change in position on angled x-ray images. The first x-ray locates the abnormality in the breast, after which two stereo views are obtained, each angled 15 degrees to either side of the initial image. The physician then marks the lesion electronically on the stereo images. The computer calculates how much the lesion's position appears to have changed on each of the stereo views, and in this way is able to determine its exact site in three-dimensional space.
One of two biopsy instruments may be used in the procedure:
- The conventional biopsy device (core needle biopsy device) consists of an inner needle. It is attached to a spring-loaded mechanism that, when fired, shoots forward, filling the trough with tissue. The outer sheath instantly moves forward to cut the tissue and keep it in the trough. Each sample is obtained in a fraction of a second. It is necessary to withdraw the needle after each sample is taken to collect the tissue.
- A newer type of instrument, the VAD, uses vacuum pressure to pull breast tissue into a needle and remove it without having to withdraw the needle after each sampling. Another advantage of this device is that biopsies are obtained in an orderly manner by rotating the probe, assuring that the entire region of interest will be sampled. Using the automated core biopsy needle, sampling is more random.