X-rays used to obtain computerized tomography images (CT or CAT scans) are a form of electromagnetic radiation similar to light, but with a shorter wavelength. X-rays are present only during the short time that the image is being taken and leave no radiation or radioactivity in the body or in milk. X-rays have no known effect on the milk in the breast at the time of imaging. No special precautions are required.
For information on contrast media used with CT scans, see the LactMed entry for the specific contrast agent.