X-ray energy is used in a similar manner to radiography, but the image is viewed as a movie, rather than a still picture. This modality is used to examine structures in motion such as food moving through the esophagus and urine flowing through the ureters. Fluoroscopy is used to guide placement of a stent in a narrowed trachea or placement of a needle for sampling of bone mass. Subtle changes in the position of structures may be examined during movement such as the head, neck and joints of the limbs.