The onset of rolling contact fatigue in an object may be detected by measuring the residual stresses in the surface of the object, and monitoring changes in their magnitude or their direction. The sterile radiological drape is designed to expand to encompass the radiological unit as it rotates into and is move through the sterile field for imaging during surgery, and then to collapse to protect the sterile portion of the drape when the radiological unit is removed from the surgical field. The cache server stores multiple files requested by the same client on the hard drive as a file bundle with meta data associating the files with one another, such that they can be accessed together. The width of the second region is sufficient to confine electromagnetic radiation within a selected wavelength range to substantially only the core region.