Multilayer Mirrors as X-ray Filters for Slit Scan Radiography

X-rays from a radiographic W-anode tube are reflected from a planar multilayer mirror into a slit-like beam of narrow energy bandwidth. The multilayer mirror functions as a tuneable energy filter which could be useful for radiological applications where a high contrast image (mammography, angiography) or tissue composition information (lesions, bone mineral loss) is desired. Low reflectivity at undesired energies permits; 1. operation of the W-anode tube at much higher voltage levels than is possible with conventional filtration, 2. the removal of most of all non-fixed beam filtration. These modifications significantly enhance the useable x-ray fluence. The slit scan imaging format also minimizes detected scattered x-rays and so improves image contrast. A simple model is used to optimize the energy dependent detected signal-to-noise ratio (SNR) with respect to the total energy absorbed by the patient. Theoretical reflectivity curves for ReW-C mirrors which include the effects of surface roughness and layer thickness errors are presented along with W-anode tube spectra reflected from a 2x4 inch ReW-C mirror (11-22Å).