A SEMIANALYTIC MODEL TO INVESTIGATE THE POTENTIAL APPLICATIONS OF X-RAY SCATTER IMAGING

Although x-ray scatter is generally regarded as a nuisance that reduce s radiographic contrast (C) and the signal-to-noise ratio (SNR) in con ventional images, many technologies have been devised to extract usefu l information from the scattered x rays. A systematic approach, howeve r, for analyzing the potential applications of x-ray scatter imaging h as been lacking. Therefore, we have formulated a simple but useful sem ianalytic model to investigate C and SNR in scatter images. Our model considers the imaging of a target object against a background material of the same dimensions when both are situated within a water phantom. We have selected biological materials (liver, fat, bone, muscle, bloo d, and brain matter) for which intermolecular form factors for coheren t scattering were available. Analytic relationships between C and SNR were derived, and evaluated numerically as the target object thickness (0.01-40 mm) and photon energy (10-200 keV) were systematically varie d. The fundamental limits of scatter imaging were assessed via calcula tions that assumed that all first-order scatter exiting the phantom, o ver 4 pi steradians, formed the signal. Calculations for a restricted detector solid angle were then performed. For the task of imaging whit e brain matter versus blood in a 15 cm thick water phantom, the maximu m SNR, over all energies, for images based on the detection of all for ward scatter within the angular range 2 degrees-12 degrees is greater than that of primary images for target object thicknesses less than or equal to 23 mm. Use of the backscattered x rays within the range 158 degrees-178 degrees to image objects 3 cm below the surface of a 25 cm thick water phantom allows the liver to be distinguished from fat wit h a SNR superior to that of primary imaging when the objects are less than or equal to 22 mm thick. Our analysis confirms the usefulness of scattered x rays, and provides simple methods for determining the regi mes of medical interest in which x-ray scatter imaging could outperfor m conventional imaging. (C) 1998 American Association of Physicists in Medicine. [S0094-2405(98)01306-6].