A semianalytic model to investigate the potential applications of x-ray scatter imaging.

Although x-ray scatter is generally regarded as a nuisance that reduces radiographic contrast (C) and the signal-to-noise ratio (SNR) in conventional images, many technologies have been devised to extract useful information from the scattered x rays. A systematic approach, however, for analyzing the potential applications of x-ray scatter imaging has been lacking. Therefore, we have formulated a simple but useful semianalytic 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, blood, and brain matter) for which intermolecular form factors for coherent 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 varied. The fundamental limits of scatter imaging were assessed via calculations that assumed that all first-order scatter exiting the phantom, over 4 pi steradians, formed the signal. Calculations for a restricted detector solid angle were then performed. For the task of imaging white brain matter versus blood in a 15 cm thick water phantom, the maximum SNR, over all energies, for images based on the detection of all forward scatter within the angular range 2 degrees-12 degrees is greater than that of primary images for target object thicknesses < or = 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 with a SNR superior to that of primary imaging when the objects are < or = 22 mm thick. Our analysis confirms the usefulness of scattered x rays, and provides simple methods for determining the regimes of medical interest in which x-ray scatter imaging could outperform conventional imaging.