Fundamental analysis and algorithms for development of a mobile fast-scan lateral migration radiography system

Lateral migration radiography (LMR) is a unique x-ray Compton backscatter imaging (CBI) technique to image surface and subsurface, or internal structure of an object. An x-ray pencil beam scans the interrogated area and the backscattered photons are registered by detectors which have varying degrees of collimation.; In early LMR applications, either the LMR systems or the imaged objects are moved on a rectangular grid, and at each node, the systems register backscattered photon energy deposition as pixel intensity in acquired images. The mechanical movement of the system or objects from pixel to pixel causes prolonged image scan time with a high percentage of system dead time. To avoid this drawback, a particular x-ray beam formation technique is proposed and analyzed. A corresponding mobile, fast-scan LMR system is designed, fabricated and tested. The results show a two orders-of-magnitude reduction in image scan time compared with those of previous systems.; The x-ray beam formation technique, based on a rotating collimator in the LMR system, implements surface line scan by sampling an x-ray fan beam. This rotating collimator yields unique imaging effects compared to those for an x-ray beam with fixed collimation and perpendicular incidence: (1) the speed of the x-ray beam spot on the scanned surface is not uniform; (2) constant movement of the x-ray beam spot changes the resolution in the image raster direction; (3) x-ray beam spot size changes with location on the scanned surface; (4) the object image shows a squeezed effect in the raster scan direction; (5) under a uniform background, the Compton scatter angular distribution causes the x-ray backscatter field to be stronger, when the x-ray beam has greater incidence angle; and (6) the x-ray illumination spot trace on the scanned surface is skewed. The physics generating these effects is analyzed with Monte Carlo computer simulations and/or measurements. Image acquisition and image processing algorithms are developed and applied to acquired images to eliminate the undesirable effects. The images acquired with this system also have the general characteristics of LMR images: (1) displacement of object image center from the true object center exists for subsurface objects in the collimated detector images; (2) shadowing effects occur for objects that protrude above the scanned surface; (3) scanned objects with air volumes present greater contrast in the acquired images than those without air volumes. Image processing and object recognition algorithms are developed and applied to the LMR images to enhance the image quality, to remove surface clutter, and to obtain depth information of subsurface objects.; The physical analysis of the x-ray beam rotating collimator and the development of the corresponding mobile fast-scan LMR system and its image acquisition and processing algorithms show that LMR is a proven technique for fast, mobile object surface and subsurface examination.