Optimization for the tradeoff of detection efficiency and absorbed dose in x-ray backscatter imaging

Abstract

Low radiation dose is mandatory in x-ray backscatter personnel screening. However, too low dose causes severe image noise and may fail to detect low-contrast contraband. Since noise depends mainly on detection efficiency, we need to improve detection efficiency and at the same time, to maintain low absorbed dose. In this study, both detection efficiency and absorbed dose were calculated based on the theory of Compton scatter and photoelectric absorption. Except multiple scattering, all other physical factors such as the realistic energy spectrum of incident x-rays and beam hardening were included. Realistic detection geometry and count density were obtained from the literature. The lower limit of detection efficiency is 1.35 % for 50 kVp x-rays and 1.99 % for 120 kVp x-rays in soft tissue. Correspondingly, the upper limit of entry dose is 0.051 μGy and 0.018 μGy. The better performance of 120 kVp x-rays is due to the highest detection efficiency and lowest absorbed dose produced by the x-rays with energy from 60 to 80 keV. Detection efficiency increases dramatically as the two detectors of the scanner are closer to each other. Also, thicker x-ray beam significantly improves low-contrast detectability at the price of higher absorbed dose and poorer high-contrast detectability. The optimal spectrum should include as much 60 to 80 keV x-rays and as little x-rays under 30 keV as possible. In the design of the personnel scanner, the two detectors should be positioned as close as possible. A thick x-ray beam should be used as long as radiation dose stays within the ANSI limit and spatial resolution is adequate for detecting small metal weapons and thin wires.