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An adapted modulation transfer function for x-ray backscatter radiography by selective detection

The Modulation Transfer Function (MTF) is a quantitative function based on frequency resolution that characterizes imaging system performance. In this study, a new MTF methodology is investigated for application to Radiography by Selective Detection (RSD), an enhanced single-side x-ray Compton backscatter imaging (CBI) technique which detects selected scatter components. The RSD imaging modality is a unique type of real-time radiography that uses a set of fin and sleeve collimators to preferentially select different components of the x-ray backscattered field. Radiography by selective detection has performed successfully in different non-destructive evaluation (NDE) applications. A customized RSD imaging system was built at the University of Florida for inspection of the space shuttle external tank spray-on foam insulation (SOFI). The x-ray backscatter RSD imaging system has been successfully used for crack and corrosion detection in a variety of materials. The conventional transmission x-ray image quality characterization tools do not apply for RSD because of the different physical process involved. Thus, the main objective of this project is to provide an adapted tool for dynamic evaluation of RSD system image quality. For this purpose, an analytical model of the RSD imaging system response is developed and supported. Two approaches are taken for the MTF calculations: one using the Fourier Transform of a line spread function and the other one using a sine function pattern. Calibration and test targets are then designed according to this proposed model. A customized Matlab code using image contrast and digital curve recognition is developed to support the experimental data and provide the Modulation Transfer Functions for RSD.