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Measurement of the Modulation Transfer Function for a Backscatter Radiography System Used in Nondestructive Examination

Traditional methods for measuring the modulation transfer function (MTF) of transmission x-ray systems cannot be directly applied to backscatter radiography units. This work presents two new methods for measuring the MTF of a backscatter radiography system based on imaging sharp edges and square-wave line pairs. Since the MTF of backscatter systems is dependent on the target material, test pieces were constructed of wood inlaid with lead line pairs. Signals from scanning the initial square-wave test pattern showed minimal contrast and high levels of noise, most likely attributable to insufficient attenuation provided by absorbing lead bars. Experimental results from the edge spread method show good agreement with theoretical data (mean difference within +/- 3%), indicating the potential use of this technique for measuring the MTF of a backscatter system. Inconclusive results from the line-pair method demonstrate the complexity of measuring the MTF for a backscatter system with this technique, most likely caused by insufficient attenuation, contributing scatter signal from material below the tool, and distortion from edge effects. We discuss a new design of an optimal line pair test tool for backscatter radiography, whose calculated MTF can then be compared to those from the edge spread method.

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