Scatter Correction by Modulation of Primary Radiation in Industrial X-ray CT: Beam-hardening Effects and their Correction

摘要

Scattered radiation presents a major cause of image degradation for industrial X-ray cone-beam CT scanners. It introduces several kinds of artifacts in reconstructed CT volumes, such as streaks, a general loss of contrast, and in-homogeneities in regions of homogeneous material, also known as cupping artifact. We present a novel experimental method for the correction of scatter artifacts which is adapted from medical application and based on primary modulation. Here, a primary modulator placed between the X-ray source and an object imprints a spatially varying pattern on primary X-rays through attenuation, e.g. a checkerboard-like pattern. This modulation pattern is only preserved in unaltered primary X-ray photons – scattered X-rays have a broad spatial distribution after the (Compton-) scattering process and, thereby, the original pattern gets lost. Thus, modulated primary signals and non-modulated scatter signals can be separated in frequency space by appropriate filtering algorithms. After demodulation of primary signals and their subtraction from total signals, we obtain a scatter estimate. For the proposed scatter correction method, we introduce a beam-hardening correction procedure and discuss its effects on the scatter estimation process. Scatter-corrected CT reconstructions show a significant improvement in image quality. Compared to other scatter correction techniques, e.g. approaches based on beam-stops, this method offers the advantage to integrate the scatter measurement into the actual CT scan which reduces scan time and effort.