Scatter correction is a major challenge in x-ray imaging using large area detectors. Recently, the authors proposed a promising scatter correction method for x-raycomputed tomography(CT) using primary modulation. Proof of concept was previously illustrated by Monte Carlo simulations and physical experiments on a small phantom with a simple geometry. In this work, the authors provide a quantitative evaluation of the primary modulation technique and demonstrate its performance in applications where scatter correction is more challenging.Methods:
The authors first analyze the potential errors of the estimated scatter in the primary modulation method. On two tabletop CT systems, the method is investigated using three phantoms: A Catphan©600 phantom, an anthropomorphic chest phantom, and the Catphan©600 phantom with two annuli. Two different primary modulators are also designed to show the impact of the modulator parameters on the scatter correction efficiency. The first is an aluminummodulator with a weak modulation and a low modulation frequency, and the second is a coppermodulator with a strong modulation and a high modulation frequency.Results:
On the Catphan©600 phantom in the first study, the method reduces the error of the CT number in the selected regions of interest (ROIs) from 371.4 to 21.9 Hounsfield units (HU); the contrast to noise ratio also increases from 10.9 to 19.2. On the anthropomorphic chest phantom in the second study, which represents a more difficult case due to the high scatter signals and object heterogeneity, the method reduces the error of the CT number from 327 to 19 HU in the selected ROIs and from 31.4% to 5.7% on the overall average. The third study is to investigate the impact of object size on the efficiency of our method. The scatter-to-primary ratio estimation error on the Catphan©600 phantom without any annulus (20 cm in diameter) is at the level of 0.04, it rises to 0.07 and 0.1 on the phantom with an elliptical annulus (30 cm in the minor axis and 38 cm in the major axis) and with a circular annulus (38 cm in diameter).Conclusions:
On the three phantom studies, good scatter correction performance of the proposed method has been demonstrated using both image comparisons and quantitative analysis. The theory and experiments demonstrate that a strong primary modulation that possesses a low transmission factor and a high modulation frequency is preferred for high scatter correction accuracy.
This project was supported by the NIH under Grant Nos. R21 EB008186 and R01 EB003524 and by the Lucas Foundation. The authors would like to thank the editor and the anonymous reviewers for their valuable and insightful comments. Thanks also go to Jared Starman for his help with the tabletop system.
II. THE PRIMARY MODULATION METHOD
II.A. The basic concept and implementation
II.B. The algorithm of scatter estimation
II.C. Errors of the scatter estimation
III. EXPERIMENTS AND RESULTS
III.A. The tabletop CBCT systems and the phantoms
III.B. The aluminum and the copper primary modulators
III.C. Experiments and evaluations
III.D. First study: Catphan©600 phantom on system I
III.E. Second study: Chest phantom on system I
III.F. Third study: Catphan©600 phantom on system II
IV. DISCUSSION AND CONCLUSIONS
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