A supporting study of the scatter correction method for x-ray CBCT using primary modulation: hypothesis validation and MTF measurement

Lei Zhu, Rebecca Fahrig

Recently, we have developed an effective scatter correction algorithm for x-ray imaging using primary modulation. A calibration sheet with spatially variant attenuating materials is inserted between the x-ray source and the object, so that the scatter and part of the primary distributions are strongly separate in the Fourier domain, with the hypothesis that the scatter is predominantly low frequency even if high-frequency components are present in the x-ray source distribution. Linear filtering and demodulation techniques suffice to obtain the scatter estimation and correction. The algorithm has been verified by physical experiments on our conebeam CT (CBCT) system. In this work, we validate the key hypothesis in the algorithm using Monte Carlo simulations. Two experiments were carried out on a water cylinder phantom, using a uniform incident x-ray spatial distribution and a nonuniform distribution with a high-frequency strip pattern. The result comparison reveals that the high-frequency x-ray distribution results in very small high-frequency components in the scatter. Using the same low-pass filter as in the scatter correction algorithm, the difference of the relative scatter estimation errors between these two setups is below 0.2%. Since a filtering-based technique is used in the algorithm, we also investigate the algorithm performance on the reconstructed image resolution using modulation transfer function (MTF) measurements. The results show that the image resolution performance of a CBCT system using the primary modulation method is better than that of without scatter correction, and it is comparable with the system using a slot-scan geometry, where the scatter is inherently suppressed.