Overview of X-ray Scatter in Cone-beam Computed Tomography and Its Correction Methods

X-ray cone-beam computed tomography (CBCT) is widely used nowadays, mainly for its large volume coverage and hardware compatibility with open-gantry x-ray imaging systems. As the size of x-ray illumination increases, an inevitable and adverse effect is the boost of scatter contamination on the x-ray images, which becomes one of the fundamental limitations of CBCT imaging. The large scatter signals in CBCT cause severe streaking and cupping artifacts in the CT images and greatly hamper the applications of CBCT due to its degraded image quality as compared to that of the conventional x-ray CT scanner. Research on scatter correction has gained heated attention in recent years. In this review, we first analyze the magnitudes of scatter in CBCT and its resultant errors in the reconstructed images. The existing CBCT scatter correction methods are then summarized in several categories: pre-processing methods, and post-processing methods including measurement-based, software-based, hardware-based decomposition and hybrid methods. An important issue related to the post-processing methods, the noise increase in the scatter corrected images, is also discussed. Although numerous scatter correction methods have been proposed in the literature, each approach has its own strengths and drawbacks and an optimal and standard method is still elusive. This review provides a comprehensive summary of the current research on scatter correction, and suggests future directions from the authors' perspective.