Correction for patient table‐induced scattered radiation in cone‐beam computed tomography (CBCT)

Abstract

Purpose: In image-guided radiotherapy, an artifact typically seen in axial slices of x-ray cone-beam computed tomography ͑CBCT͒ reconstructions is a dark region or "black hole" situated below the scan isocenter. The authors trace the cause of the artifact to scattered radiation produced by radiotherapy patient tabletops and show it is linked to the use of the offset-detector acquisition mode to enlarge the imaging field-of-view. The authors present a hybrid scatter kernel superposition ͑SKS͒ algorithm to correct for scatter from both the object-of-interest and the tabletop. Methods: Monte Carlo simulations and phantom experiments were first performed to identify the source of the black hole artifact. For correction, a SKS algorithm was developed that uses separate kernels to estimate scatter from the patient tabletop and the object-of-interest. Each projection is divided into two regions, one defined by the shadow cast by the tabletop on the imager and one defined by the unshadowed region. The region not shadowed by the tabletop is processed using the recently developed fast adaptive scatter kernel superposition ͑fASKS͒ method which employs asymmetric kernels that best model scatter transport through bodylike objects. The shadowed region is convolved with a combination of slab-derived symmetric SKS kernels and asymmetric fASKS kernels. The composition of the hybrid kernels is projection-angle-dependent. To test the algorithm, pelvis phantom and in vivo data were acquired using a CBCT test stand, a Varian Acuity simulator, and a Varian On-Board Imager, all of which have similar geometries and components. Artifact intensities and Hounsfield unit ͑HU͒ accuracies in the reconstructions were assessed before and after the correction. Results: The hybrid kernel algorithm provided effective correction and produced substantially better scatter estimates than the symmetric SKS or asymmetric fASKS methods alone. HU nonuniformities in the reconstructed pelvis phantom were reduced from 220 to 50 HU ͑i.e., 22%-5%͒. In the in vivo scans, the black hole artifact was reduced by up to 147 HU, a 73% improvement, and anatomical details in the prostate and rectum areas were made considerably more visible. Conclusions: Radiotherapy tabletops, which are generally flatter and larger than those used for diagnostic CT, can produce significant scatter-related artifacts. The proposed hybrid SKS algorithm accurately estimates scatter from both the object-of-interest and the patient tabletop, and resulting image uniformities and HU accuracies are greatly improved.

Transmission characteristics of a two dimensional antiscatter grid prototype for CBCT

“…Scatter correction methods, which refers to correcting the effects of scatter after its detection by the image receptor, are often employed to correct the residual scatter. [14][15][16][17][18][19][20][21][22][23][24] Generally, both of these approaches have been used together in CBCT systems for radiation therapy. However, the improvement in image quality is not at the desired level to achieve highly accurate CT numbers or improved visualization of low contrast objects.…”

A practical cone-beam CT scatter correction method with optimized Monte Carlo simulations for image-guided radiation therapy

“…Calculation-based methods have also been employed to compute scatter signals. Kernel methods are particularly popular in this category (Shaw et al , 1982; Hangartner, 1987; Seibert and Boone, 1988; Kruger et al , 1994; Ohnesorge et al , 1999; Star-Lack et al , 2009; Sun and Star-Lack, 2010; Sun et al , 2011). They decompose a measured projection into the scatter and primary components subject to an experiment-calibrated empirical relationship between them.…”

CT to cone-beam CT deformable registration with simultaneous intensity correction

“…Second, scatter contamination also leads to severe cupping, streak artifacts and degrade of image contrast. In a typical clinical CBCT system for radiotherapy scatter-to-primary ratio (SPR) may even exceed 100% (Siewerdsen and Jaffray, 2001), although many methods have been proposed to correct scatter artifacts (Siewerdsen et al , 2006; Rinkel et al , 2007; Maltz et al , 2008; Zhu et al , 2009b; Poludniowski et al , 2009; Yan et al , 2010; Meyer et al , 2010; Sun et al , 2011), it is still an open problem. Third, the gantry mounted bowtie filter in CBCT system may wobble, as the gantry rotates, which can result in crescent artifacts (Giles et al , 2011; Zheng et al , 2011).…”