A novel total variation based ring artifact suppression method for CBCT imaging with two‐dimensional antiscatter grids

Abstract

Purpose Two‐dimensional antiscatter grids (2DASG) for cone beam computed tomography (CBCT) is a new area of research to reduce scatter intensity, and consequently improve CBCT image quality. One of the challenges in implementation of 2DASGs is their septa shadows that are impinged on the projections. If these artifacts are not corrected, they may lead to ring artifacts in CBCT images. In this work, we present a novel method to suppress ring artifacts in FPD‐based CBCT images. Methods Briefly, our method first detects the locations of 2DASG’s septa shadows in projections and then, reduces projection pixel values in septa shadows iteratively until a residual‐based convergence criterion is met. To suppress the 2DASG’s septa shadows, we developed a total variation minimization (TVM) formulation, referred to as adaptive‐diffusive total variation minimization (adTVM), where the diffusivity regularization parameter was adaptively adjusted during each iteration based on the magnitude of the local pixel gradients. To test our method, we have acquired CBCT scans of phantoms using three 2DASG prototypes with different grid geometries. Projections were acquired with a linac mounted CBCT system, operated in offset detector geometry. These projections were then corrected in the following steps: first, projections were corrected using a gantry angle‐specific gain correction map; next, projections were corrected by applying our adTVM method. CBCT images were reconstructed using FDK filtered backprojection algorithm. To evaluate adTVM’s performance, pixel value statistics and contrast‐to‐noise ratio (CNR) were compared in CBCT images corrected with and without our adTVM method. Results Without our adTVM method, all three 2DASG prototypes introduced ring artifacts with varying intensities in CBCT images. With our method, significant reduction in ring artifacts was observed in all test cases. Standard deviation of CT numbers was reduced by 7–74% in uniform density phantom CBCT images, CNR was increased by 8–67%, and CT number accuracy of contrast objects embedded in the phantom was preserved. Conclusion We propose a new method to suppress ring artifacts caused by the 2DASG's septa shadows in CBCT images. Our initial investigations indicated that adTVM method could substantially reduce such ring artifacts while preserving CT number accuracy and maintaining good spatial resolution. Therefore, our method may potentially play an important role in enabling the implementation of 2DASGs in flat panel detector based CBCT systems.

Evaluation of scatter rejection and correction performance of 2D antiscatter grids in cone beam computed tomography

“…A final issue is the suppression of image artifacts induced by 2D ASG's septal shadows or footprint. Our prior work 23,24,40 on grid-induced artifact suppression provided promising results, and can help to address this challenge.…”

Feature-Sensitive Ring Artifact Correction for Computed Tomography

“…Yousuf et al perform ring correction in the already Feldkamp-Davis-Kress (FDK) filtered projections [7]. Alexeev et al correct ring artifacts that occur specifically with antiscatter grids [13]. Croton et al combat normalization errors in phase contrast tomography by correcting both gain and offset of each detector pixel [14].…”

3D‐printed large‐area focused grid for scatter reduction in cone‐beam CT

“…The combination of both effects—reduced fluence and signal leakage—renders conventional pixel‐to‐pixel variation correction methods ineffective 47,48 . In such a case, the CBCT reconstruction can be affected by severe ring artifacts that impair radiological interpretation and analysis 33,47 . Another limitation of recently investigated 2D‐ASG prototypes relates to their coverage area of the detector.…”