Scatter correction in cone-beam CT via a half beam blocker technique allowing simultaneous acquisition of scatter and image information
ABSTRACT X-ray scatter incurred to detectors degrades the quality of cone-beam computed tomography (CBCT) and represents a problem in volumetric image guided and adaptive radiation therapy. Several methods using a beam blocker for the estimation and subtraction of scatter have been proposed. However, due to missing information resulting from the obstruction of the blocker, such methods require dual scanning or dynamically moving blocker to obtain a complete volumetric image. Here, we propose a half beam blocker-based approach, in conjunction with a total variation (TV) regularized Feldkamp-Davis-Kress (FDK) algorithm, to correct scatter-induced artifacts by simultaneously acquiring image and scatter information from a single-rotation CBCT scan.
A half beam blocker, comprising lead strips, is used to simultaneously acquire image data on one side of the projection data and scatter data on the other half side. One-dimensional cubic B-Spline interpolation/extrapolation is applied to derive patient specific scatter information by using the scatter distributions on strips. The estimated scatter is subtracted from the projection image acquired at the opposite view. With scatter-corrected projections where this subtraction is completed, the FDK algorithm based on a cosine weighting function is performed to reconstruct CBCT volume. To suppress the noise in the reconstructed CBCT images produced by geometric errors between two opposed projections and interpolated scatter information, total variation regularization is applied by a minimization using a steepest gradient descent optimization method. The experimental studies using Catphan504 and anthropomorphic phantoms were carried out to evaluate the performance of the proposed scheme.
The scatter-induced shading artifacts were markedly suppressed in CBCT using the proposed scheme. Compared with CBCT without a blocker, the nonuniformity value was reduced from 39.3% to 3.1%. The root mean square error relative to values inside the regions of interest selected from a benchmark scatter free image was reduced from 50 to 11.3. The TV regularization also led to a better contrast-to-noise ratio.
An asymmetric half beam blocker-based FDK acquisition and reconstruction technique has been established. The proposed scheme enables simultaneous detection of patient specific scatter and complete volumetric CBCT reconstruction without additional requirements such as prior images, dual scans, or moving strips.
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ABSTRACT: A beam stop array (BSA) is widely used to estimate the scatter distribution of cone-beam computed tomography (CT) experimentally. A rectangular BSA, which is the most generally used, produces a penumbra region at the edge of the grid on the projection image. The penumbra region can be the cause of an inaccurate scatter estimate and loss of image data. In this study, two types of BSAs have been designed and developed, a linear BSA (l-BSA), which is a simple rectangular type, and a curved BSA (c-BSA) to avoid the penumbra effect. A Monte Carlo simulation was performed to determine the thickness and the material of beam stop arrays. The particle tracking technique of MCNP5 code was used to analyze the accuracy of the scatter distribution acquired by the beam stop arrays. The improvement in the image quality was analyzed by using an in-house phantom, a simple type CT phantom, and a humanoid phantom. How the curved beam stop array improved the penumbra effect was also analyzed in a large field of view. The additional dose caused by a dual scan was analyzed by using a Monte Carlo simulation and experiment. The accuracies of the scatter estimates were 83.4% for the c-BSA and 90.4% for the l-BSA. The c-BSA compensated the penumbra effect effectively while the l-BSA had a severe penumbra region in the partial projection image in a large field of view. Contrasts were improved by 24.8 (air-PMMA), 55.8 (Teflon-PMMA), and 81.7% (water-PMMA) for the c-BSA and 11.1 (air-PMMA), 44.1 (Teflon-PMMA), and 82.2% (water-PMMA) for the l-BSA by scatter correction. After noise suppression, the contrast-to-noise ratios were improved by 2.7–4.1 times for the c-BSA and 3.5–5.3 times for the l-BSA. Uniformities were also improved by up to 10 times for the c-BSA and 12 times for the l-BSA. The quality of the humanoid phantom image was also improved after the scatter correction. Consequently, the c-BSA can improve the image quality of cone-beam CT and compensate for the penumbra effect without an additional dose of radiation.Journal- Korean Physical Society 04/2014; 64(8):1220-1229. DOI:10.3938/jkps.64.1220 · 0.43 Impact Factor
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ABSTRACT: In conventional digital radiography (DR) using a dual energy subtraction technique, a significant fraction of the detected photons are scattered within the body, making up the scatter component. Scattered radiation can significantly deteriorate image quality in diagnostic X-ray imaging systems. Various methods of scatter correction, including both measurement- and non-measurement-based methods, have been proposed in the past. Both methods can reduce scatter artifacts in images. However, non-measurement-based methods require a homogeneous object and have insufficient scatter component correction. Therefore, we employed a measurement-based method to correct for the scatter component of inhomogeneous objects from dual energy DR (DEDR) images. We performed a simulation study using a Monte Carlo simulation with a primary modulator, which is a measurement-based method for the DEDR system. The primary modulator, which has a checkerboard pattern, was used to modulate the primary radiation. Cylindrical phantoms of variable size were used to quantify the imaging performance. For scatter estimates, we used discrete Fourier transform filtering, e.g., a Gaussian low-high pass filter with a cut-off frequency. The primary modulation method was evaluated using a cylindrical phantom in the DEDR system. The scatter components were accurately removed using a primary modulator. When the results acquired with scatter correction and without scatter correction were compared, the average contrast-to-noise ratio (CNR) with the correction was 1.35 times higher than that obtained without the correction, and the average root mean square error (RMSE) with the correction was 38.00% better than that without the correction. In the subtraction study, the average CNR with the correction was 2.04 (aluminum subtraction) and 1.38 (polymethyl methacrylate (PMMA) subtraction) times higher than that obtained without the correction. The analysis demonstrated the accuracy of the scatter correction and the improvement of image quality that could be obtained by using a primary modulator and showed the feasibility of introducing the primary modulation technique into dual energy subtraction. Therefore, we suggest that the scatter correction method with a primary modulator will be useful for use with the DEDR system.Journal- Korean Physical Society 08/2014; 65(4):541-552. DOI:10.3938/jkps.65.541 · 0.43 Impact Factor
Conference Paper: A patient-specific scatter artifacts correction method[Show abstract] [Hide abstract]
ABSTRACT: This paper provides a fast and patient-specific scatter artifact correction method for cone-beam computed tomography (CBCT) used in image-guided interventional procedures. Due to increased irradiated volume of interest in CBCT imaging, scatter radiation has increased dramatically compared to 2D imaging, leading to a degradation of image quality. In this study, we propose a scatter artifact correction strategy using an analytical convolution-based model whose free parameters are estimated using a rough estimation of scatter profiles from the acquired cone-beam projections. It was evaluated using Monte Carlo simulations with both monochromatic and polychromatic X-ray sources. The results demonstrated that the proposed method significantly reduced the scatter-induced shading artifacts and recovered CT numbers.SPIE Medical Imaging; 03/2014