Article

Scatter correction for cone-beam CT in radiation therapy

Department of Radiation Oncology, Stanford University, Stanford, California 94305, USA.
Medical Physics (Impact Factor: 2.64). 07/2009; 36(6):2258-68. DOI: 10.1118/1.3130047
Source: PubMed

ABSTRACT

Cone-beam CT (CBCT) is being increasingly used in modern radiation therapy for patient setup and adaptive replanning. However, due to the large volume of x-ray illumination, scatter becomes a rather serious problem and is considered as one of the fundamental limitations of CBCT image quality. Many scatter correction algorithms have been proposed in literature, while a standard practical solution still remains elusive. In radiation therapy, the same patient is scanned repetitively during a course of treatment, a natural question to ask is whether one can obtain the scatter distribution on the first day of treatment and then use the data for scatter correction in the subsequent scans on different days. To realize this scatter removal scheme, two technical pieces must be in place: (i) A strategy to obtain the scatter distribution in on-board CBCT imaging and (ii) a method to spatially match a prior scatter distribution with the on-treatment CBCT projection data for scatter subtraction. In this work, simple solutions to the two problems are provided. A partially blocked CBCT is used to extract the scatter distribution. The x-ray beam blocker has a strip pattern, such that partial volume can still be accurately reconstructed and the whole-field scatter distribution can be estimated from the detected signals in the shadow regions using interpolation/extrapolation. In the subsequent scans, the patient transformation is determined using a rigid registration of the conventional CBCT and the prior partial CBCT. From the derived patient transformation, the measured scatter is then modified to adapt the new on-treatment patient geometry for scatter correction. The proposed method is evaluated using physical experiments on a clinical CBCT system. On the Catphan 600 phantom, the errors in Hounsfield unit (HU) in the selected regions of interest are reduced from about 350 to below 50 HU; on an anthropomorphic phantom, the error is reduced from 15.7% to 5.4%. The proposed method is attractive in applications where a high CBCT image quality is critical, for example, dose calculation in adaptive radiation therapy.

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Available from: Lei Zhu, Aug 22, 2014
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    • "Besides the strip placement, two more parameters are needed in the blocker design: sampling period (í µí±†) and strip width (í µí±Š). To guarantee the measurement of at least one ray from its conjugate pair, we choose a relatively large sampling period (í µí±† ≈ 52 mm on the detector) based on the observed maximum spatial frequency of scatter signals in our previous studies [15] [17] [38] as well as in the literature [39]. The strip width cannot be too small since the penumbra effects on the strips limit scatter measurement accuracy [38]. "
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    ABSTRACT: Excessive imaging dose from repeated scans and poor image quality mainly due to scatter contamination are the two bottlenecks of cone-beam CT (CBCT) imaging. Compressed sensing (CS) reconstruction algorithms show promises in recovering faithful signals from low-dose projection data but do not serve well the needs of accurate CBCT imaging if effective scatter correction is not in place. Scatter can be accurately measured and removed using measurement-based methods. However, these approaches are considered unpractical in the conventional FDK reconstruction, due to the inevitable primary loss for scatter measurement. We combine measurement-based scatter correction and CS-based iterative reconstruction to generate scatter-free images from low-dose projections. We distribute blocked areas on the detector where primary signals are considered redundant in a full scan. Scatter distribution is estimated by interpolating/extrapolating measured scatter samples inside blocked areas. CS-based iterative reconstruction is finally carried out on the undersampled data to obtain scatter-free and low-dose CBCT images. With only 25% of conventional full-scan dose, our method reduces the average CT number error from 250 HU to 24 HU and increases the contrast by a factor of 2.1 on Catphan 600 phantom. On an anthropomorphic head phantom, the average CT number error is reduced from 224 HU to 10 HU in the central uniform area.
    Full-text · Article · Nov 2013 · Computational and Mathematical Methods in Medicine
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    • "The image quality for the CBCT is significantly degraded due to excessive scattered photons [5] [6] [7] [8] as well as suboptimal performance of the flat panel detector [9]. These issues limit the use of CBCT for certain advanced radiation therapy techniques such as online adaptive radiotherapy [8] [10]. It is also well known that at large cone angles, there are artifacts caused by using approximate reconstruction methods that appear in CBCT reconstructions [11], but this issue has largely been ignored in IGRT because the scatter and detector issues are the dominant factors in the degradation of CBCT image quality. "
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    ABSTRACT: Tetrahedron beam computed tomography (TBCT) performs volumetric imaging using a stack of fan beams generated by a multiple pixel X-ray source. While the TBCT system was designed to overcome the scatter and detector issues faced by cone beam computed tomography (CBCT), it still suffers the same large cone angle artifacts as CBCT due to the use of approximate reconstruction algorithms. It has been shown that iterative reconstruction algorithms are better able to model irregular system geometries and that algebraic iterative algorithms in particular have been able to reduce cone artifacts appearing at large cone angles. In this paper, the SART algorithm is modified for the use with the different TBCT geometries and is tested using both simulated projection data and data acquired using the TBCT benchtop system. The modified SART reconstruction algorithms were able to mitigate the effects of using data generated at large cone angles and were also able to reconstruct CT images without the introduction of artifacts due to either the longitudinal or transverse truncation in the data sets. Algebraic iterative reconstruction can be especially useful for dual-source dual-detector TBCT, wherein the cone angle is the largest in the center of the field of view.
    Full-text · Article · May 2013 · International Journal of Biomedical Imaging
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    • "The geometry is inversely analogous to x-ray cone beam CT with the diffuser sheet replacing the flat-panel CCD array detector and the image forming aperture of a camera substituting for the point source. Improving scatter corrections for x-ray cone beam CT is an active area of academic and commercial research [2]. "
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    ABSTRACT: Acceptance of stray light in transmission images is a general imaging problem. A commercial optical cone beam CT scanner, Vista10 was modified to reduce stray light. Further stray light reduction was accomplished by placing vertical slot aperture arrays between the object and the diffusive light source. Comparisons of single and seven aperture arrays demonstrated that transmissions equivalent to scatter free conditions can be achieved with a multiple array collimator. Results for uniform liquids and finger gel phantoms demonstrate that small objects in a bright background are a more stringent test of stray light rejection.
    Full-text · Article · Dec 2010 · Journal of Physics Conference Series
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