Browsing by Subject "cross scatter"
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Item Open Access Cross-Scatter in Dual-Cone X-ray Imaging: Magnitude, Avoidance, Correction, and Artifact Reduction(2012) Giles, WilliamOnboard cone beam computed tomography (CBCT) has become a widespread means of three-dimensional target localization for radiation therapy; however, it is susceptible to metal artifacts and beam-hardening artifacts that can hinder visualization of low contrast anatomy. Dual-CBCT provides easy access to techniques that may reduces such artifacts. Additionally, dual-CBCT can decrease imaging time and provide simultaneous orthogonal projections which may also be useful for fast target localization. However, dual-CBCT will suffer from large increases in scattered radiation due to the addition of the second source.
An experimental bench top dual CBCT system was constructed so that each imaging chain in the dual CBCT system mimics the geometry of gantry-mounted CBCT systems commonly used in the radiation therapy room. The two systems share a common axis of rotation and are mounted orthogonally. Custom control software was developed to ensure reproducible exposure and rotation timings. This software allows the implementation of the acquisition sequences required for the cross scatter avoidance and correction strategies studied.
Utilizing the experimental dual CBCT system cross scatter was characterized from 70-145 kVp in projections and reconstructed images using this system and three cylindrical phantoms (15cm, 20cm, and 30cm) with a common Catphan core. A novel strategy for avoiding cross-scatter in dual-CBCT was developed that utilized interleaved data acquisition on each imaging chain. Contrast and contrast-to-noise-ratio were measured in reconstructions to evaluate the effectiveness of this strategy to avoid the effects of cross scatter.
A novel correction strategy for cross scatter was developed wherein the cross scatter was regularly sampled during the course of data acquisition and these samples were used as the basis for low- and high- frequency corrections for the cross-scatter in projections. The cross scatter sampling interval was determined for an anthropomorphic phantom at three different sites relevant to radiation therapy by estimating the angular Nyquist frequency. The low frequency portion of the cross scatter distribution is interpolated between samples to provide an estimate of the cross scatter distribution at every projection angle and was then subtracted from the projections.
The high-frequency portion of the correction was applied after the low-frequency correction was applied. The novel high-frequency correction utilizes the fact that a direct estimate of the high-frequency components was obtained in the cross scatter samples. The high-frequency components of the measured cross scatter were subtracted from the projections in the Fourier domain, a process referred to as spectral subtraction. Each projection is corrected using the cross scatter sample taken at the closest projection angle. In order to apply this correction in the Fourier domain the high-frequency component of the cross scatter must be approximately stationary. To improve the stationarity of the high-frequency cross scatter component a novel two-dimensional, overlapping window was developed. The spectral subtraction was then applied in each window and the results added to form the final image.
The effectiveness of the correction techniques were evaluated by measuring the contrast and contrast-to-noise-ratio in an image quality phantom. Additionally, the effect of the high-frequency correction on resolution was measured using a line pair phantom.
Cross scatter in dual CBCT was shown for large phantoms to be much higher than forward scatter which has long been known to be one of the largest degrading factors of image quality in CBCT. This results in large losses of contrast and CNR in reconstructed images. The interleaving strategy for avoiding cross scatter during projection acquisition showed similar performance to cross scatter free acquisitions, however, does not acquire projections at the maximum possible rate. For those applications in which maximizing the acquisition rate of projections is important, the low- and high-frequency corrections effectively mitigated the effects of cross scatter in the dual CBCT system.
Item Open Access Scatter Correction for Dual-source Cone-beam CT Using the Pre-patient Grid(2014) Chen, YingxuanPurpose: A variety of cone beam CT (CBCT) systems has been used in the clinic for image guidance in interventional radiology and radiation therapy. Compared with conventional single-source CBCT, dual-source CBCT has the potential for dual-energy imaging and faster scanning. However, it adds additional cross-scatter when compared to a single-source CBCT system, which degrades the image quality. Previously, we developed a synchronized moving grid (SMOG) system to reduce and correct scatter for a single-source CBCT system. The purpose of this work is to implement the SMOG system on a prototype dual-source CBCT system and to investigate its efficacy in scatter reduction and correction under various imaging acquisition settings.
Methods:A 1-D grid was attached to each x-ray source during dual-source CBCT imaging to acquire partially blocked projections. As the grid partially blocked the x-ray primary beams and divided it into multiple quasi-fan beams during the scan, it produced a physical scatter reduction effect in the projections. Phantom data were acquired in the unblocked area, while scatter signal was measured from the blocked area in projections. The scatter distribution was estimated from the measured scatter signals using a cubic spline interpolation for post-scan scatter correction. Complimentary partially blocked projections were acquired at each scan angle by positioning the grid at different locations, and were merged to obtain full projections for reconstruction. In this study, three sets of CBCT images were reconstructed from projections acquired: (a) without grid, (b) with grid but without scatter correction, and (c) with grid and with scatter correction to evaluate the effects of scatter reduction and scatter correction on artifact reduction and improvements of contrast-to-noise ratio index (CNR') and CT number accuracy. The efficacy of the scatter reduction and correction method was evaluated for CATphan phantoms of different diameters (15cm, 20cm, and 30cm), different grids (grid blocking ratios of 1:1 and 2:1), different acquisition modes (simultaneous: two tubes firing at the same time, interleaved: tube alternatively firing and sequential: only one tube firing in one rotation) and different reconstruction algorithms (iterative reconstruction method vs Feldkamp, Davis, and Kress (FDK) back projection method).
Results: The simultaneous scanning mode had the most severe scatter artifacts and the most degraded CNR' when compared to either the interleaved mode or the sequential mode. This is due to the cross-scatter between the two x-ray sources in the simultaneous mode. Scatter artifacts were substantially reduced by scatter reduction and correction. CNR's of the different inserts in the CATphan were enhanced on average by 24%, 13%, and 33% for phantom sizes of 15cm, 20cm, and 30cm, respectively, with only scatter reduction and a 1:1 grid. Correspondingly, CNR's were enhanced by 34%, 18%, and 11%, respectively, with both scatter reduction and correction. However, CNR' may decrease with scatter correction alone for the larger phantom and low contrast ROIs, because of an increase in noise after scatter correction. In addition, the reconstructed HU numbers were linearly correlated to nominal HU numbers. A higher grid blocking ratio, i.e. with a greater blocked area, resulted in better scatter artifact removal and CNR' improvement at the cost of complexity and increased number of exposures. Iterative reconstruction with total variation regularization resulted in better noise reduction and enhanced CNR', in comparison to the FDK method.
Conclusion:Our method with a pre-patient grid can effectively reduce the scatter artifacts, enhance CNR', and modestly improve the CT number linearity for the dual-source CBCT system. The settings such as grid blocking ratio and acquisition mode can be optimized based on the patient-specific condition to further improve image quality.