Scatter Reduction and Correction for Dual-Source Cone-Beam CT Using Prepatient Grids

Abstract

Purpose: Scatter significantly limits the application of the dual-source cone-beam computed tomography by inducing scatter artifacts and degrading contrast-to-noise ratio, Hounsfield-unit accuracy, and image uniformity. Although our previously developed interleaved acquisition mode addressed the cross scatter between the 2 X-ray sources, it doubles the scanning time and doesn't address the forward scatter issue. This study aims to develop a prepatient grid system to address both forward scatter and cross scatter in the dual-source cone-beam computed tomography.

Methods: Grids attached to both X-ray sources provide physical scatter reduction during the image acquisition. Image data were measured in the unblocked region, while both forward scatter and cross scatter were measured in the blocked region of the projection for postscan scatter correction. Complementary projections were acquired with grids at complementary locations and were merged to form complete projections for reconstruction. Experiments were conducted with different phantom sizes, grid blocking ratios, image acquisition modes, and reconstruction algorithms to investigate their effects on the scatter reduction and correction. The image quality improvement by the prepatient grids was evaluated both qualitatively through the artifact reduction and quantitatively through contrast-to-noise ratio, Hounsfield-unit accuracy, and uniformity using a CATphan 504 phantom.

Results: Scatter artifacts were reduced by scatter reduction and were removed by scatter correction method. Contrast-to-noise ratio, Hounsfield-unit accuracy, and image uniformity were improved substantially. The simultaneous acquisition mode achieved comparable contrast-to-noise ratio as the interleaved and sequential modes after scatter reduction and correction. Higher grid blocking ratio and smaller phantom size led to higher contrast-to-noise ratio for the simultaneous mode. The iterative reconstruction with total variation regularization was more effective than the Feldkamp, Davis, and Kress method in reducing noise caused by the scatter correction to enhance contrast-to-noise ratio.

Conclusion: The prepatient grid system is effective in removing the scatter effects in the simultaneous acquisition mode of the dual-source cone-beam computed tomography, which is useful for scanning time reduction or dual energy imaging.

Keywords: CNR enhancement; cross scatter; dual energy imaging; grid blocking ratio; scatter artifact; simultaneous acquisition; total variation regularization.

Figure 1

Experimental set up of the dual source CBCT system

Figure 2

An example to illustrate the scatter correction and projection merging procedure using grid of blocking ratio 1:1.

Figure 3

Axial view of the CTP404 module with inserts of different contrasts. The red circles are the ROIs inside the inserts while the yellow ones are the ROIs surrounding the inserts.

Figure 4

Axial view of the conventional CBCT of the CTP486 uniformity module of the CATphan. Five ROIs were defined (one is in the center and the other four are at the edge) to calculate the uniformity of the image.

Figure 5

An example illustrating the scatter estimation and correction process in (a). a blank scan and (b). a phantom scan. The blue solid line is the original 1 D profile in the longitudinal direction in a blank/phantom scan projection. The star dots are the measured scatter in the blocked areas. The black line is the interpolated scatter distribution. The red solid line is final profile after scatter correction by subtracting the black line from the blue line.

Figure 6

Profiles extracted from complementary projections acquired using 1:1 grid after blank scan normalization and negative logarithm transformation. (a) The red and blue solid lines were from two complementary projections acquired with the grid shifted by one grid inter-space. (b) Profile of the merged projection.

Figure 7

Reconstructed images of a CATphan of 20cm diameter using 1) simultaneous, 2) interleaved and 3) sequential acquisition mode. 2:1 grid was applied for scatter reduction and scatter correction. From left to right column: (a) without grid, (b) scatter reduction only and (c) scatter reduction and correction. Window width of 1000HU and window level of −200HU were used for all images. The FDK method was used for reconstruction.

Figure 8

Reconstructed images of a CATphan of 15, 20 and 30cm diameters (from 1st to 3rd row) acquired by simultaneous mode (dual-source). 1:1 grid was applied for scatter reduction and scatter correction. From left to right column: (a) without grid, (b) scatter reduction only and (c) scatter reduction and correction. Display for 1st and 2nd : window width of 1000HU and window level of −200HU were used. Display for 3rd : window width of 2200HU and window level of 100HU were used. The FDK method was used for reconstruction.

Figure 9

Reconstructed images of a CATphan of 20 cm diameter acquired by simultaneous mode (dual-source). 1:1(1st row) and 2:1grid (2nd row) was applied for scatter reduction and scatter correction. From left to right column: (a) without grid, (b) scatter reduction only and (c) scatter reduction and correction. Window width of 1000HU and window level of −200HU were used for all images. The FDK method was used for reconstruction.

Figure 10

Reconstructed images of a CATphan of 20cm diameter acquired by simultaneous mode (dual-source). 1:1 grid was applied for scatter reduction and scatter correction. Images were reconstructed from the FDK method (1st row) and iterative method (2nd row). From left to right column: (a) without grid, (b) scatter reduction only and (c) scatter reduction and correction. Window width of 1000HU and window level of −200HU were used for all images.