4D cone-beam computed tomography (CBCT) using a moving blocker for simultaneous radiation dose reduction and scatter correction

Abstract

Four-dimensional (4D) x-ray cone-beam computed tomography (CBCT) is important for a precise radiation therapy for lung cancer. Due to the repeated use and 4D acquisition over a course of radiotherapy, the radiation dose becomes a concern. Meanwhile, the scatter contamination in CBCT deteriorates image quality for treatment tasks. In this work, we propose the use of a moving blocker (MB) during the 4D CBCT acquisition ('4D MB') and to combine motion-compensated reconstruction to address these two issues simultaneously. In 4D MB CBCT, the moving blocker reduces the x-ray flux passing through the patient and collects the scatter information in the blocked region at the same time. The scatter signal is estimated from the blocked region for correction. Even though the number of projection views and projection data in each view are not complete for conventional reconstruction, 4D reconstruction with a total-variation (TV) constraint and a motion-compensated temporal constraint can utilize both spatial gradient sparsity and temporal correlations among different phases to overcome the missing data problem. The feasibility simulation studies using the 4D NCAT phantom showed that 4D MB with motion-compensated reconstruction with 1/3 imaging dose reduction could produce satisfactory images and achieve 37% improvement on structural similarity (SSIM) index and 55% improvement on root mean square error (RMSE), compared to 4D reconstruction at the regular imaging dose without scatter correction. For the same 4D MB data, 4D reconstruction outperformed 3D TV reconstruction by 28% on SSIM and 34% on RMSE. A study of synthetic patient data also demonstrated the potential of 4D MB to reduce the radiation dose by 1/3 without compromising the image quality. This work paves the way for more comprehensive studies to investigate the dose reduction limit offered by this novel 4D MB method using physical phantom experiments and real patient data based on clinical relevant metrics.

Fig. 1

Moving-blocker based 4D CBCT with respiratory gating (transverse view). Different colors of blockers and FPDs denote that the projection data at a particular angle (view) is a combination of a particular respiratory phase and a particular blocker position. (RM: respiratory motion; FPD: flat panel detector. Illustration only, not to scale.)

Fig. 2

The blocker position versus the projection view angle in the simulation. The red dot denotes the blocker position of the phase 1, where each blocker position has five occurrences. The other phases have the same blocker position distribution.

Fig. 3

NCAT phantom and reconstructed images using different 4D CBCT acquisition methods (Conventional acquisition and MB acquisition) and reconstruction methods (3D TV and 4D SMEIR). No scatter was included in simulation. (Left: transverse; middle: coronal; right: sagittal). The red dashed line in (a) transverse view is the profile position of Fig. 5. The red arrow in (a) coronal view indicates the tumor. The three dot squares in (a) are the volume of interest (VOI) for quantitative measures of SSIM and RMSE. The yellow and blue arrows indicate the fine structures lost in 3D TV with fewer views and the green arrow points the artifact likely due to use of the blocker.

Fig. 4

Reconstructed NCAT images using different 4D CBCT acquisition methods (conventional acquisition and MB acquisition) and reconstruction methods (3D TV and 4D SMEIR) with scatter included in the simulation (SC: scatter correction). Note that the radiation dose in (c) and (d) is 1/3 lower than that in (a) and (b) because of use of the moving blocker.

Fig. 5

Line profiles across the tumor in a transverse slice (denoted by a red dashed line in Fig. 3a) for different methods shown in Fig. 4. The arrow indicates the tumor location.

Fig. 6

The deviations of reconstructed tumor motion from the true tumor motion of the NCAT phantom for different methods.

Fig. 7

Convergence of 4D MB reconstruction surrogated by the motion estimation objective function defined in Eq. (8).

Fig. 8

Reconstructed images (from left to right: coronal phase 1, coronal phase 3, sagittal phase 1, and sagittal phase 3) from different reconstruction methods for real patient data at the regular dose and pseudo-real moving blocker data at 2/3 of the regular dose. The red dashed boxes are for quantitative measures in Table 4 and 5.

Fig. 9

The blocker position versus the projection view angle. The red dot denotes the blocker position of the phase 1. All phases have the same blocker position distribution.