Abstract— A moving blocker based strategy has shown
promising results for scatter correction in cone-beam computed
tomography (CBCT). Different parameters of the system design
affect its performance in scatter estimation and image
reconstruction accuracy. In this work, we evaluate the
performance in scatter estimation and image reconstruction
accuracy under various combinations of width and separation of
the lead strips at different moving speeds by Monte Carlo
simulation. The scatter estimation error varied from 0.8% to
5.8% when the combinations of width and separation of the lead
strips ranging from 5 pixels to 100 pixels at the detector plane.
CT number error in the reconstructed CBCT images can be
reduced to 24 HU, if we use strip width 10 pixels and gap width
30 pixels at a blocker moving speed over 15 pixels per projection.
The moving blocker system can achieve accurate reconstruction
if we use the optimized parameters.
I. I
NTRODUCTION
In recent years, cone-beam computed tomography (CBCT)
mounted on the gantry of the linear accelerator has become an
instrumental part of volumetric image guidance in radiation
therapy [1]. However, due to the broad beam geometry
utilized in such systems, the presence of scatter contamination
within the projection data will lead to reduction of image
quality by introducing image artifacts, reducing contrast, and
limiting CT number accuracy, especially for sites requiring
large ¿eld of view (FOV).
Various strategies have been proposed for estimating the
scatter signal in projection images including analytical
This work was supported in part by the Cancer Prevention and Research
Institute of Texas (RP130109), the American Cancer Society (RSG-13-326-
01-CCE), US National Health Institute (R01 EB020366) and the National
Natural Science Foundation of China (61401349).
Xi Chen is with the Department of Radiation Oncology, UT Southwestern
Medical Center, Dallas, TX 75235, USA, also with the Institute of Image
Processing and Pattern Recognition, Xi’an Jiaotong University, Xi’an,
Shaanxi 710049 China, and also with the Beijing Center for Mathematics and
Information Interdisciplinary Sciences, Beijing 10048 China. (e-mail:
Xi2.Chen@UTSouthwestern.edu).
Luo Ouyang, Hao Yan, Xun Jia, You Zhang and Jing Wang are with the
Department of Radiation Oncology, UT Southwestern Medical Center, Dallas,
TX 75235, USA. (e-mail: Luo.Ouyang@UTSouthwestern.edu, yhhere
@126.com , Xun.Jia@UTSouthwestern.edu, You.Zhang@UTSouthwestern.
edu, Jing.Wang@UTSouthwestern.edu).
Bin Li and Qingwen Lyu is with School of Biomedical Engineering,
Southern Medical University, Guangzhou 510515, China. (e-mail:
libin371@smu.edu.cn, gzbeer@smu.edu.cn).
calculation, Monte Carlo (MC) simulation and beam blocker-
blocker-based techniques. In our previous study [2-3], we
proposed a scatter correction strategy based on a moving
blocker system and demonstrated its effectiveness. Scatter
was estimated by interpolating values in un-blocked regions
from the scatter signal in blocker regions. Instead of
the missing primary signal of the blocked region through
interpolation, only the primary signal in the unblocked regions
was used to reconstruct the CBCT image. This method can
simultaneously estimate scatter signal and reconstruct the
complete volume within the FOV from a single scanning.
method is not limited to full-fan scan geometry and small size
of FOV.
In this work, we systematically investigate how variations
in the design and speed of moving blocker affect imaging
performance. We evaluate the performance in scatter
estimation and image reconstruction accuracy under various
combinations of width and separation of the lead strips at
different moving speed. The present investigation involves
MC simulations of projection images of a pelvis phantom
using CBCT geometry at clinically realistic radiation doses.
II. MATERIALS AND METHODS
A. Monte Carlo simulation
Fig. 1 illustrates the design of lead strips and the geometric
setup of the moving blocker for CBCT imaging. The blocker
consists of equally spaced lead strips which are 3.2 mm in
thickness. For our study, various combinations of strip width
and gap were investigated. The lead strips of the blocker are
aligned perpendicular to the gantry rotation axis z and moves
back and forth along the rotation axis z, as indicated in Fig.
1(c).
The MC simulation toolkit utilized in this study is
gDRR[4]. The accuracy of photon transport in this package
has been previously demonstrated by comparing simulation
results with those from EGSnrc, as well as indirectly by
comparing computed radiation dose with measurements [5].
The spectrum used in our study is 125kVp and the number of
photons is 5.0e+9. Phantom was generated based on a pelvis
patients CT dataset. The scan geometry is illustrated in Fig.1
(c). Projections were down sampled by a factor of 2 to yield a
size 512u512 with pixel size 0.8 mm u0.8 mm. The MC
simulated data used in this study were projection data with
lead strips of
Optimization of the Geometry and Speed of a
Moving Blocker System for Cone-beam
Computed Tomography Scatter Correction
Xi Chen, Luo Ouyang, Hao Yan, Xun Jia, Bin Li, Qingwen Lyu, You Zhang, and Jing Wang
The 4th International Conference on Image Formation in X-Ray Computed Tomography
553