TY - JOUR
T1 - Bow-tie wobble artifact
T2 - Effect of source assembly motion on cone-beam CT
AU - Zheng, Dandan
AU - Ford, John C.
AU - Lu, Jun
AU - Lazos, Dimitrios
AU - Hugo, Geoffrey D.
AU - Pokhrel, Damodar
AU - Zhang, Lisha
AU - Williamson, Jeffrey F.
N1 - Funding Information:
The authors thank Dr.Douglas J. Moseley of Princess Margaret Hospital for his guidance and consultation on applying the 9-parameter geometric calibration and for lending the phantom to us. The authors also thank Dr. Kai Yang of University of California, Davis for his helpful discussion, and Mr. Chris Bartee of Virginia Commonwealth University for assisting mechanical measurements of the system. The authors would like to acknowledge the anonymous referees for their useful comments and suggestions. This project was supported in part by grants from the National Institutes of Health (P01 CA116602) and Varian Medical Systems.
PY - 2011/5
Y1 - 2011/5
N2 - Purpose: To investigate the cause of a bow-tie wobble artifact (BWA) discovered on Varian OBI CBCT images and to develop practical correction strategies.Method and Materials: The dependence of the BWA on phantom geometry, phantom position, specific system, and reconstruction algorithm was investigated. Simulations were conducted to study the dependence of the BWA on scatter and beam hardening corrections. Geometric calibration was performed to rule out other gantry-angle dependent mechanical non-idealities as BWA causes. Air scans were acquired with ball-bearing markers to study the motions of the x-ray head assembly as functions of gantry angle. Based on measurements, we developed hypothesis regarding the BWA cause. Simulations were performed to validate our hypothesis. Two correction strategies were implemented: a measurement-based method, which acquires gantry-dependent normalization projections (NPs); and a model-based method that involves numerically shifting the single-angle NP to compensate for the previously-measured bow-tie-filter (BTF) motion.Results: The BWA has a diameter of ∼15 cm, is centered at the isocenter, and is reproducible independent of phantom, position, system, reconstruction, and standard corrections, but only when the BTF is used. Measurements identified a 2D sinusoidal gantry-angle-dependent motion of the x-ray head assembly, and it was the BTF motion (3 mm amplitude projected onto the detector) resulting an intensity mismatch between the all-angle CBCT projections and a single-angle NP that caused the BWA. Both correction strategies were demonstrated effective.Conclusions: A geometric mismatch between the BTF modulation patterns on CBCT projections and on the NP causes the BWA. The BTF wobble requires additional degrees of freedom in CBCT geometric calibration to characterize.
AB - Purpose: To investigate the cause of a bow-tie wobble artifact (BWA) discovered on Varian OBI CBCT images and to develop practical correction strategies.Method and Materials: The dependence of the BWA on phantom geometry, phantom position, specific system, and reconstruction algorithm was investigated. Simulations were conducted to study the dependence of the BWA on scatter and beam hardening corrections. Geometric calibration was performed to rule out other gantry-angle dependent mechanical non-idealities as BWA causes. Air scans were acquired with ball-bearing markers to study the motions of the x-ray head assembly as functions of gantry angle. Based on measurements, we developed hypothesis regarding the BWA cause. Simulations were performed to validate our hypothesis. Two correction strategies were implemented: a measurement-based method, which acquires gantry-dependent normalization projections (NPs); and a model-based method that involves numerically shifting the single-angle NP to compensate for the previously-measured bow-tie-filter (BTF) motion.Results: The BWA has a diameter of ∼15 cm, is centered at the isocenter, and is reproducible independent of phantom, position, system, reconstruction, and standard corrections, but only when the BTF is used. Measurements identified a 2D sinusoidal gantry-angle-dependent motion of the x-ray head assembly, and it was the BTF motion (3 mm amplitude projected onto the detector) resulting an intensity mismatch between the all-angle CBCT projections and a single-angle NP that caused the BWA. Both correction strategies were demonstrated effective.Conclusions: A geometric mismatch between the BTF modulation patterns on CBCT projections and on the NP causes the BWA. The BTF wobble requires additional degrees of freedom in CBCT geometric calibration to characterize.
KW - bow-tie-filter
KW - cone-beam computed tomography
KW - geometric calibration
KW - image artifact
KW - image-guidance radiation therapy
UR - http://www.scopus.com/inward/record.url?scp=79959551744&partnerID=8YFLogxK
U2 - 10.1118/1.3582944
DO - 10.1118/1.3582944
M3 - Article
C2 - 21776785
AN - SCOPUS:79959551744
SN - 0094-2405
VL - 38
SP - 2508
EP - 2514
JO - Medical physics
JF - Medical physics
IS - 5
ER -