title: Crescent artifacts in cone-beam CT.
abstract: OBJECTIVE
  In image-guided radiation therapy, cone-beam CT has been adopted for three-dimensional target localization in the treatment room. In many of these cone-beam CT images, dark and light crescent artifacts can be seen. This study investigates potential causes of this artifact and a technique for mitigating the crescents.
  
  
  METHODS
  Three deviations from an ideal geometry were simulated to assess their ability to cause crescent artifacts: Bowtie filter sag, x-ray tube sag, and x-ray tube rotation. The magnitudes of these deviations were estimated by matching shifts in simulated projections to those observed with clinical systems. To correct the artifacts, angle-dependent blank projections were acquired and incorporated into image reconstruction. The degree of artifact reduction was evaluated with varying numbers (1-380) of blank projections. Scanner-acquired phantom and patient studies were conducted to demonstrate the effectiveness of the proposed correction method.
  
  
  RESULTS
  All three investigated causes of the crescent artifact introduced similar mismodeling of the acquired projections and similar crescent artifacts. The deviations required for these artifacts were in the range of 0.5-5 mm or 0.1 degrees. RMS error is reduced from 8.91 x 10(-4) to 5.25 x 10(-7) for 1-380 blank projections over a 200 degrees scan angle. In the patient and phantom studies, reconstructions that utilized 380 blank projections largely mitigated the crescent artifacts.
  
  
  CONCLUSIONS
  Small deviations from an ideal geometry can result in crescent artifacts due to steep gradients in the bowtie filter. Angle-dependent blank projections can largely alleviate the artifacts.
authors.affiliation: Medical Physics Graduate Program, Duke University, Durham, North Carolina 27710, USA. william.giles@duke.edu
authors.forename: William
authors.initials: W
authors.lastname: Giles
authors.forename: James
authors.initials: J
authors.lastname: Bowsher
authors.forename: Hao
authors.initials: H
authors.lastname: Li
authors.forename: Fang-Fang
authors.initials: FF
authors.lastname: Yin
mesh_terms: D016477:Artifacts; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D011856:Radiographic Image Enhancement