title: Monte Carlo evaluation of scatter mitigation strategies in cone-beam CT.
abstract: OBJECTIVE
  To investigate the efficacy of two widely used scatter mitigation methods: antiscatter grids (ASGs) and beam modulating with bowtie filters (BTFs), in combination with subtractive scatter correction or zeroth order normalization phantom calibration, for improving image noise, contrast, contrast-to-noise ratio (CNR), and image uniformity for on-board cone-beam CT (CBCT) imaging systems used for image-guided radiation therapy.
  
  
  METHODS
  PTRAN Monte Carlo CBCT x-ray projections of head and pelvic phantoms were calculated for combinations of beam-modulation and scatter rejection methods and images were reconstructed by in-house developed software. In addition, a simple one-dimensional analytic model was developed to predict scatter-to-primary ratio (SPR) and CNR as a function of cylindrical phantom thickness, ASG transmission, and beam modulation with bow-tie filters.
  
  
  RESULTS
  ASGs were found to have slightly negative or no effect on head phantom image CNR and to modestly improve CNR (10%-20%) in pelvic phantom images. However, scatter subtraction and norm-phantom calibration perform better when applied on data acquired with ASGs. Scatter subtraction improves CT number accuracy, but increases noise, and in high SPR/low primary-photon transmission scenarios can dramatically reduce CNR and introduce streaking artifacts. The BTF is found to reduce SPR and image noise, resulting in a better trade-off between CNR and imaging dose, but introduces a circular band artifact.
  
  
  CONCLUSIONS
  Our study shows that ASGs have a modest positive impact in pelvic scans and negative in head scans, scatter subtraction improves the HU accuracy but reduces CNR, while BTF has a clearly positive effect.
authors.affiliation: Department of Radiation Oncology, Beth Israel Medical Center, New York, New York 10003, USA. dlazos@chpnet.org
authors.forename: Dimitrios
authors.initials: D
authors.lastname: Lazos
authors.forename: Jeffrey F
authors.initials: JF
authors.lastname: Williamson
mesh_terms: D055592:Biophysical Phenomena; D054893:Cone-Beam Computed Tomography / Q000706:statistics & numerical data*; D006257:Head; D006801:Humans; D009010:Monte Carlo Method; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D010388:Pelvis; D019047:Phantoms, Imaging; D011857:Radiographic Image Interpretation, Computer-Assisted; D011880:Radiotherapy Planning, Computer-Assisted / Q000706:statistics & numerical data*; D012542:Scattering, Radiation; D012984:Software