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@ARTICLE{Waltrich:136931,
      author       = {N. Waltrich$^*$ and S. Sawall$^*$ and J. Maier$^*$ and J.
                      Kuntz$^*$ and K. Stannigel and K. Lindenberg and M.
                      Kachelriess$^*$},
      title        = {{E}ffect of detruncation on the accuracy of {M}onte
                      {C}arlo-based scatter estimation in truncated {CBCT}.},
      journal      = {Medical physics},
      volume       = {45},
      number       = {8},
      issn         = {0094-2405},
      address      = {New York, NY},
      reportid     = {DKFZ-2018-01368},
      pages        = {3574 - 3590},
      year         = {2018},
      abstract     = {The purpose of this study is to investigate the necessity
                      of detruncation for scatter estimation of truncated
                      cone-beam CT (CBCT) data and to evaluate different
                      detruncation algorithms. Scattered radiation results in some
                      of the most severe artifacts in CT and depends strongly on
                      the size and the shape of the scanned object. Especially in
                      CBCT systems the large cone-angle and the small
                      detector-to-isocenter distance lead to a large amount of
                      scatter detected, resulting in cupping artifacts, streak
                      artifacts, and inaccurate CT-values. If a small field of
                      measurement (FOM) is used, as it is often the case in CBCT
                      systems, data are truncated in longitudinal and lateral
                      direction. Since only truncated data are available as input
                      for the scatter estimation, the already challenging
                      correction of scatter artifacts becomes even more
                      difficult.The following detruncation methods are compared
                      and evaluated with respect to scatter estimation: constant
                      detruncation, cosine detruncation, adaptive detruncation,
                      and prior-based detruncation using anatomical data from a
                      similar phantom or patient, also compared to the case where
                      no detruncation was performed. Each of the resulting,
                      detruncated reconstructions serve as input volume for a
                      Monte Carlo (MC) scatter estimation and subsequent scatter
                      correction. An evaluation is performed on a head simulation,
                      measurements of a head phantom and a patient using a dental
                      CBCT geometry with a FOM diameter of 11 cm. Additionally, a
                      thorax phantom is measured to assess performance in a C-Arm
                      geometry with a FOM of up to 20 cm.If scatter estimation is
                      based on simple detruncation algorithms like a constant or a
                      cosine detruncation scatter is estimated inaccurately,
                      resulting in incorrect CT-values as well as streak artifacts
                      in the corrected volume. For the dental CBCT phantom
                      measurement CT-values for soft tissue were corrected from
                      -204 HU (no scatter correction) to -87 HU (no
                      detruncation), -218 HU (constant detruncation), -141 HU
                      (cosine detruncation), -91 HU (adaptive detruncation),
                      -34 HU (prior-based detruncation using a different prior)
                      and -24 HU (prior-based detruncation using the identical
                      prior) for a reference value of -26 HU measured in slit
                      scan mode. In all cases the prior-based detruncation results
                      in the best scatter correction, followed by the adaptive
                      detruncation, as these algorithms provide a rather accurate
                      model of high-density structures outside the FOM, compared
                      to a simple constant or a cosine detruncation.Our
                      contribution is twofold: first we give a comprehensive
                      comparison of various detruncation methods for the purpose
                      of scatter estimation. We find that the choice of the
                      detruncation method has a significant influence on the
                      quality of MC-based scatter correction. Simple or no
                      detruncation is often insufficient for artifact removal and
                      results in inaccurate CT-values. On the contrary,
                      prior-based detruncation can achieve a high CT-value
                      accuracy and nearly artifact-free volumes from truncated
                      CBCT data when combined with other state-of-the-art artifact
                      corrections. Secondly, we show that prior-based detruncation
                      is effective even with data from a different patient or
                      phantom. The fact that data completion does not require data
                      from the same patient dramatically increases the
                      applicability and usability of this scatter estimation.},
      cin          = {E020 / E025},
      ddc          = {610},
      cid          = {I:(DE-He78)E020-20160331 / I:(DE-He78)E025-20160331},
      pnm          = {315 - Imaging and radiooncology (POF3-315)},
      pid          = {G:(DE-HGF)POF3-315},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:29888791},
      doi          = {10.1002/mp.13041},
      url          = {https://inrepo02.dkfz.de/record/136931},
}