title: Direct and fast measurement of CT beam filter profiles with simultaneous geometrical calibration.
abstract: OBJECTIVE
  To accurately measure the beam filter profiles from a variety of CT scanner models and to provide reference data for Monte Carlo simulations of CT scanners.
  
  
  METHODS
  This study proposed a new method to measure CT beam filter profiles using a linear-array x-ray detector (X-Scan 0.8f3-512; Detection Technology Inc., Espoo, Finland) under gantry rotation mode. A robust geometrical calibration approach was developed to determine key geometrical parameters by considering the x-ray focal spot location relative to the linear-array detector and the gantry's angular increment at each acquisition point. CT beam intensity profiles were synthesized from continuously measured data during a 10° gantry rotation range with calibrated detector response and system geometry information. Relative transmission profiles of nineteen sets of beam filters were then derived for nine different CT scanner models from three different manufacturers. Equivalent aluminum thickness profiles of these beam filters were determined by analytical calculation using the Spektr Matlab software package to match the measured transmission profiles. Three experiments were performed to validate the accuracy of the geometrical calibration, detector response modeling, and the derived equivalent aluminum thickness profiles.
  
  
  RESULTS
  The beam intensity profiles measured from gantry rotation mode showed very good agreement with those measured with gantry stationary mode, with a maximal difference of 3%. The equivalent aluminum thickness determined by this proposed method agreed well with what was measured by an ion chamber, with a mean difference of 0.4%. The determined HVL profiles matched well with data from a previous study (max difference of 4.7%).
  
  
  CONCLUSIONS
  An accurate and robust method to directly measure profiles from a broad list of beam filters and CT scanner models was developed, implemented, and validated. Useful reference data was provided for future research on CT system modeling.
authors.affiliation: Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA.
authors.forename: Kai
authors.initials: K
authors.lastname: Yang
authors.affiliation: Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA.
authors.forename: Xinhua
authors.initials: X
authors.lastname: Li
authors.affiliation: Nuclear Engineering Program, Rensselaer Polytechnic Institute, Troy, New York, 12180, USA.
authors.forename: X
authors.initials: X
authors.lastname: George Xu
authors.affiliation: Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA.
authors.forename: Bob
authors.initials: B
authors.lastname: Liu
mesh_terms: D000535:Aluminum; D002138:Calibration; D009010:Monte Carlo Method; D013997:Time Factors; D015898:Tomography Scanners, X-Ray Computed
chemical_list: D000535:Aluminum
keywords: beam filtration; bowtie; computed tomography; geometrical calibration; transmission