title: Creation of an atlas of filter positions for fluence field modulated CT.
abstract: OBJECTIVE
  Fluence field modulated CT (FFMCT) and volume of interest (VOI) CT imaging applications require adjustment of the profile of the x-ray fluence incident on a patient as a function of view angle. Since current FFMCT prototypes can theoretically take on an infinite number of configurations, measuring a calibration data set for all possible positions would not be feasible. The present work details a methodology for calculating an atlas of configurations that will span all likely body regions, patient sizes, patient positioning, and imaging modes. The hypothesis is that there exists a finite number of unique modulator configurations that effectively span the infinite number of possible fluence profiles with minimal loss in performance.
  
  
  METHODS
  CT images of a head, shoulder, thorax, abdominal, wrist, and leg anatomical slices were dilated and contracted to model small, medium, and large sized patients. Additionally, the images were positioned from iso-center by three different amounts. The modulator configurations required to compensate for each image were computed assuming a FFMCT prototype, digital beam attenuator, (DBA), was set to equalize the detector exposure. Each atlas configuration should be different from the other atlas configurations. The degree of difference was quantified using the sum of the absolute differences in filter thickness between configurations. Using this metric, a set of unique wedge configurations for which no two configurations have a metric value smaller than some threshold can be constructed. Differences in the total number of incident photons between the unconstrained filters and the atlas were studied as a function of the number of atlas positions for each anatomical site and size/off-centering combination.
  
  
  RESULTS
  By varying the threshold used in creating the atlas, it was found that roughly 322 atlas positions provided an incident number of photons within 20% of using 19,440 unique filters (the number of atlas entries ranged from 7213 to 1). Additionally, for VOI applications implemented with a single VOI region, the number of required filter configurations was expressed in a simple closed form solution.
  
  
  CONCLUSIONS
  The methodology proposed in this work will enable DBA-FFMCT and DBA-VOI imaging in the clinic without the need for patient specific air-scans to be performed. In addition, the methodology proposed here is directly applicable to other modulator designs such as piecewise linear, TomoTherapy multi leaf collimators, 2D fluid arrays, and inverse geometry CT.
authors.affiliation: Departments of Radiology and Medical Physics, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, Wisconsin 53705.
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Szczykutowicz
authors.affiliation: Department of Biomedical Engineering, University of Wisconsin-Madison, 1550 Engineering Drive, Madison, Wisconsin 53706.
authors.forename: James
authors.initials: J
authors.lastname: Hermus
mesh_terms: D001271:Atlases as Topic; D002138:Calibration; D003198:Computer Simulation; D005240:Feasibility Studies; D006257:Head / Q000000981:diagnostic imaging; D006801:Humans; D007866:Leg / Q000000981:diagnostic imaging; D019047:Phantoms, Imaging; D017785:Photons; D011860:Radiography, Abdominal / Q000379:methods; D013902:Radiography, Thoracic / Q000379:methods; D012782:Shoulder / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods*; D014953:Wrist / Q000000981:diagnostic imaging; D014965:X-Rays