title: The influence of focal spot blooming on high-contrast spatial resolution in CT imaging.
abstract: OBJECTIVE
  The objective of this work was to investigate focal spot blooming effects on the spatial resolution of CT images and to evaluate an x-ray tube that uses dynamic focal spot control for minimizing focal spot blooming.
  
  
  METHODS
  The influence of increasing tube current at a fixed tube potential of 80 kV on high-contrast spatial resolution of seven different CT scanner models (scanners A-G), including one scanner that uses dynamic focal spot control to reduce focal spot blooming (scanner A), was evaluated. Spatial resolution was assessed using a wire phantom for the modulation transfer function (MTF) calculation and a copper disc phantom for measuring the slice sensitivity profile (SSP). The impact of varying the tube potential was investigated on two scanner models (scanners A and B) by measuring the MTF and SSP and also by using the resolution bar pattern module of the ACR CT phantom. The phantoms were scanned at 70-150 kV on scanner A and 80-140 kV on scanner B, with tube currents from 100 mA up to the maximum tube current available on each scanner. The images were reconstructed using a slice thickness of 0.6 mm with both smooth and sharp kernels. Additionally, focal spot size at varying tube potentials and currents was directly measured using pinhole and slit camera techniques.
  
  
  RESULTS
  Evaluation of the MTF and SSP data from the 7 CT scanner models evaluated demonstrated decreased focal spot blooming for newer scanners, as evidenced by decreasing deviations in MTF and SSP as tube current varied. For scanners A and B, where focal spot blooming effects as a function of tube potential were assessed, the spatial resolution variation in the axial plane was much smaller on scanner A compared to scanner B as tube potential and current changed. On scanner A, the 50% MTF never decreased by more than 2% from the 50% MTF measured at 100 mA. On scanner B, the 50% MTF decreased by as much as 19% from the 50% MTF measured at 100 mA. Assessments of the SSP, the bar patterns in the ACR phantom and the pinhole and slit camera measurements were consistent with the MTF calculations.
  
  
  CONCLUSIONS
  Focal spot blooming has a noticeable effect on spatial resolution in CT imaging. The focal spot shaping technology of scanner A greatly reduced blooming effects.
authors.affiliation: Department of Radiology, Mayo Clinic, Rochester, Minnesota 55905.
authors.forename: Joshua
authors.initials: J
authors.lastname: Grimes
authors.affiliation: Department of Radiology, Mayo Clinic, Rochester, Minnesota 55905.
authors.forename: Xinhui
authors.initials: X
authors.lastname: Duan
authors.affiliation: Department of Radiology, Mayo Clinic, Rochester, Minnesota 55905.
authors.forename: Lifeng
authors.initials: L
authors.lastname: Yu
authors.affiliation: Siemens Healthcare CT R&D, Malvern, Pennsylvania 19355.
authors.forename: Ahmed F
authors.initials: AF
authors.lastname: Halaweish
authors.affiliation: Siemens Healthcare GmbH, Forchheim 91301, Germany.
authors.forename: Nicole
authors.initials: N
authors.lastname: Haag
authors.affiliation: Department of Radiology, Mayo Clinic, Rochester, Minnesota 55905.
authors.forename: Shuai
authors.initials: S
authors.lastname: Leng
authors.affiliation: Department of Radiology, Mayo Clinic, Rochester, Minnesota 55905.
authors.forename: Cynthia
authors.initials: C
authors.lastname: McCollough
mesh_terms: D016477:Artifacts; D007091:Image Processing, Computer-Assisted; D019047:Phantoms, Imaging; D059629:Signal-To-Noise Ratio; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods*