title: In-plane shielding for CT: effect of off-centering, automatic exposure control and shield-to-surface distance.
abstract: OBJECTIVE
  To assess effects of off-centering, automatic exposure control, and padding on attenuation values, noise, and radiation dose when using in-plane bismuth-based shields for CT scanning.
  
  
  METHODS
  A 30 cm anthropomorphic chest phantom was scanned on a 64-multidetector CT, with the center of the phantom aligned to the gantry isocenter. Scanning was repeated after placing a bismuth breast shield on the anterior surface with no gap and with 1, 2, and 6 cm of padding between the shield and the phantom surface. The "shielded" phantom was also scanned with combined modulation and off-centering of the phantom at 2 cm, 4 cm and 6 cm below the gantry isocenter. CT numbers, noise, and surface radiation dose were measured. The data were analyzed using an analysis of variance.
  
  
  RESULTS
  The in-plane shield was not associated with any significant increment for the surface dose or CT dose index volume, which was achieved by comparing the radiation dose measured by combined modulation technique to the fixed mAs (p > 0.05). Irrespective of the gap or the surface CT numbers, surface noise increased to a larger extent compared to Hounsfield unit (HU) (0-6 cm, 26-55%) and noise (0-6 cm, 30-40%) in the center. With off-centering, in-plane shielding devices are associated with less dose savings, although dose reduction was still higher than in the absence of shielding (0 cm off-center, 90% dose reduction; 2 cm, 61%) (p < 0.0001). Streak artifacts were noted at 0 cm and 1 cm gaps but not at 2 cm and 6 cm gaps of shielding to the surface distances.
  
  
  CONCLUSIONS
  In-plane shields are associated with greater image noise, artifactually increased attenuation values, and streak artifacts. However, shields reduce radiation dose regardless of the extent of off-centering. Automatic exposure control did not increase radiation dose when using a shield.
authors.affiliation: Department of Radiology, Massachusetts General Hospital, Boston, MA 02114, USA. mkalra@partners.org
authors.forename: Mannudeep K
authors.initials: MK
authors.lastname: Kalra
authors.forename: Pragya
authors.initials: P
authors.lastname: Dang
authors.forename: Sarabjeet
authors.initials: S
authors.lastname: Singh
authors.forename: Sanjay
authors.initials: S
authors.lastname: Saini
authors.forename: Jo-Anne O
authors.initials: JA
authors.lastname: Shepard
mesh_terms: D000704:Analysis of Variance; D016477:Artifacts; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011835:Radiation Protection / Q000379:methods*; D014057:Tomography, X-Ray Computed