title: Lens dose in routine head CT: comparison of different optimization methods with anthropomorphic phantoms.
abstract: OBJECTIVE
  The purpose of this study was to study different optimization methods for reducing eye lens dose in head CT.
  
  
  METHODS
  Two anthropomorphic phantoms were scanned with a routine head CT protocol for evaluation of the brain that included bismuth shielding, gantry tilting, organ-based tube current modulation, or combinations of these techniques. Highsensitivity metal oxide semiconductor field effect transistor dosimeters were used to measure local equivalent doses in the head region. The relative changes in image noise and contrast were determined by ROI analysis.
  
  
  RESULTS
  The mean absorbed lens doses varied from 4.9 to 19.7 mGy and from 10.8 to 16.9 mGy in the two phantoms. The most efficient method for reducing lens dose was gantry tilting, which left the lenses outside the primary radiation beam, resulting in an approximately 75% decrease in lens dose. Image noise decreased, especially in the anterior part of the brain. The use of organ-based tube current modulation resulted in an approximately 30% decrease in lens dose. However, image noise increased as much as 30% in the posterior and central parts of the brain. With bismuth shields, it was possible to reduce lens dose as much as 25%.
  
  
  CONCLUSIONS
  Our results indicate that gantry tilt, when possible, is an effective method for reducing exposure of the eye lenses in CT of the brain without compromising image quality. Measurements in two different phantoms showed how patient geometry affects the optimization. When lenses can only partially be cropped outside the primary beam, organ-based tube current modulation or bismuth shields can be useful in lens dose reduction.
authors.affiliation: 1 HUS Medical Imaging Center, Helsinki University Central Hospital, POB 340 (Haartmaninkatu 4), 00290 Helsinki, Finland.
authors.forename: Ulla
authors.initials: U
authors.lastname: Nikupaavo
authors.forename: Touko
authors.initials: T
authors.lastname: Kaasalainen
authors.forename: Vappu
authors.initials: V
authors.lastname: Reijonen
authors.forename: Sanna-Mari
authors.initials: SM
authors.lastname: Ahonen
authors.forename: Mika
authors.initials: M
authors.lastname: Kortesniemi
mesh_terms: D065638:Absorption, Radiation; D003955:Diagnostic Tests, Routine; D006257:Head / Q000000981:diagnostic imaging*; D006801:Humans; D007908:Lens, Crystalline / Q000528:radiation effects*; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011835:Radiation Protection / Q000379:methods*; D011856:Radiographic Image Enhancement / Q000295:instrumentation / Q000379:methods*; D011874:Radiometry; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods*
keywords: CT optimization; MOSFET; anthropomorphic phantom; image quality; lens dose