title: Reducing radiation dose at CT colonography: decreasing tube voltage to 100 kVp.
abstract: OBJECTIVE
  To assess the effect of a decrease in tube voltage from 120 kVp to 100 kVp on dose, contrast-to-noise ratio (CNR), and three-dimensional (3D) image quality in patients undergoing computed tomographic (CT) colonography as well as to determine how these changes are affected by patient size.
  
  
  METHODS
  This HIPAA-compliant and institutional review board-approved retrospective study included 63 consecutive patients who underwent CT colonography and who waived informed consent. Scanning was performed with patients in the supine (120 kVp) and prone (100 kVp) positions, with other parameters unchanged. Volume CT dose index (CTDI(vol)), dose-length product (DLP), image noise, attenuation of selected materials, and CNR were compared with the Wilcoxon matched-pairs signed rank test. Two readers blinded to tube voltage independently assessed 3D endoluminal image quality. The k coefficients were calculated for interobserver agreement. Average image quality ratings were compared with the Wilcoxon signed rank test. All recorded data were stratified by patient anteroposterior diameter to determine effects of patient size.
  
  
  RESULTS
  Decreasing tube voltage from 120 to 100 kVp resulted in a 20% decrease in CTDI(vol) (P < .001) and a 16% decrease in DLP (P < .001). Image noise increased by 32% (P < .001). Mean attenuation of tagged fluid increased from 395 to 487 HU (P < .001). There was no change in mean CNR of tagged fluid (17.1 at 120 kVp, 16.8 at 100 kVp; P = .37), regardless of patient size. The 3D image quality decreased slightly from a median score of 5 out of 5 to 4 out of 5 (P < .001). There was substantial interobserver agreement.
  
  
  CONCLUSIONS
  A decrease in tube voltage from 120 to 100 kVp results in a significant decrease in radiation dose but only a minimal decrease in 3D image quality at all patient sizes. © RSNA, 2012.
authors.affiliation: Department of Diagnostic Imaging, Warren Alpert Medical School of Brown University, Rhode Island Hospital, 593 Eddy St, Providence, RI 02903, USA. kchang@lifespan.org
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Chang
authors.forename: Dominique B
authors.initials: DB
authors.lastname: Caovan
authors.forename: David J
authors.initials: DJ
authors.lastname: Grand
authors.forename: Walter
authors.initials: W
authors.lastname: Huda
authors.forename: William W
authors.initials: WW
authors.lastname: Mayo-Smith
mesh_terms: D000328:Adult; D000368:Aged; D001822:Body Burden; D023881:Colonography, Computed Tomographic; D006801:Humans; D008297:Male; D008875:Middle Aged; D011829:Radiation Dosage; D011835:Radiation Protection / Q000379:methods*; D011874:Radiometry; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012810:Sigmoid Diseases / Q000000981:diagnostic imaging*