title: Image noise and liver lesion detection with MDCT: a phantom study.
abstract: OBJECTIVE
  The purpose of this study was to determine the upper limit of noise for detection of small low-contrast lesions in a liver phantom.
  
  
  METHODS
  A CT liver phantom containing 21 low-contrast, low-attenuation, circular simulated lesions ranging in size from 2.4 to 10 mm was scanned 23 times at different tube current ranges (varying noise index) on a 64-MDCT scanner with automatic tube current modulation. The attenuation of the simulated lesions was 20 HU less than that of the liver-equivalent background. Three radiologists independently reviewed the resultant CT images, which contained either a low-contrast lesion or no lesion and scored certainty of lesion detection using a 4-point Likert scale. Overall performance was evaluated by sensitivity analysis with receiver operator curve and area under the curve (A(z)) computation for ranges of noise index.
  
  
  RESULTS
  The reviewers achieved 100% sensitivity with a noise index of 15 or less for lesions measuring 6.3-10.0 mm (A(z) = 0.96). Increasing noise index to the 17-21 range resulted in a minor decrease in sensitivity and overall performance (sensitivity, 92.3%; A(z) = 0.93). A further increase in noise index to the 23-27 range resulted in a moderate decrease in sensitivity (sensitivity, 81.4%; A(z) = 0.77). Beyond the noise index 23-27 range, sensitivity dropped markedly from 81.4% to 39%. Agreement between the three readers in assessing the image sets was moderate.
  
  
  CONCLUSIONS
  For detection of small low-contrast lesions in the liver phantom model used in this study, the upper limit of noise index may be in the 15-21 range for sensitivity greater than 90%.
authors.affiliation: Department of Radiology, University of Washington, Box 357987, 1959 NE Pacific St, Seattle, WA 98195-7987, USA. kkanal@u.washington.edu
authors.forename: Kalpana M
authors.initials: KM
authors.lastname: Kanal
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Chung
authors.forename: Jin
authors.initials: J
authors.lastname: Wang
authors.forename: Puneet
authors.initials: P
authors.lastname: Bhargava
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Kohr
authors.forename: William P
authors.initials: WP
authors.lastname: Shuman
authors.forename: Brent K
authors.initials: BK
authors.lastname: Stewart
mesh_terms: D000465:Algorithms; D019540:Area Under Curve; D006801:Humans; D008113:Liver Neoplasms / Q000000981:diagnostic imaging*; D019047:Phantoms, Imaging; D012372:ROC Curve; D011857:Radiographic Image Interpretation, Computer-Assisted; D012680:Sensitivity and Specificity; D036542:Tomography, Spiral Computed / Q000379:methods*