title: Histogram analysis for characterization of indeterminate adrenal nodules on noncontrast CT.
abstract: OBJECTIVE
  To determine the effectiveness of the CT histogram method to characterize indeterminate adrenal nodules above 10 Hounsfield units (HU) on noncontrast CT.
  
  
  METHODS
  Retrospective review of clinical CT data from January 2005 through 2008 identified 194 indeterminate adrenal nodules (>10 HU on noncontrast CT) in 175 patients. 20 nodules in 18 patients were excluded due to large standard deviation (SD > 30) of HU values. Of the remaining 174 nodules, 131 were classified as benign lipid-poor nodules based on size stability for ≥1 year (104), in- and opposed-phase MRI (17), adrenal washout CT (3), or biopsy (7). 43 were classified as malignant by size increase over a short time (30), avid FDG uptake on PET/CT (15), or biopsy (5). Histogram analysis was performed by drawing a circular region of interest on all adrenal nodules. Mean attenuation, total number of pixels, number of negative pixels, and percentage of negative pixels were recorded for each nodule.
  
  
  RESULTS
  At the threshold value of >10% negative pixels, 59/131 benign nodules were correctly characterized, but 1/43 malignant nodules was falsely characterized as benign (sensitivity 45%, specificity 98%, positive predictive value 98%). With a slightly higher threshold value of >15% negative pixels, there were no false benign judgments. 36 nodules had more than 15% negative pixels, all of which were benign (sensitivity 27%, specificity 100%, positive predictive value 100%). In the subgroup of benign nodules measuring 11-20 HU, 80% and 54% were identified with threshold values of >10% and >15% negative pixels, respectively.
  
  
  CONCLUSIONS
  The CT histogram method with a threshold value of >10% negative pixels can identify many benign adrenal nodules with attenuation values >10 HU on unenhanced CT with extremely high specificity. A threshold of >15% negative pixels can achieve 100% specificity. This method is highly robust provided very "noisy" CT examinations (SD > 30) are eliminated.
authors.affiliation: Mallinckrodt Institute of Radiology, Washington University in St. Louis, 510 South Kingshighway, Box 8131, St. Louis, MO, 63131, USA, linm@mir.wustl.edu.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Lin
authors.forename: Lauren Q
authors.initials: LQ
authors.lastname: Chang-Sen
authors.forename: Jay P
authors.initials: JP
authors.lastname: Heiken
authors.forename: Thomas K
authors.initials: TK
authors.lastname: Pilgram
authors.forename: Kyongtae T
authors.initials: KT
authors.lastname: Bae
mesh_terms: D000310:Adrenal Gland Neoplasms / Q000000981:diagnostic imaging*; D000311:Adrenal Glands / Q000000981:diagnostic imaging*; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*