title: Detection of the normal appendix with low-dose unenhanced CT: use of the sliding slab averaging technique.
abstract: OBJECTIVE
  To determine the frequency of normal appendix visualization at low-dose (LD) unenhanced computed tomography (CT) performed with a 16- or 64-detector row scanner when images are reviewed by using the sliding slab averaging technique.
  
  
  METHODS
  The institutional review board approved the study and waived the informed consent requirement. A total of 259 patients, 37 (14.3%) of whom had previously undergone appendectomy, underwent LD unenhanced CT (mean effective dose, 1.7 mSv) performed with a 16- or 64-detector row scanner to assess urinary colic. Three readers used the sliding slab averaging technique to retrospectively review the thin-section (0.67- or 2.00-mm section thickness) images and grade the appendix as absent, unsurely or partly visualized, or clearly and entirely visualized. Interobserver agreement was measured with weighted kappa statistics. McNemar tests were used to compare sensitivity between the readers. Logistic regression analysis was performed to assess the effects of body mass index, patient sex, and type of CT scanner on appendiceal visualization.
  
  
  RESULTS
  The kappa statistics for each reader pair were as follows: 0.97 for agreement between readers 1 and 2, 0.93 for agreement between readers 2 and 3, and 0.92 for agreement between readers 1 and 3. Each reader clearly identified the entire appendix in 213 (96.0%), 209 (94.1%), and 205 (92.3%) of the 222 patients without a history of appendectomy. When unsurely or partly visualized appendices were included, the frequencies increased to 99.1% (n = 220), 98.7% (n = 219), and 97.3% (n = 216), respectively, for readers 1, 2, and 3. These frequencies rarely differed between the readers. (P values ranged from .021 to greater than .99.) The three readers consistently reported that the appendix was not visualized in the 37 patients who had undergone appendectomy. None of the tested variables significantly affected appendix visualization.
  
  
  CONCLUSIONS
  Most normal appendices are visualized on thin-section LD unenhanced CT images reviewed with the sliding slab averaging technique.
authors.affiliation: Department of Radiology, Seoul National University Bundang Hospital, Republic of Korea.
authors.forename: Seung-Moon
authors.initials: SM
authors.lastname: Joo
authors.forename: Kyoung Ho
authors.initials: KH
authors.lastname: Lee
authors.forename: Young Hoon
authors.initials: YH
authors.lastname: Kim
authors.forename: So Yeon
authors.initials: SY
authors.lastname: Kim
authors.forename: Kyuseok
authors.initials: K
authors.lastname: Kim
authors.forename: Kil Joong
authors.initials: KJ
authors.lastname: Kim
authors.forename: Bohyoung
authors.initials: B
authors.lastname: Kim