title: Accuracy of cross-table lateral knee radiography for evaluation of joint effusions.
abstract: OBJECTIVE
  The purpose of our study was to investigate the efficacy of cross-table lateral knee radiography in the diagnosis of knee effusions compared with an MRI reference standard, to evaluate reader experience in effusion assessment, and to establish a new threshold for suprapatellar pouch measurement for the diagnosis of effusion.
  
  
  METHODS
  First- and third-year radiology residents and an attending musculoskeletal radiologist retrospectively assessed 108 cross-table lateral knee radiographs for qualitative grading of joint fluid and quantitative measurement of the suprapatellar pouch. Qualitative and quantitative evaluation of ipsilateral knee MRI examinations performed within 1 week of radiography was performed by two attending musculoskeletal radiologists as a reference standard.
  
  
  RESULTS
  Qualitative visual grading of cross-table lateral radiographs had a sensitivity of 90-92%, specificity of 39-54%, and accuracy of 69-76% for joint effusion. Extrapolating from previous work showing 4 mL of fluid distends the suprapatellar pouch to 4 mm on midline sagittal MRI, the corresponding measurement on cross-table lateral radiographs was predicted to be 7 mm. Using this new criterion of effusion, sensitivity, specificity, and accuracy compared with an MR midline sagittal reference standard were 76%, 83%, and 81%, respectively. Historical data for overhead lateral radiographs had a sensitivity of 78%, specificity of 80%, and accuracy of 79%.
  
  
  CONCLUSIONS
  Qualitative visual assessment of cross-table lateral knee radiographs is highly sensitive for the detection of joint effusion. By performing quantitative evaluation with a new 7-mm criterion for suprapatellar pouch measurement, sensitivity, specificity, and accuracy are equivalent to that of overhead lateral radiography.
authors.affiliation: Department of Radiology, Brigham and Women's Hospital, Boston, MA 02115, USA. awtai@partners.org
authors.forename: Angela W
authors.initials: AW
authors.lastname: Tai
authors.forename: Hatice L
authors.initials: HL
authors.lastname: Alparslan
authors.forename: Brent A
authors.initials: BA
authors.lastname: Townsend
authors.forename: Tamara N
authors.initials: TN
authors.lastname: Oei
authors.forename: Usha S
authors.initials: US
authors.lastname: Govindarajulu
authors.forename: Piran
authors.initials: P
authors.lastname: Aliabadi
authors.forename: Barbara N
authors.initials: BN
authors.lastname: Weissman
mesh_terms: D005122:Exudates and Transudates / Q000166:cytology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007592:Joint Diseases / Q000175:diagnosis*; D007719:Knee Joint / Q000473:pathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015588:Observer Variation; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016683:Supine Position