title: Use of computed tomography scout film and Hounsfield unit of computed tomography scan in predicting the radio-opacity of urinary calculi in plain kidney, ureter and bladder radiographs.
abstract: OBJECTIVE
  The objective of this study is to determine the diagnostic utility of computed tomography (CT)- scout film with an optimal non-contrast helical CT scan Hounsfield unit (HU) in predicting the appearance of urinary calculus in the plain kidneys, ureter, urinary bladder (KUB)-radiograph.
  
  
  METHODS
  A prospective cross-sectional study was executed and data were collected from June 2007 to June 2012 at a tertiary hospital. The included subjects were diagnosed to have <10mm urolithiasis with non-contrast helical CT scan and KUB X-ray, which were carried out on the same day. Both KUB radiographs and CT-scout film were read by two qualified radiologists with inter-observer standardization prior to the study. Urolithiasis characteristics such as stone location, CT attenuation value, CT-scout film and KUB radiograph appearance were recorded independently by two observers. Univariate logistic analysis with receiver operating characteristic curve was generated to determine the best cut-off HU value of urolithiases not identified in CT-scout film, but determined radio-opaque in KUB X-ray. Subsequently, its sensitivity, specificity, predictive values and likelihood ratios were calculated. Statistical significance was set at P value of 0.05 or less.
  
  
  RESULTS
  Two hundred and three valid cases were included. 73 out of 75 CT-scout film detected urolithiasis were identified on plain radiograph and determined as radio-opaque. The determined best cut off value of HU utilized for prediction of radiographic characteristics was 630HU at which urinary calculi were not seen at CT-scout film and were KUB X-ray radio-opaque. The set HU cut-off was established of ideal accuracy with an overall sensitivity of 82.2%, specificity of 96.9% and a positive predictive value of 96.5% and negative predictive value of 83.5%.
  
  
  CONCLUSIONS
  Urolithiases identified on the CT-scout film were also seen as radiopaque on the KUB radiograph while those stones not visible on the CT-scout film, but above the optimal HU cut-off value of 630 are also likely to be radiopaque.
authors.affiliation: St. Luke's Medical Center, Institute of Urology, Philippines.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Chua
authors.affiliation: St. Luke's Medical Center, Institute of Radiology, Philippines.
authors.forename: Odina R
authors.initials: OR
authors.lastname: Gomez
authors.affiliation: St. Luke's Medical Center, Institute of Urology, Philippines.
authors.forename: Lorelei D
authors.initials: LD
authors.lastname: Sapno
authors.affiliation: St. Luke's Medical Center, Institute of Urology, Philippines.
authors.forename: Steve L
authors.initials: SL
authors.lastname: Lim
authors.affiliation: St. Luke's Medical Center, Institute of Urology, Philippines ; National Kidney and Transplant Institute, Department of Urology, Philippines.
authors.forename: Marcelino L
authors.initials: ML
authors.lastname: Morales
keywords: Computed tomography scout film; Hounsfield units; kidneys; non-contrast helical computed tomography scan; ureter; urinary bladder radiograph; urolithiasis