title: Dual energy can accurately differentiate uric acid-containing urinary calculi from calcium stones.
abstract: OBJECTIVE
  To retrospectively evaluate the accuracy of dual-energy CT (DECT) in the detection of the chemical composition of urinary calculi in correlation with infrared spectroscopic stone analysis.
  
  
  METHODS
  We reviewed the CT scans of 255 patients who underwent DECT due to a clinical suspicion of urolithiasis. Out of this group, we included 64 patients with clinically symptomatic urolithiasis requiring stone removal. After surgical removal of the stone by ureterorenoscopy, chemical composition was analyzed with infrared spectroscopy. We correlated DECT stone characterization results with chemical stone composition based on dual-energy indices (DEI). A total of 213 renal and ureteral stones could be removed and chemically analyzed.
  
  
  RESULTS
  A total of 213 calculi were evaluated. Thirty eight out of sixty four (59 %) patients had >1 stone. DECT was used to differentiate stones by using DEI. Stones harboring calcium (CA) were color-coded in blue, while stones containing uric acid (UA) were colored red. Median DEI in UA-containing stones were 0.001. Non-UA-containing stones had a DEI between 0.073 for pure CA stones and 0.077 containing CA and other substances (p = 0.001; p = 0.03, respectively). Sensitivity of DECT was 98.4 % for differentiation of UA from non-UA-containing calculi. Specificity was 98.1 %. Mean effective radiation dose of DECT was 4.18 mSv (0.44-14.27 mSv), thus comparable to conventional CT scans of the abdomen. Conventional measurement of Hounsfield units did not correlate with stone composition.
  
  
  CONCLUSIONS
  DECT with image post-processing reliably discriminates UA-containing calculi from all other stones, but the study offered limitations. Discrimination within the non-UA stones cannot be reliably achieved but is clinically insignificant.
authors.affiliation: Department of Urology, University Hospitals Munich, Campus Grosshadern, Munich, Germany. Annabel.spek@med.uni-muenchen.de.
authors.forename: A
authors.initials: A
authors.lastname: Spek
authors.affiliation: Department of Urology, University Hospitals Munich, Campus Grosshadern, Munich, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Strittmatter
authors.affiliation: Department of Radiology, University Hospitals Munich, Campus Grosshadern, Munich, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Graser
authors.affiliation: Department of Urology, University Hospitals Munich, Campus Grosshadern, Munich, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Kufer
authors.affiliation: Department of Urology, University Hospitals Munich, Campus Grosshadern, Munich, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Stief
authors.affiliation: Department of Urology, University Hospitals Munich, Campus Grosshadern, Munich, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Staehler
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002118:Calcium / Q000032:analysis*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed; D014527:Uric Acid / Q000032:analysis*; D014545:Urinary Calculi / Q000737:chemistry* / Q000000981:diagnostic imaging; D055815:Young Adult
chemical_list: D014527:Uric Acid; D002118:Calcium
keywords: Chemical composition; Dual-energy CT; Uric acid stones; Urolithiasis