title: Absolute Hounsfield unit measurement on noncontrast computed tomography cannot accurately predict struvite stone composition.
abstract: BACKGROUND
  The purpose of our study was to determine, in vivo, whether single-energy noncontrast computed tomography (NCCT) can accurately predict the presence/percentage of struvite stone composition.
  
  
  METHODS
  We retrospectively searched for all patients with struvite components on stone composition analysis between January 2008 and March 2012. Inclusion criteria were NCCT prior to stone analysis and stone size ≥4 mm. A single urologist, blinded to stone composition, reviewed all NCCT to acquire stone location, dimensions, and Hounsfield unit (HU). HU density (HUD) was calculated by dividing mean HU by the stone's largest transverse diameter. Stone analysis was performed via Fourier transform infrared spectrometry. Independent sample Student's t-test and analysis of variance (ANOVA) were used to compare HU/HUD among groups. Spearman's correlation test was used to determine the correlation between HU and stone size and also HU/HUD to % of each component within the stone. Significance was considered if p<0.05.
  
  
  RESULTS
  Fourty-four patients met the inclusion criteria. Struvite was the most prevalent component with mean percentage of 50.1%±17.7%. Mean HU and HUD were 820.2±357.9 and 67.5±54.9, respectively. Struvite component analysis revealed a nonsignificant positive correlation with HU (R=0.017; p=0.912) and negative with HUD (R=-0.20; p=0.898). Overall, 3 (6.8%) had <20% of struvite component; 11 (25%), 25 (56.8%), and 5 (11.4%) had 21% to 40%, 41% to 60%, and 61% to 80% of struvite, respectively. ANOVA revealed no difference among groups regarding HU (p=0.68) and HUD (p=0.37), with important overlaps. When comparing pure struvite stones (n=5) with other miscellaneous stones (n=39), no difference was found for HU (p=0.09) but HUD was significantly lower for pure stones (27.9±23.6 v 72.5±55.9, respectively; p=0.006). Again, significant overlaps were seen.
  
  
  CONCLUSIONS
  Pure struvite stones have significantly lower HUD than mixed struvite stones, but overlap exists. A low HUD may increase the suspicion for a pure struvite calculus.
authors.affiliation: Glickman Urological and Kidney Institute, Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA.
authors.forename: Giovanni Scala
authors.initials: GS
authors.lastname: Marchini
authors.forename: Surafel
authors.initials: S
authors.lastname: Gebreselassie
authors.forename: Xiaobo
authors.initials: X
authors.lastname: Liu
authors.forename: Cindy
authors.initials: C
authors.lastname: Pynadath
authors.forename: Grace
authors.initials: G
authors.lastname: Snyder
authors.forename: Manoj
authors.initials: M
authors.lastname: Monga
mesh_terms: D003287:Contrast Media; D006801:Humans; D007669:Kidney Calculi / Q000000981:diagnostic imaging*; D017616:Magnesium Compounds / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D000069877:Struvite; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D003287:Contrast Media; D017616:Magnesium Compounds; D010710:Phosphates; D000069877:Struvite