In vitro evaluation of contrast medium concentration and depth effects on the radiographic appearance of specific canine urolith mineral types.

Nine pure mineral types of canine uroliths (bladder or urethral origin only identified in a chronologic sample from the Minnesota Urolith Center mere compared to sequential dilutions of iodinated radiographic contrast medium in vitro. The uroliths studied mere those composed of 100% magnesium ammonium phosphate, calcium oxalate monohydrate, calcium oxalate dihydrate, calcium phosphate appatite, calcium hydrogen phosphate dihydrate (brushite), ammonium acid orate, sodium acid urate, cystine, and silica. The radiopacity of the uroliths was classified as radiolucent, isopaque, or radiopaque, as compared to the radiopacity of the contrast medium solutions in which they were placed, using 2.0 mm and 5.0 mm depths in petri dishes radiographed using a table-top technique. A statistically significant relationship was found between the effective atomic number of the uroliths and the effective atomic number of the contrast medium solutions to which they were compared for the endpoints of isopacity, first lucency tin increasing iodine concentration sequence), and optimal visualization of internal architecture. In general, uroliths isopaque or radiolucent in contrast medium solutions weaker than 23.5 mgI(2)/ml are most likely ammonium acid urate or sodium acid urate. Uroliths isopaque or radiolucent in contrast medium solutions between 23.5 mgI(2)/ml and 44.4 mgI(2)/ml are probably magnesium ammonium phosphate, cystine, or silica. Uroliths that remained radiopaque in solutions stronger than 44.4 mgI(2)/ml, and particularly those radiopaque in contrast medium solutions stronger than 80 mgI(2)/ml, almost always contained calcium. This relative opacity assessment is proposed for use in double contrast cystography as an aid in differentiating urolith mineral types clinically to facilitate appropriate use of medical protocols to dissolve uroliths or to present their growth or recurrence.