Characterization of urinary calculi: in vitro study of "twinkling artifact" revealed by color-flow sonography.

Abstract

The "twinkling artifact" is a color-flow sonographic artifact described behind calcifications and presenting as a random color encoding in the region where shadowing would be expected on gray-scale images. Our purpose was to study the relationship between this twinkling artifact seen behind urinary stones on color-flow sonography and the morphology or biochemical composition of these urinary stones. Forty-seven urinary stones were studied in vitro with color-flow sonography. Transmit frequency, color gain, velocity range, color filters, focal depth, and depth of field were changed during scanning. The twinkling artifact was graded 0 when absent, 1 when present but occupying a portion of acoustic shadowing, and 2 when occupying the entire acoustic shadowing. Stones were studied under a binocular magnifying glass to characterize the surface, and infrared spectrophotometry was used to determine the chemical composition. Calculi of calcium oxalate dihydrate and calcium phosphate always produced a grade 1 or grade 2 twinkling artifact. Absence of artifact was noted only for calcium oxalate monohydrate and urate stones. In 100% of grade 0 calcium oxalate stones, the monohydrate compound was predominant (>93%). In 100% of grade 2 calcium oxalate stones, the dihydrate compound was predominant (>75%). For calcium oxalate stones, the surface pattern was correlated with their composition. Sensitivity and specificity for absence of artifact, as indicative of calcium oxalate monohydrate, were 60% and 83%, respectively, for all stones and 56% and 100%, respectively, only for radiopaque stones. An in vitro relationship exists between the twinkling artifact and the morphology of urinary stones. Color-flow sonography could play a role in detecting dense calcium oxalate monohydrate calculi, which in turn may help predict fragmentability.