Helical computed tomography of urinary tract stones: Accuracy and diagnostic value of stone size and density measurements

Abstract

This study was carried out to determine the accuracy and diagnostic utility of ureteral stone size and density measurements at helical computed tomography (CT). Sixteen calciumcontaining and eight non-calciumcontaining ureteral stones of varying size and composition were examined in vitro with helical CT. Scans were obtained with 10-,5-,3-, and 1-mm collimation. Stone size was determined with CT calipers at each collimation thickness. Stone density was determined by using the maximum value obtained at pixelgram analysis, measured in Hounsfield units. Stone size measured with CT cursors closely matched actual stone size with 10-, 5-, 3-,and 1-mm collimation. Pixelgram density measurements steadily increased as slice thickness decreased. There was better separation of calciumcontaining from non-calcium-containing stones with thinner slices. CT stone size measurement does not vary significantly with slice thickness. CT stone density measurement in Hounsfield units does vary with slice thickness; the highest density numbers were recorded with the thinnest slice thicknesses. Density measurements did not reliably separate calciumcontaining from non-calcium-containing stones, but the amount of density overlap between the two stone types was diminished on the thinnest slices.