Effect of radiographic contrast material exposure on spiral CT attenuationof renal calculi

Rationale and Objectives. The authors performed this, study to determine wh ether exposure of renal calculi to radiographic contrast material has an ef fect on the attenuation values at computed tomography (CT) performed with v arying collimation widths. Materials and Methods. Renal calculi (23 stones of various composition) wer e scanned with 1-, 3-, and 10-mm collimation. Stones were then exposed to a solution of radiographic contrast material for 5 minutes, washed with wate r, and rescanned 36 hours later. The reproducibility of the CT attenuation measurements on different days was evaluated by obtaining measurements in a subset of 16 renal stones on 4 different days. Results. There was no statistically significant change in attenuation after contrast material exposure at narrow collimation. At wider collimation, st atistically significant increases were noted in both attenuation and standa rd deviation. A small amount of variability between readings was noted on d ifferent days, with a minimal increase in attenuation each day. Correlation between readings remained very high. Conclusion. Exposure of stones to a radiographic contrast material had a st atistically significant effect on CT attenuation values only at wide collim ation. This may be related to technical factors including volume averaging. Absence of an effect at narrow collimation suggests that the attenuation v alues of renal stones do not significantly change after exposure to contras t material.