Background: Non-contrast CT scan (NCCT) is becoming the standard imaging modality in urinary stone disease. Radiation dose remains an issue, especially for those patients who may need to undergo several CT scans for this indication during their lifetime. Low-dose and ultra-low-dose protocols exist, but there is limited data on the relationship between the minimum radiation dose capable of detecting stone fragments and stone composition.
Methods: Seven different kinds of human kidney stone were selected. Fragments of 1, 2, 4 and 7 mm were obtained for each stone. Four fragments of the same material were placed in a porcine kidney. A CT scan was then used to scan the kidney at decreasing dosages of 140, 70, 30, 15 and 7mAs. The scans were repeated for each type of stone. Images were reviewed by two radiologists independently with the intent of identifying the stone composition and providing information on its position, dimensions and Hounsfield units (HU).
Results: All types of stone were visible at all settings. Only the 1-mm uric-acid fragment was not detected by both radiologists at 7 and 15 mAs. Dose Length product (DLP) decreased with the reduction in mAs. In terms of HU a statistically significant difference was observed between calcium-based and non-calcium-based stones. Stone dimensions and HU were not affected by the reductions in mAs.
Conclusions: Ultra-low-dose CT has a good detection rate for all kinds of stone, even when the fragment size is small. Only small uric acid fragments need higher energy settings in order to be detected. When the stone composition is known after surgery for urolithiasis, the most appropriate CT scan setting could be suggested by the urologist during their follow-up.