Characterisation of urinary stones in the presence of iodinated contrast medium using dual-energy CT: A phantom study

Objective: To develop a dual-energy CT (DECT) method for differentiating uric acid (UA) from non-UA stones in the presence of iodinated contrast medium. Methods: Thirty UA and 45 non-UA stones were selected after infra-red spectroscopic analysis and independently placed in a 1.5-ml vial, which was filled first with saline and then with increasing concentrations of iodine. For each condition, tubes were put in a 35-cm water phantom and examined using a dual-source CT system at 100 and 140 kV. Virtual unenhanced images created from CT data sets of the stones in iodine-containing solutions provided position and volume information. This map was used to calculate a CT number ratio to differentiate stone type. A region-growing method was developed to improve the ability to differentiate between UA and non-UA stones with iodinated contrast medium. Results: The sensitivity for detecting UA stones was 100 % for unenhanced images but fell to 18 % with 20 mgI/ml iodine solution and 0 % for higher concentrations. With region growing, the sensitivity for detecting UA stones was increased to 100 %, 82 %, 57 %, 50 % and 21 % for iodine solutions of 20, 40, 60, 80 and 100 mgI/ml. Conclusion: The region-growing method improves differentiation of UA from non-UA stones on contrast-enhanced DECT urograms. Key Points: • Computed tomography is widely used to assess renal tract calculi • Dual-energy CT can assess stone composition and provide virtual unenhanced images • However, iodinated contrast medium affects the volume estimation for urinary stones. • CTR of stones is altered by the surrounding iodine in CT urograms. • The region-growing method improves the identification of uric acid stones.