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Objective: To estimate the minimum iodine concentrations detectable in simulated vessels of various diameters for both subtraction computed tomography (CT) and dual‐energy CT systems. Methods: Fillable tubes (diameters: 1, 3, and 5 mm) were filled with a variety of iodine concentrations (range: 0–20 mg/ml), placed in the center of 28‐mm cylindrical rods and surrounded with water. Rods with and without fillable tubes were placed in a 20‐cm cylindrical solid‐water phantom to simulate administration of iodine in blood vessels. The phantom was scanned with clinical subtraction CT (SCT) and dual‐energy CT (DECT) head protocols to assess the detection of minimum iodine concentrations in both systems. The SCT and DECT images were evaluated quantitatively with a MATLAB script to extract regions of interest (ROIs) of each simulated vessel. ROI measurements were used to calculate the limit of detectability (LOD) and signal‐to‐noise ratio of Rose criteria for the assessment of the contrast thresholds. Results: Both SNRRose and LOD methods agreed and determined the minimum detectable iodine concentration to be 0.4 mg/ml in the 5‐mm diameter vessel for SCT. However, the minimum detectable concentration in the 5‐mm vessel with DECT was 1 mg/ml. The 3‐mm vessel had a minimum detectable concentration of 0.8 mg/ml for SCT and 2 mg/ml for DECT. Lastly, the minimum detectable iodine concentration for the 1‐mm vessel was 10 mg/ml for SCT and 10 mg/ml for DECT. Conclusion: In this phantom study, SCT showed the capability to detect lower iodine concentrations compared to DECT. Contrast thresholds varied for vessels of different diameters and the smaller vessels required a higher iodine concentration for detection. Based on this knowledge, radiologists can modify their protocols to increase contrast enhancement. [ABSTRACT FROM AUTHOR]