OBJECTIVE: To investigate the effect of low-concentration contrast medium on spectral computed tomography (CT) image quality for portal venography CT. METHODS: 150 patients with suspected portal diseases were divided into three groups and had spectral CT examination using a GE Discovery CT 750 HD scanner. The patients in three groups were injected with different concentrations of iodine (350 mgI/mL, 315 mgI/mL and 280 mgI/mL) at an injection rate of 4.0-5.0 mL/s with 1.2 mL/kg (body weight) of contrast medium, respectively. During the portal vein imaging phase, 0.625 mm-slice-thickness monochromatic images and optimal monochromatic images were obtained. Optimal keV mono-energy was achieved using the optimal contrast-to-noise ratio (CNR) in the portal vein relative to the erector spinae muscle. Volume rendering and maximum intensity projection methods were applied to generate portal venography. The CT values and standard deviations were measured at the portal vein, the erector spinae muscle, and the abdomen fat, respectively. These values were used to calculate the signal-to-noise ratio (SNR); while CNR was calculated using CT values of the portal vein and erector spinae muscle. The overall imaging quality was evaluated on a five-point scale by two radiologists with at least five years' experience. Comparisons among the three groups were performed using One-Way ANOVA test. RESULTS: Monochromatic images at 50-53 keV demonstrated the best CNR for both the portal vein and erector spinae muscle. SNR and CNR of images with different contrast medium concentrations were similar (P>0.05). The five-point scores were also similar (P>0.05) for the three groups. The total iodine intake at 280 mgI/mL was 25.4% lower than that at 350 mgI/mL. CONCLUSIONS: Spectral CT with monochromatic images at 50-53 keV allows significant reduction in iodine load while improving portal vein signal intensity and maintaining image quality.