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To establish a curve of dosage gradient-liver signal intensity of super-paramagnetic iron oxide (SPIO) in normal rats and to find an appropriate enhancement dosage. Methods: Seventy-two SD rats were randomized into 18 groups (n=4), received MR enhancement of 0, 2, 5, 8, 10, 12, 15, 20, 30, 40, 50, 60, 70, 80, 100,140, 210, 280 μmol/kg SPIO after plane examination. The signal-to-noise ratio of liver was measured to make a curve of dosage gradient-liver signal intensity. Results: (1) The signal intensity of both T1 and T2 weighted images of liver declined with the increase of SPIO. (2) T2 weighted images were more sensitive than T1 weighted images in small dosage, the ED50 of T1 and T2 weighted were 8 μmol/kg and 5 μmol/kg respectively. (3) When the dose was greater than 15 μmol/kg, the signal intensity of T1 weighted images declined more rapidly than that of T2 weighted,the effect of enhanced T1 weighted images was similar to T2 weighted ones, and its images were with nicer resolution.(4) At the dose of 40 μ mol/kg, the signal intensity of T1 weighted images approached to the background noise; at the dose of 15 μmol/kg, the signal intensity of T2 weighted images approached to the background noise. Conclusion:Good effect of T1 and T2 enhanced MR imaging can be acquired at the dose of 20 to 10 μmol/kg SPIO respectively,the best contrast-to-noise ratio is found on T2-weighted enhanced MR image.