AbstractMRI has the potential of providing a noninvasive assessment of liver pathology. This work introduces a portal pressure gradient (PPG) model derived from fluid mechanics, where the PPG is proportional to the average velocity and inversely proportional to the vessel area in the upper part of portal vein. Using a phase‐contrast spoiled gradient echo sequence, the PPG model was verified in a phantom study and was tested in an animal study using 35 rats with various degrees of hepatic fibrosis induced by carbon tetrachloride (CCl4). Histological examination was conducted to determine the severity of hepatic fibrosis. The fibrosis score monotonically increased with the duration of CCl4 treatment. The PPG was highly correlated with nonzero fibrosis scores (r2 = 0.90, P < 0.05). There was a significant difference between control and cirrhosis groups (P < 0.0006, α < 0.0018). The difference between control and fibrosis (noncirrhosis) groups (P < 0.002, α < 0.006) was also significant. Without the administration of any contrast agent, the MRI‐PPG approach shows promise as a noninvasive means of evaluating liver fibrosis. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.