Objective:To investigate the changes of liver spin-lattice relaxation time (T 1rho) values in the rotating frame in the progression and regression of carbon tetrachloride (CCl 4)-induced model rats with liver fibrosis and the diagnostic values for staging liver fibrosis. Methods:Eighty rats were prospectively enrolled and randomly divided into the CCl 4 group ( n=49), the regression group ( n=20) and the control group ( n=11). All rats were labeled and then examined using MRI at baseline. The liver fibrosis model was established by subcutaneous injection of 40% CCl 4 in hackles. The CCl 4 group underwent black-blood T 1rho imaging at the end of the 4th, 6th, 8th, 10th, 12th week post CCl 4 injection. The regression group underwent black-blood T 1rho imaging at the end of the 4th, 6th week post CCl 4 injection and the end of 1st, 2nd, 4th, 6th week post CCl 4 withdrawal (the injection was stopped at the end of the 6th week). The control group was injected with the same amount of corn oil at the same time point and underwent black-blood T 1rho imaging at the end of 4th, 6th, 8th, 10th, 12th week. The liver T 1rho values were measured in each group over time. Independent-samples t test was used to analyze the differences of liver T 1rho values in adjacent time points. The experimental mice were divided into no liver fibrosis group (S0), mild liver fibrosis group (S1, 2) and moderate or severe liver fibrosis group (S3, 4). The differences of liver T 1rho values were analyzed in different fibrosis stages by Kruskal-Wallis H test. The receiver operating characteristic (ROC) curve analysis was used to evaluate the diagnostic ability of T 1rho values in staging liver fibrosis. The correlation between liver T 1rho values and fibrosis stages was analyzed using Spearman correlation coefficient. Results:Fifty-nine rats completed the whole experiment, including 28 rats in the CCl 4 group, 20 rats in the recovery group and 11 rats in the control group. In the CCl 4 group, the liver T 1rho values gradually increased, reached the maximum at the end of week 8, and then gradually decreased. There was statistically significance in liver T 1rho values at the adjacent time points ( P<0.05) except at the 4th to 6th week ( P=0.112) and 10th to 12th week ( P=0.487) in the CCl 4 group. In regression group, the liver T 1rho values gradually increased post CCl 4 injection and decreased post CCl 4 injection withdrawal. There was statistically significance in liver T 1rho values at the adjacent time points ( P<0.05) in regression group. There was no statistically significance in liver T 1rho values at the adjacent time points ( P>0.05) in control group. The T 1rho values in the no liver fibrosis group (S0, n=15), the mild liver fibrosis group (S1, 2, n=23) and the moderate or severe liver fibrosis group (S3, 4, n=21) were [36.3(34.4,41.4)], (47.2±8.4), (48.8±9.0) ms, respectively. The liver T 1rho values increased with the aggravation of the liver fibrosis, and there was a low positive correlation between them ( r=0.402, P=0.001). There were statistically significant differences in T 1rho values among the three groups ( P<0.01).The area under the curve values to distinguish no liver fibrosis (S0) from liver fibrosis (S1 to 4) and no or mild liver fibrosis (S0 to 2) from moderately or severe liver fibrosis (S3,4) were 0.825 (95% confidence intervals is 0.720 to 0.931) and 0.668 (95% confidence intervals is 0.540 to 0.796), separately. Conclusion:The liver T 1rho values are useful for evaluating the progression and regression of liver fibrosis. It has a moderate diagnostic value to assess the presence of liver fibrosis, but a low diagnostic value to differentiate no or mild liver fibrosis from moderately to severe liver fibrosis.