To investigate MR Elastography (MRE)–assessed mechanical properties of shear stiffness and phase angle to distinguish hepatic inflammation and fibrosis in several in vivo animal models with progressively developed chronic liver diseases that involve inflammation, fibrosis, congestion and portal hypertension.All activities related to animal subjects were reviewed and approved by our institutional animal care and use committee. A total of 188 animals, including 183 mice and 5 pigs that represent five different liver diseases with a varying combination and extent of hepatic inflammation, fibrosis, congestion and portal hypertension. 1) 55 wild type male mice with fast food diet (FFD) induced nonalcoholic fatty liver disease or steatohepatitis (NAFLD/NASH) and 40 gender and age-matched control mice; 2) 19 knockout mice with autosomal recessive polycystic kidney disease (ARPKD) and 18 gender and age-matched control mice; 3) 12 wild type mice with toxic chemical carbon tetrachloride (CCl₄) induced hepatic fibrosis and 12 gender and age-matched control mice; 4) 16 wild type mice with partial inferior vena cava ligation (pIVCL) surgery induced venous congestion and 11 gender and age-matched sham mice; 5) 5 inborn fumarylacetoacetate hydrolase (FAH)-deficient pigs. An in vivo multifrequency 3D MRE examination was performed on each animal monthly (models #1&5) or one time (models #2, 3& 4) before euthanization¹. The complex shear modulus was calculated with outputs of liver stiffness and phase angle at multiple mechanical frequencies (80-200 Hz in mice, 40-80 Hz in pigs).

Results validated the well-established fact that both shear storage modulus and shear stiffness of the liver increased progressively with the severity of chronic liver diseases (p<0.05). However, it was not able to detect very early onset of inflammation at high frequencies. Results also demonstrated that phase angle increased significantly at the early onset of inflammation (p<0.05), even before histologically detectable macrophage transformation or migration. Moreover, it has better discrimination ability at lower frequencies. With the progressively developed fibrosis with coexisting/diminishing inflammation, phase angle decreased and lost its sensitivity to inflammation in models #1, 2&3 (p>0.1). In the congestive liver model without significant fibrosis, phase angle was inversely proportional to portal pressure (r=-0.6129, p<0.001, model #4), but increased significantly in response to cirrhosis induced spontaneous portal hypertension (model #5).

As shown in Figure-1 of the NAFLD/NASH model, monthly MRE results demonstrated that liver stiffness successfully detected the onset of inflammation (16 weeks, p=0.04) and the onset of fibrosis (24 weeks, p=0.04). Both loss modulus and phase angle increased significantly as early as 4 weeks (80Hz, p=0.005) before histologically detectable inflammation and stayed significantly high from 12 until 40 weeks. From 44 to 48 weeks, there were no significant change observed in phase angle (p>0.1).

As shown in Figure-2 of the ARPKD model, fibrosis extent increased progressively with age; while the portal inflammation extent increased significantly and stayed consistently mild in the ARPKD mice of 3 and 6 months old, but not detectable in the histologic analysis of 1 month old mice. Liver stiffness successfully distinguished ARPKD mice for all age groups (80Hz only, not for higher frequencies). Phase angle showed significant augmentation in 1 month old mice (80Hz, p<0.0001), but no change in older mice (p>0.1).

As shown in Figure-3 of the CCl₄ model, peak necroinflammation was observed at week 1, but decreased at week 2 and 4 then diminished at week 6; the onset of fibrosis was at week 2 and increased thereafter. Liver stiffness successfully detected the fibrosis at week 4 and after (200Hz, p<0.03); while phase angle discriminated early inflammation as early as week 1 (200Hz, p=0.03).

As shown in Figure-4 of the pIVCL model, portal pressure increased significantly for all time groups with insignificant inflammation and fibrosis developed in the congestive liver. Liver stiffness positively correlated with the portal pressure (best fit at 200Hz, r=0.8738, p<0.0001), while phase angle negatively correlated with portal pressure (best fit at 200Hz, r=-0.6129, p<0.001).

As shown in Figure-5 of the FAH model, both liver stiffness and phase angle increased and reached their maximum value through the animal’s lifespan at the time of spontaneous portal hypertension with gastrointestinal bleeding. In pigs without GI bleeding, at one month of age with onset of abnormality, the liver stiffness maintained its lowest value, while the phase angle reached its maximum values.

Results in five animal models indicated that the phase angle, defined by the complex shear modulus, is useful for discriminating between inflammation and fibrosis alone in the early stage of liver diseases. It may also distinguish venous congestion and portal hypertension associated with cirrhosis in the advanced stage of liver diseases.