  Hepatic fibrosis causes an increase in liver stiffness, a parameter measured
by elastography and widely used as a diagnosis method. The concomitant presence
of portal vein thrombosis (PVT) implies a change in hepatic portal inflow that
could also affect liver elasticity. The main objective of this study is to
determine the extent to which the presence of portal occlusion can affect the
mechanical properties of the liver and potentially lead to misdiagnosis of
fibrosis and hepatic cirrhosis by elastography. Portal vein occlusion was
generated by insertion and inflation of a balloon catheter in the portal vein
of four swines. The portal flow parameters peak flow (PF) and peak velocity
magnitude (PVM) and liver mechanical properties (shear modulus) were then
investigated using 4D-flow MRI and MR elastography, respectively, for
progressive obstructions of the portal vein. Experimental results indicate that
the reduction of the intrahepatic venous blood flow (PF/PVM decreases of
29.3%/8.5%, 51.0%/32.3% and 83.3%/53.6%, respectively) measured with 50%, 80%
and 100% obstruction of the portal vein section results in a decrease of liver
stiffness by $0.8\%\pm0.1\%$, $7.7\%\pm0.4\%$ and $12.3\%\pm0.9\%$,
respectively. While this vascular mechanism does not have sufficient influence
on the elasticity of the liver to modify the diagnosis of severe fibrosis or
cirrhosis (F4 METAVIR grade), it may be sufficient to attenuate the increase in
stiffness due to moderate fibrosis (F2-F3 METAVIR grades) and consequently lead
to false-negative diagnoses with elastography in the presence of PVT.
