The creation of a transjugular intrahepatic portosystemic shunt (TIPS) influences hepatic blood flow dynamics. TIPS-related changes of diffusion-weighted imaging based intravoxel incoherent motion (IVIM) parameters at 1.5 T were investigated in liver parenchyma of cirrhotic patients. An increase of the IVIM perfusion fraction was found, indicating improved microvascular flow within the liver tissue after decompression of the portal vein. Diffusion parameters were not significantly influenced by TIPS-creation.
Purpose
Portal hypertension in patients with end-stage liver disease is associated with severe complications. It can be treated by creation of a transjugular intrahepatic portosystemic shunt (TIPS), leading to decompression of the portal venous system1-4. TIPS-creation influences both portal venous and arterial blood flow dynamics which can be observed with Doppler-US or 4-dimensional flow MRI5-8. However, flow changes within the microvascular system of the liver parenchyma after TIPS-creation have so far not been investigated.
The aim of this study was therefore to investigate perfusion changes within the liver parenchyma after TIPS-creation by using intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI).
Methods
16 patients (m/f: 13/3, mean age 53 years) with cirrhosis and portal hypertension underwent liver-MRI at 1.5T including DWI (b0=0, b1=50, b2=800s/mm2) before and after TIPS-creation. Apparent diffusion-coefficient ADC(0,800), estimated diffusion coefficient D’ and estimated perfusion fraction f’ were determined voxelwise using a simplified IVIM-approach as previously described9-11. Hereby, D’ = ADC(50,800) = (ln(S(b1))-ln(S(b2)))/(b2-b1) and f’ = 1–S(b1)/(S(0)▪exp(-b1▪ADC(50,800)), S(b) and S(0) are the signal intensities with and without motion-probing gradients.
In each patient, three hand-drawn regions of interest (ROIs) were analyzed (1. ventral right, 2. dorsal right and 3. left liver-lobe) before and after TIPS-creation at the same anatomical location. Additionally, subgroups of ROIs were defined according to contrast enhanced MRI as (A) ROIs with completely preserved and (B) absent contrast enhancement of portal-venous branches within the ROI after TIPS-creation.
Results
TIPS-creation was successful in all patients. The portosystemic gradient which was elevated pre-TIPS was successfully lowered to less than 12mmHg in all cases post-TIPS.
Overall IVIM analysis showed that f’-values increased significantly from 9.3±3.8% to 11.0±5.0% (p<0.028) after TIPS-creation, while ADC(0,800) and D’ (both given in 10-6mm2/s) did not change (1263.9±195.8 vs. 1274.2±167.8, p=0.461; 1151.1±174.9 vs. 1133.7±152.6, p=0.490, respectively).
After TIPS-creation 8/48 ROIs showed no contrast enhancement of portal venous branches (2 right ventral, 5 right dorsal, 1 left). In those ROIs, a significant drop in f’-values from 11.1±4.5% to 5.3±1.8% (p=0.003) was found. In ROIs with patent portal venous branches after TIPS-creation, f’-values increased significantly from 9.0±3.6% to 12.1±4.7% (p<0.00002). In none of these groups, a significant ADC(0,800) or D’ change was found. No patient showed impaired enhancement of the hepatic artery and its branches.
Discussion
For DWI acquired with more than two b-values, analysis based on the intravoxel incoherent motion (IVIM) model was proposed by Le Bihan12. By assuming a bi-exponential behavior of the signal intensity, diffusion and perfusion effects on the DWI signal can be separated13 leading to the determination of true diffusion coefficient D, perfusion fraction f (reflecting the relative contribution of microvascular perfusion to the DWI signal), and pseudodiffusion coefficient D* (related to blood flow velocity and vessel architecture). In the simplified IVIM model used in this study9-11, the estimated perfusion fraction f' may deviate from the exact f value because it is also influenced by D*; one should therefore look at f’ as an empirical global perfusion parameter9.
Generally perfusion related IVIM parameters are known to be decreased in diffuse liver disease (fibrosis and cirrhosis) compared to normal liver tissue14-16. After TIPS-creation both blood flow volume and velocity within the portal vein and its tributaries as well as the hepatic artery are known to increase by a factor of 2 to 4 6-8. The net increase of hepatic perfusion (combined arterial and portal venous perfusion) has recently been described as 424 ml/min in a 4-D flow MRI study5.
The results of our study demonstrate that microvascular flow within the liver parenchyma is generally improved after TIPS-creation while diffusion parameters are not significantly affected. The fact, that f’ decreased in branches with no portal venous enhancement (due to hemodynamic changes or thrombotic obstruction), suggests that IVIM perfusion may be preferentially influenced by portal venous rather than by arterial blood flow. Further research, however, is needed to clarify this issue.
Conclusion
The IVIM perfusion parameter f’ may be suitable to assess the changes in microvascular perfusion in the liver after TIPS-creation. Further investigation into a possible dominant influence of the portal venous compared to arterial blood flow on IVIM perfusion parameters is warranted.1. Brensing KA, Textor J, Perz J, Schiedermaier P, Raab P, et al. Long term outcome after transjugular intrahepatic portosystemic stent-shunt in nontransplant cirrhotics with hepatorenal syndrome: a phase II study. Gut. 2000;47:288–295.
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