2012 Proceedings - International Tissue Elasticity Conference

047 IN VIVO MEASUREMENT OF VOLUMETRIC STRAIN FOR THE ASSESSMENT OF
INTRAHEPATIC PRESSURE ALTERATIONS.
Sebastian Hirsch 1 , Thomas J. Kroencke 1 , Jing Guo 1 , Rolf Reiter 1 , Sebastian Papazoglou 1 ,
Ingolf Sack 1 , Jürgen Braun 2 .
1 Radiology Department, 2 Institute of Medical Informatics, Charité–University Medicine Berlin,
Berlin, GERMANY.
Background: Liver cirrhosis often causes portal hypertension which frequently leads to intestinal bleeding
or the buildup of fluid within the abdomen. The distortion of hepatic blood flow due to increased portal
pressure can be treated by Transjugular Intrahepatic Portosystemic Stent Shunting (TIPSS). This
intervention creates an artificial channel within the liver that establishes communication between the
inflow portal vein and the outflow hepatic vein. Successful TIPSS can significantly reduce flow obstructions
in the portal area of the liver and therewith reduce portosystemic pressure gradients. In contrast to the
classical assumption that soft tissue is incompressible, recent studies report on poroelastic characteristics
of brain tissue [1,2], thus motivating us to apply compression–sensitive elastography to the human liver.
Aims: TIPSS is not expected to affect the shear modulus of the liver. Instead, the alteration of
intrahepatic pressure conditions may influence mechanical constants related to compression and
pressure. As shown by 3D vector–field MRE of lung [3], volumetric strain is sensitive to different pressure
states of tissue. In this work, we aimed to test the sensitivity of volumetric strain to TIPSS.
Methods: MRE of the liver was performed in 10 patients with liver cirrhosis before and after TIPSS
implantation using a 1.5T MRI scanner with a piezo–electrical vibration generator. The vibration generator
was operated with time–harmonic signals of 25 and 50Hz frequency. 3D vector fields were acquired by
means of a single–shot EPI sequence in 10 transversal slices of 5mm thickness and 2.73x2.73mm² in–plane
resolution. Shear strain and volumetric strain were derived from complex wave fields obtained by Fourier
transformation using the curl and divergence operators based on multi–dimensional gradients [4]. The
magnitudes of the complex oscillatory divergence and the corresponding curl components were averaged
within the entire liver considering the 5 most central slices.
Results: On average, TIPSS reduced the portal pressure by 11.0±2.8mmHg. This pressure change was not
translated to the shear modulus (5.5±2.4kPa prior to the intervention and 4.9±2.2kPa post treatment,
P=0.46). Shear strain given by the rms of curl components was not significantly altered either (0.13±0.05% vs.
0.12±0.05%, P=0.28). In contrast, volumetric strain showed a significant correlation with TIPSS treatment.
The divergence of the harmonic field increased in all 10 patients from mean values of 0.028±0.011% to
0.033±0.014% (P=0.00064), prior and post intervention, respectively. Figure 1 demonstrates the display of
shear strain and volumetric strain in a patient and displays the alteration of volumetric strain in both groups.
Conclusions: Volumetric strain is sensitive to the change in hepatic pressure due to TIPSS treatment.
The observed reciprocal correlation of volumetric strain with pressure is opposed to the reported increase
of volumetric strain in the lungs upon inhalation [3]. This inverse behavior may indicate the intrinsically
different nature of pressure development in lung and liver. While lung pressure can be modeled by an
air–cavity phantom under external load [3], intrahepatic pressure is communicated by the vascular tree
which obeys complex autoregulatory mechanisms [5].
References:
[1] Klatt et al.: Proc. 10th ITEC, Arlington, Texas, p. 115, 2011.
[2] Mcgarry et al.: Proc. of the 20th Conference of the ISMRM, Melbourne, p. 2519, 2012.
[3] Hirsch et al.: Magn Reson Med, doi:10.1002/mrm.24294, 2012.
[4] Anderssen & Hegland: Math Comput, 68, pp. 1121–1141, 1999.
[5] Roadbard: Am Heart J, 65, pp. 648–655, 1963.
a) shear strain (pre TIPPS) shear strain (post TIPPS) b) vol. strain (pre TIPPS) vol. strain (post TIPPS) c)
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Figure 1: Shear strain and volumetric strain at 50Hz harmonic excitation in human liver. Two stages were examined, once with hepatic
hypertension due to flow obstruction in the portal vein and a second time after TIPSS implantation leading to a reduction of
hepatic pressure. While shear strain is not altered due to TIPSS (a), volumetric strain is significantly higher in post–TIPPS patients
indicating a higher compressibility of liver tissue (b). TIPSS–induced increase in volumetric strain was observed in all patients (c).
indicates Presenter 53
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