2012 Proceedings - International Tissue Elasticity Conference

031 MULTIFREQUENCY VS MONOFREQUENCY MR ELASTOGRAPHY FOR THE
CHARACTERIZATION OF LIVER FIBROSIS AND INFLAMMATION.
P Garteiser 1,2 , G D’Assignies 1,2 , H Leitao 1,2,3 , RS Sahebjavaher 1,2,4 , SA Lambert 1,2 , F Mouri 1,2 ,
V Vilgrain 1,2 , BE Van Beers 1,2 , R Sinkus 1,2 .
1 INSERM, U773, F–75205, Paris, FRANCE; 2 University Paris Diderot, Sorbonne Paris Cité,
UMRS773, F–75205, Paris, FRANCE; 3 University of Coimbra, Coimbra, PORTUGAL; 4 University of
British Columbia, Vancouver, BC, CANADA.
Background: Magnetic Resonance Elastography (MRE) is a non–invasive modality that quantifies the
mechanical properties of tissues [1,2]. Theoretical and physical considerations indicate that the presence
of microscopic obstacles may influence not only the absolute value of viscoelastic parameters, but also
their relationship with frequency [3,4]. In particular, the frequency behavior of mechanical parameters
may be modeled as a power law, in which case the exponent parameter represents a mechanical property
inherent to a given material. Yet, most studies use a single excitation frequency thus knowledge about
frequency related mechanical properties is very limited.
Aims: Our aim is to determine which of the monofrequency parameters G’ (kPa), the storage modulus, G’’
(kPa), the loss modulus, Gabs (kPa), the absolute value of the shear modulus, β (mm-1 , 2π over the
wavelength) and α (mm-1 the attenuation coefficient) or their corresponding multifrequency parameters
(the exponent coefficients γ G',
γ G,
" γ , γ Gabs
β , γ α of their response to frequency as modeled with a power law;
γ
γ G
γ
G '
γ G"
abs
β
γ α
i.e. G'=
ω , G"
= ω , Gabs
= ω , β = ω , α = ω with ω the frequency) correlates best with the METAVIR
score.
Methods: All patient studies were performed under informed consent and within the agreement of local
ethical guidelines. Twenty patients with viral hepatitis B (n=5), C (n=14) and nonalcoholic fatty liver
disease (n=1) underwent MRE. Liver inflammation/fibrosis was assessed with METAVIR scoring of
percutaneous biopsies. For MRE, a gradient–echo sequence was used (TR/TE=112ms/9.6ms, 4mm
resolution). Three–directional motion encoding (eight phase offsets) was performed on simultaneous 28,
56 and 84Hz mechanical waves transduced by an electromechanical actuator. Complex viscoelastic
parameters were calculated by demodulation and local inversion of the linear viscoelastic 3D wave
equation. Monofrequency and multifrequency parameters were compared among inflammatory grades
and fibrosis stages, using the Kruskal–Wallis test and Dunn’s post–test.
Results: At 28Hz Gabs and β discriminated between F1 vs F3 (P