2012 Proceedings - International Tissue Elasticity Conference

060 VISCOELASTICITY PARAMETERS OF HIFU ABLATED CANINE LIVER TISSUES EX VIVO.
Danial Shahmirzadi 1 , Gary Y. Hou 1 and Elisa E. Konofagou 1 .
1 Columbia University, VC 12–232, 622 West 168th Street, New York, NY, 10032, USA.
Background: The feasibility of using elasticity imaging techniques for high–intensity–focused–ultrasound
(HIFU) ablation monitoring and assessment has been previously demonstrated both in vitro and in vivo
(e.g. [1,2]). Nonetheless, effective use of these techniques requires quantitative assessment of the changes
in the mechanical properties of the ablated tissue during the treatment process. The present study
focuses on the ablation effects on the liver tissue viscoelasticity ex vivo.
Aims: To quantify the viscoelastic shear properties of thermally–ablated canine live tissue ex vivo.
Methods: Canine liver specimens (n=6) were freshly excised immediately after sacrifice and kept
immersed in de–ionized and degassed phosphate–buffered saline (PBS) at room temperature during the
entire in vitro experimentation process. A therapeutic transducer (fcenter = 4.755MHz, fAM=25Hz,
Riverside Research Inst, NY) was used to thermally ablate the liver test samples under 4 increasing
HIFU energy levels of 36W×10s=360J (n=9; Group 1), 25W×30s=750J (n=15; Group 2),
30W×30s=900J (n=18; Group 3), and 36W×30s=1080J (n=11; Group 4). After completion of the
thermal ablation, the same test samples were tested regarding their viscoelastic parameters using a
rheometer (TA Instrument, DE). The complex shear modulus (G*) and shear phase shift (δ) were
obtained under a shear strain of ε=1% at a sweeping frequency range of f=0.1–10Hz. The
measurements were also used to compute the tissue dynamic shear viscosity coefficient (η').
Results: Figure 1 shows an example of a very large HIFU lesion in a liver sample. The shear complex
modulus and dynamic viscosity for the unablated and ablated samples are shown in Figure 2 as a
function of shear strain frequency (logarithmic scale). Most notably, the results indicate that all of the
ablated samples (Groups 1–4) possess a shear complex modulus and a dynamic viscosity of about an
order of magnitude higher than those of the unablated samples. However, the stiffening of ablated
samples in Groups 1 and 2 (i.e. lower thermal energies) is greater than that of samples in Groups 3 and 4
(i.e. higher thermal energies), all compared to the unablated tissue. The complex modulus and the
dynamic viscosity were also found to hold direct and reverse relationships, respectively, to the frequency.
Conclusions: The findings in this study indicated that raising the thermal ablation energy does not
guarantee a monotonic increase in stiffness or viscosity of the ablated tissue with HIFU exposure. The
shift from high stiffening to reduced stiffening that occurs at the higher ablation energies could possibly
be explained in terms of the tissue thermo–mechanical effects occurring at very high temperatures.
Specifically, in situ peak temperature measurements of higher than 107ºC in samples under similar
experimental parameters [3] indicate that tissue pulverization is likely to occur during boiling of the
tissue thereby altering both the structural and mechanical properties of the tissue. Quantification of such
phenomena plays a significant role in proper adjustment of the elasticity imaging experimental set up in
order to generate the target ablated lesion effectively. Future studies are needed to examine the tissue
viscoelastic behavior under a wider range of thermal ablation energies (i.e. including lower levels) in order
to better differentiate between high tissue stiffening due to coagulation at lower ablation energies and
reduced tissue stiffening due to pulverization at higher ablation energies.
References:
[1] Maleke et al.: Phys Med Biol, 53, pp. 1773–1793, 2008.
[2] Maleke et al.: IEEE Trans Biomed Eng, 57, pp. 7–11, 2010.
[3] Hou et al.: IEEE Int Ultras Symp, in press, 2012.
Figure 1: An example image of a very
large HIFU lesion in a canine
liver sample ex vivo.
Complex shear modulus (kPa)
10 2
(A)
10 1
10 0
10 -1
10 0
Frequency (Hz)
indicates Presenter 91
10 1
Dynamic shear viscosity (kPa.s)
10 1
10 0
10 -1
10 -2
(B)
10 -1
10 0
Frequency (Hz)
10 1
Unablated
Group 1
Group 2
Group 3
Group 4
Figure 2: Dynamic viscoelastic mechanical properties of canine liver tissue ex vivo
as a function of temperature: (A) Complex shear modulus (G*); (B) Shear
dynamic viscosity coefficient (η'). The error bars (either upper or lower)
indicate the standard deviations in each group.