2012 Proceedings - International Tissue Elasticity Conference

043 ROLE AND OPTIMISATION OF THE INDENTER SIZE FOR QUANTITATIVE
POROELASTOGRAPHY.
Jérémie Fromageau 1 , Nigel Bush 1 , Jeffrey C. Bamber 1 .
1 The Institute of Cancer Research and Royal Marsden Hospital, 15 Cotswold Road, Sutton,
Surrey, England, UK.
Background: It is known that subjecting a poroelastic object that has free and permeable boundaries to a
sustained unidirectional compression results in a 3D internal volumetric strain that varies with position
and time, which can be used to estimate tissue properties such as permeability, related to interstitial
pressure and the capacity of fluid to diffuse through the elastic porous matrix. Experimentally, it is
desirable to apply a uniform compression, but this has the drawback that the response is dependent on
the size of the poroelastic object, and this is unknown in vivo.
Aims: In this study we investigated a technique to solve this problem by applying the compression with
an indenter on a very limited surface of the tissue (much smaller than the size of the object), which
produces a spatially-limited region of fluid motion and to determine the optimum indenter size for which
the effect of sample size become negligible.
Methods: Simulations using MARC Mentat (MSC Software) were used to study the spatio–temporal
dependence of volumetric strain from cylindrical samples compressed with an indenter of size varying from
0mm to the sample diameter. The results were quantified in terms of relaxation time and compared with
those from experiments using agarose ultrasound–scattering phantoms with similar geometry and
boundary conditions. Experimental observations were made with an ultrasound scanner (DIASUS, Dynamic
Imaging), using a 3D 7MHz probe. The probe was connected to a test instrument (Instron 3342), permitting
control of its displacement and recording of the applied pressure for the duration of the experiment. For the
uniform compression, the probe itself was used as the compressor. The volumetric strain in the phantom
was estimated from the radiofrequency (RF) echo signals using a 2×2D cross–correlation elastography
method.
Results:
90
Volumetric compression x 10 3
(a) (b)
Time (s)
Figure 1: Simulated volumetric strain relaxations at the centre of a reference cylindrical poroelastic
sample. (a) For a uniform compression applied on the reference cylinder (dot line), a cylinder of
double diameter (dashed line) and a cylinder of same reference size but lower permeability
(solid line). Changes of either scale or permeability lead to an increase of the relaxation time,
and it can be challenging to discriminate the two effects. (b) For an indentation compression,
the relaxation time to depend only on the permeability (same coding).
Conclusions: The results showed that when compressing with a small indenter, the strain relaxation rate
depends only on permeability, unlike the uniform compression where it also depends on the size of the
object. It should therefore be possible to use the technique for a reliable quantitative characterization.
The time constant for strain relaxation is also much shorter than for the uniform compression
experiment, making it more suitable for clinical implementation.
Acknowledgements: This work was supported by grants from EPSRC (EP/E030505/1).
Volumetric compression x 10 3
Time (s)
indicates Presenter