2012 Proceedings - International Tissue Elasticity Conference

070 MEASURING BLADDER VISCOELASTICITY USING ULTRASOUND.
IZ Nenadic 1 , B Qiang 1 , MW Urban 1 , A Nabavizadeh 1 , JF Greenleaf 1 , M Fatemi 1 .
1 Mayo Clinic College of Medicine, Rochester, MN, USA.
Background: Changes in bladder mechanical properties are associated with various diseases. A
technique capable of quantifying bladder elasticity and viscosity could be useful in diagnosis and
progression of pathology and has the potential to improve the quality of patient care.
Aims: In this study, we propose an ultrasound technique for quantifying elasticity and viscosity of the
bladder wall. The approach uses radiation force to excite Lamb waves in the bladder wall and track the
motion using pulse–echo ultrasound for the purpose of measuring bladder elasticity and viscosity. The
method is applied to measure elasticity and viscosity of an excised and in vivo pig bladder wall.
Methods: Focused ultrasound radiation force excites impulsive Lamb waves in the bladder wall and
pulse–echo methods measure the motion at several points along the line of wave propagation. Cross–spectral
analysis of the echoes is used to calculate wall motion as a function of time. A two–dimensional fast Fourier
transform (2D–FFT) of the bladder wall motion as a function of time yields the k–space whose coordinates
are frequency (f) and wave number (k). Since the wave velocity c=f/k, the phase velocity at each frequency
can by calculated by searching for peaks at the given frequency and dividing the frequency (f) coordinate
by the wave number (k) coordinate for the given peak. The Lamb wave dispersion equation is fit to the
dispersion data to estimate bladder viscoelasticity. The use of the Lamb wave dispersion equation for flat
plate–like organs has been extensively validated in our previous work [1].
Due to curvature of the bladder wall, finite element analysis (FEA) of viscoelastic plates surrounded by a
liquid was used to study the effect of bladder curvature on the velocity dispersion. FEA studies were
performed using ABAQUS 6.8–3 (SIMULIA, Providence, RI). A flat and a curved plate with the radius of
curvature of 4cm, both 2mm thick and assumed to obey the Voigt model where the Young’s modulus was
25kPa and viscosity 2Pa⋅s, were excited with an impulse using a line source. The k–space analysis was
used to obtain the Lamb wave velocity dispersion.
In ex vivo studies, an excised pig bladder was filled with water until the surface was taut and placed
inside a water tank. The inside of the urethra was covered with industrial glue and attached to rubber
tubing. A digital pressure gauge (Omegadyne, Inc., Sunbury, OH, USA) was used to measure the pressure
inside the bladder. A programmable ultrasound imaging platform (Verasonics, Inc. Redmond, WA, USA)
operating a linear array L7–4 transducer (Phillips Healthcare, Andover, MA, USA) was used to excite
200–400μs impulse in the bladder wall and track the motion. Detection pulses were transmitted at a
pulse repetition frequency of 4kHz and center frequency of 5MHz. The k–space method was used to
calculate the velocity dispersion and the Lamb wave dispersion equation was fit to estimate tissue
elasticity and viscosity. The same approach was used for in vivo animal studies.
Results: The FEA simulations show that the velocity dispersion is affected by the curvature.
Compensating for the angle of curvature in the displacement vector in the curved plate simulations
produces the same dispersion as in the flat plate. The ex vivo bladder elasticity and viscosity were
48.7kPa and 3.5Pa⋅s. The in vivo bladder elasticity and viscosity were 26.1kPa and 0.9Pa⋅s.
Conclusions: The FEA results demonstrate that, if accounted for the curvature, the Lamb wave
dispersion equation for the flat plate can be used to estimate elasticity and viscosity of the bladder. The
ex vivo and in vivo porcine bladder studies demonstrate the feasibility of the proposed technique to
quantify elasticity and viscosity of the bladder tissue.
Acknowledgements: The authors would like to thank to Thomas Kinter and Randall Kinnick for their technical
expertise and Jennifer Milliken for administrative support. This project is supported by grant number R01EB002167
and R01EB002640 from the National Institute of Biomedical Imaging and Bioengineering. The content is solely the
responsibility of the authors and does not necessarily represent the official views of the National Institute of
Biomedical Imaging and Bioengineering or the National Institutes of Health.
References:
[1] I. Z. Nenadic, M. W. Urban, S. A. Mitchell, J.F. Greenleaf: Lamb Wave Dispersion Ultrasound Vibrometry (LDUV)
Method for Quantifying Mechanical Properties of Viscoelastic Solids. Phys Med Biol, 56(7), pp. 2245–2264, 2011.
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