Damage mechanisms for ultrasound- induced cavitation in tissuemwarnez/ISTU2014.pdf · 2014. 4....
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Damage mechanisms for ultrasound-
induced cavitation in tissue 2014 April 4
International Symposium for Therapeutic Ultrasound, Las Vegas NV
Matt Warnez1 and Eric Johnsen2
In collaboration with Eli Vlaisavljevich3 and Zhen Xu3
1 Engineering Physics, 2 Mechanical Engineering, 3 Biomedical Engineering
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Background • Cavitation occurs in various ultrasound therapies (e.g.,
histotripsy, lithotripsy)
• High-amplitude pressure oscillations cause violent bubble response
and potentially damage
• Many cavitation bioeffect mechanisms have been proposed
• Tissue is viscoelastic, which significantly effects bubble behavior
• Numerical models allow insight into high-speed bubble dynamics
• Objective: to understand cavitation damage
mechanisms in viscoelastic media using
numerical simulations
Micro- and macroscopic histotripsy-induced ablation –
University of Michigan Therapeutic Ultrasound Group
Large gas bubble in tissue-mimicking gel –
University of Michigan Therapeutic Ultrasound Group
1 mm
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Possible damage mechanisms
Minimum radius
Collapse phase
• High temperatures
• High pressures
• Shockwaves, free radicals, microjets
• Large strain rates, viscous stresses
Maximum radius
Growth phase
• Large strains, elastic stresses
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Theoretical model Assumptions:
• Spherical bubble
• Uniform bubble pressure
• Zero mass transfer
• Incompressible near field with compressible correction
𝜏
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Governing equations • Generalized Keller-Miksis equation
• Energy equations, solved via Chebyshev collocation (Kamath et al. 1989)
• Internal bubble pressure
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Constitutive model • Past approaches include: Allen & Roy (2000), Yang & Church (2005)
• The Zener (standard linear solid) model is the simplest viscous- elastic model to include relaxation effects
• A Chebyshev spectral method was developed to solve for the stresses and temperatures in the entire domain
• Model agrees well with experimental data:
• Laser-induced cavitation experiments in water
• Histotripsy experiments in gels (see presentation by Eli Vlaisavljevich)
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Sinusoidal forcing:
Viscoelasticity reduces the violence of the collapse (and thus pressure)
Water
Zener tissue properties
Water properties
Zener tissue
Radius versus time Pressure
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External temperature
Water
Temperatures external to bubble remain cool, despite viscous heating
Zener tissue
Internal temperature
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Finite strain rate dγθθ/dt
Water
Deformation and deformation rates large, but similar between media
Zener tissue
Finite strain γθθ
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Stress at particle
Water
Geometrical effects amplify stresses experienced at Lagrangian points
Zener tissue
Radial stress τrr
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Stress at particle
Viscoelastic properties induce higher stress even for low forcing amplitudes
Zener tissue
Radial stress τrr
Water
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Causes for increased stress • Viscoelastic properties amplify stress values; relaxation
effects allow for large oscillations
• Out-of-equilibrium oscillations means Lagrangian points are nearer to bubble wall at collapse
Viscoelastic collapse
Bubble at maximum
radius
Bubble at collapse
Newtonian collapse
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Conclusions • In tissue, viscoelastic stresses may be important damage mechanisms
in therapeutic ultrasound
• Large stresses may be an important bioeffect mechanisms due to:
• Viscoelastic properties
• Geometry
• Future work:
• Simulating more complex constitutive models for tissue
• Studying resilience of cells to viscoelastic stresses
• High time-resolution ultrasound-induced cavitation experiments (collaboration with Zhen Xu, U-Michigan)
Matt Warnez ([email protected])
Eric Johnsen ([email protected])
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Azimuthal strain γθθ
Water
Comparison of radial and azimuthal finite strain
Zener tissue
Radial strain γrr
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Azimuthal stress τθθ
Water
Comparison of radial and azimuthal stress
Zener tissue
Radial stress τrr