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Ultrasound measurements on tissue
Penny Probert SmithInstitute of Biomedical EngineeringDepartment of Engineering Science University of Oxford
(also Professors Alison Noble, Harvey Burd; Dr Fares Mayia, Russ ShannonChris Haw, Emma Crowley, Jon Dennis)
Mechanical model of tissue
Viscoelastic properties
Non-linear Almost incompressible G,E<<K
)1(2
EG
)21(3
EK
Kelvin or Voigt model
Maxwell Model
GE 3;5.0
Why ultrasound?
Possibility of in-vivo measurements Compared with MRI:
Cheaper Faster (so possibility of measurements
during muscle action)
BUT LESS ACCURATE
Propagation of ultrasound in tissueRelevant material properties
Wave propagation velocity depends mainly on elasticity, density:
Independent of frequency
Attenuation (longitudinal and transverse waves) depends on shear viscosity Also frequency dependent BUT also affected by scattering
Multimode operation
sec/mU
c
Spectral response
Stokes-Navier eqn inherently non-linear; normally make linear assumption Reasonable assumption for propagation in water Poor assumption in tissue – exploited in e.g.
harmonic imaging. Non-linearity coefficient: B/A
proportion of second to first harmonic excited Depends on tissue composition, orientation Can measure through taking spectrum of echo
signals
Measurements
Compression, shear velocity measurements – ex vivo Leads to estimation of K,G
Elastography (in-vivo) Strain visualisation
Shear elastography (in- vivo) Leads to estimation of G
Compression measurements on fish muscle
To assess lipid content
22
21
2
)1(1
c
x
c
x
c
Mixture rule: relates volume fraction, x , to changes in material propertiese.g. velocity
Experimental rig
sample
flightoftime
LLvelocity calibratesample TX RX
Correlation with tissue composition
High repeatability in measurement system Good repeatability and correlation with elastic properties
in phantom (normally a gel) or water
Height of water column
Spe
ed o
f so
und
Fat content (from chemical analysis)
Spe
ed o
f so
und
But not so good in tissue ..
Causes of error in samples
Region of muscle Region of fat (myosepta)
Structure
Shape and orientationLoading: 0.2% compressive strain - but hard to judge 0% strain
Specimen preparation: Degassing – air bubbles have huge effect
Velocities in other tissues
Important issue in ultrasound imaging
Fat composition very important Data mixed; poor repeatability
between different people/tissues In-vivo the fat layer causes most
distortion
Measuring shear velocity – the eye lens
Low frequency vibration excites shear waveTime of flight measurement gives velocity
Pressure from motor? Time dependent effects?
Oscilloscope
For eye lens ..
High attenuation at ultrasound frequencies Mechanical (or low frequency) wave excitation
Results compare well with other estimates(spinning lens, deformation)
In-vivo methods
Can monitor the tendon/muscle etc in use and under different (real) loading
Limited in ultrasound windows Signal may be affected by other tissue – eg
fat layer Possible to probe particular parts of the
anatomy
Elastography
Ultrasound modality becoming standard Designed for in-vivo use – used mainly in
tumour detection Measures tissue displacement – either
through B-mode or r.f. image
Soft tissue biomechanics Elasticity imaging
P = P0
P = P0+P
Window
Length
Beam Width
Sample Volume
…
vv
Prof. Alison Noble
Measurements of tissue strain .. in-vivo
No absolute measure of length Measure changes at different strains Correlation of successive traces
Displacement from strain (induced by temperature change in this case)
Strain estimation
Ultrasound image Strain estimation
(from embedded heat source)
Based on coherent (r.f.) ultrasound data
Strain imaging – pilot study results
Fibroadenoma
Blue=high strain “ok”
Red =low strain “suspect”
DCIS
CancerCyst
Prof. Alison Noble
Tendon elastography
Revell et al, IEEE Trans Medical Imaging, 24 6 2006http://www.cs.bris.ac.uk/Research/Digitalmedia/cve/invivo.html
Uses B-mode image; tracks speckle pattern
BUT ..
Inverse problem (local strain to elastic constants) very hard to solve
Effect of surrounding tissue Orientation – limited number of
ultrasound windows
Shear measurements
Generate a low frequency shear wave Through differential movement Through interference pattern from two
transducers From ‘pushing pulse’
Watch propagation of wave with hgih frequency ultrasound
Shear measurements on musclesDifferential movement
Hoyt et al, 2008
Muscle Shear modulus (relaxed) Shear modulus (contracted)Rectus femoris 5.87kPa 11.17kPaBiceps brachii 6.09 8.42
ARFI (Acoustic Radiation Force) imaging
‘Pushing pulse’ acting locally – can be high frequency for good focal volume control. Longitudinal wave
Excites shear waveHigh speed image acquisition to capture shear velocity
Adapted from Melodelima et al,Ultrasound in Medicine & Biology
Volume 32, Issue 3, March 2006, Pages 387-396 ‘pushing pulse’
Tissue
Shearwave generation
With thanks to Chris Haw, Alison Noble
Conclusions
Ex-vivo Holding tissue – end effects? Artificial loading conditions Effect of neighbouring structure
In-vivo Quantitative shear measurements Displays of compression Possibility of measuring under real loading Limitation of viewing windows
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