mr elastography: measuring brain mechanical properties in...
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MR Elastography: Measuring brain mechanical properties in
vivo
A Practical GuideLynne E. Bilston
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Magnetic Resonance Elastography
viscosity
Amplitude of waveis reduced due to viscosity
Diagram from R. Sinkus
Wavelength changes accordingto local stiffness elasticity
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Brain MRE• Vibration
propagated into the brain through the skull
• Human and animal studies have been performed
• Isotropic and anisotropic properties have been measured
• Grey and white matter
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Normal Brain MRE
Green et al, NMR Biomed 2008
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MRE in NPH• One group has
measured brain elasticity in NPH patients (Streitberger et al, NMR Biomed 2010; Sack et al ISMRM 2011) N
orm
alN
PH
• Substantial variability, limited specificity
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What do you need to do MRE?
• MR pulse sequence – To drive the MR scanner
• Transducer – To create the vibration
• Analysis software– To calculate the brain mechanical properties
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MR Pulse Sequence for elastography• Some different types
– Spin echo– FFE etc
• Motion encoding gradients, synchronised with mechanical vibration measure displacement at specific points during vibration cycle
• There are sequences available for Philips, Siemens, GE scanners
• Implementations still in the ‘research’ phase for brain.
• MRE can be done on 1.5T or 3T scanners
• Scan time varies depending on sequence type (~4mins-15mins)
• Can add DTI for anisotropy
Sinkus et al, Phys Med Biol 2000
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Transducer and hardwareEquipment needed:• Programmable
function generator• Amplifier• Connections to
scanner & transducer
• Filters for pass-through into scan room
• Transducer• No commercial
packages for brain elastography
Trigger from MRI
Signal Generator
Amplifier Cathode Ray Oscilloscope (CRO)
FilterTransducer
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Transducers for Brain MRE• Two main types
– Mouthguard– Head rocker
• Neither are ideal for all subjects
Berlin group head rocker transducerSydney Brain Transducer – coil driven mouthguard
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Analysis• Many implementations of
analysis• Outputs include:
– Shear modulus (G) – a measure of stiffness
– Shear elastic modulus (G’, μ)– Shear viscous modulus (G”, ωη)– Various other implementation
specific parameters• Vendors can supply software to
read specific scanner formatted files
• Most vendors have software to do basic analysis (from their liver packages)
• One freely available application (MRE/wave) does elasticity only
Elastography analysis package (from R. Sinkus)
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Summary• To do MR elastography, you need:
– MR pulse sequence – Transducer & associated hardware– Analysis software
• Brain elastography is not yet a commercial product– Transducers are mostly ‘custom made’– Software often implementation-specific
• Limited comparison has been done across sites and scanners/implementations
• Lots of different details – frequencies, sequence, transducer, analysis