Diffusion weighted MRI
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Transcript of Diffusion weighted MRI
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
Diffusion weighted MRI
Brian Hansen, [email protected]
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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• Background: The physics of diffusion Fickian diffusion Brownian motion Self diffusion
• Diffusion measurements PGSE pulse sequence and spin dynamics Interpreting the diffusion weighted signal Diffusion in biological tissues
• Diffusion MRI in neuroimaging / neuroscience Diffusion weighted MRI and the ADC map Other applications: Fibre tracking
Lecture outline
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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Background and motivation
• Diffusion processes are everywhere in Nature Gases Solids (semi-conductors, alloys) Liquids (chemical reactions, biology, physiology)
• Diffusion Weighted (DW) MRI is a non-invasive method for measuring diffusion: Diffusion coefficient (physical or apparent) Direction of diffusion (preferred direction)
• From these parameters the state of e.g. tissue can be estimated.
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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Clinical Application
• Ischemic infarction is not visible on conventional MRI (T1,T2, PD)
• DW MRI introduces new sensitivity:
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Physical Principles
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Fickian diffusion
Fick’s two laws describe diffusion driven by a difference in concentration.
Fick’s 1st:
Fick’s 2nd:
C(x,t)
x
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
Self-diffusion
• All water molecules perform a thermally driven random walk.
• We can only describe this motion statistically:
H2O
For Brownian motion z = 2
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Brownian Motion
• Named after scottish botanist Robert Brown (1773-1858).
• Explained by Einstein in 1905.
• The thermal motion of the molecules cause them to collide. Random motion follows.
Described by the Stokes-Einstein relation:
D is diffusion coefficient, kB is the Boltzmann constant, T is absolute temperature, is liquid viscosity and r is particle radius.
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
Diffusion in biological tissue
Diffusion in tissue is resticted by cell membranes, organelles etc:
These random trajectories will in time fill the plane and reveal the structure.
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Measuring Diffusion
• The Pulsed Gradient Spin Echo (PGSE) sequence
90
180
time
g
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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No diffusion:
x
Spin 1 Spin 2 Spin 3
B
Stationary spins are unaffected by diffusion gradient.
time
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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With diffusion:
x
Spin 1 Spin 2 Spin 3
B
time
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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Vector sum
+ + =
+ + =
No loss of signal in areas with no diffusion.
Diffusion introduces a signal loss.
High diffusion gives strong signal attenuation.
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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DW MRI parameters
Parameters g, , are combined in the b-factor:
Here is the proton gyromagnetic ratio.
The b-factor can be varied by varying one of g, and .
For the PGSE sequence the case b = 0 corresponds to the simpel SE sequence.
A large b-factor gives a large signal loss in areas with high diffusion. This is called strong diffusion weighting.
90
180
g
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Signal and b-factor I
The DWMR signal from simple free diffusion is described by:
By ”simple free diffusion” we mean that
applies for all times.
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Signal and b-factor IIOn a log-plot this yields a straight line:
log(S(b)/S(b=0)) = -bD
The slope of the curve gives us the physical diffusion coefficient.
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Measuring D
Remember:
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Diffusion in tissue
Diffusion in biological tissue is not free:
Cell membranes, organelles etc. restrict the diffusion of the water molecules.
Simple signal behaviour
breaks down due to complex tissue structure:
Grey matter, ECS in red.
This means that
no longer applies for all times.
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Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
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The Apparent Diffusion Coefficient
• The value we measure is no longer the physical diffusion coefficient: Instead we get an average over many restricted random walks
• We introduce the term Apparent Diffusion Coefficient (ADC)
• Two measurements at b = 0 and b = 1000 s/mm2 are made:
The slope gives the ADC – not the physical diffusion coefficient.
Typical ADC values in brain (mm2/s):
Normal gray matter: 0.8-1.010-3
Normal white matter: 0.2-1.0 10-3
Free water (CSF): 2.9 10-3
Review and references in Journal of Computer Assisted Tomography 25(4):515-519.
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Increasing b-values
b = 2000 b = 4000 b = 7000
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DWI and ADC maps
DW MRI provides two new image types:
The Diffusion Weighted Image (DWI) The ADC map (a calculated image)
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Stroke DWI
Acute
DWI
tPA + 2h tPA + 24h
DWIDWI
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
Possible cause of the bright areas in the DWI:
Cells in normal tissue
Cells in infarct (stroke)
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Summary: Strength of DWI
T2 MRI:
• Infarct is not visible – brain appears normal
DWI:
• Infarct clearly visible
• Scan time: 30 sec, EPI
• No IV contrast agent needed
• Infarct detectable after few minutes
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Summary: Image types
Two image types: DWI and ADC map:
DWI:
• Signal is diffusion weighted.
• High diffusion: signal loss
• Low diffusion: no signal loss
• Infarcts are bright
ADC map:
• Calculated image
• Contrast opposite to DWI
• Low intensity: low ADC value (low diffusion)
• High intensity: high ADC (high diffusion)
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DWI
ADC
MTT
Osvd
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Diffusion Tensor Imaging
Diffusion is often directional – e.g. along fibers:
Instead of measuring many b-values we measure along many different directions.
Instead of the ADC we obtain the Diffusion Tensor which describes the diffusion coefficient in space.
This is the basis of fibre tracking.
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
Images courtesy of Jesper Frandsen, CFIN
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The Danish National Research Foundation’s
Center of Functionally Integrative NeuroscienceAarhus University / Aarhus University Hospital
Image courtesy of Jesper Frandsen, CFIN