DTI Basics – Water Diffusion
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Transcript of DTI Basics – Water Diffusion
Einstein on Brownian Motion
1905 five important papers
DTI Basics – Water DiffusionDTI Basics – Water Diffusion
Conventional TConventional T22 WI WI DW-EPIDW-EPI
Why MRI : Detection of Acute StrokeWhy MRI : Detection of Acute Stroke
“Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center
Sodium ion pumps fail, water goes in cells and can not diffuse.
TumorTumor
T2 (bright water)DWI (x direction)
(T2 (bright water)+(diffusion))
T1 + Gadolinium
Why MRI (more recently): Fiber TrackingWhy MRI (more recently): Fiber Tracking
Diffusion Weighted Image X directionDiffusion Weighted Image X direction
David Porter - November 2000
Artifact or Abnormality
Higher diffusion in X direction lower signal
1st level of complexity
Pulse Sequence: Gradient-EchoPulse Sequence: Gradient-EchoDiffusion Weighting in X directionDiffusion Weighting in X direction
90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
G
-G EPI (T2)
diffusion gradients
GradientCoils
RFGxGyGz
-
Time
NO DIFFUISION
with DIFFUISION
y
x
protons (Hydrogen)
signal loss
RFGxGyGz
-
Time
(gradient strength)
T2T2 + diffusion
T2
DIFFUSION MAPSDIFFUSION MAPS
(used to remove spin density, T1, T2, TR, and TE effects)(used to remove spin density, T1, T2, TR, and TE effects)S=SS=S00ee(-bDxx)(-bDxx)
Ln(S) = Ln(SLn(S) = Ln(S00) – bD) – bDxxxx or D or Dxxxx = (Ln(S = (Ln(S00) – Ln(S))/b) – Ln(S))/b
S0 (T2* EPI) S (T2* EPI + weak Diffusion in X direction)
Dxx
Most Important image
weak
2nd Level of complexity2nd Level of complexityDWI : 3 Direction DWI : 3 Direction
courtesy of Dr Sorensen, MGH, Boston
David Porter - November 2000
single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions Higher diffusion lower signal
Dzz Dxx Dyy
X Diffusion-WeightingX Diffusion-Weighting Y Diffusion-Y Diffusion-WeightingWeightingZ Diffusion-WeightingZ Diffusion-Weighting
GGFEFE
GGPEPE
GGSSSS
RFRF
A Little More DetailA Little More Detail
SS
PE
FEx
z
y
y
x
z
Apparent Diffusion Coefficient ADC (AKA TRACE)
ADC = (Dxx + Dyy + Dzz)/3
Orientation independent
used in clinical stroke, tumor, etc
No directional information (ie direction of greatest diffusion)
3rd level of complexity3rd level of complexityDiffusion Tensor Imaging BasicsDiffusion Tensor Imaging Basics
Measures water diffusion in Measures water diffusion in at least 6 at least 6 directionsdirections – we use 12 – we use 12Echo-planar imaging (fast acquisition)Echo-planar imaging (fast acquisition)Collecting small voxels (1.8x1.8x3mm), Collecting small voxels (1.8x1.8x3mm), scanning takes about 10 minutesscanning takes about 10 minutes
• Useful for following white matter tracts
Higher diffusion lower signal
Isotropic Anisotropic
Adapted from: Beaulieu (2002). NMR in Biomed; 15:435-455
Higher diffusion lower signal
x
yz
DTI ellipsoidDTI ellipsoidmeasure 6 directions to describemeasure 6 directions to describe
no diffusion
Pierpaoli and Basser, Toward a Quantitative Assessment of Diffusion Anisotropy, Magn. Reson. Med, 36, 893-906 (1996)
Ellipsoid Image
Tract
Information available through DTI
Tract tracing problems (fiber crossing)Tract tracing problems (fiber crossing)
TractographyTractography
Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf.
Superior view color fiber maps Lateral view color fiber maps
Diffusion Tensor Imaging data forcortical spinal tract on right side
blue = superior – inferior fibersgreen = anterior – posterior fibersred = right – left fibers
Note tumor is darker mass on leftside of axial slice
axial
sag
corMRISC
But what is a diffusion But what is a diffusion tensortensor?? It is a mathematical description of It is a mathematical description of the ellipsoid. the ellipsoid.
y
x
z
no diffusion
Dxx Dxy Dxz
Dyx Dyy Dyz
Dzx Dzy Dzz
z-xz
y-zxz
y-z
xy -xy
What is diffusion “Tensor” (D)?
Dxx Dxy Dxz
Dyx Dyy Dyz
Dzx Dzy Dzz
Dx’x’ 0 00 Dy’y’ 00 0 Dz’z’
S=SS=S00exp(-bD)exp(-bD)
difussion gradient direction vector
= (mathematical manipulation)
Calculate FA (fractional anisotropy)Fiber track
y
ellipsoid reference framelab reference frame
Information available through DTI
av
FA (fractional anisotropy)FA (fractional anisotropy)
FA = ((Dx’x’-Dav)2 + (Dy’y’-Dav)2 + (Dz’z’-Dav)2)0.5
(Dx’x’2+Dy’y’
2+Dz’z’2)0.5
Dx’x’
Dy’y’
Dz’z’
FA = 0
FA = 0.9
FA (fractional anisotropy) MAP
FA + colorFA + color (largest diffusion direction)(largest diffusion direction)
red = right – leftgreen = anterior – posteriorblue = superior - inferior
probably no useprobably no use
TractographyTractography
Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf.
Superior view color fiber maps Lateral view color fiber maps
G
18090
G
echoTE
At the echo time TE, NMR signal is decayed by, - T2 decay (spin-spin diffusion) - diffusive motion
ijij2 DbTTEoij eeSbTES /),(
b D G D Gij ij i ij j ( / )
b G2 ( / )
For any set of diff. gradient pulses
Signal loss Signal loss : : by intra-voxel phase dispersionby intra-voxel phase dispersion
DTI Scalar ParametersDTI Scalar Parameters
TraceTrace: Magnitude of : Magnitude of diffusion in a voxel.diffusion in a voxel.– Increases in damaged white Increases in damaged white
mattermatter
Fractional Anisotropy (FA)Fractional Anisotropy (FA): : Measure of directionally-Measure of directionally-restricted diffusion.restricted diffusion.– Decreases in damaged white Decreases in damaged white
mattermatter
Rosenbloom M, et al. (July 2004). NIAA pubs; http://www.niaaa.nih.gov/publications/arh27-2/146-152.htm
The Diffusion Tensor, DThe Diffusion Tensor, DDiffusion is not equal in all directions (anisotropic).Diffusion is not equal in all directions (anisotropic).Use this to probe brain structure!Use this to probe brain structure!Represent the diffusion pattern at each point in Represent the diffusion pattern at each point in the brain using an ellipsoid.the brain using an ellipsoid.
Diffusion Vector (Colour) MapDiffusion Vector (Colour) Map
The three magnitudes The three magnitudes of the diffusion ellipsoid of the diffusion ellipsoid can be shown using can be shown using three colours (RGB).three colours (RGB).– Red = Left – RightRed = Left – Right– Green = Ant. – Pos.Green = Ant. – Pos.– Blue = Sup. – Inf.Blue = Sup. – Inf.
Map of major directions Map of major directions of water movement in of water movement in the brain.the brain.
Fibre TractographyFibre Tractography
In principle, the In principle, the locations of major locations of major white matter fibre white matter fibre tracts in the brain tracts in the brain can be mapped can be mapped using the using the information in the information in the colour map, by colour map, by “following the “following the arrows.”arrows.”
without hindrance
with hindrance
Hindered Diffusion(diffusion ellipsoid)
WILSON
Information available through DTI – Orientation of λ1
• Useful for following white matter tracts
D)(
21A D
Information available through DTI -- Aσ
• Related to the shape of the ellipsoid
• Independent of Dav (normalized)
• Zero for a sphere, positive for other shapes
• Sensitive to myelination and cortical development
av
Diffusion Tensor Imaging (Diffusion Tensor Imaging (AA))Normal Adult Brain
(A maps)
Higher diffusion lower signal
Tissue Sample ATissue Sample A Tissue Sample BTissue Sample B
Freely Diffusing Water = DarkFreely Diffusing Water = Dark Larger DLarger D
Restricted Diffusion = BrightRestricted Diffusion = Bright Smaller DSmaller D
CELLCELL
EXTRA-CELLULAR SPACEEXTRA-CELLULAR SPACE
FREELY DIFFUSING WATER INFREELY DIFFUSING WATER INEXTRA-CELLULAR SPACEEXTRA-CELLULAR SPACE
K-space view of the spin echo imaging
Kx
Ky
123.......n
X Diffusion-WeightingX Diffusion-WeightingY Diffusion-Y Diffusion-WeightingWeighting Z Diffusion-WeightingZ Diffusion-Weighting
GGFEFE
GGPEPE
GGSSSS
RFRF
Diff. Grad. along different axisDiff. Grad. along different axis
SS
PE
FE
DTIDTI (Diffusion Tensor Imaging) (Diffusion Tensor Imaging)
courtesy of Dr Sorensen, MGH, Boston
David Porter - November 2000
single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions Higher diffusion lower signal
Conventional TConventional T22 WI WI DW-EPIDW-EPI
Detection of Acute StrokeDetection of Acute Stroke“Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center
Sodium ion pumps fail, water goes in cells and can not diffuse.
TumorTumor
T2 (bright water)DWI (x direction)
(T2 (bright water)+diffusion)
T1 + Gadolinium
90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
G
-G
Do X, Y , and Z at the same timeDo X, Y , and Z at the same time
The Diffusion Tensor, DThe Diffusion Tensor, DDiffusion is not equal in all directionsDiffusion is not equal in all directions
(anisotropic).(anisotropic).Use this to probe brain structure!Use this to probe brain structure!Diffusion ellipsoid for each voxelDiffusion ellipsoid for each voxel
x
yz
ijij2 DbTTEoij eeSbTES /),(
b D G D Gij ij i ij j ( / )b G2 ( / )
DTI Scalar ParametersDTI Scalar Parameters
TraceTrace: The magnitude of diffusion in a : The magnitude of diffusion in a voxel.voxel.
Fractional Anisotropy (FA)Fractional Anisotropy (FA): The extent to : The extent to which diffusion is directionally restricted.which diffusion is directionally restricted.