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DTI Basics – Water Diffusion
(DTI – Diffusion Tensor Imaging)
Einstein on
Brownian
Motion
1905 five
important papers
Why USE DTI 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
Conventional T2 WI
DW-EPI
Sodium ion pumps fail - water goes in cells and can not diffuse – DW image gets bright
(note – much later cells burst and stroke area gets very dark)
Why USE DTI MRI Tumor
T2 (bright water)
T2 (bright water)
DWI (x direction)
(T2 (bright water)+(diffusion))
Contrast
(T1 + Gadolinium)
Why DTI MRI (more recently): Fiber
Tracking
Diffusion Weighted Image X direction

Higher diffusion in X direction  lower signal
Artifact or Abnormality
David Porter - November 2000
T2 + diffusion
T2
Sequence
Time
RF
Gx
-
Gy
T2
Image
Gz
Excite
(gradient strength)
Measure
diffusion
Regular
T2 image
Measuring Diffusion in other directions
(examples)
 single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2
and diffusion gradients applied along three orthogonal directions

Higher diffusion  lower signal
Dxx
Dzz
Dyy

courtesy of Dr Sorensen, MGH, Boston
David Porter - November 2000
How can we track white matter fibers using DTI
• Measures water diffusion in at least 6
directions
• Echo-planar imaging (fast acquisition)
• Collecting small voxels (1.8 x 1.8 x 3mm),
scanning takes about 10 minutes

Higher diffusion  lower signal
water
Diffusion ellipsoid
Diffusion ellipsoid
White matter fibers
• Useful for following white matter tracts in
healthy brain

Higher diffusion  lower signal
White matter fibers
Isotropic
Anisotropic
Adapted from: Beaulieu (2002). NMR in Biomed; 15:435-455
DTI ellipsoid
measure 6 directions to describe
z
no diffusion
y
x
Ellipsoid represents magnitude of diffusion in all directions
by distance from center of ellipsoid to its surface.
Ellipsoid Image
Information available through DTI
Tract
Pierpaoli and Basser, Toward a Quantitative Assessment of
Diffusion Anisotropy, Magn. Reson. Med, 36, 893-906 (1996)
Tractography
Superior view color fiber maps
Lateral view color fiber maps
Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf.
axial
cor
sag
Diffusion Tensor Imaging data for
cortical spinal tract on right side
blue = superior – inferior fibers
green = anterior – posterior fibers
red = right – left fibers
Note tumor is darker mass on left
side of axial slice
MRISC
FA + color
(largest diffusion direction)
red = right – left
green = anterior – posterior
blue = superior - inferior
MRS – Magnetic Resonance Spectroscopy
• Proton spectroscopy (also can do C, O, Ph,.. Nuclei)
• Looking at protons in other molecules ( not water)
(ie NAA, Choline, Creatine, …….)
• Need
> mmol/l of substances
high gyromagnetic ratio ( )
• Just like spectroscopy used by chemist but includes
spatial localization
Just looking at Proton Spectroscopy
• Just excite small volume
• Do water suppression so giant
peak disappears
• Compare remaining peaks
precession
Frequency
Frequency
MRS – Magnetic Resonance Spectroscopy
NAA = N-acetyl aspartate, Cr = Creatine, Cho = Choline
amplitude
NAA
Cr
Cho
Frequency of precession
Multi – Voxel Spectroscopy (aka Chemical Shift Imaging – CSI)
• Do many voxels at once
• Can be some disadvantages with signal to noise (S/N) and “voxel bleeding”
Evaluate Health of Neurons (NAA level)
Normalize with Creatine (fairly constant in brain)
Red means
High NAA/CR
levels
Epilepsy Seizures (effects metabolite levels)
• find location
• determine onset time
Other Nuclei of interest for Spectroscopy
23Na in Rat Brain
(low resolution images are sodium 23 images)
(high resolution images are hydrogen images)
Note: This slide has nothing to do with Spectroscopy. It is a standard imaging
slide created with the 23Na nucleus and the hydrogen nucleus. It has been
included to show an example of imaging done with a nuclei other than hydrogen.
Common Metabolites used in Proton Spectroscopy
Important Concepts
•
•
•
•
•
•
•
•
•
What energies are used in each modality?
How does the energy interact with the tissue?
How is the image produced?
What is represented in the image?
What are important advantages and disadvantages of the
major imaging modalities?
What are the fundamental differences between the Xray
technologies (2D vs 3D, Radiography vs CT vs
Fluoroscopy)?
What are the two major types of MRI images (T1, T2), and
how are they different?
How are Angiograms produced (both Xray and MRI)?
Why are the advantages of combining imaging modalities?
Important Concepts
• What
does DTI, diffusion tensor imaging, measure?
• What structures that we are interested in effect DTI images?
• What does the DTI ellipsoid represent?
• How might DTI be useful for clinical application or research?
• What are we looking at with proton spectroscopy?
• What are the three major metabolites we typically measure?
• What do we “need” to be able to do proton spectroscopy?
• What might proton spectroscopy be used for?
probably no use
Tractography
Superior view color fiber maps
Lateral view color fiber maps
Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf.
Signal loss : by intra-voxel phase dispersion
At the echo time TE, NMR signal is decayed by,
- T2 decay (spin-spin diffusion)
- diffusive motion
G
For any set of diff. gradient pulses
G
S(TE, bij )  Soe
TE / T2
e
 bij Dij
bij Dij      (   / )Gi DijG j
90
180
b      (   / )G 2
echo
TE
DTI Scalar Parameters
• Trace: Magnitude of
diffusion in a voxel.
– Increases in damaged white
matter
• Fractional Anisotropy (FA):
Measure of directionallyrestricted diffusion.
– Decreases in damaged white
matter
Rosenbloom M, et al. (July 2004). NIAA pubs;
http://www.niaaa.nih.gov/publications/arh27-2/146-152.htm
The Diffusion Tensor, D
• Diffusion is not equal in all directions
(anisotropic).
• Use this to probe brain structure!
• Represent the diffusion pattern at each point in
the brain using an ellipsoid.
Diffusion Vector (Colour) Map
• The three magnitudes
of the diffusion
ellipsoid can be
shown using three
colours (RGB).
– Red = Left – Right
– Green = Ant. – Pos.
– Blue = Sup. – Inf.
• Map of major
directions of water
movement in the
brain.
Fibre Tractography
• In principle, the
locations of major
white matter fibre
tracts in the brain
can be mapped
using the
information in the
colour map, by
“following the
arrows.”
Hindered Diffusion
(diffusion ellipsoid)
without hindrance
WILSON
with hindrance
Information available through DTI –
Orientation of λ1
• Useful for following white matter tracts
Information available through DTI -- Aσ
1  ( D)
A 
D
2
av
• 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
Diffusion Tensor Imaging (A)
Normal Adult Brain
(A maps)

Higher diffusion  lower signal
CELL
FREELY DIFFUSING WATER IN
EXTRA-CELLULAR SPACE
EXTRA-CELLULAR SPACE
Tissue Sample A
Freely Diffusing Water = Dark
Larger D
Tissue Sample B
Restricted Diffusion = Bright
Smaller D
K-space view of the spin echo imaging
Ky
1
2
3
.
.
.
.
.
.
.
n
Kx
Diff. Grad. along different axis
X Diffusion-Weighting
PE
FE
SS
GFE
GPE
GSS
RF
Y DiffusionWeighting
Z Diffusion-Weighting
DTI
(Diffusion Tensor Imaging)
 single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2
and diffusion gradients applied along three orthogonal directions

Higher diffusion  lower signal

courtesy of Dr Sorensen, MGH, Boston
David Porter - November 2000
Detection of Acute Stroke
“Diffusion Weighted Imaging (DWI) has proven to be the most effective
means of detecting early strokes” Lehigh Magnetic Imaging Center
Conventional T2 WI
DW-EPI
Sodium ion pumps fail, water goes in cells and can not diffuse.
Tumor
T2 (bright water)
T1 + Gadolinium
DWI (x direction)
(T2 (bright water)+diffusion)
The Diffusion Tensor, D
• Diffusion is not equal in all directions
(anisotropic).
• Use this to probe brain structure!
• Diffusion ellipsoid for each voxel
TE / T2
 bij Dij
S(TE, bij )  Soe
e
bij Dij      (   / )Gi DijG j
b      (   / )G 2
DTI Scalar Parameters
• Trace: The magnitude of diffusion in a
voxel.
• Fractional Anisotropy (FA): The extent to
which diffusion is directionally restricted.
Unused slides from Ana516 lecture
Pulse Sequence: Gradient-Echo
Diffusion Weighting in X direction
Excitation
RF
90o
G
Gx
G
-
diffusion gradients
Gy
Gz
EPI (T2)
Image
Acquisition
Gradient
Coils
y
Time
RF
Gx
x
-
Gy
Gz
NO DIFFUISION
protons (Hydrogen)
with DIFFUISION
signal loss
DIFFUSION MAPS
(used to remove spin density, T1, T2, TR, and TE effects)
S=S0e(-bDxx)
Ln(S) = Ln(S0) – bDxx or Dxx = (Ln(S0) – Ln(S))/b
Dxx
Most Important image
S0 (T2* EPI)
weak
S (T2* EPI +
weak Diffusion
in X direction)
A Little More Detail
Z Diffusion-Weighting X Diffusion-Weighting
y

PE
FE
x
SS
z
GFE
x
GPE
y
GSS
z
RF
Y DiffusionWeighting
• Tract tracing problems (fiber crossing)
Do X, Y , and Z at the same time
Excitation
RF
90o
G
Gx
G
-
Image
Gy
Gz
Acquisition
But what is a diffusion tensor?
no diffusion
It is a mathematical description of
the ellipsoid.
xy
-xy
y-z
xz
Dxx Dxy Dxz
Dyx Dyy Dyz
Dzx Dzy Dzz
z
y-z
z
x
y
-xz
What is diffusion “Tensor” (D)?
difussion gradient direction vector
y
S=S0exp(-bD)
lab reference frame
Dxx Dxy Dxz
Dyx Dyy Dyz
Dzx Dzy Dzz
ellipsoid reference frame
=
(mathematical
manipulation)
Dx’x’ 0 0
0 Dy’y’ 0
0 0 Dz’z’
Calculate FA (fractional anisotropy)
Fiber track
FA (fractional anisotropy)
Information available through DTI
Dy’y’
Dx’x’
Dz’z’
FA = ((Dx’x’-Dav)2 + (Dy’y’-Dav)2 + (Dz’z’-Dav)2)0.5

av2)0.5
(Dx’x’2+Dy’y’2+Dz’z’
FA = 0.9
FA = 0
FA (fractional anisotropy) MAP
Apparent Diffusion Coefficient ADC (AKA TRACE)
used in clinical stroke, tumor, etc
ADC = (Dxx + Dyy + Dzz)/3
Orientation independent
No directional information (ie direction of greatest diffusion)