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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

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dti basics water diffusion dti diffusion tensor imaging
DTI Basics – Water Diffusion(DTI – Diffusion Tensor Imaging)

Einstein on

Brownian

Motion

1905 five

important papers

slide2

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
Why USE DTI MRITumor

T2 (bright water)

T2 (bright water)

DWI (x direction)

(T2 (bright water)+(diffusion))

Contrast

(T1 + Gadolinium)

slide5

1st level of complexity

Diffusion Weighted Image X direction

  • Higher diffusion in X direction  lower signal

Artifact or Abnormality

David Porter - November 2000

slide6

Time

T2 + diffusion

T2

Sequence

RF

Gx

-

Gy

Gz

T2

Image

Measure

diffusion

Regular

T2 image

Excite

(gradientstrength)

slide7

2nd Level of complexity

DWI : 3 Direction

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

Dyy

Dzz

courtesy of Dr Sorensen, MGH, Boston

David Porter - November 2000

3rd level of complexity diffusion tensor imaging basics
3rd level of complexityDiffusion Tensor Imaging Basics

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
slide9

Higher diffusion  lower signal

water

Diffusion ellipsoid

Diffusion ellipsoid

White matter fibers

  • Useful for following white matter tracts in healthy brain
slide10

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
DTI ellipsoidmeasure 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.

slide12

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
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.

slide14

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
FA + color(largest diffusion direction)

red = right – left

green = anterior – posterior

blue = superior - inferior

slide16

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

slide17

Just looking at Proton Spectroscopy

  • Just excite small volume
  • Do water suppression so giant peak disappears
  • Compare remaining peaks

precession

Frequency

Frequency

slide18

MRS – Magnetic Resonance Spectroscopy

NAA = N-acetyl aspartate, Cr = Creatine, Cho = Choline

amplitude

NAA

Cr

Cho

Frequency of precession

slide19

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”
slide20

Evaluate Health of Neurons (NAA level)

Normalize with Creatine (fairly constant in brain)

Red means

High NAA/CR

levels

slide21

Epilepsy Seizures (effects metabolite levels)

  • find location
  • determine onset time
slide23

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.

important concepts
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 concepts1
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?
tractography1
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.

slide30

Signal loss : by intra-voxel phase dispersion

At the echo time TE, NMR signal is decayed by,

- T2 decay (spin-spin diffusion)

- diffusive motion

For any set of diff. gradient pulses

G

G

90

180

echo

TE

dti scalar parameters
DTI Scalar Parameters
  • Trace: Magnitude of diffusion in a voxel.
    • Increases in damaged white matter
  • Fractional Anisotropy (FA): Measure of directionally-restricted 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
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
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
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.”
slide35

Hindered Diffusion

(diffusion ellipsoid)

without hindrance

WILSON

with hindrance

slide36

Information available through DTI – Orientation of λ1

  • Useful for following white matter tracts
slide37

Information available through DTI -- Aσ

s

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 s
Diffusion Tensor Imaging (As)

Normal Adult Brain

(A maps)

slide39

CELL

EXTRA-CELLULAR SPACE

FREELY DIFFUSING WATER IN

EXTRA-CELLULAR SPACE

  • Higher diffusion  lower signal

Tissue Sample A

Tissue Sample B

Freely Diffusing Water = Dark

Larger D

Restricted Diffusion = Bright

Smaller D

slide41

PE

FE

SS

Diff. Grad. along different axis

Y Diffusion-Weighting

Z Diffusion-Weighting

X Diffusion-Weighting

GFE

GPE

GSS

RF

slide42

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

slide43

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
Tumor

T2 (bright water)

DWI (x direction)

(T2 (bright water)+diffusion)

T1 + Gadolinium

the diffusion tensor d1

y

z

x

The Diffusion Tensor, D
  • Diffusion is not equal in all directions

(anisotropic).

  • Use this to probe brain structure!
  • Diffusion ellipsoid for each voxel
dti scalar parameters1
DTI Scalar Parameters
  • Trace: The magnitude of diffusion in a voxel.
  • Fractional Anisotropy (FA): The extent to which diffusion is directionally restricted.
pulse sequence gradient echo diffusion weighting in x direction
Pulse Sequence: Gradient-EchoDiffusion Weighting in X direction

Excitation

90o

RF

G

-G

Gx

EPI (T2)

Image

Acquisition

diffusiongradients

Gy

Gz

slide51

y

Time

RF

Gx

-

Gy

Gz

x

NO DIFFUISION

protons (Hydrogen)

with DIFFUISION

signalloss

diffusion maps
DIFFUSION MAPS

weak

(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

S (T2* EPI +

weak Diffusion

in X direction)

S0(T2* EPI)

slide53

PE

FE

SS

A Little More Detail

Y Diffusion-Weighting

Z Diffusion-Weighting

X Diffusion-Weighting

y

x

z

GFE

GPE

GSS

RF

x

y

z

do x y and z at the same time

Excitation

90o

RF

G

-G

Gx

Image

Acquisition

Gy

Gz

Do X, Y , and Z at the same time
slide56

DxxDxyDxz

DyxDyy Dyz

Dzx DzyDzz

But what is a diffusion tensor? It is a mathematical description of the ellipsoid.

no diffusion

xy

-xy

y-z

xz

z

-xz

y-z

y

z

x

slide57

Dxx Dxy Dxz

Dyx Dyy Dyz

Dzx Dzy Dzz

Dx’x’ 0 0

0 Dy’y’ 0

0 0 Dz’z’

Whatisdiffusion“Tensor” (D)?

difussion gradient direction vector

y

S=S0exp(-bD)

labreferenceframe

ellipsoidreferenceframe

=

(mathematical manipulation)

Calculate FA (fractional anisotropy)

Fiber track

slide58

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

(Dx’x’2+Dy’y’2+Dz’z’2)0.5

s

av

FA = 0.9

FA = 0

slide60

X Diffusion-Weighting

Z Diffusion-Weighting

Y Diffusion Weighting

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)