Download presentation
Presentation is loading. Please wait.
Published byStephen Horton Modified over 9 years ago
1
Introduction to Diffusion-weighted Imaging Joelle Sarlls, Ph.D. NIH MRI Research Facility National Institute of Neurological Disorder and Stroke National Institutes of Health
2
Motivation Magnetic resonance imaging provides information about the spatial distribution of water. Diffusion-weighted MRI (DWI) provides information about the motion of water. DWIs are sensitive to cellular architecture and tissue integrity. DWI can provide quantitative measures that are directly comparable. Diffusion imaging can be used to identify specific white matter tracts Over 1000 publications combining fMRI and DWI –120 since I gave this lecture last years ago
3
Outline What is diffusion? How do we measure diffusion in MRI? How do we extract directional information? What are the practical problems and limitations? Beyond the diffusion tensor
4
Outline
5
Diffusion Diffusion refers to the random translational (Brownian) motion of molecules that results from the thermal energy of theses molecules (for sphere) Stokes-Einstein
6
Gaussian Distribution 1D2D3D Large number of particles that are free to diffuse have a squared displacement of a Gaussian form Einstein, A. Ann Physik (1905) 4: 549-590
7
Diffusion For H 2 O at 37 o C D ≈ 3.0x10 -3 mm 2 /s T dif ≈ 30 ms r ≈ 25 μm If the motion of water is hindered by cell membranes, macromolecules, etc. the displacement will be less and D will appear lower.
8
Outline What is diffusion? How do we measure diffusion in MRI? How do we extract directional information? What are the practical problems and limitations? Beyond the diffusion tensor
9
Image Intensity in MRI Physical property of tissue water –ρproton density –T1relaxation time –T2relaxation time –T2*relaxation time –Ddiffusion coefficient Experimentally controlled parameters –Sequence Spin-echo/gradient echo –TRTime of Repetition –TETime to echo –b-valuediffusion-weighting factor Rotational motion, Magnetic field strength } Concentration of water Translational motion
10
Frequency ω0ω0 0+Z-Z GzGz Gradients make the resonance frequency a function of spatial position ω = γ B = γ B 0 + γ zG z
11
Basic Diffusion-weighting 90° +x S total -x 0 GxGx
12
Phase Twist +Z -Z 0
13
Basic Diffusion-weighting 90° +x S total -x 0 stationary moving GxGx
14
b-value = 1000 s/mm 2 b-value = 0 s/mm 2 Guess the intensity
15
Spin-echo Diffusion Preparation 90°180° RF G diff Δ G Stejskal, EO and Tanner, JE. J Chem Phys (1965) 42 : 288-292 echo
16
DWI Non-diffusion-weighted signal intensity B-value sec/mm 2 Diffusion Coefficient mm 2 /sec
17
Typical DWI Single-shot “spin-echo” Echo Planar Imaging ParameterValueComment TE50-100msLimited by b-value TR>5sFully relaxed Matrix96 x 962.5 x 2.5 mm Slice Thickness2.5 mmEqual dimensions B-value~1000 s/mm 2 For brain* *Jones D., et al. Mag Res Med (1999) 42 : 515
18
8 200400650 b (s/mm 2 ) Diffusion map S = S 0 e -bD Calculate Diffusion Parameters
19
b = 0 s/mm 2 I 0 b = 1100 s/mm 2 Gz Dz
20
8 μm EM of mouse corpus callosum Water Diffusion in Tissue D perp << D par D perpendicular D parallel X Not Free Cell membranes Myelin Organelles Extracellular matrix Anisotropy
21
ADC b = 0 s/mm 2 Gz Gy Gx
22
b = 0 s/mm 2 720 s/mm 2 Diffusion Map 0.75 x 10 -3 mm 2 /s0.43 x 10 -3 mm 2 /s Acute Stroke Warach S., et al. Ann Neurol (1995) 37:231-241
23
Outline What is diffusion? How do we measure diffusion in MRI? How do we extract directional information? What are the practical problems and limitations? Beyond the diffusion tensor
24
8 μm EM of mouse corpus callosum Water Diffusion in Tissue D perp << D par D perpendicular D parallel X Not Free Cell membranes Myelin Organelles Extracellular matrix Anisotropy
25
Anisotropic Diffusion
26
The Diffusion Tensor x y z Basser, P, et. al. J Magn Reson B (1994) 3 : 247-254
27
b = 0 s/mm 2 GxGyGz GxyGxz Gyz DTI b = 1100 s/mm 2
28
Calculate Diffusion Tensor
29
Diagonalize DT x y z ε3ε3 ε2ε2 ε1ε1 EigenvaluesEigenvectors
30
Quantitative Parameters Fractional Anisotropy 0 ≤ FA ≤ 1 Average Diffusivity x y z ε3ε3 ε2ε2 ε1ε1 <D><D>
31
isotropicanisotropic
32
FA
33
Directional Encoding for DTI z x y ε3ε3 ε2ε2 ε1ε1 Z X Y Pajevic S. and Pierpaoli C., Magn Reson Med (1999) 43 : 526-540
34
Directional Encoded Color Map
35
FA DECNo sym DEC LinearLine FieldPlanarSpherical
36
Applications of DTI Cerebral Ischemia (Stroke) Brain Cancer and Effects of Radiotherapy Multiple Sclerosis Epilepsy Metabolic Disorders Normal Brain Maturation and Aging Traumatic Brain Injury Alzheimer’s Disease Amyotrophic Lateral Sclerosis Niemann-Pick type C Disease Dementias Connectivity
37
Pediatric DIPG T2-weightedMDFADEC
38
Outline What is diffusion? How do we measure diffusion in MRI? How do we extract directional information? What are the practical problems and limitations? Beyond the diffusion tensor
39
Typical DW SSEPI Low Resolution Distortions - Field inhomogeneities Distortions - Diffusion weighting Insensitive to Bulk motion Time Efficient PROCON
40
CON: Distortions from field inhomogeneities Non-diffusion-weighted SSEPI T2-weighted FSE SSEPI corrected
41
CON: Distortions from DW FA maps DW SSEPI volumes
42
Other Common Problems in DTI Low SNR Incomplete Fat Suppression Bulk movement Cardiac pulsation
43
Low SNR 1.7mm iso1.3mm iso2.5 mm iso
44
Low SNR 1.7mm iso 4.913 mm 3 1.3mm iso 2.197 mm 3 2.5 mm iso 15.625 mm 3
45
Low SNR 1.7mm iso 4.913 mm 3 1.3mm iso 2.197 mm 3 2.5 mm iso 15.625 mm 3
46
Incomplete Fat Suppression b=1100 s/mm 2 MDFA
47
Cardiac Pulsation Diffusion weighting in Z
48
Bulk Movement
49
Outline What is diffusion? How do we measure diffusion in MRI? How do we extract directional information? What are the practical problems and limitations? Beyond the diffusion tensor
50
What isTractography? The use of orientation information from diffusion imaging to reconstruct estimates of white matter pathways in the brain.
51
Limitation to DTI comes from partial volume effects Typical resolution for SSEPI DTI 2.5 x 2.5 x 2.5 mm Cortical projection systems of left cerebral hemisphere
52
Partial Volume Effect DT ellipsoid distribution
53
Sub-millimeter DTI
54
Beyond Standard DTI High Angular Resolution Diffusion Imaging (HARDI) –Multi-tensor models –Non-parametric algorithums DSI, Qball, SD, PAS
55
Non-parametric Algorithms fODFdistributionfODFdistribution
56
b = 0 s/mm 2
57
ACKNOWLEDGEMENTS Peter Bandettini, PhD Carlo Pierpaoli, MD,PhD Ted Trouard, PhD Lindsay Walker, MS Kathy Warren, MD Emilie Steffen Dan Handwerker, PhD THANK YOU
58
Diffusion Profile IsotropicAnisotropic
59
Inherent Motion in Living Systems ei1ei1 ei2ei2 einein kyky kxkx Diffusion-Weighted MRI S n (t) = F[ k x (t), k y n ] e i n S n (t) = F[ k x (t)+ k x n, k y n + k y n ] e i n
60
kyky kxkx PRO: insensitive to bulk motion S(t) = F[ k x (t)+ k x, k y + k y ] e i
61
DWI Non-diffusion-weighted signal intensity B-value sec/mm 2 Diffusion Coefficient mm 2 /sec Take two measures of signal and solve for D.
62
Average (Trace) Image
63
Calculated the ADC I ave b-value = 0 ADC map (mm 2 /sec)
64
Trace b = 0 s/mm 2 FA DEC Gx Gy GzGxy Gxz Gyz λ 1 +λ 2 +λ 3
65
Not all processing software is created equal! TORTOISECompetitor
66
Diffusion-Weighted MRI (DWI) Sensitizes MRI image intensity to small, thermally induced random motion of water molecules The motion of water within tissue is extremely sensitive to the microscopic architecture and integrity of the tissue For H 2 O at 37 o C D ≈ 3.0x10 -3 mm 2 /s T dif ≈ 30 ms r ≈ 25 μm Einstein, A. Ann Physik (1905) 4: 549-590
67
Identical anisotropy maintained throughout the entire voxel Anisotropy a small fraction of the voxel. Not experimentally observed Complete anisotropy, but variable orientation. Experimentally isotropic. Limitation to DTI : Spatial resolution
Similar presentations
© 2024 SlidePlayer.com Inc.
All rights reserved.