diffusion weighted mriibruce/courses/ee3ba3...2 outline. y. mri quick review. y. what is diffusion...
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Diffusion Weighted MRI
Zanqi
Liang & Hendrik
Poernama
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Magnetic fieldMagnetic fieldTissue protons align with magnetic field(equilibrium state)
Tissue protons align with magnetic field(equilibrium state)
RF pulsesRF pulses
Protons absorbRF energy (excited state)
Protons absorbRF energy (excited state)
RelaxationprocessesRelaxationprocesses
Protons emit RF energy(return to equilibrium state) Protons emit RF energy(return to equilibrium state)
NMR signaldetectionNMR signaldetection
RepeatRepeat RAW DATA MATRIXRAW DATA MATRIX Fourier transformFourier transform IMAGEIMAGE
Relaxation processesRelaxation processes
Spatial encodingusing magneticfield gradients
Spatial encodingusing magneticfield gradients
1. MRI Procedure
1.Magnetic Field2.Radio-Frequency Pulse3.Relaxation
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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What is the “D” in DMRI?
Diffusion‐ The spontaneous spreading of matter (particle), heat or momentumRate of concentration change proportional to diffusion coefficient
Water makes up 60–80% of our body weight.
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Diffusion and MRIWhen the patient enters the large tunnel of a static magnetic field, nuclear spins (small magnets inside each proton nucleus) are lined up along the direction of the big magnetMagnetic field gradients of certain duration will then add a smaller magnetic field to spins located in different regions within the tissue. (Magic ink)
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Conventional MRI and DMRIConventional MRIMeasures the phase changed due to local magnetic properties of the surrounding tissueCan distinguish different types of tissue. Eg: liver, fat, muscle and water...
Diffusion MRIMeasures the phase changed due to the changed position of individual spinsMore sensitive to cellular changes than conventional MRI sequences
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Detecting DiffusionParticles move around randomly (Brownian Motion) Average displacement determined by diffusion coefficientGiven observation time and displacement, the diffusion coefficient could be calculatedMovement of particles attenuates signal in addition to T2 and T1 relaxations in a magnetic field gradient
Angular frequency depends on magnetic fieldMovement of particles decreases phase coherence much like T2 relaxation
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Diffusion Represents Molecular Events
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Magnetic Field Gradient
Field inhomogeneities exist in traditional MRI, interfering with T2 signalApplying stronger field gradient amplifies diffusion effectGradient timing
Constant gradientPulsed gradient
)(0
ADCbeSS ×−×=•Diffusion weighted image (Also called trace image)measure s S•ADC image weighted image measures ADC
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Figure 1.1 A typical pulse sequence for diffusion imaging.The shaded areas represent field gradient pulses.DW diffusion weighted,TE time evolution
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Apparent Diffusion Coefficient (ADC)
Water self‐diffusion is constantWater movement is restricted by tissuesSince brain cerebrospinal fluid (CSF) contains water that can move around freely, its ADC value is much higher than that of other brain tissues (either gray matter or white matter)
Low ADC gives strong echo, High ADC gives low echo
)(0
ADCbeSS ×−×=
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Acute Cerebral Ischemia
Ischemia: Lack of blood supplyIschemic tissues have lower ADCCauses:
Blood clotShock (excessive bleeding, heart failure)
Consequences:Stroke symptomsPermanent CNS damage (CNS does not regenerate) High risk of death
Treatments must be rapid to prevent permanent damage
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Importance of Diffusion‐weighted MRI
Required to decide whether risky treatments are necessary
Localization of AttackMeasuring SeverityDistinguish hemorrhage from Ischemia
CT and T2‐wighted MRI shows damage 5‐6 hours after attackDiffusion‐weighted MRI detects within minutes
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Acute Ischemia
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EdemaStroke can also be due to edema. Edema means water molecule transfer from extracellula to intracellula. More water molecule is restricted leads to decreasing ADC
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Edema•Stroke due to Edema•Lesioned neurons:
•Decreased ADC •Increased Trace•T2 and CT does not show the damage
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Tumor
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Diffusion Anisotropy
Cell membranes decreases diffusion rate
Diffusion rate at a point is direction dependent
Axons act like pipes, myelin amplifies this effect
Consequences:Diffusion must be scanned in all 3 axesDiffusion rate is represented as vector (modeled as ellipsoid)
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Diffusion Tensor Imaging
Scanning in 3 axes is not sufficientHead alignment causes inconsistent dataSolution:
Use a tensor matrix and scan from 6 directionsCalculate eigenvalues
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
=
zzzyzx
yzyyyx
xzxyxx
DDDDDDDDD
D
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Tensor Calculations
0)( =− XID λ
0=−
−−
λλ
λ
zzzyzx
yzyyyx
xzxyxx
DDDDDDDDD
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DTI allows researchers to quantify the diffusion of water in brain tissue
Diffusion for each image voxel is described by 3 perpendicular vectors
λ1λ2
λ3
λ1
λ2
λ3
Isotropic diffusion occurs when there is no restriction to water movement (e.g., ventricles, CSF)
Anisotropic diffusion occurs when water movement is restricted to one primary direction (e.g., myelinated fibers)
Diffusion Tensors & Anisotropy
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Diffusion Ellipsoids
Diffusion ellipsoids
reconstructed from real DTI
data.
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Fractional AnisotropyFractional Anisotropy
Difference in eigenvalues
Directional diffusion
Mean Diffusivity (ADC)Mean Diffusivity (ADC)
Addition of eigenvalues
Overall diffusion
Mean Diffusivity & Fractional Anisotropy
( ) ( ) ( )23
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λλλ
λλλλλλ
++
−+−+−=FA
3321 λλλλ ++
=
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Color
Coded Direction
Color map used to indicate dominant diffusion direction
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Left: Conventional T2W image does not show white matter fiber tracts in the brain.
Middle: Anisotropy map highlights the white matter bundles in the brain.
Right: The z-map high intensity regions correspond to large out of plane diffusion.
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Number of fibers
Size of fibers
Myelination of fibers
Directionality of Fibers
Diffusivity and FA help determine the number, size and myelination of fibers, whereas only FA gives information about directionality.
High Diffusivity Low FA
High FA Low Diffusivity
High Diffusivity Low FA
High FA Low Diffusivity
High Diffusivity Low FA
High FA Low Diffusivity
Low FA Same Diffusivity
High FA Same Diffusivity
Interpreting Diffusivity and FA
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Tumor
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Cyst
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Fiber Tracking
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Whole‐Brain Tractography
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Application of Fiber
Tracking
White matter researchWhite matter disruption due to tumorDiseases related to fiber dysfunction
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Other applications: Neurosurgery, Brain Tumors
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White matter fiber
tracts and SchizophreniaThere are widespread gray matter deficits reported in MRI structural studies, but fewer reports of white matter abnormalities.Functional abnormalities are reported in different brain regions and different systems using fMRI and PET.Several theories link schizophrenia with disconnection between different brain regions.
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Cingulum Bundle
The most prominent connection between limbic structures.Consolidates information by interconnecting thalamus, prefrontal, parietal, temporal lobes (including amygdala, hippocampus and parahippocampal gyrus) with cingulate gyrus.
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Schizophrenia related symptoms most frequently linked with cingulate
dysfunction
Thought disorder Disorganized behaviorHallucinationsFlattening of affectDelusionsLack of attention
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Coronal sections of diffusion tensor maps show cingulate fasciculi (out of plane diffusion component- coded in orange)(arrows), above the corpus callosum (in plane component- coded in blue). Patient with schizophrenia on the left, comparison subject on the right. Note the difference in area of the bundle.
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300
350
400
450
500
550
right left
RA
normal controls
schizophrenics
Diffusion anisotropy within the left cingulum bundle in schizophrenia group was 7.4 % lower than in normal comparison subjects (mean of the percentage difference for all eight slices), while diffusion anisotropy on the right side within the CB in schizophrenics was only 2 % lower than in normal comparisons.
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OutlineMRI Quick ReviewWhat is Diffusion MRI?Detecting Diffusion
Stroke and Tumor DetectionPresenting Diffusion
Anisotropy and Diffusion TensorCyst Detection
Fiber TractographyTumor Surgery and Schizophrenia
Future Research Challenges
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Research ChallengesImage Resolution. (Limited by gradient strength) White Matter Segmentation.Statistical Analysis.Current Region Of Interests:
Superior Temporal Gyrus.Uncinate Fasciculus.Cingulate Bundle.Arcuate Fasciculus.
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Fiber Clustering
Automated tools separate fibers on the basis of their shape and projections
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Conclusion
The concept behind Diffusion MRI is relatively simple, yet there are many different applications utilizing this
technology.
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ReferencesGillard, Jonathan, Adam Waldman, and Peter Barker. Clinical MR Neuroimaging. Cambridge: Cambridge University Press, 2005.http://www.uchsc.edu/neuroimaging/diffus/diffus.htmhttp://www.jubileum.lu.se/MR_physics/molecular%20motion.htmYoshiura T,Wu O, Sorensen AG (1999) Advanced MR techniques: Diffusion MR imaging,perfusion MR imaging, and Spectroscopy. Neuroimaging Clin N Am 9:439–453Chun T, Filippi CG, Zimmerman RD, Ulug AM (2000) Diffusion changes in the Edemic human brain. Am J Neuroradi-ol 21:1078–1083Engelter ST, Provenzale JM, Petrella JR, DeLong DM,Mac-Fall JR (2000) The effect of stroke on the apparent diffusion coefficient of normal-appearing white matter. Am JRoentgenol 175:425–430Helenius J, Soinne L, Perkio J (2002) Diffusion-weighted MR imaging in normal human brains in various age groups. Am J Neuroradiol 23:194–199Gideon P, Thomsen C, Henriksen O (1994) Increased self-diffusion of brain water. J Magn Reson Imaging 4:185–188