diffusion weighted mri - ece.mcmaster.caibruce/courses/ee3ba3...2 outline. y. mri quick review. y....

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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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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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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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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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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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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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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





Low FA Same Diffusivity

High FA Same Diffusivity

Interpreting Diffusivity and FA

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Tumor

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Cyst

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Fiber Tracking

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Whole‐Brain Tractography

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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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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

diffusion weighted mri - ece.mcmaster.caibruce/courses/ee3ba3...2 outline. y. mri quick review. y....

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