National Alliance for Medical Image Computing Diffusion Weighted MRI.

National Alliance for Medical Image Computing http://na-mic.org

Diffusion Weighted MRI


Diffusion Imaging

S. Maier and M. Kubicki

•In an isotropic medium (inside a glass of water for example) water molecules naturally move according to Brownian motion.

•In biological tissues however the diffusion is very often anisotropic. For example a molecule inside the axon of a neuron has a low probability to cross a myelin membrane.

•Apply gradient in a given direction dark regions correspond to where non Brownian motion has occured.


DTI allows researchers to quantify the diffusion of water in brain tissue

Diffusion for each image voxel is described by 3 perpendicular vectors

1

2

3

1

2

3

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


Anisotropy

•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.


Fractional AnisotropyFractional Anisotropy

Difference in eigenvalues

Directional diffusion

1+ 2 + 3 3

Mean DiffusivityMean Diffusivity

Addition of eigenvalues

Overall diffusion

Diffusivity & Fractional Anisotropy


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


Fiber Tracking


Axonal Fibers Using 3T MR-DTI

Hae-Jeong Park, Ph.D.

Whole-Brain Tractography


Automated tools separate fibers on the basis of their shape and projections

Fiber Clustering


Fronto-temporal connections

uncinate fasciculus (blue)

inferior occipito-frontal fasciculus (yellow)

IOCC rarely seen in the literature (from Gray’s Anatomy)


Other applications: Neurosurgery, Brain Tumors


White matter fiber tracts and Schizophrenia

• There 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.


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.

M. Kubicki


Schizophrenia related symptoms most frequently linked with cingulate dysfunction

• Thought disorder • Disorganized behavior• Hallucinations• Flattening of affect• Delusions• Lack of attention

M. Kubicki


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.

M. Kubicki

Anisotropy Analysis


300

350

400

450

500

550

right left

RA

normal controls

schizophrenics

M. Kubicki

Relative anisotropy within cingulate fasciculus

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.


Fiber Tractography

H.J. Park


Fiber Clustering

H.J. Park


Fiber Tractography

H.J. Park


Research Challenges

• Diffusion Sequences and Image Resolution.• Fiber Tractography.• White Matter Segmentation.• Statistical Analysis.• Current Region Of Interests:

– Superior Temporal Gyrus.– Uncinate Fasciculus.– Cingulate Bundle.– Arcuate Fasciculus.


STEP 1

STEP 2

Initial coarse registration done manually

with Slicer Alignment Module.

no gradient DTI gray-aligned SPGR

+

Automatic fine registration using

registration module, with Mutual

Information as the registration metric.

Coregistration of MRI and DTMRI


Non Linear Registration Module

Alexandre Guimond, Lifeng Liu (CNI), Matthan Caan (LMI).


Coregistration: Qualitative Evaluation.

Magenta: MRI (SPGR)

Green: DTMRI (T2 baseline)


Coregistration: Pilot Study Results

left rightgray white gray white

Case3 FA 0.264 0.360 0.254 0.402AA 0.383 0.516 0.379 0.574RA 0.158 0.223 0.151 0.251

ADC 0.817 0.771 0.839 0.763Case45 FA 0.226 0.350 0.261 0.338

AA 0.339 0.467 0.399 0.448RA 0.134 0.215 0.156 0.207

ADC 0.849 0.725 0.874 0.721Case117 FA 0.258 0.375 0.242 0.374

AA 0.396 0.519 0.354 0.514RA 0.154 0.232 0.144 0.232

ADC 0.850 0.743 0.820 0.737Case0590 FA 0.238 0.346 0.221 0.354

AA 0.362 0.492 0.328 0.498RA 0.141 0.212 0.131 0.218

Measurements:FA: Fractional Anisotropy, AA: Absolute Anisotropy, RA: Relative Anisotropy, ADC: Apparent Diffusion Coefficient.

Kang-Uk Lee

National Alliance for Medical Image Computing Diffusion Weighted MRI.

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