Diffusion Tensor Imaging - PowerPoint PPT Presentation

Loading...

PPT – Diffusion Tensor Imaging PowerPoint presentation | free to download - id: 414aa0-NWVmM



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Diffusion Tensor Imaging

Description:

Diffusion Tensor Imaging Johansen-Berg and Rushworth 2008 Glasser and Rilling 2008 20 April 2010 Limitations Unidentified pathways probably exist Crossing fibers add ... – PowerPoint PPT presentation

Number of Views:46
Avg rating:3.0/5.0
Slides: 42
Provided by: AlbertJ150
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Diffusion Tensor Imaging


1
Diffusion Tensor Imaging
  • Johansen-Berg and Rushworth 2008
  • Glasser and Rilling 2008
  • 20 April 2010

2
Connectivity
  • Connectivity patterns define functional networks
  • Passingham et al. (2002)
  • Connectional anatomy
  • Boiling in oil
  • Dissection
  • Myelin stains
  • Axonal stains
  • Retrograde and anterograde tracers

3
Diffusion-weighted MRI
  • Acquisition of multiple images
  • Each image is sensitized to a different direction
    so multiple measures are associated with each
    voxel
  • Fit a mathematical model such as diffusion tensor
    imaging
  • Each voxel is described by an ellipsoid, or
    tensor
  • Three orthogonal eigenvectors and associated
    eigenvalues (?1, ?2, ?3)

4
Diffusion-weighted MRI
  • Self-diffusion of water molecules
  • Water is directionally dependent in tissue with
    directional structure

5
Fractional Anisotropy
  • In the ventricles, diffusion should be similar in
    each direction which is associated with a
    spherical tensor
  • Fiber bundles have more barriers and the
    associated tensor is elongated
  • Directional dependency (Anisotropy) is quantified
    by fractional anisotropy
  • 0 (fully isotropic) to 1 (fully anisotropic)
  • White matter 0.6-0.8

6
Diffusion Anisotropy
  • Directional estimates of principal diffusion
    direction form the basis of diffusion
    tractography
  • This is NOT anatomical
  • Functional connectivity is just the correlation
    between two or more active areas

7
Deterministic vs. Probabilistic Models
  • Difficult to interpolate beyond the end of the
    fiber bundle
  • One can assign a probability to the direction of
    each voxel
  • Sampling is used to create streamlines which are
    used to represent interconnectivity

8
Strengths and Weaknesses
  • Trace cortico-cortical networks
  • Parietal and premotor cortex
  • Fine-grained spatial mapping
  • Thalamic connections
  • Difficult to determine connection strength
  • Anterograde or retrograde?
  • Accurate detection of connections
  • Validation biologically realistic phantoms

9
DTI Tractography of the Human Brains Language
Pathways
  • Glasser and Rilling 2008
  • http//cercor.oxfordjournals.org/cgi/content/full/
    bhn011/DC1

10
DTI and Language
  • Arcuate Fasciculus
  • Originates in temporal lobe
  • Curves around Sylvian fissure
  • Projects into the frontal lobe
  • Connects Brocas area and Wernickes area
  • Dejerine 1895
  • Leftward asymmetry in the arcuate fasciculus

11
Three Components of Langauge
  • Phonemes
  • Wernickes area and BA 40
  • Posterior Brocas area
  • Bilateral
  • Prosody
  • Right MTG
  • Frontal lobe
  • Bilateral?
  • Lexical-semantics
  • Concepts and meanings
  • Middle and temporal areas (BA 21 and 17)
  • Brocas area and areas more anterior and frontal
  • Left localized

12
Hypotheses
  • Bilateral arcuate connection to the superior
    temporal gyrus
  • Leftward asymmetry connection to middle temporal
    gyrus

13
Methodology
  • No behavioral tasks
  • A priori reasons to think phonology,
    lexico-semantics, and prosody have distinct
    locations
  • Determine the connections using a deterministic
    algorithm
  • Corroborate the connections using outside sources

14
Subjects
  • 20 right-handed males
  • 18-50 (23.75, 7.1)
  • Handedness and sex influence laterality

15
MRI
  • Diffusion MRI
  • TE 90 ms
  • TR 7700 ms
  • 12 diffusion directions
  • 1.7 x 1.7 x 2.0 mm voxels
  • T1-weighted
  • TE 4 ms
  • TR 2300 ms
  • 256 x 256 matrix

16
Deterministic Tractography
  • Siemens DTI Task Card Parameters
  • of samples/voxel length 8
  • Minimum FA threshold 0.15
  • Maximum turning angle 15
  • Step length between calculations 0.25 mm

17
Deterministic Tractography
  • Step 1
  • Select Arcuate Fasciculus as ROI in left and
    right hemispheres from a coronal slice
  • ROIs defined as BA 22 (Wernickes), BA 21 and BA
    37 (middle temporal gyrus)
  • Determine terminations in frontal lobe ROI
  • BA 44, 45, 6, 9 (Brocas area and surrounding
    cortex)

18
ROIs
19
Arcuate Fasciculus Tractography
20
The STG Pathway
  • Left Hemisphere
  • Found in 17 of 20 subjects
  • BA 22 (posterior STG) connected to BA 44 and BA 6
  • Right Hemisphere
  • Found in 4 of 20 subjects
  • BA 22 (posterior STG) connected to BA 44 and BA 6

21
The MTG Pathway
  • Left Hemisphere
  • Found in 20 of 20
  • BA 21 and BA 37 projected to
  • BA 44
  • and parts of BA 6, 9, 45
  • Right Hemisphere
  • Found in 11 of 20
  • BA 37 (posterior MTG)

22
Functional Activations
  • Left Hemisphere
  • Phonological Studies overlapped with STG pathway
  • Lexical-Semantic Studies overlapped with MTG,
    ITG, and AG segments
  • Right Hemisphere
  • Phonological Studies did not overlap (more
    anterior) with STG pathway
  • Prosody Studies overlapped MTG and STG segments

23
Asymmetries in Connections
  • Phonology is usually bilaterally activated more
    activation found on the left side than usual
  • Lexical-Semantics is usually activated on the
    left more activation found on the right than
    usual

24
Hickok and Poeppel (2004) and Price (2000) Model
Left
25
Hickok and Poeppel (2004) and Price (2000) Model
Right
26
Aphasia
  • Can we explain aphasia using the Hickok and
    Poeppel (2004) and Price (2000) model with
    additional information added from the DTI study?

27
Brocas Aphasia
  • Symptoms
  • Difficulty producing grammatical speech
  • Slow and halting speech
  • Able to communicate
  • Difficulty with phoneme discrimination
  • Location of lesions
  • Left inferior and premotor cortex (Dysarthria)
  • BA 44 and BA 45 (agrammatism)
  • Frontal cortical terminations

28
Wernickes Aphasia
  • Symptoms
  • Fluent speech
  • Frequent phonological and semantic errors
  • Difficulty understanding speech
  • Location of lesions
  • Left STG or subcortical connections
  • Middle and inferior temporal cortex
  • Disruption of phonological decoding

29
Conduction Aphasia
  • Symptoms
  • Difficulty with repetition
  • Phonological errors
  • Difficulty naming objects
  • Intact comprehension
  • Spontaneous speech
  • Location of lesions
  • Left supramarginal gyrus (BA 40)
  • Arcuate fasciculus?
  • Superficial to avoid damaging the MTG pathway

30
Conduction Aphasia
  • Symptoms
  • Difficulty with repetition
  • Difficulty naming objects
  • Phonological errors
  • Intact comprehension
  • Spontaneous speech preserved
  • Location of lesions
  • Left supramarginal gyrus (BA 40)
  • Superficial arcuate fasciculus

31
Transcortical Motor Aphasia
  • Symptoms
  • Limited spontaneous speech
  • Impaired naming
  • Intact repetition
  • Normal articulation
  • Good auditory comprehension
  • Location of lesions
  • Anterior and superior to Brocas area
  • White matter near MTG pathway

32
Transcortical Sensory Aphasia
  • Symptoms
  • Intact repetition
  • Lack of understanding
  • Location of lesions
  • Left temporal lobe lesion which spares Wernickes
    area
  • Lesions in middle and inferior temporal lobe
  • Electrical stimulation of LMTG and LITG causes
    transient aphasia

33
Aprosodias
  • Symptoms
  • Difficulty with comprehension of emotional
    prosody
  • Location of lesions
  • Right temperoparietal lesions
  • Right middle and temporal cortex
  • Subcortical

34
Limitations
  • Unidentified pathways probably exist
  • Crossing fibers add noise
  • Deterministic vs. Probabilistic
  • Heterogeneity of functional areas
  • Only three components of language

35
Unexpectations
  • Lexical-semantic and STG pathways project to BA 6
  • Fewer projections to BA 45 and BA 47 using a
    deterministic methodology than a probabilistic
    methodology
  • More anterior with probabilistic
  • These differences could be explained if the
    arcuate fasciculus was combined with the SLF

36
Conclusions
  • Arcuate fasciculus connects the STG and MTG with
    the inferior frontal lobe
  • Phonological processing associated with left STG
  • Lexical-semantic processing associated with left
    MTG
  • Prosody processing associated with right MTG and
    STG

37
Hickok and Poeppel (2004) and Price (2000) Model
Aphasia
38
Questions
  • Marianna
  • How can they find evidence of activation of
    different areas?
  • Lucy
  • What would these models predict for people who
    are relatively more bilateral? Just more/higher
    incidence of pathways in the right hemisphere?

39
Questions
  • Israel
  • How do they know that the connections they find
    are related to these linguistic areas and not to
    the other linguistic areas?
  • Pawel
  • Couldn't familial left-handedness explain the
    results showing that they were unable to find MTG
    pathways in the right hemisphere of some subjects
    and the STG pathway in the left hemisphere of
    others?

40
Questions
  • Lynn
  • Could the motor theory or elements of it be
    useful in interpreting any of the current results
    including visual information and the right
    hemisphere?

41
Neat Website
  • http//white.stanford.edu/brian/papers/mri/2006-W
    andell-NIPS-Tutorial.pdf
About PowerShow.com