Diffusion Tensor Imaging and Reading Development

Transcription

1 Diffusion Tensor Imaging and Reading Development Bob Dougherty Stanford Institute for Reading and Learning

2 Reading and Anatomy Every brain is different... Not all brains optimized for highly proficient reading?

3 Background Diffusion of water through a temporoparietal white matter region is correlated with reading skill in adults Reading score (Klingberg et al., Neuron, 2000) Diffusion Anisotropy (FA)

4 Two Outstanding Questions Cause or result of reading? Measure children before reading experience differs Lesion or displacement? Measure tract positions Measure diffusion along major tracts

5 Diffusing Water Probes Microscopic Tissue Structure Tissue structures affect diffusion MR diffusion measure (DTI) hints at microscopic structure within voxel Diffusion through white matter probes: density of axons degree of myelination average fiber diameter directional similarity of axons

6 Diffusion in the Brain Apparent Diffusion Coefficient (ADC) is measured in 6 directions Unimpeded direction- large ADC White matter fibers H2O Impeded direction- smaller ADC H2O

7 DTI- Scalar Indices Fractional Anisotropy Normalized variance of elipsoid axis magnitudes (Basser & Pierpaoli, 1996) FA=0 for sphere, FA=1 for tube Coherence Index Variance in principal direction measured across array of voxels Random: CI=0, aligned: CI=1

8 DTI: White Matter Structure T1 Fractional Anisotropy

9 Behavioral Measures of Reading Normal readers vs. Poor readers Groups Matched on: Nonverbal IQ SES, handedness, sex Differed on: Reading (W-J letter-word id & word attack, Passage comp., reading fluency) Rapid naming Phonological processing (CTOPP)

10 Data Analysis Spatially normalize FA & CI images Normalize inplane to MNI EPI template Apply params to FA & CI Search in volume of interest Map significant regions back to unnormalized brains Analyze diffusion direction maps

11 FA & CI Correlated with Reading Skill in Children Deutsch, Dougherty, Bam m er, Siok, Gabrieli, Wandell (in press). Cortex.

12 Reading & FA in Children r = 0.62 (p = 0.017) Poor readers Normal readers

13 Two Outstanding Questions Cause or result of reading? Difference present in children- cause Lesion or displacement? Measure tract positions Measure diffusion along major tracts

14 Lower FA & CI Could Mean Microstructure ( lesion ) Less myelination Lower fiber density More disorganized fibers Macrostructure ( displacement ) Displaced fiber-bundle paths Known anatomical differences related to reading: sulcal patterns, corpus callosum...

15 Two Outstanding Questions Cause or result of reading? Difference present in children- cause Lesion or displacement? Measure tract positions Measure diffusion along major tracts

16 Lesion or Displacement?

17 H1: Intra-pathway Lesion

18 H1: Intra-pathway Lesion

19 H2: Pathway Displacement

20 H2: Pathway Displacement

21 Principal Diffusion Direction is Related to FA r=0.5 Poor readers Normal readers Elevation: angle from ac-pc plane

22 Two Outstanding Questions Cause or result of reading? Difference present in children- cause Lesion or displacement? Tract positions differ FA & CI along major tracts do not differ displacement

23 Tracing Virtual Fibers

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46 Conclusions White matter differences between good and poor readers are present in children- important for reading Differences likely due to differing organization of neural pathways rather than tissue microstructure Involving: Superior Longitudinal Fasiculus, Corona Radiata and perhaps posterior Corpus Callosum

47 Thank You! Gayle Deutsch- SIRL & Neurology Roland Bammer- Radiology Mark Eckert- Psychiatry Wai Ting Siok- Psychology John Gabrieli- Psychology Brian Wandell- SIRL & Psychology

48 Ongoing & Future Efforts Combine DTI & fmri T1 anatomical measurements Tractography Validation Longitudinal Study of Reading development LR, 11yrs old

49 Reading and Lateriazation Corpus callosum differences Poor readers have: smaller bending angle Habib, 1998) (Robichon & Larger isthmus & splenium et. al. 1996) (Rumsey Laterialization? Intra-hemispheric connections? splenium isthmus truncus genu rostrum

50 DTI: How it works Bipolar gradient pulse ( diffusion-weighting ) Pulse pair has no net effect on stationary spins Second pulse undoes first Spins moving along gradient are not rephased by second pulse and end up phase-shifted Phase-shift distance moved during time T acquisition RF G t Gradient T