MR Diffusion Tensor Imaging, Tractography

MR Diffusion Tensor Imaging, Tractography Richard Watts, D.Phil. Citigroup Biomedical Imaging Center Weill Medical College of Cornell University Box 234, 1300 York Avenue, New York, NY 10021 Email riw2004@med.cornell.edu, Telephone 212 746-5781

Acknowledgements • Weill Medical College of Cornell University • Department of Radiology • Aziz Ulug, Linda Heier. • Citigroup Biomedical Imaging Center • Doug Ballon, Jon Dyke, Katherine Kolbert. • Sackler Institute • BJ Casey, Matt Davidson, Katie Thomas.

Outline • Background • Diffusion • Restricted Diffusion and Anisotropy • Methods • Data Acquisition • Display of Diffusion Tensor Data • Fiber Tracking • Problems and Limitations • Examples

Diffusion

Diffusion Equation r = Displacement (mm) D = Diffusion constant (mm2/s) t = Time (mm)

Distance Scales Question: What distance do protons travel during an EPI readout time? Assume: Diffusion constant ~ 10-3 mm2/s Time ~ 100 ms = 0.1s The root mean square (RMS) distance is ~0.02mm = 20μm Such an experiment is sensitive to changes in diffusion caused by structures on this scale or smaller

Diffusion Imaging of Leukemia

Spin Echo

180º 90º RF time  TE Diffusion Gradients g Gx  where Data Acquisition – Spin Echo Echo

Restricted Diffusion

Diffusion Ellipsoid in White Matter

Anisotropy Isotropic: Having the same properties in all directions Anisotropic: Not isotropic; having different properties in different directions Webster’s Dictionary

180º 90º RF time TE Gx Gy Gz Data Acquisition – Spin Echo Echo Linear combination of gradients - measure component of diffusion in any direction

Diffusion Tensor Imaging • Tensor is a mathematical model of the directional anisotropy of diffusion • Represented by a 3x3 symmetric matrix  6 degrees of freedom • Fit experimental data to the tensor model • From the tensor, we can calculate • Direction of greatest diffusion • Degree of anisotropy • Diffusion constant in any direction

* Various definitions Calculated Quantities… T2-Weighted Image “Average” Diffusion* Degree of Anisotropy* Diffusion along X Diffusion along Y Diffusion along Z

1. (Approximately) Isotropic Diffusion How a blob of ink would spread out

2. Anisotropic Diffusion How a blob of ink would spread out

Vector Plot In-plane Through-plane

Direction of Greatest Diffusion + + + X-component Y-component Z-component Anisotropy Color (Hue) = Direction of highest diffusion Brightness = Degree of anisotropy =

Diffusion Tensor – Colour Map Left-Right Anterior-Posterior Superior-Inferior

DTI – Color Map

Diffusion Tensor – 3D Colour Map Left-Right Anterior-Posterior Superior-Inferior

How Many Measurements?

Which Directions? Isotropic resolution diffusion tensor imaging with whole brain acquisition in a clinically acceptable time D.K. Jones, S.C.R. Williams, D. Gasston, M.A. Horsfield, A. Simmons, R. Howard Human Brain Mapping 15, 216-230 (2002)

Fiber Tracking – Discrete Case Direction of Greatest diffusion

Fiber Tracking – Continuous Case Direction of Greatest diffusion Mori et al, 1999

Fiber Tracking – Where to Start • Everywhere: Seed points distributed evenly throughout volume

DTI Tractography

Fiber Tracking – Where to Start • Within a plane: All fibers within or crossing a selected plane are tracked

Fiber Tracking – Corpus Callosum

Fiber Tracking – Where to Start • Within a small volume

Fiber Tracking - CST

“Human Neuroanatomy” Carpenter & Sutin 1981 Upper Extremity Trunk Lower Extremity

“Human Neuroanatomy” Carpenter & Sutin 1983 Upper Extremity Trunk Lower Extremity

Fiber Tracking - CST

Combining DTI and fMRI

fMRI – Feet Movement

fMRI – Finger Tapping

fMRI – Tongue Movement

Results – fMRI – Feet, Fingers, Tongue

“Images of Mind”, Posner and Raichie, 1999

Fiber Tracking - CST Subject 1 Subject 2 Subject 3 Subject 4

Cingulum Feet movement Tongue movement Corpus Callosum Longitudinal Fasciculus Corticospinal Tract Crossing Fibers

MR Diffusion Tensor Imaging, Tractography

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