Introduction to Diffusion-weighted Imaging

1. Introduction to Diffusion-weighted Imaging Joelle Sarlls, Ph.D. NIH MRI Research Facility National Institute of Neurological Disorder and Stroke National Institutes of Health

2. Motivation • Magnetic resonance imaging provides information about the spatial distribution of water. • Diffusion-weighted MRI (DWI) provides information about the motion of water. • DWIs are sensitive to cellular architecture and tissue integrity. • DWI can provide quantitative measures that are directly comparable. • Diffusion imaging can be used to identify specific white matter tracts • Over 1000 publications combining fMRI and DWI • 120 since I gave this lecture last years ago

3. Outline • What is diffusion? • How do we measure diffusion in MRI? • How do we extract directional information? • What are the practical problems and limitations? • Beyond the diffusion tensor

4. Outline • What is diffusion? • How do we measure diffusion in MRI? • How do we extract directional information? • What are the practical problems and limitations? • Beyond the diffusion tensor

5. (for sphere) Stokes-Einstein Diffusion • Diffusion refers to the random translational (Brownian) motion of molecules that results from the thermal energy of theses molecules

6. Gaussian Distribution • Large number of particles that are free to diffuse have a squared displacement of a Gaussian form 1D 2D 3D Einstein, A. Ann Physik (1905) 4: 549-590

7. Diffusion For H2O at 37o C D ≈ 3.0x10-3mm2/s Tdif ≈ 30 ms r ≈ 25 μm • If the motion of water is hindered by cell membranes, macromolecules, etc. the displacement will be less and D will appear lower.

8. Outline • What is diffusion? • How do we measure diffusion in MRI? • How do we extract directional information? • What are the practical problems and limitations? • Beyond the diffusion tensor

9. Physical property of tissue water ρ proton density T1 relaxation time T2 relaxation time T2* relaxation time D diffusion coefficient Experimentally controlled parameters Sequence Spin-echo/gradient echo TR Time of Repetition TE Time to echo b-value diffusion-weighting factor Image Intensity in MRI Concentration of water } Rotational motion, Magnetic field strength Translational motion

10. Gradients make the resonance frequency a function of spatial position Gz ω0 Frequency -Z 0 +Z ω = γB = γB0 + γzGz

11. Basic Diffusion-weighting 90° Gx +x 0 -x S total

12. Phase Twist +Z 0 -Z

13. Basic Diffusion-weighting 90° Gx +x 0 -x stationary moving S total

14. Guess the intensity b-value = 1000 s/mm2 b-value = 0 s/mm2

15. Spin-echo Diffusion Preparation 90° 180° echo RF δ G Gdiff Δ Stejskal, EO and Tanner, JE. J Chem Phys (1965) 42 : 288-292

16. DWI Diffusion Coefficient mm2/sec Non-diffusion-weighted signal intensity B-value sec/mm2

17. Typical DWI • Single-shot “spin-echo” Echo Planar Imaging *Jones D., et al. Mag Res Med (1999) 42 : 515

18. S = S0 e-bD Diffusion map Calculate Diffusion Parameters b (s/mm2) 8 200 400 650

19. b = 0 s/mm2 I0 b = 1100 s/mm2 Gz Dz

20. D perpendicular D parallel X Water Diffusion in Tissue EM of mouse corpus callosum Not Free Anisotropy Cell membranes Myelin Organelles Extracellular matrix Dperp<< Dpar 8 μm

21. Gx Gy Gz ADC b = 0 s/mm2

22. 0.75 x 10-3 mm2/s 0.43 x 10-3 mm2/s Diffusion Map Acute Stroke b = 0 s/mm2 720 s/mm2 Warach S., et al. Ann Neurol (1995) 37:231-241

23. Outline • What is diffusion? • How do we measure diffusion in MRI? • How do we extract directional information? • What are the practical problems and limitations? • Beyond the diffusion tensor

24. D perpendicular D parallel X Water Diffusion in Tissue EM of mouse corpus callosum Not Free Anisotropy Cell membranes Myelin Organelles Extracellular matrix Dperp<< Dpar 8 μm

25. Anisotropic Diffusion

26. z y x The Diffusion Tensor Basser, P, et. al. J Magn Reson B (1994) 3 : 247-254

27. DTI Gx Gy Gz Gyz Gxy Gxz b = 0 s/mm2 b = 1100 s/mm2

28. Calculate Diffusion Tensor

29. z ε3 y ε2 x ε1 Diagonalize DT Eigenvalues Eigenvectors

30. z ε3 y ε2 x ε1 Quantitative Parameters Average Diffusivity <D> Fractional Anisotropy 0 ≤ FA ≤ 1

31. isotropic anisotropic

32. <D> FA

33. y ε3 x ε1 ε2 z Y X Z Directional Encoding for DTI Pajevic S. and Pierpaoli C., Magn Reson Med (1999) 43 : 526-540

34. Directional Encoded Color Map <D>

35. <D> FA DEC No sym DEC Line Field Linear Planar Spherical

36. Applications of DTI • Traumatic Brain Injury • Alzheimer’s Disease • Amyotrophic Lateral Sclerosis • Niemann-Pick type C Disease • Dementias • Connectivity • Cerebral Ischemia (Stroke) • Brain Cancer and Effects of Radiotherapy • Multiple Sclerosis • Epilepsy • Metabolic Disorders • Normal Brain Maturation and Aging

37. MD FA DEC Pediatric DIPG T2-weighted

38. Outline • What is diffusion? • How do we measure diffusion in MRI? • How do we extract directional information? • What are the practical problems and limitations? • Beyond the diffusion tensor

39. Typical DW SSEPI PRO CON Time Efficient Low Resolution Insensitive to Bulk motion Distortions - Field inhomogeneities Distortions - Diffusion weighting

40. CON: Distortions from field inhomogeneities T2-weighted FSE Non-diffusion-weighted SSEPI SSEPI corrected

41. CON: Distortions from DW DW SSEPI volumes FA maps

42. Other Common Problems in DTI • Low SNR • Incomplete Fat Suppression • Bulk movement • Cardiac pulsation

43. Low SNR 2.5 mm iso 1.7mm iso 1.3mm iso

44. Low SNR 2.5 mm iso 15.625 mm3 1.7mm iso 4.913 mm3 1.3mm iso 2.197 mm3

45. Low SNR 2.5 mm iso 15.625 mm3 1.7mm iso 4.913 mm3 1.3mm iso 2.197 mm3

46. Incomplete Fat Suppression b=1100 s/mm2 MD FA

47. Cardiac Pulsation Diffusion weighting in Z

48. Bulk Movement

49. Outline • What is diffusion? • How do we measure diffusion in MRI? • How do we extract directional information? • What are the practical problems and limitations? • Beyond the diffusion tensor

50. What isTractography? The use of orientation information from diffusion imaging to reconstruct estimates of white matter pathways in the brain.