DTI Basics – Water Diffusion (DTI – Diffusion Tensor Imaging)

1. DTI Basics – Water Diffusion(DTI – Diffusion Tensor Imaging) Einstein on Brownian Motion 1905 five important papers

2. Why USE DTI MRI : Detection of Acute Stroke “Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center Conventional T2 WI DW-EPI Sodium ion pumps fail - water goes in cells and can not diffuse – DW image gets bright (note – much later cells burst and stroke area gets very dark)

3. Why USE DTI MRITumor T2 (bright water) T2 (bright water) DWI (x direction) (T2 (bright water)+(diffusion)) Contrast (T1 + Gadolinium)

4. Why DTI MRI (more recently): Fiber Tracking

5. 1st level of complexity Diffusion Weighted Image X direction • Higher diffusion in X direction  lower signal Artifact or Abnormality David Porter - November 2000

6. Time T2 + diffusion T2 Sequence RF Gx - Gy Gz T2 Image Measure diffusion Regular T2 image Excite (gradientstrength)

7. 2nd Level of complexity DWI : 3 Direction Measuring Diffusion in other directions (examples) • single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions • Higher diffusion  lower signal Dxx Dyy Dzz  courtesy of Dr Sorensen, MGH, Boston David Porter - November 2000

8. 3rd level of complexityDiffusion Tensor Imaging Basics How can we track white matter fibers using DTI • Measures water diffusion in at least 6 directions • Echo-planar imaging (fast acquisition) • Collecting small voxels (1.8 x 1.8 x 3mm), scanning takes about 10 minutes

9. Higher diffusion  lower signal water Diffusion ellipsoid Diffusion ellipsoid White matter fibers • Useful for following white matter tracts in healthy brain

10. Higher diffusion  lower signal White matter fibers Isotropic Anisotropic Adapted from: Beaulieu (2002). NMR in Biomed; 15:435-455

11. DTI ellipsoidmeasure 6 directions to describe z no diffusion y x Ellipsoid represents magnitude of diffusion in all directions by distance from center of ellipsoid to its surface.

12. Ellipsoid Image Information available through DTI Tract Pierpaoli and Basser, Toward a Quantitative Assessment of Diffusion Anisotropy, Magn. Reson. Med, 36, 893-906 (1996)

13. Tractography Superior view color fiber maps Lateral view color fiber maps Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf.

14. axial cor sag Diffusion Tensor Imaging data for cortical spinal tract on right side blue = superior – inferior fibers green = anterior – posterior fibers red = right – left fibers Note tumor is darker mass on left side of axial slice MRISC

15. FA + color(largest diffusion direction) red = right – left green = anterior – posterior blue = superior - inferior

16. MRS – Magnetic Resonance Spectroscopy • Proton spectroscopy (also can do C, O, Ph,.. Nuclei) • Looking at protons in other molecules ( not water) (ie NAA, Choline, Creatine, …….) • Need > mmol/l of substances high gyromagnetic ratio ( ) • Just like spectroscopy used by chemist but includes spatial localization

17. Just looking at Proton Spectroscopy • Just excite small volume • Do water suppression so giant peak disappears • Compare remaining peaks precession Frequency Frequency

18. MRS – Magnetic Resonance Spectroscopy NAA = N-acetyl aspartate, Cr = Creatine, Cho = Choline amplitude NAA Cr Cho Frequency of precession

19. Multi – Voxel Spectroscopy (aka Chemical Shift Imaging – CSI) • Do many voxels at once • Can be some disadvantages with signal to noise (S/N) and “voxel bleeding”

20. Evaluate Health of Neurons (NAA level) Normalize with Creatine (fairly constant in brain) Red means High NAA/CR levels

21. Epilepsy Seizures (effects metabolite levels) • find location • determine onset time

22. Other Nuclei of interest for Spectroscopy

23. 23Na in Rat Brain (low resolution images are sodium 23 images) (high resolution images are hydrogen images) Note: This slide has nothing to do with Spectroscopy. It is a standard imaging slide created with the 23Na nucleus and the hydrogen nucleus. It has been included to show an example of imaging done with a nuclei other than hydrogen.

24. Common Metabolites used in Proton Spectroscopy

25. Important Concepts • What energies are used in each modality? • How does the energy interact with the tissue? • How is the image produced? • What is represented in the image? • What are important advantages and disadvantages of the major imaging modalities? • What are the fundamental differences between the Xray technologies (2D vs 3D, Radiography vs CT vs Fluoroscopy)? • What are the two major types of MRI images (T1, T2), and how are they different? • How are Angiograms produced (both Xray and MRI)? • Why are the advantages of combining imaging modalities?

26. Important Concepts • What does DTI, diffusion tensor imaging, measure? • What structures that we are interested in effect DTI images? • What does the DTI ellipsoid represent? • How might DTI be useful for clinical application or research? • What are we looking at with proton spectroscopy? • What are the three major metabolites we typically measure? • What do we “need” to be able to do proton spectroscopy? • What might proton spectroscopy be used for?

27. probably no use

29. Tractography Superior view color fiber maps Lateral view color fiber maps Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf.

30. Signal loss : by intra-voxel phase dispersion At the echo time TE, NMR signal is decayed by, - T2 decay (spin-spin diffusion) - diffusive motion For any set of diff. gradient pulses G G 90 180 echo TE

31. DTI Scalar Parameters • Trace: Magnitude of diffusion in a voxel. • Increases in damaged white matter • Fractional Anisotropy (FA): Measure of directionally-restricted diffusion. • Decreases in damaged white matter Rosenbloom M, et al. (July 2004). NIAA pubs; http://www.niaaa.nih.gov/publications/arh27-2/146-152.htm

32. The Diffusion Tensor, D • Diffusion is not equal in all directions (anisotropic). • Use this to probe brain structure! • Represent the diffusion pattern at each point in the brain using an ellipsoid.

33. Diffusion Vector (Colour) Map • The three magnitudes of the diffusion ellipsoid can be shown using three colours (RGB). • Red = Left – Right • Green = Ant. – Pos. • Blue = Sup. – Inf. • Map of major directions of water movement in the brain.

34. Fibre Tractography • In principle, the locations of major white matter fibre tracts in the brain can be mapped using the information in the colour map, by “following the arrows.”

35. Hindered Diffusion (diffusion ellipsoid) without hindrance WILSON with hindrance

36. Information available through DTI – Orientation of λ1 • Useful for following white matter tracts

37. Information available through DTI -- Aσ s av • Related to the shape of the ellipsoid • Independent of Dav (normalized) • Zero for a sphere, positive for other shapes • Sensitive to myelination and cortical development

38. Diffusion Tensor Imaging (As) Normal Adult Brain (A maps)

39. CELL EXTRA-CELLULAR SPACE FREELY DIFFUSING WATER IN EXTRA-CELLULAR SPACE • Higher diffusion  lower signal Tissue Sample A Tissue Sample B Freely Diffusing Water = Dark Larger D Restricted Diffusion = Bright Smaller D

40. K-space view of the spin echo imaging Ky 1 2 3 . . . . . . . n Kx

41. PE FE SS Diff. Grad. along different axis Y Diffusion-Weighting Z Diffusion-Weighting X Diffusion-Weighting GFE GPE GSS RF

42. DTI (Diffusion Tensor Imaging) • single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions • Higher diffusion  lower signal  courtesy of Dr Sorensen, MGH, Boston David Porter - November 2000

43. Detection of Acute Stroke “Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center Conventional T2 WI DW-EPI Sodium ion pumps fail, water goes in cells and can not diffuse.

44. Tumor T2 (bright water) DWI (x direction) (T2 (bright water)+diffusion) T1 + Gadolinium

45. y z x The Diffusion Tensor, D • Diffusion is not equal in all directions (anisotropic). • Use this to probe brain structure! • Diffusion ellipsoid for each voxel

47. DTI Scalar Parameters • Trace: The magnitude of diffusion in a voxel. • Fractional Anisotropy (FA): The extent to which diffusion is directionally restricted.

48. Unused slides from Ana516 lecture

49. Pulse Sequence: Gradient-EchoDiffusion Weighting in X direction Excitation 90o RF G -G Gx EPI (T2) Image Acquisition diffusiongradients Gy Gz

50. Gradient Coils