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Biomedical Imaging II

Biomedical Imaging II. Class 9 – Magnetic Resonance Imaging (MRI) Functional MRI (fMRI): Magnetic Resonance Angiography (MRA), Diffusion-weighted MRI (DWI) Blood Oxygen Level Dependent (BOLD) MRI 04/10/06. MRI vs. fMRI. MR Imaging Principles. Basic MRI measurement:

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Biomedical Imaging II

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  1. Biomedical Imaging II Class 9 – Magnetic Resonance Imaging (MRI) Functional MRI (fMRI): Magnetic Resonance Angiography (MRA), Diffusion-weighted MRI (DWI) Blood Oxygen Level Dependent (BOLD) MRI 04/10/06

  2. MRI vs. fMRI

  3. MR Imaging Principles

  4. Basic MRI measurement: Homogeneous static magnetic field (B0) RF pulse generator Antenna (coil) for sending and receiving Free induction decay (FID) signal Free: No external RF field during detection Exponential decay at rate T2* due to spin-spin relaxation (dephasing) and local field inhomogeneities Free induction decay (FID)

  5. Inversion pulse after time t phase recovery at 2t Corrects for dephasing due to static B inhomogeneities x x y y Spin echo 180 degree spin flip

  6. Spin echo sequence • Multiple p pulses create “Carr-Purcell-Meiboom-Gill (CPMG)” sequence • Decays with time constant T2

  7. Gradient fields in MRI 1 • Strength of Bz component varies linearly in space

  8. Gradient fields in MRI 2 • Larmor frequency varies linearly in space:

  9. 1st Dimension (z): Slice selection • Slice position: z0 ~ f0 • Slice thickness: • Slice profile: profile ~ FT (pulse shape) (Frequency f0 bandwidth B, pulse length T) d

  10. Slice selection cont. • Pulse sequence (PS) for slice selection (TR = repetition time, TE = echo time)

  11. Frequency encoding • The NMR signal from each x-position contains a specific center frequency • The over-all NMR signal is the sum of signals along x • FT recovers signal contribution at each frequency, i.e. x-location • Resulting spectrum is a projection Frequency spectrum

  12. Frequency encoding cont. • Pulse sequence: two gradients for x and z

  13. 3rd Dimension (y) • How to achieve y-localization? Frequency encoding will always produce iso-lines of resonance frequencies • Solution: • Reconstruction from projections • Phase encoding

  14. Phase encoding • Pulse sequence: TP

  15. 2D FT pulse sequence (spin warp) • Most commonly employed pulse sequence

  16. k-space map • The 2D array of NMR signals obtained from repeated pulse sequences is referred to as the k-space map. FT FT ky (Phase encoding) K-space kx (Frequency encoding)

  17. From Structure to Function

  18. Magnetic Resonance Angiography (MRA)

  19. Arterial Spin Labeling (Perfusion MRI) HOW?

  20. and/or this one and/or this one Step 1: Select this slice Step 2: Saturate this slice (TR <<T1) Step 3: Excite this region Step 1: Select this slice Step 2: Saturate this slice (TR <<T1) Step 1: Select this slice Step 2: Saturate this slice (TR <<T1) Step 3: Excite this region Step 4: Apply phase- and frequency-encoding gradients, record FIDs Step 1: Select this slice Basic Idea #1: Time-of-Flight (TOF)

  21. Basic Idea #2: Phase Contrast Bipolar Field Gradients

  22. Static (not moving) stuff Stuff that moves Bipolar Gradient Effects First gradient Second gradient

  23. Examples of Phase-Contrast MRA

  24. fMRI vs. Nuclear Imaging (PET)

  25. Diffusion-weighted MRI (DWI) • Stronger bipolar gradients → lower tissue velocities detectable • Blood flow velocities: ~(0.1 – 10) cm-s-1 • Water diffusion velocity: ~200 μm-s-1 • Using the same basic strategy as phase-contrast MRA, can image “apparent diffusion coefficient” (ADC) • Useful for diagnosing and staging conditions that significantly alter the mobility of water • e.g., cerebrovascular accident (“stroke,” apoplexy)

  26. Examples of Diffusion-weighted images

  27. Examples of Diffusion-weighted images

  28. Magnetic Susceptibility-based Imaging

  29. Magnetic Susceptibility Effects I

  30. Magnetic Susceptibility Effects II

  31. Magnetic interaction of Hb  Image local field inhomogeneities (T2* weighted)

  32. Reminder: Neuro-vascular coupling

  33. Blood vessels

  34. Hemoglobin-Oxygen Interaction

  35. Hemoglobin-Oxygen Interaction

  36. Hemoglobin-Oxygen Interaction

  37. Effect of Oxygen Binding Deoxyhemoglobin: “puckered” heme; paramagnetic Oxyhemoglobin: planar heme; diamagnetic

  38. Actual k-space

  39. Gradient Echo “Gradient echo”

  40. Gradient Echo cont.

  41. Fast Sensitive to T2 (Pulse) Echo Planar Imaging (EPI)

  42. K-space sampling EPI Conventional (spin warp)

  43. Gradient Echo Imaging • No refocusing pulse • Short TR (<10 ms) • Spin tip angle a< 90° • Sensitive to T2* (Spin-spin relaxation + field inhomogeneities)  very short TE (1-10 ms) FID + Echo: Gradient- Recalled Acquisition in the Steady State Fast Imaging with Steady State Precession FID only: Fast Low-Angle Shot Spoiled GRASS

  44. (Gradient) Echo planar imaging • “Single shot” imaging  fast! • Spatial resolution limited by gradient switching time

  45. MRI contrast mechanisms

  46. fMRI investigation of hemodynamics

  47. T2* weighted images rest activation

  48. Subtraction

  49. Average for multiple stimulations Spatial mean over 426 activated voxels Spatial mean over 426 non-activated voxels

  50. Example for visual stimulation

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