1 / 69

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:

takoda
Download Presentation

Biomedical Imaging II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related