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  1. Contrast Mechanism and Pulse Sequences Allen W. Song Brain Imaging and Analysis Center Duke University

  2. Part III.1 Image Contrast Mechanisms

  3. The Concept of Contrast (or Weighting) • Contrast = difference in RF signals — emitted by water protons — between different tissues • T1 weighted example: gray-white contrast is possible because T1 is different between these two types of tissue

  4. MR Signal MR Signal T2 Decay T1 Recovery Static Contrast Imaging Methods 1 s 50 ms

  5. Optimal TR and TE for Proton Density Contrast TR TE MR Signal MR Signal T1 Recovery T2 Decay t (s) t (ms)

  6. Proton Density Contrast • Technique: use very long time between RF shots (large TR) and very short delay between excitation and readout window (short TE) • Useful for anatomical reference scans • Several minutes to acquire 256256128 volume • ~1 mm resolution

  7. Proton Density Weighted Image

  8. Optimal TR and TE for T2* and T2 Contrast TR TE T1 Recovery MR Signal MR Signal T2 Decay T1 Contrast T2 Contrast

  9. T2* and T2 Contrast • Technique: use large TR and intermediate TE • Useful for functional (T2* contrast) and anatomical (T2 contrast to enhance fluid contrast) studies • Several minutes for 256  256  128 volumes, or second to acquire 64  64  20 volume • 1mm resolution for anatomical scans or 4 mm resolution [better is possible with better gradient system, and a little longer time per volume]

  10. T2* Cars on different tracks T2

  11. Can we compensate the field term? … Fast Spin Fast Spin TE/2 t=0 180o turn t = TE/2 Fast Spin Fast Spin TE/2 t=TE Slow Spin Slow Spin TE/2 t=0 180o turn t = TE/2 Slow Spin TE/2 Slow Spin t=TE

  12. T2 Weighted Image

  13. TR TE T1 Recovery T2 Decay MR Signal MR Signal T1 contrast T2 contrast Optimal TR and TE for T1 Contrast

  14. T1 Contrast • Technique: use intermediate timing between RF shots (intermediate TR) and very short TE, also use large flip angles • Useful for creating gray/white matter contrast for anatomical reference • Several minutes to acquire 256256128 volume • ~1 mm resolution

  15. T1 Weighted Image

  16. Inversion Recovery for Extra T1 Contrast S = So * (1 – 2 e –t/T1) So S = So * (1 – 2 e –t/T1’) -So

  17. Inversion Recovery (CSF Attenuated) T2

  18. In summary, TR controls T1 weighting and TE controls T2 weighting. Short T2 tissues are dark on T2 images, but short T1 tissues are bright on T1 images.

  19. Motion Contrast Imaging Methods • Can “prepare” magnetization to make readout signal sensitive to different motion properties • Flow weighting (bulk movement of blood) • Diffusion weighting (scalar or tensor) • Perfusion weighting (blood flow into capillaries)

  20. MR Angiogram • Time-of-Flight Contrast • Phase Contrast

  21. Acquisition Excitation Saturation No Flow Medium Flow High Flow No Signal Medium Signal High Signal Vessel Vessel Vessel Time-of-Flight Contrast

  22. Time to allow fresh flow enter the slice 90o 90o RF Excitation Gx Saturation Image Acquisition Gy Gz Pulse Sequence: Time-of-Flight Contrast

  23. Blood Flow v Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G T 2T 0 Time Phase Contrast (Velocity Encoding)

  24. Pulse Sequence: Phase Contrast 90o RF Excitation G Gx Phase Image Acquisition -G Gy Gz

  25. MR Angiogram

  26. Diffusion Weighted Imaging Sequences Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G T 2T 0 Time

  27. Excitation 90o RF G -G Gx Image Acquisition Gy Gz Pulse Sequence: Gradient-Echo Diffusion Weighting

  28. Pulse Sequence: Spin-Echo Diffusion Weighting 180o 90o RF G G Excitation Gx Image Acquisition Gy Gz

  29. Advantages of DWI • The absolute magnitude of the diffusion • coefficient can help determine proton pools • with different mobility • 2. The diffusion direction can indicate fiber tracks

  30. Diffusion Anisotropy

  31. Determination of fMRI Using the Directionality of Diffusion Tensor

  32. Display of Diffusion Tensor Using Ellipsoids

  33. Diffusion Contrast

  34. Perfusion/Flow Weighted Arterial Spin Labeling Coil Tagging Imaging Plane Transmission

  35. Perfusion/Flow Weighted Arterial Spin Labeling with Pulse Sequences Pulse Tagging Imaging Plane Alternating Inversion Alternating Inversion EPISTAR EPI Signal Targeting with Alternating Radiofrequency FAIR Flow-sensitive Alternating IR

  36. Pulse Sequence: Perfusion Imaging 180o 180o 90o RF Gx Image Gy Alternating Proximal Inversion Odd Scan Even Scan Gz 90o 180o 180o RF Gx Image Gy Odd Scan Alternating opposite Distal Inversion Gz Even Scan EPISTAR FAIR

  37. Advantages of ASL Perfusion Imaging • It can non-invasively image and quantify • blood delivery • Combined with proper diffusion weighting, • it can assess capillary perfusion

  38. Perfusion Contrast

  39. Diffusion and Perfusion Contrast Perfusion Diffusion

  40. Part III.3 Some fundamental acquisition methods And their k-space view

  41. k-Space Recap Equations that govern k-space trajectory: Kx = g/2p 0tGx(t) dt Ky = g/2p 0tGx(t) dt These equations mean that the k-space coordinates are determined by the area under the gradient waveform

  42. Gradient Echo Imaging • Signal is generated by magnetic field refocusing mechanism only (the use of negative and positive gradient) • It reflects the uniformity of the magnetic field • Signal intensity is governed by S = So e-TE/T2* where TE is the echo time (time from excitation to the center of k-space) • Can be used to measure T2* value of the tissue

  43. MRI Pulse Sequence for Gradient Echo Imaging Excitation Slice Selection Frequency Encoding Phase Encoding digitizer on Readout

  44. K-space view of the gradient echo imaging Ky 1 2 3 . . . . . . . n Kx

  45. Multi-slice acquisition Total acquisition time = Number of views * Number of excitations * TR Is this the best we can do? Interleaved excitation method

  46. readout readout readout TR Excitation …… Slice Selection …… Frequency Encoding …… Phase Encoding Readout

  47. Spin Echo Imaging • Signal is generated by radiofrequency pulse refocusing mechanism (the use of 180o pulse ) • It doesn’t reflect the uniformity of the magnetic field • Signal intensity is governed by S = So e-TE/T2 where TE is the echo time (time from excitation to the center of k-space) • Can be used to measure T2 value of the tissue

  48. MRI Pulse Sequence for Spin Echo Imaging 180 90 Excitation Slice Selection Frequency Encoding Phase Encoding digitizer on Readout

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

  50. Fast Imaging How fast is “fast imaging”? In principle, any technique that can generate an entire image with sub-second temporal resolution can be called fast imaging. For fMRI, we need to have temporal resolution on the order of a few tens of ms to be considered “fast”. Echo-planar imaging, spiral imaging can be both achieve such speed.