Contrast Mechanism and Pulse Sequences

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

2. III.1 Image Contrasts

3. The Concept of Contrast Contrast = difference in signals emitted by water protons between different tissues For 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. Most Common Static Contrasts • Weighted by the Proton Density • Weighted by the Transverse Relaxation Times (T2 and T2*) • Weighted by the Longitudinal Relaxation Time (T1)

6. The Effect of TR and TE on Proton Density Contrast TR TE MR Signal MR Signal T1 Recovery T2 Decay t (s) t (ms)

7. Optimal 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

8. Proton Density Weighted Image

9. T2* Cars on different tracks Transverse Relaxation Times T2

10. Since the Magnetic Field Factor is always present, how can we isolate it to achieve a singular T2 Contrast? 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

11. The Effect of TR and TE on T2* and T2 Contrast TR TE T1 Recovery MR Signal MR Signal T2 Decay T1 Contrast T2 Contrast

12. Optimal 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]

13. T2 Weighted Image

14. T2* Weighted Image T2* Images PD Images

15. TR TE T1 Recovery T2 Decay MR Signal MR Signal T1 contrast T2 contrast The Effect of TR and TE on T1 Contrast

16. Optimal 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

17. T1 Weighted Image

18. Inversion Recovery to Boost T1 Contrast S = So * (1 – 2 e –t/T1) So S = So * (1 – 2 e –t/T1’) -So

19. IR-Prepped T1 Contrast

20. 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.

21. Motion Contrast Imaging Methods Prepare magnetization to make signal sensitive to different motion properties • Flow weighting (bulk movement of blood) • Diffusion weighting (scalar or tensor) • Perfusion weighting (blood flow into capillaries)

22. Flow Weighting: MR Angiogram • Time-of-Flight Contrast • Phase Contrast

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

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

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

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

27. MR Angiogram

28. Diffusion Weighting Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G T 2T 0 Time

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

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

31. Diffusion Anisotropy

32. Determination of fMRI Using the Directionality of Diffusion Tensor

33. 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 ADC Anisotropy

34. Fiber Tractography

35. D A B C DTI and fMRI

36. Perfusion Weighting: Arterial Spin Labeling Imaging Plane Labeling Coil Transmission

37. Arterial Spin Labeling Can Also Be Achieved Without Additional Coils Pulsed Labeling Imaging Plane Alternating Inversion Alternating Inversion EPISTAR EPI Signal Targeting with Alternating Radiofrequency FAIR Flow-sensitive Alternating IR

38. 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

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

40. Perfusion Contrast

41. Diffusion and Perfusion Contrast Perfusion Diffusion

42. III.2 Some of the fundamental acquisition methods and their k-space view

43. 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

44. 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

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

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

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

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

49. 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

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