Basic Principles MRI related to Neuroimaging

1. Basic Principles MRI related to Neuroimaging Xiaoping Hu Department of Biomedical Engineering Emory University/Georgia Tech xhu@bme.emory.edu

2. Outline • Basic NMR/MRI Physics • Imaging sequences • Contrast Mechanisms • Pitfalls and Limitations

3. In the absence of magnetic field

4. In the presence of magnetic field

5. B0 M Bulk Nuclear Magnetization in the Presence of a Static Magnetic Field

6. gyroscope influenced by gravity Precession nuclear spin inside a magnetic field

7. Larmor Frequency  is frequency of precession and resonance usually in the radiofrequency (RF) range

8. Resonance Resonance occurs when the external influence exerted to a system matches the system’s natural frequency. E.g., pushing a swing In MRI, the natural frequency, called the Larmor frequency, is proportional to the applied magnetic field. At 1.5 T, it is ~64 Mhz (1Mhz=1000,000 hz; FM radio uses 88-106 Mhz).

9. Generation of NMR signal • Excitation • an RF pulse is applied to tip the magnetization such that it has a transverse component • Reception • precessing transverse component of M induces an emf in a receiving RF coil • Relaxation • The processes with which the magnetization returns to equilibrium. They determine the intensity/contrast of the image

10. Spatial discriminationachieved with magnetic field gradients B0 x

11. Selective Excitation Application of a band-limited RF pulse in the presence of a gradient along the direction perpendicular to the desired slice B0 w RF power

12. Lauterbur, 242, 190, Nature, 1973.

13.  w B0

14. phase frequency

15.   FT

16. RF Gss Gpe Gro Signal timing diagram of a spin-echo sequence

17. frequency encoding • phase encoding k-space traversal of a spin-echo sequence

18. Temporally interleaved multislice imaging slice #1 acquisition slice #2 acquisition • • • slice #n acquisition TR

19. Effects of Slice Spacing and Order nominal thickness with gap or skip no interleave 1 2 3 4 5 6 7 8 9 10 11 12 interleave 1 7 2 8 3 9 4 10 5 11 6 12

20. timing diagram of a blipped EPI sequence RF Gss Gpe Gro Signal

21. frequency encoding • phase encoding k-space traversal of an EPI sequence

22. Spiral Pulse Sequence

23. Spiral k-space trajectory i(t) k = k(t) e k(t) = C t (t) = C k(t) (Archimedian) 1 2

24. CONTRAST MECHANISMS in MRI • T1(Spin-lattice Relaxation time) relaxation along Bo • T2 (Spin-spin relaxation time) relaxation perpendicular to Bo • T2* (Signal decay perpendicular to Bo ) due to dephasing plus T2

25. Relaxation and Contrast z T1-relaxation y T2-relaxation x

26. T1 relaxation TR ••••• ••••• 90° pulse 90° pulse M0 M TR

27. Signal decay due to transverse relaxation •Irreversible processes (T2) • Dephasing due to different frequency of precession in the presence of magnetic field inhomogeneities (reversible) (T2’). 1/T2*=1/ T2 + 1/T2’ Characterizes decay due to both processes.

28. TE 180° pulse 90° pulse

29. TE 90° pulse time -TE/T2* S(TE) = So e

30. Relaxation and Contrast T1-relaxation: Growth of magnetization for next nutation T2-relaxation: decay of magnetization being detected

31. T1w Imaging at 3 Tesla

32. Brain Tumor Imaging T2W Pre-contrast T1W Pre-contrast T1W Post-contrast MRI for brain tumor

33. Spatial resolution • Signal-to-noise ratio • Imaging time • Gradient performance parameters • Physics • Diffusion • Signal decay

34. State of the Art • Structural imaging of human subjects • 1mm× 1mm× 1mm • Anatomic imaging of rodents • 50m× 50 m × 50 m • NMR microscopy (of samples) • 10m× 10 m × 10 m • Functional studies • Humans: 3mm× 3mm × 5mm • Animals: 100m× 100 m × 500 m • In vivo proton spectroscopy • Human: 7mm × 7mm × 7mm • Animal: 1mm × 1mm × 1mm

35. Temporal resolution • Signal-to-noise ratio • Image resolution • Gradient performance parameters • Physics • Relaxation

36. State of the Art • High resolution 3-D structural imaging • 10-20 min • Multislice imaging • minutes • Anatomic imaging of animals • hours • NMR microscopy (of samples) • hours to days • Functional studies • Sec/image, minutes/study • In vivo proton spectroscopy • Human: 10s of minutes • Animal: hours

37. High-resolution imaging with reduced FOV Zoomed imaging by outer volume saturation

38. Limitations of ultrafast sequences • EPI • Nyquist ghost • Spatial distortion • Spiral • Blurring • EPI and Spiral • Signal dropout • Resolution degradation due to T2* decay

39. Nyquist ghost k-space data image

40. image k-space data

41. B0 inhomogeneity induced distortion • Several possible causes • Static field inhomogeneity • Subject-dependent susceptibility • Field inhomogeneity disturbs the conditions of Fourier imaging • Image distortion and artifacts are encountered with severe inhomogeneity

42. EPI image distortion due to field inhomogeneity

43. Phase map original corrected flash Single-Shot EPI Segmented EPI

44. Spiral (before correction)

45. Spiral (after correction)

46. Problems in both EPI and Spiral • signal loss due to T2* decay • resolution degraded and limited by T2*

48. 7 Tesla T2*-weighted images (TE: 15 msec) 5-mm  1-mm z-shim

49. RF Gx Gy Gz TE1 TE2 Compensatory Gradient Pulse Sequence for a Single-Shot EPI with Susceptibility Compensation Song, MRM 46, 407, 2001.

50. New Single-shot Two partial-k TE1: 36 ms TE2: 44 ms Conventional Single-shot One full-k TE: 40 ms Combined images from the single-shot acquisitioncompared with conventional single-shot acquisition at 4T Song, MRM 46, 407, 2001.