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. B0B0 M Bulk Nuclear Magnetization in the Presence of a Static Magnetic Field

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

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 discrimination achieved with magnetic field gradients B0B0 x

11. B0B0  RF power Selective Excitation Application of a band-limited RF pulse in the presence of a gradient along the direction perpendicular to the desired slice

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

13. B0B0  

14. frequency phase

15.   FT

16. RF G ss G pe G ro Signal timing diagram of a spin-echo sequence

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

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

19. nominal thickness 127891011123456 174105116122839 with gap or skip no interleave interleave Effects of Slice Spacing and Order

20. RF G ss G pe G ro Signal timing diagram of a blipped EPI sequence

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

22. Spiral Pulse Sequence

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

24. CONTRAST MECHANISMS in MRI T 1 (Spin-lattice Relaxation time) relaxation along B o T 2 (Spin-spin relaxation time) relaxation perpendicular to B o T 2 * (Signal decay perpendicular to B o ) due to dephasing plus T 2

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

26. T1 relaxation TR 90° pulse TR M0 M

27. Signal decay due to transverse relaxation Irreversible processes (T 2 ) Dephasing due to different frequency of precession in the presence of magnetic field inhomogeneities (reversible) (T 2 ’). 1/T 2 *=1/ T 2 + 1/T 2 ’ Characterizes decay due to both processes.

28. 180° pulse 90° pulse TE

29. time TE S(TE) = S o e -TE/T 2 * 90° pulse

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

31. T 1 w Imaging at 3 Tesla

32. Brain Tumor Imaging T1W Pre-contrast T1W Post-contrast T2W Pre-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. k-space dataimage Nyquist ghost

40. k-space data image

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. Single-Shot EPISegmented EPI flash corrected original Phase map

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. Pulse Sequence for a Single-Shot EPI with Susceptibility Compensation TE 1 RF Gx Gy Gz Compensatory Gradient TE 2 Song, MRM 46, 407, 2001.

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

51. Acquisitions - spiral-out (A) - spiral-in (B) - combined spiral-in/out (C) Spiral-In/Out Experiments TE Glover & Law, MRM, 45, 515, 2001

52. Spiral-In/Out Combination spiral-out spiral-in  wtd ave Glover & Law, MRM, 45, 515, 2001