Principles of MRI

1. Principles of MRI Ari Borthakur, PhD Associate Director, Center for Magnetic Resonance & Optical Imaging Department of Radiology Perelman school of Medicine, University of Pennsylvania

2. A brief history… Isidor Rabi wins Nobel prize in Physics (1944) for for his resonance method for recording the magnetic properties of atomic nuclei. Felix Bloch and Edward Purcell share Nobel Prize in Physics (1952) for discovery of magnetic resonance phenomenon. Richard Ernst wins Nobel Prize in Chemistry (1991) for 2D Fourier Transform NMR. His post-doc, Kurt Wuthrich awarded a Nobel Prize in Chemistry (2002) for 3D structure of macromolecules. Nobel Prize in Physiology & Medicine awarded to Sir Peter Mansfield and Paul Lauterbur (2003) for MRI. Who’s next? Seiji Ogawa for BOLD fMRI?

3. Outline • Hardware: • MRI scanner • RF coil • Gradients • Image contrast • Software: • Pulse sequences • Fourier Transform

4. How does it work? http://www.med.harvard.edu/AANLIB/cases/case17/mra.mpg

5. Original Concept* • Rotating Gradient • Filtered Back-projection *Lauterbur, Nature (1973)

6. Fourier Imaging* • Three orthogonal gradients: • Slice Selection • Phase Encoding • Frequency Encoding • k-space • Representation of encoded signal *Kumar, et al. J. Magn. Reson. (1975)

7. MRI scanners 19 Tesla 4.7 Tesla 1.5 Tesla To generate B0 field and create polarization (M0)

8. MRI coils Mouse MRI platform Head coil Torso coil To transmit RF field (B1) and receive signal

9. Gradients To spatially encode the MRI signal

10. Image Contrast Contrast in MRI based on differences in: • Relaxation times (T1, T2, T2*, T1r) • Local environment • Magnetization = spin density • Concentration of water, sodium, phosphorous etc. • Macromolecular interactions • Chemical-exchange, dipole-dipole, quadrupolar interactions • Contrast agents • Gadolium-based, iron-oxide, manganese

11. Magnetization Bitar et al.RadioGraphics(2006) “spin” “spin ensemble” spin-up or spin-down = Boltzmann distribution Net Magnetic Moment

12. Energy levels N+/N- = exp (-E/kBT) Boltzmann distribution Energy gap Larmor Equation

13. MR experiment RF pulse applied in x-y “transverse” plane RF off M0 aligned along B0 Mz=M0 M “nutates” down to x-y plane Mz=M0cos and Mxy=M0sin Detect Mxy signal in the same RF coil

14. Equation of motion Bloch Equation (simple form)

15. Choosing a frame of reference rotating at rf, makes B1 appear static: Effect of RF pulse The 2nd B field

16. Nutation Lab frame of reference e.g. B1 oscillating in x-y plane Rotating frame of reference e.g. B1 along y-axis http://www-mrsrl.stanford.edu/ ~brian/mri-movies/

17. Spin-Lattice relaxation time=return to thermal equilibrium M0 Spin-spin relaxation with reversible dephasing Spin-spin relaxation without “reversible” dephasing After RF pulse-Relaxation T1>T2>T2*

18. Signal detection http://www-mrsrl.stanford.edu/ ~brian/mri-movies/

19. NMR signal MR Image Water Fat 4.7 ppm 1.2 ppm How does MRI work? ? = 1) Spatial encoding gradients 2) Fourier transform

20. Less MRI time/low image quality 90 180 180 90 90 180 180 RF RF RF TE TE slice slice slice phase phase phase freq freq freq TR TR Less MRI time/low image quality Spin-echo Echo planar imaging Fast spin-echo  (<90) RF TE slice phase freq TR Gradient-echo Pulse Sequence (timing diagram)

21. B1 M0 Gy Gz Gx MRI B0

22. Gradient-echo  RF TE slice phase freq TR Slice Selection* *Mansfield, et al. J. Magn. Reson. (1976) *Hoult, J. Magn. Reson. (1977)

23. Mz Gz•z z{ RF Mxy Mz BWRF= Gz•z Slice Selection B0 Mz

24. Gradient-echo  RF TE slice phase freq TR Frequency Encoding* a.k.a “readout” *Kumar, et al. J. Magn. Reson. (1975)

25. Gx•x FOVx BWread= Gx•FOVx Frequency Encoding Mz

26. Gradient-echo  RF TE slice phase pe freq TR Phase Encoding* *Kumar, et al. J. Magn. Reson. (1975) *Edelstein, et al. Phys. Med. Biol. (1980)

27. FOVy 1/pe= Gy•FOVy Phase Encoding Gy2•y Gy1•y Gy0•y Each PE step imparts a different phase twist to the magnetization along y

28. ky Gradient-echo  RF TE slice kx phase freq TR Traversing k-space Spin-warp imaging* *Edelstein, et al. Phys. Med. Biol. (1980)

29. image FT Fourier Transform k-space phase freq

30. freq freq freq freq Spiral phase Radial freq Echo planar Traversing k-space Spiral Radial Echo-planar

31. Fourier Transform

32. Typical FT pairs

33. K-space The signal a coil receives is from the whole object: replace: K-space signal: The image is a 2D FT of the k-space signal*: *Kumar, et al. J. Magn. Reson. (1975)

34. K-space weighting High-frequency data=edges Low-frequency data=contrast/signal

35. MRI Examples

36. 3D Neuro MRI

37. Susceptibility-Weighted Imaging (SWI)

38. BOLD fMRI

39. Phase Contrast MR Angiography

40. Knee Imaging TSE TE = 25 ms TSE Fat Saturated Resolution:200x200µm2

41. 7T Project: Hyper-polarized 3He MRI K Emami, et al., MagReson Med2009

42. Control Image TAG 7T Project: ASL-Perfusion MRI MRI Arterial Spin Labeling (ASL) technique MRI “head coil” Williams et al.,PNAS 1994

43. 18-FDG ASL MRI & PET of Alzheimer’s Disease PET Scan ASL MRI Scan CONTROL AD Detre et al., Neuroimage 2009

44. Take home… • Magnet • Create B0 • Produce M0 • RF coil • Transmit B1 field • Detect Signal • Image contrast • Relaxation, concentration, interactions etc. • Gradients • Spatial encoding • Signal in k-space • Fourier Transform • From k-space to image space • Pulse sequences • Traverse k-space • Image Artifacts