1. MRI Physics Dr Mohamed El Safwany, MD.

2. MRI Magnetic Resonance Imaging Magnetic Resonance Imaging

3. Intended learning outcome ► The student should learn at the end of this lecture MRI physics.

4. M is for Magnetic ► Atomic structure:  Proton  Neutron  Electron

5. M is for Magnetic ► Motion within the atom: SPIN

7. M is for Magnetic ► MR active nuclei:  H1  C13  F19  P31  N15  O17

8. M is for Magnetic ► Why H1?  1000 Billion Billion Atom in adult  Solitary Proton gives it a large magnetic moment

11. M is for Magnetic "NMV" Net Magnetization Vector

12. M is for Magnetic Precession

14. Larmor Equation Frequency α Magnetic Field Frequency α Magnetic Field ω α β

15. M ► Atomic structure ► SPIN ► MR active nuclei H1 ► What happen when placed in external magnetic field ( Spin Up and Spin Down ) ► NMV ► Precession ► Larmor Equation

16. R is for Resonance Def: Energy transition that occur when object is subjected to frequency the same as its own

18. R is for Resonance Here; Radio Frequency "RF" Same Frequency of H nuclei At 90 degree to B0

19. R is for Resonance ► RF Removed  Signal decreased  Amplitude of MR Signal decreased

20. I is for Imaging ► Areas of High Signal ► Areas of Low Signal ► Areas of Intermediate Signal

21. I is for Imaging NMV can be separated in to Individual Vectors of tissue present in the patient ► Such as Fat, CSF & Muscle

22. I is for Imaging Intermediate Low Signal High Signal GreyBlackWhite

23. I is for Imaging Gradient Magnets ► Used to vary magnetic field in known manner ► Each point has slightly different rate of precession & Larmor Frequency. ► Variety of signal released by Protons returning to z-plane can used to determine the composition of exact location of each point.

24. Contrast Mechanisms Intermediate Low Signal High Signal GreyBlackWhite

25. Contrast Mechanisms (Relaxation Process) Relaxation Process after removal of RF pulse Signal induced in Receiver Coil decrease

26. Contrast Mechanisms (Definitions) Repetition Time "TR" Time from application of one RF pulse To the application of the next (it affects the length of relaxation period after application of one RF excitation pulse to the beginning of the next)

27. Contrast Mechanisms (Definitions) Echo Time "TE" Time between RF excitation pulse and collection of signal (it affects the length of relaxation period after removal of RF excitation pulse and the peak of signal received in receiver coil)

28. Contrast Mechanisms (Definitions) Flip Angle Angle throw which the NMV moved as result of a RF excitation pulse

29. Contrast Mechanisms (Parameters) Image contrast controlled by: 1- Extrinsic Contrast parameters: TR, TE & Flip Angle 2- Intrinsic Contrast parameters: T1 Recovery, T2 Decay, Proton Density, Flow & Apparent Diffusion Coefficient

32. Contrast Mechanisms ( T1 Recovery) Short TR  T1 contrast ( T1 Weighted ) ( T1 Weighted ) ► TR 300-600 ms ► TE 10-30 ms

33. Contrast Mechanisms ( T2 Decay) Fat much better at energy exchange than Water Because this T2 depend on: 1-How closely molecular motion of atoms matches Larmor Frequency 2-Proximity of other spins So; Fat's T2 time is very short compared to water FAT WATER

35. Contrast Mechanisms ( T2 Decay) Long TE  T2 contrast (T2 Weighted) ► TR 2000 ms ► TE 70 ms

36. Contrast Mechanisms ( Proton Density) ► Proton Density Long TR  Proton density ► TR 2000 ms ► TE 10-30 ms

37. Contrast Mechanisms ( Contrast Media) ► Contrast Media ► as Gadolinium ► local magnetic field fluctuation occur near Larmor frequency ► T1 Relaxation times of nearby protons are reduced ► So they appear brighter in T1 weighted Image

38. Text Book ► David Sutton’s Radiology ► Clark’s Radiographic positioning and techniques

39. Assignment ► Two students will be selected for assignment.

40. Question ► Define echo time TE in MRI?

41. Thank You 41