Nuclear Magnetic Resonance (NMR) Magnetic Resonance Imaging (MRI)

1. Nuclear Magnetic Resonance (NMR)Magnetic Resonance Imaging (MRI)

4. What is the source and how is it working? • How is contrast created? • How is direction created? • How is the image constructed?

5. The source

6. Nuclear Magnetism Nucleons (protons, neutrons) have a quantum property known as spin (1/2 Bohr magneton, spin=1/2)

7. Why are protons important? • They are plenty in the body • Positively charged • They spin about a central axis • A spinning charge creates a magnetic field • The arrow indicates the direction of the magnetic field

8. Nuclear Magnetism Normally the nuclear spins of the protons cancel out and there is no net magnetic field to observe.

9. Nuclear Magnetism In a homogeneous magnetic field, the proton spin will orient themselves. Most spin will be oriented along the external field giving a net magnetic field.

13. z z Mo x x y y Bo Bo

14. Energy and populations • An external magnetic field create an energy difference between nuclei aligned and against Bo: • Each level has a different population (N), and the difference between the two is related to the energy difference by the Boltzman distribution: • Na / Nb = e DE / kT • The DE for 1H at 400 MHz (Bo = 9.5 T) is 3.8 x 10-5 Kcal /mol b Bo > 0 DE = h n a Bo = 0 Na / Nb = 1.000064

16. z B1= C * cos (wot) Mo x B1 Bo y i Transmitter coil (y) y y y +wo -wo = + x x x

18. Once again Mo is composed by individual atomic spin with the Larmour frequency wo. If a RF field of the same frequency is applied a magnetic vector B1with the same frequency wo is created that interact with Mounder a resonant condition. The alternating magnetic field generates a torque on Mo, and the system absorbs energy : Since the system absorbed energy, the equilibrium of the system is altered. We modified the populations of the Na and Nbenergy levels. Again, keep in mind that individual spins flipped up or down (a single quanta), but Mo can have a continuous variation. z z Mo B1 off… (or off-resonance) x x B1 Mxy wo y y wo

19. ENERGY ABSORPTION MRASUREMENT OF SIGNAL RELAXATION

20. B no change RF in 0 M0=x M0=x Energy Absorption equilibrium

21. B energy absorption RF in 0 M0=y M0=x Energy Absorption equilibrium

22. M0=x M0=y B Energy Absorption tip ““ RF pulse 0

23. M0=x B Energy Absorption 900 tip M0=z 900 RF pulse 0

24. M0=x B Energy Absorption 1800 tip M0=-x 1800 RF pulse 0

25. What happens to the released energy? • released as heat • exchanged and absorbed by other protons • released as radio waves

26. z B y x Measuring the MR Signal • cannot easily record ML • the signal is measured in the transverse (x, y) plane

27. z z z 900 RF y x y x y x Rotation in the xy-plane t=t0 t=t0+ t=t0++ motion in the xy-plane =   >> T1 relaxation, MHz vs seconds

28. z z y x y x 900 RF Mxy and ML • the magnitude of Mxy depends on the size of MLimmediatelyprior to the 900 RF pulse fat protons - short T1 water protons - long T1

29. z y x Measuring the MR Signal RF signal from precessing protons RF antenna

30. Relaxation • relaxation is the release of energy from an excited state to a lower energy state • protons absorb and release energy in MRI

31. CONTRAST

32. Basic Principle • The signal in MR images vary due to • proton density • T1 relaxation • T2 relaxation • flow • contrast

33. T1 Relaxation • synonyms • longitudinal relaxation • thermal relaxation • spin-lattice relaxation

35. T2 Relaxation • immediately after the absorption of the RF energy and the initiation of precession, the process of T2 relaxation begins • results from loss of phase coherence among groups of protons rotating in the transverse plane

36. y y y x x x y x Bo Spin Dephasing after excitation

44. 180o 180o 180o TE Time Echo returned RF Signal

45. T2 and T2* Relaxation • T2* is the observed T2 relaxation time • T2 is the natural relaxation time • T2* < T2

46. FID=Free Induction Decay TE=Time Echo TR=Time Repetition