1. BE 581
Lecture 3- Intro to MRI

2. BE 581
Lecture 3 - MRI

3. Block Equation - T1 decay
90 pulse
180 pulse

4. T1 relaxation (slow) (longitudinal or spin-lattice)
Fat ms 260
Liver
Kidney
White m
Grey m
Cerebrospinal fluid 2,000 2,400

5. T2 relaxation (quick) 1.5T Fat 60-80 Liver 40 Kidney 60 White m. 90
Grey m
Cerebrospinal fluid 160

6. How to measure T1 & T2? Sequence of RF pulses with a specific
TE: Echo Time- time after 90o RF pulse until readout. Determines how much spin-spin relaxation will occur before reading one row of the image.
TR: Repetition Time– time between successive 90o RF pulses. Determines how much spin-lattice relaxation will occur before constructing the next row of the image

7. Measuring T1 Magnetization Mz A 90o RF pulse Mz->My
Wait for a t time
Send a new 90o RF
How long does it take for Mz to recover?
Generate the Mz recovery curve

8. Measuring T1 Large molecules small molecules
Energy transfer works when the frequency of precession of the protons overlaps with vibrational freq. of lattice
Large molecules->low vibrational freq->longT1
Small molecules->broad vibrational freq->long T1
Medium/viscous fluid-> intermediate freq ->short T1
Large molecules
small molecules

9. Measuring T1 Large molecules small molecules
Large molecules->low vibrational freq -> small overlap with o
Small molecules->broad vibrational freq-> larger overlap with o
Medium/viscous fluid->intermediate freq.->largest overlap with o
Large molecules
small molecules

10. T1 and T2
Molecule size T1 T2
Small
Long
Medium
Large
Longest

11. T1 and T2 relaxation time

12. Spin echo First 90o nutate magnetization Second 180 re-phasing pulse
spin in phase T2 and T2* impact signal
Second 180 re-phasing pulse
applied at time T ->re-phases spins

13. Spin echo
The 180o pulse has the function of rotating the magnetization vector to the opposite direction of the first 90o pulse.
Spins experience OPPOSITE magnetic field inhomogeneities -> cancel its effect
T2* is cancelled

14. Spin echo contrast h proton density Using the same pulse seq.
TR repetition time
TE echo time
Using the same pulse seq.
We get different S depending on T1 and T2

15. Inversion recovery Emphasizes T1 relaxation time
Extends longitudinal recovery time by a factor of 2

16. Inversion recovery 180 pulse Mz => -Mz wait TI (time of inversion)
90 pulse -Mz => Mxy => FID
Wait TE/2
180 pulse produces echo at TE

17. Inversion recovery
No T2
A factor of 2 (-Mz to Mz)

18. How do you generate images?
Spatial Encoding
Generate magnetic gradient across the patient
B decreases

19. Spatial encoding Frequency of precession vary with B
Resonance frequency will also vary
A wise choice of RF frequency can give just one slice
f1 f2 Bo
B decreases

20. Spatial encoding You can do this in all 3 planes
The intersection of all planes gives us a location (voxel)
A voxel becomes a value of intensity on the MRI image

21. Sensitive point technique (se)
Apply slice select gradient
No effect everywhere else
The location is established by RF central frequency
Slice thickness is established by RF bandwidth

22. Phase encoding
Protons at the end of a gradient (strong B) go faster than the one at the other end (weak B).
Protons where B was higher are ahead of protons where B was slower B ON B
OFF
WE GET A PHASE GRADIENT

23. Frequency encoding

24. Gradients

25. Spatial encoding You can do this in all 3 planes
The intersection of all planes gives us a location (voxel)
A voxel becomes a value of intensity on the MRI image
Fourier transforms are used to go from time to frequency

26. Spatial encoding
Apply slice select gradient while transmitting an RF pulse
Apply phase encoding gradient
Apply frequency encoding gradient
Fourier transform received signal
Repeat with different phase

27. Spatial encoding Slice -> Z axis
Frequency of returned RF signal -> x axis
Phase of returned RF signal -> y axis
The intersection of all planes gives us a location (voxel)

28. MRI instrument

29. MRI

30. Magnets and coils

31. Main magnet 1 Tesla = 10,000 Gauss Earth 0.5 µT - 0.5G Magnet can be
Resistive -can be turned on and off, consume a lot of electricity (0.35T)
Permanent-cannot be turned off (0.5T)
Superconducting - best performance need to be cooled

32. Superconducting magnet
Several tesla
Conduct electrical current with little resistance
Wire- wrapped cylinder (solenoid)
Need high cooling (4.2K)

33. Gradient coils Up to 60 mT/m
In the z direction are called Helmholtz coils
X and y are Saddle coils
Fast switch on/of 500 µs

34. RF coils Frequencies 1 MHz - 10GHz Transmitter coil - sends RF pulse
Receive coils (can be same as transmitter) - receive RF signal

35. Magnetic Shielding Layers of steel plates around the magnets
RF shielding - faraday cage (copper sheet metal all around the MRI room.

36. Homework Please write a short description of
T1 Weighting
T2 Weighting
Spin Proton Weighting
(Matlab should be used to generate graphs that will help your description)

37. Images References The essential physics of medical imaging (Bushberg)
Lucas Parra CCNY

38. Matlab exercise

39. Pulse effect
We start by assuming that the equilibrium magnetization vector is
[0, 0, 1]'
If we had a perfect 90-degree excitation, about the y axis, then the vector becomes [1, 0, 0]'
Try defining M=[1, 0, 0]' in Matlab, and notice the result.

40. Transverse relaxation
Exponential decay process of the x and y components of magnetization
Mathematically this means
Mx(t)=Mx(0)exp(-t/T2)
My(t)=My(0)exp(-t/T2).

41. Transverse relaxation
Assume M consists of only an x component.
Let's say that T2=100 ms.
Ignoring other effects, what is the magnetization vector due to T2-decay after 50 ms?

42. Answer
[ ];

43. Transverse relaxation
What matrix do you need to do this in vector form? (remember your homework)

44. Answer
[ exp(-50/100) ;
exp(-50/100) 0; ];