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Physical Principles of MRI Signal Generation
- 1. PHYSICAL PRINCIPLES Signal Generation
- 2. nuclei, protons and spins longitudinal relaxation T1 transverse relaxation T2 exitation and relaxation relation between T1 and T2 signal generation
- 3. transverse relaxation T2 Spin - Spin relaxation signal generation
- 4. X Y Z B o signal generation transverse relaxation T2
- 5. X Z B o Y signal generation transverse relaxation T2
- 6. X Y Z B o signal generation transverse relaxation T2
- 7. X Y Z B o signal generation transverse relaxation T2
- 8. X Y Z B o signal generation transverse relaxation T2
- 9. X Y Z B o signal generation transverse relaxation T2
- 10. X Y Z B o signal generation transverse relaxation T2
- 11. X Y Z B o signal generation transverse relaxation T2
- 12. X Y Z B o signal generation transverse relaxation T2
- 13. X Y Z B o signal generation transverse relaxation T2
- 14. X Y Z B o signal generation transverse relaxation T2
- 15. X Y Z B o Signal signal generation transverse relaxation T2
- 16. X Y Z B o Signal signal generation transverse relaxation T2
- 17. X Y Z B o Signal signal generation transverse relaxation T2
- 18. X Y Z B o Signal signal generation transverse relaxation T2
- 19. X Y Z B o Signal dephasing signal generation transverse relaxation T2
- 20. S time F ree I nduction D ecay signal FID T2 signal generation transverse relaxation T2
- 21. T2 S T2 definition T2: the time that passes untill 63% of the original signal has been lost time 63 % signal generation transverse relaxation T2
- 22. T2 S T2* time signal generation transverse relaxation T2* Due to magnetic inhomegeneities the transverse relaxation is lost faster !! T2 T2*
- 23. X Y Z B o signal generation transverse relaxation T2
- 24. X Y Z B o transmit coil radio frequency transmitter signal generation transverse relaxation T2
- 25. X Y Z B o transmit coil radio frequency transmitter 180 º 180 º signal generation transverse relaxation T2
- 26. X Y Z B o 180 º signal generation transverse relaxation T2
- 27. X Y Z B o 180 º signal generation transverse relaxation T2
- 28. X Y Z B o 180 º signal generation transverse relaxation T2
- 29. X Y Z B o 180 º signal generation transverse relaxation T2
- 30. X Y Z B o 180 º signal generation transverse relaxation T2
- 31. X Y Z B o 180 º signal generation transverse relaxation T2
- 32. X Y Z B o 180 º signal generation transverse relaxation T2
- 33. X Y Z B o 180 º signal generation transverse relaxation T2
- 34. X Y Z B o 180 º signal generation transverse relaxation T2
- 35. X Y Z B o 180 º signal generation transverse relaxation T2
- 36. 180 º refocusing pulse signal generation transverse relaxation T2
- 37. refocusing pulse 180 º 180 º signal generation transverse relaxation T2
- 38. T2 envelope signal generation transverse relaxation T2
- 40. nuclei, protons and spins longitudinal relaxation T1 transverse relaxation T2 excitation and relaxation relation between T1 and T2 signal generation
Notas del editor
- THIS IS THE
- Transverse relaxation is also called Spin-Spin relaxation. The speed of rotation is influenced by the neighbouring protons. Compare with walking in a shopping mall on a Saturday afternoon. While walking you have to change your speed and direction constantly to avoid collisions whith other shoppers. The only difference is that protons do collide.
- When we look at one single proton, we can see that it makes one 360 degree rotation in a certain time...
- ...
- ... But in the same time the red proton has covered more than 360 degrees, and the blue proton less.
- After the next rotation these differences are bigger.
- ..and bigger
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- Back to the first rotation and look at the signals that belong to the protons..
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- ...
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- Now we add the summed signal, and you can see that the signal is getting weaker the more the protons are pointing in different directions.
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- The signal shift relative to each other, cancelling each other out. This is called dephasing.
- The signal that is generated after the 90 degree pulse is called FID or T2*.
- And is different for the various tissues. The time untill 63% of the signal has ´´died´´ is called the T2 time.
- And is different for the various tissues. The time untill 63% of the signal has ´´died´´ is called the T2 time.
- At the dawn of MR this signal could not be measured, because the hardware did not allow the fast switching. So we had to find a trick to overcome this problem.
- For that we need the help of the transmit coil again.
- When we transmit a signal for so long time that the protons are flipped over 180 degrees, look what happens..
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- ...
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- The slow protons will be overtaken by the faster ones, thus generating a new strong signal.
- ...
- ...
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- In other words. The signal is restored with the help of a 180 degree pulse, resulting in a strong echo.
- Thats why we call it a refocussing pulse.
- As you saw, after the rephasing of the protons they will dephase again. And again we can apply a 180 degree pulse in order to rephase them, resuting in new signal or echo...
- The curve that we can draw through the tops of the echos is called the T2-envelope.
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