Principles of MRI

1. Principles of MRI

2. Principles of MRI • Some terms: • Nuclear Magnetic Resonance (NMR) • quantum property of protons • energy absorbed when precession frequency matches radio frequency • Magnetic Resonance Imaging (MRI) • uses spatial differences in resonance frequencies to form an image • basis of anatomical MRI • functional Magnetic Resonance Imaging (fMRI) • exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

3. Principles of MRI • Some terms: • Nuclear Magnetic Resonance (NMR) • quantum property of protons • energy absorbed when precession frequency matches radio frequency • Magnetic Resonance Imaging (MRI) • uses spatial differences in resonance frequencies to form an image • basis of anatomical MRI • functional Magnetic Resonance Imaging (fMRI) • exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

4. Principles of MRI • Some terms: • Nuclear Magnetic Resonance (NMR) • quantum property of protons • energy absorbed when precession frequency matches radio frequency • Magnetic Resonance Imaging (MRI) • uses spatial differences in resonance frequencies to form an image • basis of anatomical MRI • functional Magnetic Resonance Imaging (fMRI) • exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

5. Principles of MRI • Some terms: • Nuclear Magnetic Resonance (NMR) • quantum property of protons • energy absorbed when precession frequency matches radio frequency • Magnetic Resonance Imaging (MRI) • uses spatial differences in resonance frequencies to form an image • basis of anatomical MRI • functional Magnetic Resonance Imaging (fMRI) • exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

6. Principles of NMR • Protons are like little magnets • they orient in magnetic fields like compass needles • what way do they normally point?

7. Principles of NMR • Protons are like little magnets • they orient in magnetic fields like compass needles • what way do they normally point? • normally aligned with Earth’s magnetic field

8. Principles of NMR • Protons are like little magnets • they orient in magnetic fields like compass needles • what way do they normally point? • normally aligned with Earth’s magnetic field • NMR uses a big magnet to align all the protons in a sample (e.g. brain tissue)

9. Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field

10. Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field • once out of alignment, the protons begin to precess

11. Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field • once out of alignment, the protons begin to precess • protons gradually realign with field (relaxation)

12. Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field • once out of alignment, the protons begin to precess • protons gradually realign with field (relaxation) • protons “echo” back the radio frequency that originally tipped them over • That radio “echo” forms the basis of the MRI image

13. Principles of NMR • Protons are like little magnets • The following simple equation explains MRI image formation

14. MRI Image Formation • First you need a scanner: • The first MRI scanner

15. MRI Image Formation • Modern Scanners

16. MRI Image Formation • Our Scanner

17. MRI Image Formation • Our Scanner

18. MRI Image Formation • Our Scanner

19. MRI Image Formation • Our Scanner

20. MRI Image Formation • MRI Image formation • resonance frequency depends on field strength • gradient coils alter resonance frequency over distance • slight differences in the “echo” frequency indicate the location of each proton • second-dimension of a slice is coded by the phase of the protons Increasing Field Strength field gradient = frequency gradient

21. Functional Imaging • Functional Imaging must provide a spatial depiction of some process that is at least indirectly related to neural activity • in most imaging (i.e. PET, fMRI) that process is change in blood oxygenation related to changes in regional cerebral blood flow • Why should we measure blood oxygenation?

22. Functional Imaging • Why should we measure blood oxygenation? • Onset of a stimulus (or cognitive task) changes local blood oxygenation • first with a decrease • then with an “overshoot”

23. Functional Imaging • Why should we measure blood oxygenation? • Onset of a stimulus (or cognitive task) changes local blood oxygenation • first with a decrease • then with an “overshoot” • How do we measure changes in blood oxygenation?

24. Measuring Blood Oxygenation in the Brain

25. Functional Imaging • Recall that precessing protons give off a radio “echo” as they realign with the magnetic field

26. Functional Imaging • Recall that precessing protons give off a radio “echo” as they realign with the magnetic field • We pick up the combined echo from many protons that are in phase

27. Functional Imaging • recall that the precession frequency depends on the field strength • anything that changes the field at one proton will cause it to de-phase

28. Functional Imaging • recall that the precession frequency depends on the field strength • anything that changes the field at one proton will cause it to de-phase • The de-phased region will give off less echo

29. Functional Imaging • Oxygenated hemoglobin is diamagnetic - it has no magnetic effects on surrounding molecules • Deoxygenated hemoglobin is paramagnetic - it has strong magnetic effects on surrounding molecules! Hemoglobin Heme

30. Functional Imaging • Oxygenated hemoglobin is diamagnetic - it has no magnetic effects on surrounding molecules • Deoxygenated hemoglobin is paramagnetic - it has strong magnetic effects on surrounding molecules! • Thus deoxygenated tissue gives of less MR echo because the protons de-phase quickly

31. Functional Imaging • blood flow overshoots baseline after a brain region is activated • More oxygenated blood in that region increases MR signal from that region (other regions de-phase faster)

32. Functional Imaging • It is important to recognize that fMRI “sees” changes in the ratio of oxygenated to deoxygenated blood - nothing more • BOLD: Blood Oxygenation Level Dependant contrast • How do we create those pretty pictures?