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Principles of MRI

Principles of MRI. 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

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Principles of MRI

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  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. 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

  15. Cognitive Neuroscience

  16. Cognitive Neuroscience

  17. Cognitive Neuroscience

  18. Cognitive Neuroscience

  19. Cognitive Neuroscience

  20. Cognitive Neuroscience

  21. Cognitive Neuroscience

  22. Cognitive Neuroscience

  23. Cognitive Neuroscience

  24. Cognitive Neuroscience

  25. Cognitive Neuroscience

  26. Cognitive Neuroscience

  27. Cognitive Neuroscience

  28. Cognitive Neuroscience

  29. Cognitive Neuroscience

  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! Hemoglobin

  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

  32. 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

  33. 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

  34. 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

  35. Functional Imaging • How do we create those pretty pictures? • We ask the question “When the subject engages in this cognitive task, where does blood oxygenation change significantly?” “where does it change randomly?”

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

  37. MRI Image Formation • Modern Scanners

  38. MRI Image Formation • Our Scanner

  39. MRI Image Formation • Our Scanner

  40. MRI Image Formation • Our Scanner

  41. MRI Image Formation • Our Scanner

  42. Experimental Design in fMRI • Experimental Design is crucial in using fMRI • Simplest design is called “Blocked” • alternates between active and “rest” conditions Active Rest Active Rest 60 sec 60 sec 60 sec 60 sec

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