9. Principles of MRI

10. 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

11. 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

12. 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

13. 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

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

15. 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

16. 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)

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

18. 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

19. 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)

20. 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

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

22. 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

23. Cognitive Neuroscience

38. 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

39. Functional Imaging blood flow overshoots baseline after a brain region is activated More oxygenated blood in that region increases MR signal from that region

40. 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

41. 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

42. 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

43. 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?”

44. MRI Image Formation First you need a scanner: – The first MRI scanner

45. MRI Image Formation Modern Scanners

46. MRI Image Formation Our Scanner

47. MRI Image Formation Our Scanner

48. MRI Image Formation Our Scanner

49. MRI Image Formation Our Scanner

50. Experimental Design in fMRI Experimental Design is crucial in using fMRI Simplest design is called “Blocked” – alternates between active and “rest” conditions ActiveRestActiveRest 60 sec

51. Experimental Design in fMRI Experimental Design is crucial in using fMRI Simplest design is called “Blocked” – alternates between active and “rest” conditions ActiveRestActiveRest 60 sec

52. Experimental Design in fMRI A voxel in tissue insensitive to the task demands shows random signal change ActiveRestActiveRest 60 sec Signal

53. Experimental Design in fMRI A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulus ActiveRestActiveRest 60 sec Signal

54. Experimental Design in fMRI A real example of fMRI block design done well: – alternate moving, blank and stationary visual input MovingBlankStationaryBlank 40 sec

55. Experimental Design in fMRI Voxels in Primary cortex tracked all stimuli

56. Experimental Design in fMRI Voxels in area MT tracked only the onset of motion

57. Experimental Design in fMRI Voxels in area MT tracked only the onset of motion How did they know to look in area MT?

58. PET: another way to measure blood Oxygenation Positron Emission Tomography (PET) Injects a radioisotope of oxygen PET scanner detects the concentration of this isotope as it decays

59. Advantages of fMRI Advantages of MRI: 1.Most hospitals have MRI scanners that can be used for fMRI (PET is rare) 2.Better spatial resolution in fMRI than PET 3.Structural MRI is usually needed anyway 4.No radioactivity in MRI 5.Better temporal resolution in MRI

60. Advantages of PET Advantages of PET: 1.Quiet 2.A number of different molecules can be labeled and imaged in the body

61. Limitations of fMRI All techniques have constraints and limitations A good scientist is careful to interpret data within those constraints

62. Limitations of fMRI Limitations of MRI and PET: 1.BOLD signal change does not necessarily mean a region was specifically engaged in a cognitive operation 2.Poor temporal resolution - depends on slow changes in blood flow 3.expensive