1. 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

2. 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

3. 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

4. Functional Imaging
blood flow overshoots baseline after a brain region is activated
Deoxygenated blood in some region causes relatively less signal from that region
More oxygenated blood in some region causes relatively more signal from that region

5. 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

6. 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?”

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

8. MRI Image Formation
Modern Scanners

9. MRI Image Formation
Our Scanner

10. MRI Image Formation
Our Scanner

11. MRI Image Formation
Our Scanner

12. MRI Image Formation
Our Scanner

13. Experimental Design in fMRI
Experimental Design is crucial in using fMRI
Simplest design is called “Blocked”
alternates between active and “rest” conditions
Active
Rest
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Rest
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14. Experimental Design in fMRI
Experimental Design is crucial in using fMRI
Simplest design is called “Blocked”
Cognitive state alternates between “active” and “rest” conditions
Scanner takes a sequence of “volume” images
Active
Rest
Active
Rest
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15. Experimental Design in fMRI
A voxel in tissue insensitive to the task demands shows random signal change
Signal
Active
Rest
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16. Experimental Design in fMRI
A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulus
Signal
Active
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Rest
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17. Experimental Design in fMRI
A real example of fMRI block design done well:
alternate moving, blank and stationary visual input
Moving
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Stationary
Blank
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18. Experimental Design in fMRI
Voxels in Primary cortex tracked all stimuli

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

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

21. Structural and Functional Imaging
What you really want is an image, not a squiggly line
Make a map of a statistic (like t-score or z-score) that describes how well each voxel tracked the cognitive task:
Set all “non-significant” voxels to be transparent

22. Structural and Functional Imaging
What you really want is an image, not a squiggly line
Make a map of a statistic (like t-score or z-score) that describes how well each voxel tracked the cognitive task:
Set all “non-significant” voxels to be transparent

23. 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

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

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

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

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

28. Next Time
Electrophysiology