Opportunity to Participate
• EEG studies of
vision/hearing/decision
making – takes about 2
hours
• Sign up at www.tatalab.ca
– Keep checking back there
for more time slots
• Two extra points added to
your final grade!
Structural and Functional Imaging
• This is a Functional MRI Image !?
Structural and Functional Imaging
• This is a structural MRI image (an “anatomical” image)
Structural and Functional Imaging
• What you really want is both images co-registered
Structural and Functional Imaging
• What you really want is both images co-registered
But what is this really an image of!?
MRI Image Formation
• First you need a scanner:
– The first MRI scanner
MRI Image Formation
• Our Scanner
MRI Image Formation
• Our Scanner
Structural and Functional Imaging
• 2-dimensional structural and functional images are comprised of pixels
Pixels
Structural and Functional Imaging
• Brain scan images (CAT, PET, MRI, fMRI) are all
made up of pixels (stands for picture
elements)
• But of course the brain is 3-D not 2-D
Pixels
Structural and Functional Imaging
• “Slices” are assembled into “volumes”
Pixels
Structural and Functional Imaging
• Volumes are composed
of “volume elements”
or voxels
Voxels
Structural and Functional Imaging
• Another thing you want: the ability to tell
other people where something is
– “the activity was centered on voxel #653” will not
work in a scientific journal
Structural and Functional Imaging
• MRI anatomical spaces
– Talairach Space:
• Based on detailed analysis of one elderly woman
• Talairach & Tournoux (1988)
– Montreal Neurological Institute Template (MNI)
• based on average of 152 different brains, each normalized to
Talairach space
• advantage: gyri and sulci are more representative
• disadvantage: it’s blurry
– MNI “Representative Brain”
• the one brain from the 152 in the MNI Template set that is most
like the average
• advantage: it’s not blurry
• disadvantage: it’s still just one person’s brain
Structural and Functional Imaging
• Reasons for normalizing to standard
stereotaxic space (templates)
Structural and Functional Imaging
1. structural and functional
volumes may not otherwise
be coregistered due to
These pertain to a
single subject’s
data
• movement
• distortion
2. results can be described in
standard coordinates
3. data across sessions can be
averaged
Structural and Functional Imaging
These pertain to
dealing with
multiple subjects
4. Volumes will not match
because of variability across
individuals
5. data across participants can be
averaged
The Talairach Coordinate System
PC
AC
The Talairach Coordinate System
-y
AC - PC line
defines y-axis
+y
The Talairach Coordinate System
-y
+x
x-axis perpendicular to
interhemispheric plane
-x
+y
The Talairach Coordinate System
+z
-y
+x
z-axis perpendicular to xy plane
-z
+y
-x
Structural and Functional Imaging
•
Cortical Flattening
– Software such as
BrainVoyager can “inflate”
the cortex like a balloon so
that sulci and gyri are
“flattened”
– functional data can be
transformed with the same
complex function
– functional and structural
data can be overlaid so that
distribution on cortical sheet
can be visualized
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
Functional Imaging
• blood flow overshoots baseline
after a brain region is activated
• More oxygenated blood in that
region increases MR signal
from that region
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
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?”
Experimental Design in fMRI
• Experimental Design is crucial in using fMRI
• Simplest design is called “Blocked”
– alternates between active and “rest” conditions
Active
60 sec
Rest
60 sec
Active
60 sec
Rest
60 sec
Experimental Design in fMRI
• Experimental Design is crucial in using fMRI
• Simplest design is called “Blocked”
– alternates between active and “rest” conditions
Active
60 sec
Rest
60 sec
Active
60 sec
Rest
60 sec
Experimental Design in fMRI
Signal
• A voxel in tissue insensitive to the task demands
shows random signal change
Active
60 sec
Rest
60 sec
Active
60 sec
Rest
60 sec
Experimental Design in fMRI
Signal
• A voxel in tissue that responds to the task shows
signal change that matches the timecourse of the
stimulus
Active
60 sec
Rest
60 sec
Active
60 sec
Rest
60 sec
Experimental Design in fMRI
• A real example of fMRI block design done
well:
– alternate moving, blank and stationary visual
input
Moving
40 sec
Blank
40 sec
Stationary
40 sec
Blank
40 sec
Experimental Design in fMRI
• Voxels in Primary
cortex tracked all
stimuli
Experimental Design in fMRI
• Voxels in area MT tracked only the onset of
motion
Experimental Design in fMRI
• Voxels in area MT tracked only the onset of
motion
• How did they know to look in area MT?
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
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
Advantages of PET
• Advantages of PET:
1. Quiet
2. A number of different molecules can be labeled
and imaged in the body
Limitations of fMRI
• All techniques have constraints and limitations
• A good scientist is careful to interpret data
within those constraints
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