Measuring Blood Oxygenation in the
Brain
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?
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”
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?
Functional Imaging
• Recall that precessing protons
give off a radio “echo” as they
realign with the magnetic field
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
Functional Imaging
• recall that the precession
frequency depends on the field
strength
– anything that changes the field
at one proton will cause it to dephase
Functional Imaging
• recall that the precession
frequency depends on the field
strength
– anything that changes the field
at one proton will cause it to dephase
• The de-phased region will give
off less echo
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!
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Hemoglobin
Heme
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
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)
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?
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?
• 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
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Active
60 sec
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Rest
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Rest
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Experimental Design in fMRI
Signal
• A voxel in tissue insensitive to the task demands
shows random signal change
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Rest
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Rest
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Experimental Design in fMRI
Signal
• A voxel in tissue that responds to the task shows
signal change that matches the timecourse of the
stimulus
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Rest
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Experimental Design in fMRI
• A real example of fMRI 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
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Experimental Design in fMRI
• Voxels in area MT tracked only the onset of motion
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Experimental Design in fMRI
• Voxels in area MT tracked only the onset of motion
• How did they know to look in area MT?
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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
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PET: another way to measure blood
Oxygenation
• Although oxygenation is measured differently, the
logic of PET and fMRI are similar: compare active
and “rest” conditions
Advantages of fMRI
• All techniques have certain advantages
• A good scientist leverages these advantages
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