measuring blood oxygenation in the brain. functional imaging functional imaging must provide a...

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


• Onset of a stimulus (or cognitive task) changes local blood oxygenation– first with a decrease

– then with an “overshoot”

Functional Imaging


• 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 de-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 de-phase

• 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 Rest Active Rest

60 sec 60 sec 60 sec 60 sec


• Experimental Design is crucial in using fMRI

• Simplest design is called “Blocked”– alternates between active and “rest” conditions




























• A voxel in tissue insensitive to the task demands shows random signal change



























Sig

nal


• A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulus



























Sig

nal


• A real example of fMRI design done well:– alternate moving, blank and stationary visual input

Moving Blank Stationary Blank



• Voxels in Primary cortex tracked all stimuli




• Voxels in area MT tracked only the onset of motion




• 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



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

measuring blood oxygenation in the brain. functional imaging functional imaging must provide a...

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oxygenated blood

functional imaging recall

echo slide

measuring blood oxygenation

magnetic field slide

brain slide

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overshoot slide