Lecture 19- Functional MRI

Question 1

What is the basic principle of functional MRI?

Answer 1

How does MRI signal change with time. How does it change for example when someone is doing a task versus when they are at rest.

Question 2

How does functional MRI compare to structural and diffusion MRI?

Answer 2

-Structural MRI takes ~5 minutes to acquire one image
-Diffusion and functional MRI take lots of fast images (one every 1-3 seconds) for a total of ~5 minutes (100-300 images+), which are more susceptible to artefacts (mistakes/ things appearing that aren’t actually there e.g. hole)
-To help with fast scanning, diffusion and fMRI have lower
spatial resolution (1-3mm) than structural MRI (<1mm)

Question 3

How does the physiology of a fMRI work?

Answer 3

• Neurons require more oxygen when active
• This results in the hemodynamic response: vasodilation of blood vessels lead to faster blood flow + more blood volume (more oxygen rich blood coming in). Due to this you also get more oxygen in the output/ venules.
• Oxygenated hemoglobin has different properties to deoxygenated hemoglobin. Therefore an increase in neuron activation and thus blood oxygen results in an increase in the functional MRI signal

Question 4

How would you set up an fMRI experiment?

Answer 4

Get participant in fMRI machine to carry out three events:

• 1st type: word generation
• 2nd type: word shadowing
• 3rd type: null event

repeat

Question 5

What is the importance of the null event?

Answer 5

Null events act as the ‘baseline’ to compare the word events to:

• Contains a visual component
• No word component

Question 6

What does the scan show the functional MRI?

Answer 6

Can see what areas light up (are active) during certain tasks and thus infer what different parts of the brain do

Question 7

What are some limitations of functional MRI?

Answer 7

-Does not measure electrical activity or metabolic activity directly instead the
changes in blood oxygen levels resulting from electrical and metabolic activity of active neutrons
-BOLD-FMRI is qualitative (change from baseline important, not baseline itself: remember baselines change from time to time and from person to person)
-Sensitive to fast imaging artefacts (like diffusion)

Question 8

What are two complimentary techniques to functional MRI? How do these alternative techniques compare?

Answer 8

Positron Emission Tomography (PET):

• Measures brain metabolism directly
• Can use radioactive tracers
• Slower temporal resolution vs. fMRI
• Reduced spatial resolution vs. fMRI

Electroencephalography (EEG):

• Measures electrical brain activity directly
• Much faster temporal resolution vs. fMRI (milliseconds)
• Reduced spatial resolution vs. fMRI