Functional MRI Methods

Question 1

What does fMRI measure

Answer 1

Measures the blood oxygen level in specific regions

Question 2

Explain the biology of fMRI

Answer 2

when the brain region needs energy, needs adenosine triphosphate to generate energy
Need oxygen to metabolise glucose to generate adenosine triphosphate
oxygenated haemoglobin carries oxygen to the brain region and leaves as deoxygenated haemoglobin
Therefore, more oxygenated blood in the region = more active of the region

Question 3

Explain BOLD Signals and HRF

Answer 3

Blood Oxygen-Level Dependent (BOLD) Signals = the signal that measures the oxygenated blood level in the region
The higher the BOLD Signal = the whiter the region is = the more active the region is
Oxygenated blood = diamagnetic = better signals

Hemodynamic Response Function (HRF) = the function that describes the change of BOLD Signals
small dip > peak (oversupplying ) (4-8 seconds) > Negative overshoot > baseline (16 secs)
HRF is similar across every region, BUT NOT THE SAME. THEREFORE CANNOT DIRECTLY COMPARE THE HRF ACROSS DIFFERENT REGIONS
The peak of the HRF = 4-8 secs after the peak activity level of the brain
the signals add up linearly

Question 4

What do the BOLD Signals reflect

Answer 4

complicated
earlier research says reflecting the local potential fields (i.e., the input of signals of a neuron)
later research says also reflecting the action potentials (output of signals)
in sum, mixture of the local cortical exhibition-inhibition network

Question 5

Explain how the fMRI results can be sparse

Answer 5

Since it is impossible to measure the every timepoint of the HRF, therefore have to predict the rest of the signals (sparse)

with the statistical parametric mapping system for analysis
using the general linear modelling approach> search for brain regions that fit the model of the HRF

Question 6

Rough steps of measuring fMRI results

Answer 6

1. Measure the brain many times
2. Average the results and the results should appear in grey and white with the T2* decay and the T2* weighted pulse sequences (white points indicate high oxygen level = high activation)
3. Do test on whether the white brain region aligns with the GLM, and t-test whether the white region is significantly different with other brain regions
4. If fits GLM and statistically different = orange in the brain structural image

Question 7

Explain the technical of fMRI (i.e., T2* decay)

Answer 7

fMRI uses the T2* weighted pulse sequence to measure the decay of protons
since oxygenated blood are diamagnetic, they have lower disruption to the protons (i.e., the protons in the region with high oxygenated blood will precess longer at the transversal plane than neurons in regions with deoxygenated blood)
this means the protons in the more active areas will provide stronger signals to the fMRI
T2* weighted pulses provides signals that are only due to the change of blood oxygen level
T2*weighted pulse consequences = mostly used by BOLD fMRI