16. Functional MRI

Question 1

How does MRI work?

Answer 1

• Hydrogen atoms have a property called spin - acts like a tiny magnet.
  *These spins will align with MRI magnetic field.
  *RF pulses from scanner causes spins to change position.
  *When RF pulses stop, spins return to original position by giving off energy.
  *Energy is detected and then used to form an image.
  *Different tissues give off energy at different rates which creates contrast in images.

Question 2

What is FMRI

Answer 2

A tool for measuring neural activity in the brain.

Question 3

How does FMRI work?

Answer 3

Standard maps of activation only true for ‘standard’ people.
Detects changes in oxygen of blood in brain.

Question 4

Describe how the haemodynamic response relates to fMRI.

Answer 4

Changes in blood flow and oxygen are closely linked to neural activity.
1. Task starts
2. Active nerve cells consume oxygen
3. Initial decrease in blood oxygen
4. Increase in blood flow, increased blood oxygen
5. Increase in MRI signal

Question 5

Explain the relevant properties of Haemoglobin in fMRI.

Answer 5

1. Hb carries oxygen around body, (oxy Hb is diamagnetic and deoxy is paramagnetic.
   Changes in their concentrations caused local magnetic field changes which affects the speed at which the spins return to baseline position. Hence it affects the amplitude of the MRI signal detected.

Question 6

Describe resting blood flow and haemoglobin.

Answer 6

Normal blood flow
Normal ratio of Hbo2 to Hbr
Normal MRI signal

Question 7

Describe activated blood flow and haemoglobin.

Answer 7

Increased blood flow
Increased ratio of HbO2 to Hbr
Increased MRI signal

Question 8

What functions can be tested using fMRI.

Answer 8

Motor - finger tapping/toe curling
Language - generate word beginning with the letter…
Memory - remembering words

Question 9

How would the signal of the brain in fMRI change with time.

Answer 9

Signal would fluctuate in sync with the task.
Signal will not change in uninvolved parts.

Question 10

Describe 2 properties of fMRI images.

Answer 10

Relatively poor resolution
Numerous slices - 25 slices per volume, 110 volumes per dataset - lots of disk space

Question 11

What are the preprocessing steps before analysis?

Answer 11

1. Motion correction.
2. Align with high resolution ‘structural’ image.
3. Warp to match template.

Question 12

Describe the statistical analysis of fMRI images.

Answer 12

regression analysis to determine the extent to which each voxel fits the expected response over time.

Question 13

Equation
Regression example

Answer 13

1. Calculate t or F statistic to see whether a is statistically significant for that pixel. Repeat for all pixels.
   Observed = Constant (a) x expected response if activated + mean pixel value

Question 14

What are display sections?

Answer 14

Light up pixels that have statistically significant activation.

Question 15

What are the criticisms of fMRI?

Answer 15

BOLD signal is only indirect measure of neural activity - susceptible to influence of non-neural changes.
Different brain regions have different haemodynamic response function
Poor temporal response.

Question 16

What are the uses of fMRI?

Answer 16

Pre surgical brain mapping (epilepsy)
Study of normal brain function
Clinical neuroscience research
Pharmacological research