Task 5 - MRI, PET, fMRI

Question 1

What is the main disadvantage of PET?

Answer 1

That you have to inject a radioactive tracer.

Question 2

Describe the basic functionality of a PET scanner on a molecular level.

Answer 2

When the positrons emitted from the tracer’s nuclei collide with an electron (indicating electric activity), gamma rays are created. The PET scanner is then basically a gamma ray detector, that reconstructs the location of the collision.

Question 3

What is the advantage of using a tracer?

Answer 3

There are many tracers with different properties. Some bind to certain neurodegenerative disorder molecules and can thus be used as a biomarker in the brain.

Question 4

Which experimental designs can be used with PET?

Answer 4

Only blocked designs because of the need to average across a longer time.

Question 5

What form of Hemoglobin has magnetic properties?

Answer 5

Deoxyhemoglobin

Question 6

What is represented by the BOLD level?

Answer 6

The ration of deoxygenated to oxygenated Hemoglobin

Question 7

What has higher spatial accuracy - fMRI or PET?

Answer 7

fMRI

Question 8

What are the three main disadvantages of fMRI?

Answer 8

Its temporal resolution is not capable of capturing the fine-grained effects like in EEG.

The BOLD signal only indirectly corresponds to brain activity.

Scanning in an fMRI can only be done while lying down in a narrow tube with a lot of acoustic noise.

Question 9

What is the rough physical process in MRI?

Answer 9

Magnetic field aligns protons in the brain. A radio pulse causes them to change orientation. This creates changes in the magnetic field. The time it takes the protons to go back into their relaxed state is called T1 relaxation time. This can be used to differentiate between tissues.

Question 10

Why is there a critical time window for PET measurements?

Answer 10

Because the tracer takes 30sec to enter the brain and is then effective for only a certain time.

Question 11

What does HRF stand for?

Answer 11

Hemodynamic Response Function

Question 12

What is the temporal resolution of fMRI?

Answer 12

~4sec

Question 13

What is meant by the process of stereotactic normalization?

Answer 13

Mapping a brain onto a standard reference

Question 14

What is done in the smoothing process and why is it done?

Answer 14

Differences in activation is distributed over multiple voxels. This improves signal-to-noise ratio since single random activations are smoothed out.

Question 15

What is the rough outline of performing functional MRI imaging?

Answer 15

Data Acquisition -> Correction for Head Movements -> Stereotactic Normalization -> Smoothing -> Application of Experimental design (block vs ER) -> Statistical comparison

Question 16

When it comes to what sort of inferences can be made, where are the limitations of brain imaging experiments?

Answer 16

They cannot provide causal evidence.

Question 17

What is used in fMRI to infer the BOLD level?

Answer 17

Dephasing - The speed of return to randomness from precession.

Question 18

What is the parameter that regulates temporal resolution in fMRI?

Answer 18

Time to repetition

Question 19

What can be done to improve both spatial and temporal aspects without requiring a stronger scanner?

Answer 19

Jittering - Varying the inter-stimulus-interval such that it does not always line up with the time to repetition

Question 20

On a methodological level, what kind of study goals can investigated with fMRI?

Answer 20

• Commonalities of Brain activation
• Distinctiveness in Brain Activation
• Individual Differences
• Testing Psychological Models

Question 21

What are some advantages of PET measurements?

Answer 21

• We can trace specific pathways

- It is less susceptible to signal distortion around air cavities

Question 22

How can one prevent many brain regions being active by chance in brain imaging?

Answer 22

Using a strict Bonferroni corrected alpha level

Using the False Discovery Rate (based on number of positive results)