Task 5 - MRI, PET, fMRI Flashcards
What is the main disadvantage of PET?
That you have to inject a radioactive tracer.
Describe the basic functionality of a PET scanner on a molecular level.
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.
What is the advantage of using a tracer?
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.
Which experimental designs can be used with PET?
Only blocked designs because of the need to average across a longer time.
What form of Hemoglobin has magnetic properties?
Deoxyhemoglobin
What is represented by the BOLD level?
The ration of deoxygenated to oxygenated Hemoglobin
What has higher spatial accuracy - fMRI or PET?
fMRI
What are the three main disadvantages of fMRI?
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.
What is the rough physical process in MRI?
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.
Why is there a critical time window for PET measurements?
Because the tracer takes 30sec to enter the brain and is then effective for only a certain time.
What does HRF stand for?
Hemodynamic Response Function
What is the temporal resolution of fMRI?
~4sec
What is meant by the process of stereotactic normalization?
Mapping a brain onto a standard reference
What is done in the smoothing process and why is it done?
Differences in activation is distributed over multiple voxels. This improves signal-to-noise ratio since single random activations are smoothed out.
What is the rough outline of performing functional MRI imaging?
Data Acquisition -> Correction for Head Movements -> Stereotactic Normalization -> Smoothing -> Application of Experimental design (block vs ER) -> Statistical comparison
When it comes to what sort of inferences can be made, where are the limitations of brain imaging experiments?
They cannot provide causal evidence.
What is used in fMRI to infer the BOLD level?
Dephasing - The speed of return to randomness from precession.
What is the parameter that regulates temporal resolution in fMRI?
Time to repetition
What can be done to improve both spatial and temporal aspects without requiring a stronger scanner?
Jittering - Varying the inter-stimulus-interval such that it does not always line up with the time to repetition
On a methodological level, what kind of study goals can investigated with fMRI?
- Commonalities of Brain activation
- Distinctiveness in Brain Activation
- Individual Differences
- Testing Psychological Models
What are some advantages of PET measurements?
- We can trace specific pathways
- It is less susceptible to signal distortion around air cavities
How can one prevent many brain regions being active by chance in brain imaging?
Using a strict Bonferroni corrected alpha level
Using the False Discovery Rate (based on number of positive results)