Task 5 - MRI, PET, fMRI Flashcards
PET (Positron Emission Tomography)
- A radioactive substance (usually a form of glucose or other molecules) is injected into the body.
- These tracers emit positrons as they decay.
- The positrons emitted by tracer collide with electrons in the body, leading to annihilation.
- This process produces gamma rays.
fMRI (Functional Magnetic Resonance Imaging)
A functional imaging technique that detects changes in blood oxygenation level (BOLD response) to infer brain activity associated with specific tasks.
MRI (Magnetic Resonance Imaging)
- MRI machine sends a radiofrequency (RF) pulse that temporarily disrupts the alignment of hydrogen protons.
- This pulse “flips” the protons from their aligned state to a higher energy state.
- Once pulse is turned off, the hydrogen protons relax back to their original alignment
- As they relax, they emit energy (radio waves) which is detected by MRI sensors
Relaxation Times
Times it takes for protons in the brain to return to equilibrium after being disturbed by the magnetic field of an MRI machine.
Signal-to-Noise Ratio (SNR) in Brain Imaging
The ratio between the desired signal and the background noise in imaging data. A higher SNR leads to clearer images.
Preprocessing
3 steps
The steps taken to clean and adjust neuroimaging data before it can be analyzed like:
Motion correction: Correcting for participant head movement during scanning.
Normalization: Aligning individual brain images to a standard template for group comparisons.
Smoothing: Applying filters to reduce noise and enhance signal detection.
Hemodynamic Response Function (HRF)
HRF models how the brain’s BOLD signal changes in response to stimuli, providing insight into neural activation patterns.
Usually looks like:
1. Initial dip
2. Peak (circa 3 seconds after stimulus presentation)
3. Overshoot
4. Decay
Time to Repetition (TR)
fMRI
TR is the time interval between successive image acquisitions. A shorter TR leads to higher temporal resolution, capturing quicker changes in brain activity.
Spatial resolution of fMRI
1-3 mm – fMRI provides high spatial resolution, allowing detailed imaging of large brain regions (better than EEG/MEG).
Temporal resolution of fMRI
~2-3 seconds – fMRI’s temporal resolution is lower compared to EEG/MEG
* Based on the hemodynamic response (blood flow changes), which takes seconds to develop.
* Also influenced by time to repetition (TR)
BOLD signal
- BOLD is a signal used in fMRI that detects changes in blood oxygenation as an indirect measure of brain activity.
- Oxygenated hemoglobin is less magnetic, while deoxygenated hemoglobin is more magnetic, creating a detectable signal.
Berman’s (5) fMRI approaches/applications
- Studies of Localization
- Studies of Commonalities in Brain Activation
- Studies of Distictiveness on Brain Activation
- Studies of individual differences
- Testing psychological models
