Measuring brain activity/MRI
13 important questions on Measuring brain activity/MRI
What is the normal situation and what happens with fMRI?
Normal situation: atomic nuclei are randomly oriented: net signal is 0.
The magnetic field inside the scanner causes the nuclei to align with the direction of the field
When pointing in the same direction, the tiny magnetic signals from individuals nuclei add up resulting in a signal large enough to measure
What is measured in MRI?
The signal from hydrogen in water
Magnetic field cause protons to align to the B0 in one of two states. Which ones?
Parallel (low-energy!)
Anti-parallel (high energy!)
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How is RF (radio frequency) used in MRI?
by sending in a RF pulse at a specific frequency we can selectively energize 1H
What is the reaction with Gradients?
field stronger at one location compared to another.
Larmor frequency different along this dimension.
RF pulse only energizes slice where field strength matches Larmor frequency.
How is a left-right regulated?
Frequency encode
How is a top-bottom regulated?
Phase encode
What are the steps of MRI?
- Transmit radio frequency pulse: atom absorb energy
- Wait
- Listen to radio frequency emission due to relaxation
- Wait, go to 1
What does the fMRI BOLD signal show?
A relative level of neuronal activity compared to an earlier situation: NOT an absolute level.
What is meant by the Sternberg task?
Three conditions: novel (set of consonants is novel), practiced (set of consonants is practiced prior to scanning), rest (subjects fixate passively)
Subjects become faster+more accurate after practice.
Contrast novel - practiced = effect of practice
What are the pros and cons of a blocked design?
Pro: maximal statistical power, simple and easy to implement.
Con: No mixing of trial types within block, habituation, some events cannot be blocked e.g. oddball
What is meant by the anatomical image and the functional images?
T1 (high resolution, different tissue types and used for normalization, localization)
and T2* (time series, lower spatial resolution, grey value represents the BOLD signal)
What is meant by first level analysis
Single subject analysis (activation over scans; within-subjects)
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