Lecture 20 MRI Flashcards
(30 cards)
Why do psychologists use neuroimaging?
To understand cognitive processes by studying their brain-based implementation.
What is fMRI?
Functional Magnetic Resonance Imaging; a method to study brain activity using a magnetic field.
What is the typical field strength of fMRI magnets?
3 Tesla for research
What does the RF coil do?
generates signal
Why is being in an MRI scanner generally safe?
Strong magnetic fields are harmless
What are hydrogen protons also called?
Spins
How do protons behave in a strong magnetic field?
They align parallel or anti-parallel to the B0 field and precess at the Larmor frequency.
What is the Larmor frequency?
The frequency at which protons precess in a magnetic field
What is the role of the B0 field?
- The B0 field is oriented into the direction of the Z-axis in the scanners coordinate system
Why can’t we measure a signal before RF pulses?
Protons are randomly out of phase and aligned with B0
What does an RF pulse do?
It excites protons
- The first effect of the RF pulse is that all protons will start precessing (spinning) in phase, meaning that their magnetisation will all point to same location in space at the same time
- The second effect of the RF pulse is that the magnetisation vector (i.e. the net magnetisation that the protons have together) is tilted away from the Z-axis
- This means, the magnetisation is tilted from the longitudinal direction (of the field B0) into the transversal plane (the X-Y-plane)
Why do we get a signal after RF excitation?
Because protons precess in phase in the transverse plane
What is T1 relaxation?
Longitudinal/spin-lattice relaxation; recovery of magnetisation along B0.
What is T2 decay?
Transverse/spin-spin relaxation; loss of phase coherence and decay of transverse magnetisation.
How do T1 and T2 differ across tissues?
They vary by tissue type
What is a T1-weighted image?
An MRI image emphasizing T1 differences; good for anatomical detail.
What is a T2-weighted image?
An MRI image emphasizing T2 decay differences; useful for highlighting fluids like CSF.
What is the role of gradient coils?
They manipulate the magnetic field to localize signal spatially in the brain.
What does the slice selection gradient do?
Selects a brain slice by matching RF pulse frequency to local precession frequency (Z-axis).
What does the phase encoding gradient do?
- Phase Encoding (Y-axis):
A phase gradient is applied to shift the phase of spins across a slice.
Degree of phase shift varies across locations—used to infer spatial position in one plane.
What does the frequency encoding gradient do?
- Frequency Encoding (X-axis):
Spins are made to precess at slightly different frequencies depending on their location.
Why are 3 gradient directions used?
To reconstruct 3D spatial information from the MRI signal.
What does the voxel refer to?
A 3D measurement point in the brain; spatial resolution unit in MRI.
What is the field of view (FOV)?
The total area/volume covered in one scan
