Lecture 8 - MRI Flashcards
How MRI works
- Protons in everyday life just spinning around in any direction
- When static magnetic field is turned on (always on in MRI) protons align and are spinning
- Add a radio-frequency pulse and protons become relaxed in the orientation they want to be
2 types of relaxation after a radiofrequency pulse
T1 and T2
As proton relaxes what happens to T1 and T2
T1 gets longer and T2 gets shorter
_____ of T2 and _____ of T1
Decay of T2
Recovery of T1
MRI steps
- Protons align with a strong static magnetic field
- Radiofrequency pulse is applied to excite protons and make them spin together
- Radiofrequency pulse is turned off and the protons relax
- Coils detect the energy released during relaxation
T1 recovery is slowest for protons in _____ and ______ and fastest for protons in ____
Water and CSF
Fat
T2 recovers the slowest for ______ and the fastest for _______
CSF
White matter
Things you can do with structural MRI
Measure volume of subcortical structures, cortical thickness, and area
Why do structural MRI
- ## Correlation between size of brain area and cognitive function
Fractional anisotropy (FA)
Preferential diffusion of water in one direction compared to 2 orthogonal directions
Isotropic
Ventricles, gray matter
Anisotropic
White matter
(more diffusion in one direction than compared with the other 2 orthogonal directions)
Brightness in DTI
How directional water diffusion is (gray matter and CSF are dark)
Color in DTI
Which direction is the principle direction
Myelination and DTI
As you develop and myelinate, water is more constrained by fatty myelin and you get brighter signal
Functional imaging relies on what
T2 signal differences between deoxygenated and oxygenated blood
Deoxygenated hemoglobin has a ________ magnetic susceptibility and causes what for the T2 signal
Greater magnetic susceptibility and causes a faster decay of the T2 signal
More deoxygenated hemoglobin =
Lower magnetic resonance signal
More oxygenated hemoglobin =
Higher magnetic resonance signal
Structural vs functional imaging
Structural
– Only collect one image/volume
– Good resolution
– T1 weighted
Functional
– Collect multiple images every 2/3s
– Coarse images
– T2 weighted
Astrocyte role in fMRI
Astrocytes sense synaptic activity and dilate blood vessels
Low neural activity
Less oxygenated blood
More constricted vessels
High neural activity
More oxygenated blood
More dilated vessels
Whole brain analysis
- Registration to a common space
- Average across participants
- Correct for multiple comparisons
Cognitive subtraction
The control condition should involve all processes except the one you’re interested in
Middle frontal gyrus involved in what
Reasoning
Striatum
Choosing response most practiced or more likely to be rewarded
If you want to see water or blood on an image what type do you use
T2
Time is on what axis
X
Signal is on what axis
Y
Resting state fMRI
- BOLD signal fluctuates spontaneously
- Anatomically connected areas show correlated BOLD fluctuations
