Lecture 8 - MRI

Question 1

How MRI works

Answer 1

• 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

Question 2

2 types of relaxation after a radiofrequency pulse

Answer 2

T1 and T2

Question 3

As proton relaxes what happens to T1 and T2

Answer 3

T1 gets longer and T2 gets shorter

Question 4

_____ of T2 and _____ of T1

Answer 4

Decay of T2
Recovery of T1

Question 5

MRI steps

Answer 5

1. Protons align with a strong static magnetic field
2. Radiofrequency pulse is applied to excite protons and make them spin together
3. Radiofrequency pulse is turned off and the protons relax
4. Coils detect the energy released during relaxation

Question 6

T1 recovery is slowest for protons in _____ and ______ and fastest for protons in ____

Answer 6

Water and CSF
Fat

Question 7

T2 recovers the slowest for ______ and the fastest for _______

Answer 7

CSF
White matter

Question 8

Things you can do with structural MRI

Answer 8

Measure volume of subcortical structures, cortical thickness, and area

Question 9

Why do structural MRI

Answer 9

• ## Correlation between size of brain area and cognitive function

Question 10

Fractional anisotropy (FA)

Answer 10

Preferential diffusion of water in one direction compared to 2 orthogonal directions

Question 11

Isotropic

Answer 11

Ventricles, gray matter

Question 12

Anisotropic

Answer 12

White matter
(more diffusion in one direction than compared with the other 2 orthogonal directions)

Question 13

Brightness in DTI

Answer 13

How directional water diffusion is (gray matter and CSF are dark)

Question 14

Color in DTI

Answer 14

Which direction is the principle direction

Question 15

Myelination and DTI

Answer 15

As you develop and myelinate, water is more constrained by fatty myelin and you get brighter signal

Question 16

Functional imaging relies on what

Answer 16

T2 signal differences between deoxygenated and oxygenated blood

Question 17

Deoxygenated hemoglobin has a ________ magnetic susceptibility and causes what for the T2 signal

Answer 17

Greater magnetic susceptibility and causes a faster decay of the T2 signal

Question 18

More deoxygenated hemoglobin =

Answer 18

Lower magnetic resonance signal

Question 19

More oxygenated hemoglobin =

Answer 19

Higher magnetic resonance signal

Question 20

Structural vs functional imaging

Answer 20

Structural
– Only collect one image/volume
– Good resolution
– T1 weighted

Functional
– Collect multiple images every 2/3s
– Coarse images
– T2 weighted

Question 21

Astrocyte role in fMRI

Answer 21

Astrocytes sense synaptic activity and dilate blood vessels

Question 22

Low neural activity

Answer 22

Less oxygenated blood
More constricted vessels

Question 23

High neural activity

Answer 23

More oxygenated blood
More dilated vessels

Question 24

Whole brain analysis

Answer 24

1. Registration to a common space
2. Average across participants
3. Correct for multiple comparisons

Question 25

Cognitive subtraction

Answer 25

The control condition should involve all processes except the one you’re interested in

Question 26

Middle frontal gyrus involved in what

Answer 26

Reasoning

Question 27

Striatum

Answer 27

Choosing response most practiced or more likely to be rewarded

Question 28

If you want to see water or blood on an image what type do you use

Answer 28

T2

Question 29

Time is on what axis

Answer 29

X

Question 30

Signal is on what axis

Answer 30

Y

Question 31

Resting state fMRI

Answer 31

• BOLD signal fluctuates spontaneously
• Anatomically connected areas show correlated BOLD fluctuations