MRI/fMRI

Question 1

what does the magnet do in MRI

Answer 1

creates main magnetic field

Question 2

what do the gradient coils do in an MRI

Answer 2

distort the magnetic field in a way that depends on position
allows spatial encoding of MRI signal

Question 3

what do the RF coils do in an MRI

Answer 3

perpendicular to main magnetic field
turned on briefly to excite proton spin system

Question 4

what is the unit of measurement for an MRI

Answer 4

voxel/volumetric pixel
size varies with the strength of the magnet and scan sequence

Question 5

what kinds of tissues can an MRI measure

Answer 5

anything with water content

Question 6

what are the pros and cons of structural MRI

Answer 6

pros
- non invasive
- allows visualisation of brain structure/anatomy
- good spatial resolution

cons
- static pictures (not function)
- expensive

Question 7

what is the difference between fMRI and structural MRI

Answer 7

fMRI uses magnetic field to measure brain activity during functional tasks - not just structure

Question 8

what does fMRI measure

Answer 8

BOLD signals
- blood oxygenation level dependent signals

Question 9

how does fMRI work

Answer 9

measures BOLD signals
- increased activity = increase in regional blood flow
- causes an increase in oxygenated hemoglobin to fuel brain activity
- deoxygenated hemoglobin is more magnetic than oxygenated
leads to different MR signals in active vs inactive areas of the brain

Question 10

how long does the BOLD response take

Answer 10

10-12sec
- finding out which structure was used 10-12sec ago

Question 11

what are the pros and cons of fMRI

Answer 11

pros
- non invasive
- allows quantification of brain activity in humans
- good spatial resolution
- best tool for imaging and quantifying brain function

cons
- poor temporal resolution
- permits only small mvmts (large mvmts distort the magnetic field - motion artifact)
- expensive

Question 12

what part of tissue can influence the MR signal and images

Answer 12

water content of tissue

Question 13

what is the original arrangement of protons

Answer 13

random arrangement

Question 14

how does proton alignment change when the magnetic field is introduced

Answer 14

realignment into an up or down state
magnetic field creates a torque that causes the proton to spin on a skew axis around the magentic field
- line up parallel or anti parallel to magnetic field
- also have precession on theri own axis

Question 15

what is precession

Answer 15

proton spinning on skew axis (not straight up and down)
looks like top wobbling before it falls over

Question 16

what happens to protons when radio waves are pulsed

Answer 16

radio waves = perpendicular to magentic field
- all protons align into down spin state
- protons precess together in phase before they go back and release energy

Question 17

what happens to protons when the radio wave pulse stops

Answer 17

some protons transition back to up state and release energy

Question 18

what field strength do most scanners use

Answer 18

3T - measured in teslas
(50000x earths magnetic field)