MRI overview

Question 1

Imaging in MRI relies on the quantum properties of the hydrogen atom

Question 2

what is SPIN in MRI

Answer 2

’ the intrinsic angular momentum of subatomic particles’

• spin of the singular proton in hydrogen atoms act as a bar magnet, creating a magnetic field around it

’ proton is the magnet and the spin is the property of the magnet (spinning) ‘

Question 3

what is the orientation of the magnetic north in MRI described as?

Answer 3

probabilistically

Question 4

combined magnetic field of hydrogen atoms in body cancels out

Question 5

what happens when hydrogen atoms within body are inserted into a large external magnetic field

Answer 5

• distribution of ‘tiny magnets’ changed, augmenting the number of atoms aligned with the external magnetic field

Question 6

how is an MRI signal created/what is the source of the MRI signal

Answer 6

• distribution of ‘ tiny magnets’ change (from 0/neutral) as hydrogen atoms from body enter large external magnetic field (MRI)
• augmentation of number of atoms aligned with external magnetic field
• imbalance is what generates the MRI signal
  ( tiny magnets get manipulated into make signals that can become images)

Question 7

what is used to ‘nudge’ the protons that are aligned with the external magnetic field?

Answer 7

magnetic radio frequency pulse

Question 8

what and how generates the magnetic radio frequency pulse

Answer 8

set of coils within machine
- alternation of current though coils at specific frequencies

Question 9

how does the magnetic radio frequency affect the alignment of the protons

Answer 9

• nudge misaligns hydrogen bar magnets to shift their magnetic field properties perpendicular to large magnetic field made by machin

Question 10

how do the hydrogen bar magnets / protons induce current

Answer 10

• after being misaligned by magnetic radio frequency, they naturally want to align their orientation back to their og position ( with external magnetic field)
• hence they decay in a spiralling motion which causes a changing magnetic field
• with nearby coils, this changing magnetic field induced a current within the coils and induce clear signals

( MRI machines can use the same coils to send the ‘ nudge pulses’ and read signals)

Question 11

what is done for higher contrast and resolution in terms of coils in MRI?

Answer 11

some machines use separate coils to transmit pulses and receive signals

• receiver coils can be placed much closer to the body maximising the strength of the signal

Question 12

how can you increase the number of hydrogen atoms in the body to align with external magnetic field?

Answer 12

increase strength of external magnetic field

Question 13

how does current, strength of magnetic field and number of aligned hydrogen atoms relate?

Answer 13

• increase external magnetic field —> increases number of aligned hydrogen atoms —–> induced a larger current as it spirals back/decays

Question 14

how does increase current, signal and image quality relate

Answer 14

increase current from decay —> increase signal strength —> increased/ improved image quality recieved

Question 15

what is a superconductor

Answer 15

resistance drops to 0 at temperatures close to absolute zero / -273k

Question 16

theoretically, how can superconductor material affect an electric current?

Answer 16

• an electric current could travel in a loop of superconducting material indefinitely without needing a power source

Question 17

with use of a superconducting material, where would the main consumption of energy fall to

Answer 17

to keep the coiled cooled down so the current can travel endlessly

Question 18

what is the most commonly superconducting material in MRI

Answer 18

Nb-Ti (niobium titanium)

Question 19

what is used to cool down the coils in MRI

Answer 19

liquid helium

Question 20

due to heat, liquid helium can evaporate, what is used nowadays to prevent the helium evaporating and escaping

Answer 20

vacuum - sealed chamber

’ zero-boil off ‘ machine

Question 21

what type of cycle is used in a vacuum sealed chamber

Answer 21

• electric refrigerant cycle
• keeps helium in liquid phase and magnets cool enough to maintain them in superconducting state

Question 22

what frequency do the protons in the hydrogen atoms rotate at in a 1.5/3.0 Tesla

Answer 22

1.5 T = 64 MHz
3.0 T = 128 MHz

Question 23

how can a bar magnet/proton be ‘nudged’ by a magnetic radiofrequency

Answer 23

by a magnetic radio frequency at the same frequency of the proton in a 1.5/3 T so at 64/128 MHZ

Question 24

how does field strength and frequency corrolate

Answer 24

stronger field = higher frequency

Question 25

what is gradient coil and how is it used in MRI

Answer 25

- separate set of electromagnets that form gradient of magnetic field strength (y= field strength x= length along tube) - allows advantage in imaging individual slices as using the gradient can allow you to selectively nudge atoms along gradient by applying corresponding frequency

Question 26

what is T1 relaxation

Answer 26

- how quickly atoms re-align themselves with large magnetic field after a nudging pulse

Question 27

what is T2 relaxation

Answer 27

- hydrogen atoms dont realign with magnetic field uniformly - in tissue, hydrogen interacts with each other and surroundings - after nudging pulse is sent, interaction causes spin to fall out of uniformity - so T2 is the sum of the decaying signals

Question 28

t1 and t2 ARE NOT EQUAL,and dependant on the tissue

Question 29

you can use T1 and T2 properties to your advantage, how?

Answer 29

- to emphasise T1, send pulses rapidly and listen to signal immediatly (T2 doesnt have enough time to take place) (e.g fatty tissue) - to emphasise T2, send pulses slowly and listen longer allowing dephasing to occur (e.g spinal fluid)

Question 30

what do MRIs sample to form imaged instead of pixels

Answer 30

striped patterns

Question 31

what is the physical interpretation of the striped patterns used to for MRI images

Answer 31

- slice seen as a grid of rotating hydrogens, rotated phases coloured by greyscale - when in unison, greyscale is all white - when out of phase 180, greyscale is all black - different phases = different shades of grey - alternate set of gradient coils precisely change phase of rotating hydrogen to create striped patterns in all directions etc - by adding increasing patterns, image begins to form