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RAD413 > MRI overview > Flashcards

MRI overview Flashcards

1
Q

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

A
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2
Q

what is SPIN in MRI

A

’ 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) ‘

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3
Q

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

A

probabilistically

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4
Q

combined magnetic field of hydrogen atoms in body cancels out

A
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5
Q

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

A
  • distribution of ‘tiny magnets’ changed, augmenting the number of atoms aligned with the external magnetic field
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6
Q

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

A
  • 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)
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7
Q

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

A

magnetic radio frequency pulse

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8
Q

what and how generates the magnetic radio frequency pulse

A

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

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9
Q

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

A
  • nudge misaligns hydrogen bar magnets to shift their magnetic field properties perpendicular to large magnetic field made by machin
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10
Q

how do the hydrogen bar magnets / protons induce current

A
  • 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)

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11
Q

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

A

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
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12
Q

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

A

increase strength of external magnetic field

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13
Q

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

A
  • increase external magnetic field —> increases number of aligned hydrogen atoms —–> induced a larger current as it spirals back/decays
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14
Q

how does increase current, signal and image quality relate

A

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

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15
Q

what is a superconductor

A

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

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16
Q

theoretically, how can superconductor material affect an electric current?

A
  • an electric current could travel in a loop of superconducting material indefinitely without needing a power source
17
Q

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

A

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

18
Q

what is the most commonly superconducting material in MRI

A

Nb-Ti (niobium titanium)

19
Q

what is used to cool down the coils in MRI

A

liquid helium

20
Q

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

A

vacuum - sealed chamber

’ zero-boil off ‘ machine

21
Q

what type of cycle is used in a vacuum sealed chamber

A
  • electric refrigerant cycle
  • keeps helium in liquid phase and magnets cool enough to maintain them in superconducting state
22
Q

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

A

1.5 T = 64 MHz
3.0 T = 128 MHz

23
Q

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

A

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

24
Q

how does field strength and frequency corrolate

A

stronger field = higher frequency

25
Q

what is gradient coil and how is it used in MRI

A
  • 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
26
Q

what is T1 relaxation

A
  • how quickly atoms re-align themselves with large magnetic field after a nudging pulse
27
Q

what is T2 relaxation

A
  • 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
28
Q

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

A
29
Q

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

A
  • 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)
30
Q

what do MRIs sample to form imaged instead of pixels

A

striped patterns

31
Q

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

A
  • 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
32
Q
A

RAD413 (24 decks)

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