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Image science principles > MRI > Flashcards

MRI Flashcards

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1
Q

what properties do hydrogen-1 nuclei have that make them ideal for MRI

A
  • they have 1 proton and 0 neutrons in their nucleus
  • these are abundant in the body
  • have magnetic properties
  • generates its own magnetic field when moving
  • has a spinning charged particle - creating a current
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2
Q

what happens to the hydrogen nuclei when placed in a magnetic field

A
  • the magnetic moments align in ‘spin up’ or spin down
  • spin up = low energy state
  • spin down = High energy state
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3
Q

longitudinal magnetization

A
  • when the overall magnetisation of the patient is in the longitudinal direction along the Z axis
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4
Q

what is Torque

A

rotational force that causes a ferrous object to turn and align with the direction of the magnetic field

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5
Q

what happens to the precessional frequency of our hydrogen nuclei as we increase the magnetic field strength?

A
  • the precessional frequency will increase, as they are directly proportional
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6
Q

what happens when we apply a radiofrequency at the resonant frequency of our processing Hydrogen nuclei?

A

Excitation occurs

  • we can excite some of the protons into the high energy state
  • reduction in longitudinal magnetisation
  • RF pulse must be at the same frequency as the precessig moments
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7
Q

what else happens when the RF is applied

A

the spins start to precess in phase with each other

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8
Q

what happens to the net magnetisation when the RF pulse is applied

A
  • the NMV changes to transverse

- due to roughly equal hydrogen spins in the low and high energy states and spinning in phase

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9
Q

longitudinal relaxation/recovery

A
  • this when the magnetic moments relax back into the low energy state
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10
Q

what happens to the longitudinal magnetisation, after the RF is turned off

A

longitudinal magnetisation increases/ recovers

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11
Q

in T1 imaging does fat recover fast or slow

A

fast

- creates high signal

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12
Q

T1 weighted imaing appearances

A

fat is bright

water/fluid is dark

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13
Q

Transverse relaxation/decay

A
  • where the spins start to dephase and overall transverse magnetisation is lost
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14
Q

T2

A

the time constant for decay/dephasing of transverse magnetisation

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15
Q

T2 imaging appearances

A
  • fat dephases quickly - giving low signal and appearing grey/dark
  • water dephases slowly - giving high signal and appearing white/bright
  • pathologies are shown well on T2
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16
Q

what happens during the pulse sequence ‘spin echo’

A

2 RF pulses

  • 90 degree excitation pulse
  • 180 degree re-phasing pulse - produces an echo which gives the signal for our image
  • 90 degree excitation pulse is applied first and the ‘spins’ align and process in phase.
  • the pulse is switched off , and the ‘spins’ dephase
  • the 180 degree pulse is switched on , resulting in the spins rephasing and an echo is produced.
17
Q

TE

A

time to echo

- time between excitation pulse and the echo being generated

18
Q

TR

A

time to repetition

- time between the excitation pulse and the next excitation pulse

19
Q

TE and TR influence…..

A

the tissue weighting

20
Q

which tissue weighting is best at showing pathologies

A
  • T2, because pathologies tend to be fluid filled and water dephases slowing , thus giving a high signal - resulting in bright areas where water/fluid is present
21
Q

which tissue weighting achieved by using a long TE and long TR

A

T2

22
Q

TE and TR for T1 weighting

A

short TE and short TR

23
Q

TE and TR for PD ( Proton density)

A

short TE, long TR

24
Q

proton density weighting image

A
  • tissues with high proton density will have signal and be bright
25
Q

Advantages of supercondcting magnets

A
  • good SNR
  • the coils are kept in helium - allowing us to reach high field strengths without coils overheating. less resistance when passing currents
  • good field uniformity - images more consistent and reliable
26
Q

disadvantages of supercondcting magnets

A
  • produce high magnetic fields which could be dangerous to safety
  • cannot be turned off, they must be quenched
27
Q

risks of the main magnetic field

A
  • projectile effect
  • pacemaker patients - field can alter function
  • Can have a Torque effect on metal implants - tries to align itself with the magnetic field, can damage soft tissue and blood vessels
28
Q

what do gradient coils do

A
  • produce a varying magnetic field localise where the signals are coming are coming from

-

29
Q

risks of gradient coils

A
  • peripheral nerve stimulation - since the magnetic field is constantly changing - it can induce electrical currents within our nerves - may feel like pins and needles
  • Acoustic noise - very loud noise
30
Q

what do radiofrequency coils do

A
  • they produce the RF and receive the signal
31
Q

risks of radiofrequency coils

A
  • can cause heating and burns - surface of skin if there’s metal
  • there are limits to specific absorption rate - watts/Kg
32
Q

what are the main concerns with gadolinium

A
  • kidney function- gadolinium cannot be excreted and can become unstable if not filtered out
  • Nephrogenic systemic Fibrosis (NSF) - can lead to multiple organ failure
  • check EGFR
33
Q

what is meant by quench

A
  • all the helium is boiled and pushed out of the room
  • this leads t the loss of the magnet’s superconductivity
  • only pressed in emergencies
34
Q

advantages of MRI

A
  • non IR
  • Multi- planar and has 3D capabilities
  • excellent tissue characterisation- particularly for the brain, spine, MSK
  • gives functional and anatomical information
  • gadolinium contrast Is better tolerated than iodinated contrast
35
Q

disadvantages of MRI

A
  • scans are slow
  • high cost and limited availability
  • not suitable for all patients e.g.
  • risks of gadolinium
  • Artefacts
  • excellent spatial resolution
  • lungs are seen poorly
36
Q

what questions would you expect to find on a patient safety checklist

A
  • any previous surgery to the brain or spine
  • any metal clips
  • any metal fragments in the eyes - yes, were they removed successfully - check previous x-ray
  • any replacements
  • any pacemakers
  • chances of pregnancy
  • recent surgeries or implants in the last 6 weeks
  • any artificial heart valves
  • wearing any hearing aids
  • any tattoos
  • whether they have had an MRI before and if they had contrast + reaction
  • any coils

Image science principles (12 decks)

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