MRI

Question 1

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

Answer 1

• 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

Question 2

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

Answer 2

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

Question 3

longitudinal magnetization

Answer 3

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

Question 4

what is Torque

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

what else happens when the RF is applied

Answer 7

the spins start to precess in phase with each other

Question 8

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

Answer 8

• the NMV changes to transverse

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

Question 9

longitudinal relaxation/recovery

Answer 9

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

Question 10

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

Answer 10

longitudinal magnetisation increases/ recovers

Question 11

in T1 imaging does fat recover fast or slow

Answer 11

fast

- creates high signal

Question 12

T1 weighted imaing appearances

Answer 12

fat is bright

water/fluid is dark

Question 13

Transverse relaxation/decay

Answer 13

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

Question 14

T2

Answer 14

the time constant for decay/dephasing of transverse magnetisation

Question 15

T2 imaging appearances

Answer 15

• 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

Question 16

what happens during the pulse sequence ‘spin echo’

Answer 16

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.

Question 17

TE

Answer 17

time to echo

- time between excitation pulse and the echo being generated

Question 18

TR

Answer 18

time to repetition

- time between the excitation pulse and the next excitation pulse

Question 19

TE and TR influence…..

Answer 19

the tissue weighting

Question 20

which tissue weighting is best at showing pathologies

Answer 20

• 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

Question 21

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

Answer 21

T2

Question 22

TE and TR for T1 weighting

Answer 22

short TE and short TR

Question 23

TE and TR for PD ( Proton density)

Answer 23

short TE, long TR

Question 24

proton density weighting image

Answer 24

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

Question 25

Advantages of supercondcting magnets

Answer 25

- 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

Question 26

disadvantages of supercondcting magnets

Answer 26

- produce high magnetic fields which could be dangerous to safety - cannot be turned off, they must be quenched

Question 27

risks of the main magnetic field

Answer 27

- 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

Question 28

what do gradient coils do

Answer 28

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

Question 29

risks of gradient coils

Answer 29

- 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

Question 30

what do radiofrequency coils do

Answer 30

- they produce the RF and receive the signal

Question 31

risks of radiofrequency coils

Answer 31

- can cause heating and burns - surface of skin if there's metal - there are limits to specific absorption rate - watts/Kg

Question 32

what are the main concerns with gadolinium

Answer 32

- 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

Question 33

what is meant by quench

Answer 33

- 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

Question 34

advantages of MRI

Answer 34

- 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

Question 35

disadvantages of MRI

Answer 35

- 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

Question 36

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

Answer 36

- 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