MRI Flashcards
what properties do hydrogen-1 nuclei have that make them ideal for MRI
- 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
what happens to the hydrogen nuclei when placed in a magnetic field
- the magnetic moments align in ‘spin up’ or spin down
- spin up = low energy state
- spin down = High energy state
longitudinal magnetization
- when the overall magnetisation of the patient is in the longitudinal direction along the Z axis
what is Torque
rotational force that causes a ferrous object to turn and align with the direction of the magnetic field
what happens to the precessional frequency of our hydrogen nuclei as we increase the magnetic field strength?
- the precessional frequency will increase, as they are directly proportional
what happens when we apply a radiofrequency at the resonant frequency of our processing Hydrogen nuclei?
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
what else happens when the RF is applied
the spins start to precess in phase with each other
what happens to the net magnetisation when the RF pulse is applied
- the NMV changes to transverse
- due to roughly equal hydrogen spins in the low and high energy states and spinning in phase
longitudinal relaxation/recovery
- this when the magnetic moments relax back into the low energy state
what happens to the longitudinal magnetisation, after the RF is turned off
longitudinal magnetisation increases/ recovers
in T1 imaging does fat recover fast or slow
fast
- creates high signal
T1 weighted imaing appearances
fat is bright
water/fluid is dark
Transverse relaxation/decay
- where the spins start to dephase and overall transverse magnetisation is lost
T2
the time constant for decay/dephasing of transverse magnetisation
T2 imaging appearances
- 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
what happens during the pulse sequence ‘spin echo’
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.
TE
time to echo
- time between excitation pulse and the echo being generated
TR
time to repetition
- time between the excitation pulse and the next excitation pulse
TE and TR influence…..
the tissue weighting
which tissue weighting is best at showing pathologies
- 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
which tissue weighting achieved by using a long TE and long TR
T2
TE and TR for T1 weighting
short TE and short TR
TE and TR for PD ( Proton density)
short TE, long TR
proton density weighting image
- tissues with high proton density will have signal and be bright
Advantages of supercondcting magnets
- 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
disadvantages of supercondcting magnets
- produce high magnetic fields which could be dangerous to safety
- cannot be turned off, they must be quenched
risks of the main magnetic field
- 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
what do gradient coils do
- produce a varying magnetic field localise where the signals are coming are coming from
-
risks of gradient coils
- 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
what do radiofrequency coils do
- they produce the RF and receive the signal
risks of radiofrequency coils
- can cause heating and burns - surface of skin if there’s metal
- there are limits to specific absorption rate - watts/Kg
what are the main concerns with gadolinium
- 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
what is meant by quench
- 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
advantages of MRI
- 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
disadvantages of MRI
- 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
what questions would you expect to find on a patient safety checklist
- 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
