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
