Lecture 2 Flashcards
Free induction decay (FID)
Most basic MRI experiment
- Place sample inside scanner (person / beaker of water)
- Excite with a RF pulse at resonant frequency wiring in-built transmitter coil (lamar freq)
- Remove the RF pulse after desired flip angle achieved
- Detect the signal wiry receiver coil
FID signal
decay rate equation
FID decay equation
Sinusoidally decreasing voltage
decay rate = e ^ (-t/T2*)
FID decay = T2 * decay
Once time goes on, the magnetisation in the x-y plane becomes smaller and smaller which causes….
- spins dephase wrt eachother
- x-y componentt of magnetisation reduces to zero bc phases of spins distributed uniformly around the x-y plane
T2* dephasing happens due to
energy exchange between spins (T2 decay = spin-spin relaxation
static spatial inhomogeneities in magnetic field (T2’ decay)
Spin echo summary
- 90 degree excitation pulse applied + spins precession phase
NMV + FID at maximum but FID decays fast
- Spins comprising isochormat start to dephase
leaders get ahead of laggers
FID + NMV decay with time
- 180 degree refocusing pulse applied
tips spin from spin up to spin down
all spins comprising NMV inverted 180 degrees
leaders become laggers + vice versa and signal + NMV small
spins rotate in transverse plane until they rephase
as faster spins approach slower the NMV + FID grow in magnitude (180 degree pulse eliminates effect of static field inhomogenities
- After time, t, the spins are once more in phase
magnetic resonance + NMV both max
After this point the spins become out of phase again + signal decays
Why does the 180 degree refocusing pulse eliminate the effect of static field inhomogeneities?
- If a spin has an inhomogeneity (say its at 3 OIT) then it will gain. ore phase than the spins at 3T wrt the other spins
refocussing pulse rephases the spins
Spin gains same amount pf phase so all inhomogeneities will be undone
assuming spins havenet moved
therefore 180 degree refocussing pulse undoes any phasing effects from static field inhomogeneity
effect of T2* removed
Spatial localisation (3 steps)
- Slice selection
- Phase encoding
- Frequency encoding
How does slice selection work?
- Direct current (DC) sent into pair of gradient coils for set time + produces magnetic field gradient along z-axis
- total magnetic field (B_o) reduced at head and increased at toe but same at isocentre (40 mTm^-1) so spins much slower at head
- protons slice precess with narrow range of frequencies
- Gradient of resonant frequency of proton along z-axis + frequency can localise signal
How do you change the slice thickness?
Alter the transmit bandwidth or gradient itselfs strengths
Transverse =
(x, y) plane
slice select gradient is z gradient
Coronal
(x,z) plane
= y gradient slice select
Sagital =
(y, z) plane
= x gradient
In which applications do we particularly wish to reduce geometric distortion
- radio therapy planning
- Surgical planning
- Neurosurgical planning
Eddy currents (gradients)
gradients dont come on instantaneously so they appear as trapezoids rather than rectangles –> ramp up + ramp downtime
T2 contrast
1 / T2* = 1 / T2 + 1/T2’
Transverse magnetisation decays via
T2 decay via spin-spin interactions
spins exchange energy resulting in dephasing + reduction in transverse magnetisation
Different tissues have different magnetic environments leading to different T2 times
brighter pixel = higher T2
