Week 2 Flashcards
what does magnet produce
static magnetic field (B0) & fringe field outside of Bore
what does magnet influence
signal / image quality; higher magnetic field = higher SNR
types of magnets
permanent, resistive electromagnets, superconducting electromagnets
advantages of permanent magnet
low cost, no cooling needed, negligible fringe field
disadvantage of permanent magnet
- limited use = only extremity imaging
- poor magnetic homogeneity = decreased image quality
- cannot ‘switch off’
- heavy
advantage of resistive electromagnet
- low cost due to superconducting magnet
- water used for cooling
- easily switched off
- easier to site compared to permanent & superconducting scanners
disadvantage of resistive electromagnet
- high operating costs
- low field strength to superconducting scanners
in superconducting electromagnets, current passes through ___ to create magnetic field
superconducting solenoid windings
advantage of superconducting electromagnets
high max B0 & variety of clinical application
disadvantage of superconducting electromagnets
high costs, high fringe field, cryogenic cooling needed
gradients disturb B0 __
linearly
gradient coils used for ___
spatial localization, slice selection, gradient echo production
RF coil functions
transmits & receives electromagnetic radiation via transceiver & smaller detachable coils
MR room nearby room for ___
air-condition, magnetic compressor unit, electricity supply
phase are ___
shifts in time
what happens when in-phase
spins precess together = strong signal
what happens when out-of-phase
spins randomly = weak signal
T1 relaxation & T2 decay occurs __
simultaneously & indepndently
T1 relaxation energy loses to ___
surroundings (spin-lattice relaxation)
T1 relaxation is the time taken for ___
Mz to recover to 63% of original level
T1 relaxation time is ___
200 - 2000ms
T2 decay energy loses to ___
surrounding spins (spin-spin relaxation)
T2 decay time is ___
10 - 100 ms
T2 decay is the time taken for ___
Mxy to decay to 37% of original level
why do spins dephase
inhomogeneity & susceptibility
decay due to spin-spin interactions
magnetic inhomogeneity
magnetic field stronger / weaker at different regions
susceptibility
paramagnetic / diamagnetic materials react different to magnetism
T2’ effect
fixed + reversible
T2 effect
random + irreversible
what is Free induction decay
T2*W dephasing
time to echo (TE)
time between excitation & signal readout
time to repetition (TR)
time between consecutive RF excitation pulses
TE signal is from
transverse (Mxy)
TR signal is from
longitudinal (Mz)
T1 = ___ TE + TR
short TE + TR
T2 = ___ TE + TR
long TE + TR
PD = __ TE + __ TR
short TE + long TR
what determines slice thickness
range of frequencies / bandwidth
readout changes frequency ___
linearly along readout / frequency direction
phase gradients changes frequency ___
linearly along phase encode direction
what contributes to TE signal
transverse magnetization; T2 decay time
what contributes to TR signal
longitudinal magnetization; T1 recovery time
what are the 3 different gradients
Slice, Readout & Phase gradients
what affects slice thickness
bandwidth / range of frequencies
why is T2* always shorter than T2 time
due to 3 effects acting on it (inhomogeneity, susceptibility, spin-spin)
what will increase T2 signal
more water
