Week 3 Flashcards
intrinsic / uncontrollable contrast parameters
T1 recovery time, T2 decay time, proton density
extrinsic / controllable contrast parameters
TR, TE, TI, ETL
what controls T1W
TR
what controls T2W
TE
contrast weighting of long TR
2000+ ms
contrast weighting of short TR
300 - 700 ms
contrast weighting of long TE
60 ms
contrast weighting of short TE
10 - 25 ms
characteristics of T1W
bright fat + dark water
T1W is used to demonstrate __
anatomy
characteristics of T2W
bright water + dark fat
T2W is used to demonstrate __
pathology as water is often associated with it
T1W = __ TE + TR
short TE + TR
T2W = __ TE + TR
long TE + TR
PD = __ TR + __ TE
long TR + TE
T2 relaxation is __
natural decay of transverse magnetization
T2* relaxation is faster than T2 due to
magnetic inhomogeneity + field distortion / susceptibility
number of phase & frequency encoding determines __
resolution
total MR signal due to sum of FIDs from voxels with different __
amplitudes, frequencies, phases
signal received by coil is ___ & digitized using ___
pre-amplified; analog digital converters
where is signal stored in
K-space which is raw data matrix for every frequency & phase combo
each point in K-phase has defined particular combo of ___
frequency & phase shifts
K-space does not correspond to __
spatially
central K-space provides __
low frequency details / contrast
outer K-space provides ___
high frequency details / edges
order of filling K-space is __
K-space trajectory
signal which is phase-shifted by 90 degrees to determine ___
direction of magnetization rotation
In Fourier Transform, any signal / waveform made up by adding ___
- pure tones / sine waves
- appropriate frequency
- amplitude
- phase
spatial / coil encoding is used for ___
parallel imaging
time encoding is used for ___
multi-slice imaging
spin-echo cancels out __
fixed magnetic effects of inhomogeneity & susceptibility
spin-echo sequence characterized by __
90 degrees RF pulse > 180 degrees RF rephasing pulse at TE/2
spin-echo mainly used for __
T1 imaging
HASTE
half-former acquisition single shot turbo spin echo
HASTE preparation, excitation, readout
preparation: none
excitation: spin-echo
readout: single shot TSE
HASTE AKA
single shot fast spin echo
2D TSE w/ high turbo factor
all lines of fourier plane in single TR
radial TSE
turbo spin echo w/ motion correction
radial TSE preparation, excitation, readout
preparation: none
excitation: spin-echo
readout: radial TSE
radial TSE AKA
PROPELLAR
spin-echo cancels __
fixed magnetic field effects of inhomogeneity & susceptibility
gradient pulse applied to __
accelerate spin dephasing
gradient echo features an RF pulse at an angle followed by __
bipolar reading gradient
since gradient choes don’t have refocusing plane, it is ___
more sensitive to B0 magnetic field inhomogeneity
gradient echo preparation, excitation, readout
preparation: none
excitation: gradient-echo
readout: spin-echo
gradient echo AKA
fast low angle shot (FLASH)
spoiled gradient echo (SPGR)
GRE T2*W preparation, excitation, readout
preparation: none
excitation: gradient-echo
readout: segmented echoes
balanced steady state free-precession preparation, excitation, readout
preparation: none
excitation: gradient-echo
readout: steady state
echo planar imaging preparation, excitation, readout
preparation: fat preparation
excitation: spin-echo (diffusion), gradient- echo (fMRI)
readout: Echo planar
echo planar imaging is __
single shot ultra fast acquisition; 1 slice = 50 ms
what is fat saturation
removal of fat signal for better visualization of lesions & oedemas
different methods of fat saturation
- spectral fat saturation
- inversion recovery
- water excitation
- dixon
inversion recovery is ___
180 degrees inversion pulse before excitation
Pulse sequence determines __
number, strength, timing of RF & gradient pulses which determines image contrast & K-space trajectory
Number of points on echo determines __
resolution in frequency direction
Lines in center of K-space determines __
contrast
spin-echo sequences are mainly used for
T1 imaging
what is TOF
- Tissues saturated with constant RF pulses
- Vessels traversing perpendicular through are transiently affected and appear bright
- Saturation bands can block either arterial or venous flow
what are the disadvantages of GRE T2*W
Increases susceptibility artifacts
Bad for implants but good for paramagnetic substances
what is fat saturation
Removes fat signal = prevents fat ghosting
Improves visualization of lesions esp post contrast & oedemas in fatty areas
what are the preparation, excitation and readout for echo planar imaging
Preparation = Fat saturation, excitation = spin echo for diffusion / gradient echo for fMRI, readout = echoplanar
DWI is highly sensitive and specific for __
acute stroke detection
what does DWI assume
haemorrhage excluded
what is used to localize signals
gradients
High spatial frequencies in K-space are stored at __
periphery, defines sharp edges of an image, acquired before or after the center
what cannot produce T2*w
spin echo sequences
what is the key difference between spin & gradient echoes
presence of 180 degrees refocusing pulse, excitation RF pulse less than 90 degrees and gradient echo is faster
gradient echo can be modified by __
RF, TE, TR
why does early bleeding appear darker in T1w
higher susceptibility
why does MRI take a long time
due to phase encoding
