MRI in practice chapter 2 Flashcards
parameters that cannot be changed because they are inherent to the body’s tissues
intrinsic contrast parameters
parameters that can be changed
extrinsic contrast parameters
T1 recovery
T2 decay
proton density
flow
apparent diffusion coefficient
intrinisc contrast parameters
TR
TE
flip angle
TI
turbo factor/echo train lenght
b value
extrinisc contrast factors
molecular lattice is more able to absorb energy from hydrogen nuclei
in herent energy of tissue is low
molecular lattice does not absorb energy from hydrogen nuclei as easily
inherent tissue energy is high
good energy match between bydrogen nuclei and molecular lattice is efficient
good match in Larmor frequency
good interaction between magnetic fields of neighboring hydrogen nuclei
good interaction important in T2 decay
time for 63% of total magnetization to be regained in longitudinal plane
T1 relaxation
63% of total magnetization lost in transverse plane
T2 decay
T1 time in fat
short
bright contrast
T1 recovery time in water
Long
dark contrast
T2 time of fat
short
dark contrast
T2 time of water
long
bright contrast
difference in signal intensity between tissues that are a consequence of their relative number of mobile protons per unit volume
proton density contrast
tissues with high proton density
large transverse component
bright contrast on PD weighted image
tissues with low proton density
small transverse component
dark on PD weighted images
large component of coherent magnetization in transverse plane
produce high signal
bright contrast
small component of coherent magnetization in transverse plane
low signal
dark contrast
characterized by
bright fat
dark water
T1 images
characterized by
bright water
dark fat
T2 images
although inherent to tissue are dependent on magnet field strength
T1 and T2 relaxation times
tissues take longer to recover
as field strength increases
_______controls how far each vector recovers before slice is excited by next RF pulse
TR
TR pulse length in T1 weighting
short
so neither has time to fully return to B(o)
______controls the amount of decay of T2 taht is allowed to occur before the signal is received
TE
TE length in T2 weighting
long
enough to give both fat and water time to decar
effects of T1 and T2 contrast must be diminished so that ___________ can dominate
proton density
TR must be ________ and TE must be ___________ for a PD weighted image
long
short
when the RF excitation pulse is removed the relaxation and decay process start immediately
T2*
is the decay of the FID following RF pulse removal
T2*
comination of two effects
T2 decay
dephasing due to magnetic field inhomgeneities
T2* faster than T2
NMV pushed beyond 90°
parially saturated
NMV pushed to full 180°
fully saturated
partial saturation of fat and water
T1 weighting occurs
saturation of fat and water does NOT occur
PD weighting occurs
predominant factor in T2* decay
inhomogeneities in magnetic field
has significant effect on saturation effects
flip angle
amount that RF pulse moves NMV via resonance
flip angle
sequences that use a 180° pulse to regeneratesignal
spin echo pulse
sequences that use a gradient
gradient echo pulse
uses a 90° pulse to flip the NMV
then a 180° pulse when that one is removed
spin echo pulse sequence
occurs after a 180° pulse when the magnetic moments rephase and align
spin echo
contains T1 and T2 information
reduces T2* dephasing
spin echo signal
Time to rephase after the application of a 180° pulse equals the time to dephase after the 90° pulse was removed
TAU
twice the TAU
TE
short TE ensures the 180° pulse and echo occur early (little T2 decay)
differnce in T2 times of tissues do not dominate the echo and contrast
short TR ensures fat and water do not fully recover so their T1 times dominate the echo and contrast
spin echo with one echo
used to produce both PD and T2 weghting
spin echo with 2 echoes
first spin echo short TE minimizing T2 decay
second echo long TE (significant T2 decay)
Long TR so T1 differences are minimized
spin echo using 2 echoes
controls T1 weighting
TR
maximizes T1 weighting
short TR
maximizes PD weighting
long TR
controls T2 weighting
TE
miminizes T2 weighting
short TE
maximizes T2 weighting
long TE
rephase the magnetic moments so that a signal is received by the coil which contains T1 and T2 information
gradient echo
generated by coils of wire situated with the bore of the magnet
magnetic field gradients
middle axis of magnetic field strength
magnetic isocenter
gradient filed adds or subtracts from main mag field depending on direction of current flow through gradient coils
polarity
increase when mag field strength increase
decreases when mag field strength decrease
precessional freq
precessional freq slow down
less gradient
precessional freq increase
more gradient
gradients taht dephase
spoilers
gradients that rephase
rewinders
associated with shorter scan times than spin echo pulses sequences
gradient echo pulse sequence
TR decreased (less than 90° flip angles)
low flip angles means less full recovery time
advantage of gradient echo pulse sequence
no compensation for mag field inhomogeneities
very susceptible to mag field inhomogeneities
contain mag susceptibility artefact
disadvantages of gradient echo pulse sequences
control T1 relaxation in gradient echo pulse sequence
TR and clip angle
short TR
high flip angle
short TE
T1 weighting in gradient echo
long TE
long TR
small flip angle
T2* weighting in gradient echo
short TE
long TR
small flip angle
PR weighting in gradient echo
gradient echo pulse uses a _______to rephase the magnetic moments
gradient
variable _________are used
flip angles
can be shorter than in spin echo imaging
TE
do not eliminate effects from mag filed inhomogeneities
gradients
