Sequence parameters and options Flashcards
the time between excitation pulses is known as the
1/2 TE
presaturation pulses are often used to
reduce flow artifacts
the presaturation pulses usually occur
prior to the excitation pulse
gradient echo sequences use flip angles
to control saturation effects
complete saturatoin is a condition where
longitudinal magnetization is not allowed to recover between excitations
increasing the TE
increases the contrast based on T2 relazatoin times of the tissue
reducing the TR down to or below the T1 relaxatoin time of the tissue
decreases the SNR of the image
increases saturatoin effects
reducing the TE
reduces contrast base on T2 relaxatoin times
as the TR increases
SNR increases up to a point
as the TE increases
SNR decreases
in a gradient echo sequence reducing the fip angle while holding the TR constant reduces
saturation
in a 2D conventional SE multislipce pulse sequence scan time is given by the equation
TR x NSA x #PEs
in an inversion recovery pulse sequence image contrast is controlled by
TR
TE
TI
in an inversion recovery pulse sequence the time between the initializing 180 pulse and the 90 pulse is known as
TI
another name for TI
Tau
a short T1 inversion recovery (STIR) sequence can suppress the signal from
fat
gadolinium enhancing lesion
decreaseing the receiver bandwidth
increase the SNR
decreaseing the receiver bandwidth
increases readout time
decreaseing the receiver bandwidth
increases susceptibility artifact
decreaseing the receiver bandwidth
decreases the number of slices
increaseing the receiver bandwidth
has no effect on the available ETL
the time during which the freq encoding gradient is on
increases with a reduction in reciever bandwidth
in a conventional spin echo multi echo sequence it is possible to create multiple images each with a different amount of
T2 weighting
the SNR will increase in a 3D sequence with an increase in
FOV
number of slices
between slices 2D acquisitions generally require
gaps
doubling the number of NSA will
increase the SNR by the square root of 2
increasing the number of phse encodings will produce an image with
decreased voxel volume
reducing NSA will reduce the scan time and
decrease the SNR
doubling the NSA will increase the SNR by a factor of
1.41
reducing the FOV by a factor of 2 will reduce the voxel volume by a factor of
4
if a STIR sequence using a TR of 3000, TE of 20 and a TI of 140 produces an image with dark fat and birght water, the contrast is such an image is primarily based on
T1
in choosing the direction of phase encoding the technologist usually consider the direction in which the
motoin artifacts traverse the least tissue or areas of interest
a chemical or spectral fat suppression sequence will suppress the signal from fat based on the
precessional freq of fat
increasing slice thickness from 5 to 10 mm (by a factor of 2 x thicker) the SNR
increases by a factor of 2
in creaseing the number of phase encodings (matrix) from 128 to 256 the SNR
decreases
gradient moment nulling is most effective when correcting for motion induced signal loss from
slow constant flow
to rephase the signal from moving spins gradient moment nulling techniques us a
gradient
using a conventional spin echo multislice sequence the number of slices allowed when increasing TR
increases by a factor of TR/TE
using a conventional SE multislice sequence the number of slices allowed when increasing the TE from 20 to 40ms
decreases
using a 3D acquisition the number of slices allowed when in creasing the TR
is not affected
using a 3D acquisition increasing the number of slices from 64 to 128
doubles the scan time
increasing the matrix in the freq direction from 256 to 512 will
doubles the scan time
the effective TE in a FSE sequence determines the
image contrast
in an FSE sequence the central lines of k space are associated witoh the
effective TE
when triggering a scan from the patients ECGH the TR of the sequence is determined by the
patients heart rate
increasing TR
increases the scan time
increasing TE
decreases scan time
increasing the number of slices in a 2D acquisition
increases scan time
for a given tissue with a given T1 relaxation time and TR, the flip angle which will result in the maximum signal for that tissue is
the ernst angle
increasing the FOV
does not affect scan time
increasing the phase matrix
increases scan time
increasing slice thickness
does not affect scan time
increasing NSA
increases scan time
increasing slice thickness
increase SNR
increasing the matrix
decreases SNR
increasing the flip angle
increases SNR up to the Ernst angle
reducing the ETL
increases scan time
reducing the TE
increases SNR
reducing the TE yeilds images with
less T2 information
increasing the TR yeilds images with
less T1 information
increasing the TR yeilds images with
less T1 information
increasing the TE yeilds images with
more T2 information
reducing the TR yeilds images with
more T1 information
reducing the flip angle yeilds images with
less T1 information
increasing the flip angle yeilds images with
more T1 information
