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ANSWERS (VERIFIED ANSWERS) ALREADY GRADED
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How does TR and TE affect image weighting in GE imaging? -
ANSWER-Like spin echo imaging the basic rules for TR and TE
still apply. That being short TR and TE for T1 weighting and
Long TR and TE for T2 weighting, however the actual values of
TR and TE drop considerably in GRE imaging due to the fact
that we use a variable flip angle no longer the 90 degrees that is
used in SE imaging.
What are some disadvantages of GE sequences? - ANSWER-
With no 180 degree pulse, unfortunately GRE sequences are
very sensitive to magnetic field inhomogeneities. This may
result in unwanted susceptibility artefacts at eg. Tissue-air
interfaces. Due to the lack of a 180 degree refocusing pulse T2
contrast in GRE imaging is referred to as T2* contrast as
dephasing due to inhomogeneity will affect the image contrast.
What are the key differences between SE and GRE imaging? -
ANSWER-SPIN
ECHO
,▪ 90 excitation pulse transfers all available Mz into the
transverse plane.
▪ 180 refocusing pulse gives largest possible signal▪ Therefore
high SNR and
reduced inhomogeneity
artefact
▪ Long scan
time
GRADIENT ECHO
▪ Utilises a gradient reversal to rephase spins
▪ Reduced flip angle therefore less signal in Mxy plane
▪ No 180 refocusing therefore susceptibility artefacts
▪ Lower SNR
▪ Shorter scan time
To achieve T1 weighting in a GRE sequence, what parameters
should be set? -
ANSWER-• T1 Weighting in GRE
Large Flip angle 70-110 degrees
Short TE 5-10ms
Short TR < 50ms
To achieve T2 weighting in a GRE sequence, what parameters
should be set? -
ANSWER-T2* Weighting
in GRE
,Small flip angle 5-20
degrees
Long TE 15-
25ms
Short TR minimum possible depending on number of slices
as the small flip
angle ensures rapid T1
recovery
To achieve PD weighting in a GRE sequence, what parameters
should be set? -
ANSWER-PD Weighting in GRE Small flip angle 5-20 degrees
Short TE 5-10ms
Short TR minimum possible depending on number of slices
as the small flip
angle ensures rapid T1
recovery
What is steady state? - ANSWER-This is the condition that
applies when the TR used is shorter than the T1 and the T2 of
the tissue. There will not be enough time for the transverse
magnetisation to have decayed away before the next RF pulse
is applied. Each RF pulse produces a FID, each pair of pulses
produces a Hahn echo, and stimulated echoes arise from each
set of three or more pulses. If the TR is short enough, the tails
of the FID's and the spin echoes merge to produce a
continuous signal of varying amplitude. The signal contains
both FID and spin-echo components, with the TR being equal to
the TAU of the spinecho
, Explain how the residual transverse magnetisation is can be
removed in a steady state sequence? - ANSWER-The use of
short TR intervals creates the steady state condition, but with
these sequences the residual transverse magnetisation is
deliberately removed or 'spoiled' before the next excitation
pulse. Spoiling is done by;
• Gradient spoiling, where a gradient of varying amplitude
is turned on just before the next RF pulse. (Siemens' Flash)
• RF spoiling, where the phase of the RF is changed with
each excitation and the receiver locks on only to the phase of
the preceding pulse. (GE's SPGR
What are gradient echo incoherent/spoiled gradient echo
sequences usually used to for in terms of weighting - ANSWER-
These sequences are used to produce rapid T1 weighted
images. eg. breath-hold liver studies. Only the FID is re-phased
- the spin-echo is spoiled and not sampled.
For GRE incoherent T1 weighted scans the following parameters
are suggested
- ANSWER-•Flip angle 30-45
degree
•Short TE : minimum possible: often two are used for in
and out of phase
(minimum TE ensures minimal T2*) •Short TR
: 20-50ms