MEDRADSC 3DE3 Lecture Notes - Lecture 7: Spin Echo, Stoping, Transverse Plane
18 Jun 2018
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Spin Echo Pulse Sequences- Spins are rephased by the 180 degree pulse
ď‚·Conventional spin echo
oonly one phase encoding step is applied per TR on each slice and therefore only one line
of K space is filled per TR
oScan time is a function of TR, NEX, and phase encoding steps
oAt each rephasing – an echo is produced and a different phase encoding step is
performed
o256 matrix is selected – 256 phase encodings must be applied that means TR time
must be applied 256 times
ď‚·Fast Spin Echo (FSE) or turbo spin echo
o
oUses multiple RF pulses for rephasing
oShorter scan times
In FSE with an ETL of 16 – the scan time will be reduced to 1/16th of the
original time
oAchieved by filling more than one phase encoding step is performed per TR
This is done by generating several echoes (180` rephasing pulses) to produce a
train of echoes
oNumber of echoes generated will define how much shorter the sequence time will be
oThe echoes generated with FSE are done at different TEs
Variable weighting
oEffective TE
Phase encoding at effective TE – placed in the center of K-Space
With FSE we select an effective TE because we are generating multiple TE’s
meaning the weighting is going to be variable (want to accentuate the weighting
we want)
very steep phase encoding slopes reduced the signal amplitude
shallow phase encoding stopes result in an echo that has max signal amplitude
oEchoes produced outside of effective TE - ordered such that they are placed at the
periphery
o
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oData from all echoes are placed in K space
oShallow echoes are placed around effective TE
oStep echoes are placed at edges of K space
Less impact on image contrast – does impact spatial resolution
oEXAMPLE
TR – 4000
TE – 100
ETL – 16
Where will phase encodings performed at 100 ms be placed?
oThe resultant image contains data from all the echoes in the train … data collected
around the effective TE will have more of an impact on image contrast as they fill the
central lines of K space – which produce the greatest signal amplitude
oData collected from the wrong TEs will have much less of an impact on image contrast
as they fill the outer lines of K space and therefore have smaller signal amplitude and
greater spatial resolution
oUSES
Similar to CSE
Most clinical applications
Has replaced CSE to a large extent – FOR T2w
Caution – _____ remains bright due to repeated excitation
Can also reduce magnetic susceptibility – bad for bleeds
ď‚·artifacts from metal are significantly reduced when using FSE
Can also increase image blur at the edge of tissues with different T2 decay
values… bc each line of K space filled during an echo train contains data from
echoes with a different TE
when using long echo trains late echoes have a low signal amplitude
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Spin echo pulse sequences- spins are rephased by the 180 degree pulse. Fast spin echo (fse) or turbo spin echo o: uses multiple rf pulses for rephasing, shorter scan times. In fse with an etl of 16 the scan time will be reduced to 1/16th of the original time: achieved by filling more than one phase encoding step is performed per tr. Phase encoding at effective te placed in the center of k-space. Less impact on image contrast does impact spatial resolution: example. Has replaced cse to a large extent for t2w. Caution _____ remains bright due to repeated excitation. Can also reduce magnetic susceptibility bad for bleeds artifacts from metal are significantly reduced when using fse. Can also increase image blur at the edge of tissues with different t2 decay values bc each line of k space filled during an echo train contains data from echoes with a different te.
