Spin Echo Refresher
• 900 RF pulse followed by 1800
RF pulse
• least artifact prone sequence
• moderately high SAR
Multi Echo Spin Echorationale
• conventional imaging uses a multi-slice 2D technique– at a given TR time, number of slices depends
on the TE time
T2 weighted imaging:long TR
long TE
PD weighted imaging:long TR
short TE
Multi Echo Spin Echo
• designed to obtain simultaneously multiple echos
• generally used for PD and T2 weighted imaging
• no time penalty for first echo
– inserted before second echo
• can do multiple echos (usually 4) to calculate T2 relaxation values
• Summary– simultaneously generates PD
and T2 weighted images
– no time penalty for acquisition of PD weighted image
– no mis-registration between echos
Multi Echo Spin Echo
Fast Spin Echo
• Rationale – importance of T2 weighted
images• most clinically useful• longest to acquire• lowest S/N
– need for higher spatial resolution
Fast Spin Echohistorical perspective
• faster T2 weighted imaging– gradient echo (T2*)
– reduced data acquisition• “half-NEX”, “half-Fourier” imaging• rectangular FOV• S/N or spatial resolution penalty
– altered flip angle SE imaging• “prise”, “thrift”
Fast Spin Echo
• single most important time limiting factor is the acquisition of enough data to reconstruct an image
• at a given image resolution, the number of phase encodings determines the imaging time
Fast Spin Echo
• each phase encoding is obtained as a unique echo following a single excitation with a 90 degree RF pulse
Spin Echopulse timing
…..…..echo
echo
phase encode nphase encode n phase encode n+1phase encode n+1
echo necho n echo n+1echo n+1
TR …..
TR …..
Spin Echo
Spin Echo Imaging Time =
time-between-90-degrees times
total-number-of-unique-echos times
number-of-signal-averages
Spin Echoscan time
• time-between-90-degrees = TR• total-number-of-unique-echos = phase encodings• number-of-averages = NEX, NSA
scan timeTR phaseencodings NEX
utesm
minsec
,=
* *60000
Fast Spin Echoimplementation
• collect multiple echos per TR– similar to multi-echo SE
– number of echos per TR referred to as the “echo train”
• re-sort the data collection order to achieve the desired image contrast (effective TE time)
Multi Echo Spin Echopulse timing
echo 2
RF
signal
readout
phase
slice
echo 1
only 1 phase encode per TR
Fast Spin Echoscan time
• time-between-90-degrees = TR• total-number-of-unique-echos = phase encodings• number-of-averages = NEX, NSA• echo-train-length = ETL
scan timeTR phaseencodings NEX
ETLutes
mmin
sec
,=
* **60000
Fast Spin Echoadvantages
• acquisition time reduced proportional to echo train length (ETL)
• can trade-off some of the time savings to improve images– increased NEX
– increased resolution
Fast Spin Echoadvantages
• image contrast similar to SE
• scan parameters– TR
– TE
– echo train length
Fast Spin Echodisadvantages
• new hardware required
• ear protection may be necessary
• higher SAR– many 1800 flips closely spaced
• motion sensitive
Fast Spin Echodisadvantages
• reduced number of slices for equivalent TR SE scan
• MT effects alter image contrast
• TE time imprecise
• image blurring may occur
• fat remains relatively bright on long TR/long TE scans
• “J-coupling”
computercomputer
Fast Spin EchodisadvantagesTE 20TE 20
TE 40TE 40
TE 60TE 60
TE 80TE 80TE 70efTE 70ef
Want: TR 3000, TE 80
Do:TR 3000, ET 420 msec IES
Get:TR 3000, TE 70ef
Fast Spin Echodisadvantages
• each echo “belongs” to a different TE image
• combining the echos to form a single image creates artifacts– worse with shorter effective TE
times
Fast Spin Echolimitations
• solutions:– use mainly for T2 weighted imaging
– limit the ET length (~ 8)
– many phase encodes (192 +)
Fast Spin Echolimitations
• solutions:– choose long TE times (> 100 msec)– choose long TR times (> 4000 msec)
• increases fat-fluid contrast
– for PD imaging, –use shorter echo trains (4) and wider
receive bandwidths (32 kHz)–alternatively, use fatsat
• interecho spacing is the time between echos, ~ 16 msec minimum on current equipment
• echo trains vary from 2 on up on current equipment
• little signal is available with long echo train imaging
Fast Spin Echointerecho spacing
• 16 ETL, 16 msec IES results in echos at the following:– 16, 32, 48, 64, 80, 96, 112, 128,
144, 160, 176, 192, 208, 224, 240, 256 msec
– last 5 or 6 echos have so little signal that there is little contribution to the final image
Fast Spin Echointerecho spacing, example
Fast Spin Echointerecho spacing, example
• time of last echo determines the number of slices per TR
• long echo trains greatly reduce the number of slices per TR, even if the effective TE is short
Fast Spin Echointerecho spacing
• hardware upgrade (echo-planar capable) will decrease interecho spacing (6-8 msec)– better image quality for same
echo train lengths
– more slices per TR for identical echo train lengths
Fast Spin Echoconclusions
• should be called “faster” spin echo
• produces superior T2 weighted images in a shorter time than conventional SE
• great innovation
• artifact prone
Inversion Recovery
• initially used to generate heavily T1 weighted images
• popular in U.K. for brain imaging
• 1800 inversion pulse followed by a spin echo or fast spin echo sequence
Inversion Recovery
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0
25
0
50
0
75
0
10
00
12
50
15
00
TI (msec)
MZ/M0long T1
short T1
STIR
• Short time-to-inversion inversion recovery imaging
• “fat nulling”• exploits the zero crossing effect of
IR imaging– all signal is in XY plane after TI time
and subsequent 900 pulse produces no signal
Inversion Recovery
-1
-0.8
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-0.2
0
0.2
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0
25
0
50
0
75
0
10
00
12
50
15
00
TI (msec)
MZ/M0long T1
short T1
STIRadvantages
• robust technique– works better than fat saturation over
a large FOV (>30 cms)
– better at lower field strengths
• high visibility for fluid– long T1 bright on STIR
– long T2 bright on STIR, given long enough TE
• poor S/N– improved with multiple averages
• FSE
– improved with shorter TE times
• incompatible with gadolinium– shorter T1 relaxation post-contrast
STIRdisadvantages
• red marrow signal can obscure subtle edema– use TE=48 to knock signal down from
marrow
• modified IR– TE=70-100– TI=110 @ 1.5T– excellent fluid sensitivity in soft tissues
STIRdisadvantages
Summary
• Spin echo– 90 degree pulse, dephase, 180 degree pulse, rephase-
echo• Multi-echo spin echo
– 90 degree pulse, dephase, 180 degree pulse, rephase-1st-echo, 180 degree pulse, rephase-2nd-echo
• Fast spin echo– obtain multiple phase encoded echos with a single 90
degree pulse– echo train length determines “turbo” factor
• Inversion recovery– 180 degree pulse, inversion time, then SE or FSE
sequence– STIR enables fat suppression over large FOVs or for open
magnets
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