Imaging Sequencespart II

• Gradient Echo

• Spin Echo

• Fast Spin Echo

• Inversion Recovery

Spin Echo Refresher

• 900 RF pulse followed by 1800

RF pulse

• least artifact prone sequence

• moderately high SAR

Spin Echo

FID spinecho

RF pulse

readoutfrequency encode

signal

gradient

RF pulse

Spin Echopulse timing

echo

RF

signal

readout

phase

slice

TE

Spin Echo Contrast

T1 weightedT1 weighted T2 weightedT2 weighted

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

Multi Echo Spin Echo

RF pulse

signal

gradient

RF pulses

TE 1TE 2

Multi Echo Spin Echopulse timing

echo 2

RF

signal

readout

phase

slice

echo 1

Spin Echo Contrast

PD weightedPD weighted T2 weightedT2 weighted

• 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 Echopulse timing

RF

signal

readout

phase

slice

echo train

multiple phase encodes 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 Echoblurring

SE TE 20SE TE 20 FSE TE 20FSE TE 20

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

• three image parameters– TI

– TR

– TE

Inversion Recovery

TI

TE

conventional SE or FSE sequenceinversion recovery

TR

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

Initial 1800 Flipinversion

z

y x

z

y x

0 RF

t=t0 t=t0+

0

AfterML=-MMXY=0

BeforeML=MMXY=0

T1 Relaxationrecovery

z

y x

z

y x

t=t0+ t=TI

AfterML=-MMXY=0

900 Flip

z

y x

z

y x

0 RF

t=t0 t=t0+

0

AfterMXY= ML

BeforeML=Msin()

z

y xz

y x

z

y x

z

y x

t=TE/2 t=TEt=0

900 RF

dephased

rephased

Second 1800 Flip

1800 RF1800 RF

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

-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

• optimal inversion time for fat nulling dependent on T1 relaxation time

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