Imaging Sequences part 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 gradient frequency encode readout  RF pulse  RF pulse signal FID spin echo

Spin Echo pulse timing   RF slice phase readout echo signal TE

Spin Echo Contrast T1 weighted T2 weighted

Multi Echo Spin Echo rationale 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: 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 gradient  RF pulses  RF pulse signal TE 1

Multi Echo Spin Echo pulse timing    RF signal readout phase slice echo 1 echo 2

Spin Echo Contrast PD weighted T2 weighted

Multi Echo Spin Echo Summary simultaneously generates PD and T2 weighted images no time penalty for acquisition of PD weighted image no mis-registration between echos

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 Echo historical 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 Echo pulse timing TR ….. ….. phase encode n phase encode n+1   echo   ….. phase encode n phase encode n+1 echo n echo n+1 TR …..

Spin Echo Spin Echo Imaging Time = time-between-90-degrees times total-number-of-unique-echos times number-of-signal-averages

Spin Echo scan time time-between-90-degrees = TR total-number-of-unique-echos = phase encodings number-of-averages = NEX, NSA

Fast Spin Echo implementation 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 Echo pulse timing    RF signal readout phase slice only 1 phase encode per TR echo 1 echo 2

Fast Spin Echo pulse timing      RF signal readout phase slice multiple phase encodes per TR echo train

Fast Spin Echo scan time time-between-90-degrees = TR total-number-of-unique-echos = phase encodings number-of-averages = NEX, NSA echo-train-length = ETL

Fast Spin Echo advantages 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 Echo advantages image contrast similar to SE scan parameters TR TE echo train length

Fast Spin Echo disadvantages new hardware required ear protection may be necessary higher SAR many 1800 flips closely spaced motion sensitive

Fast Spin Echo disadvantages 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”

Fast Spin Echo disadvantages TE 20 Want: TR 3000, TE 80 Do: TR 3000, ET 4 20 msec IES TE 40 computer TE 60 TE 80 TE 70ef Get: TR 3000, TE 70ef

Fast Spin Echo disadvantages 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 Echo blurring SE TE 20 FSE TE 20

Fast Spin Echo limitations solutions: use mainly for T2 weighted imaging limit the ET length (~ 8) many phase encodes (192 +)

Fast Spin Echo limitations 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

Fast Spin Echo interecho spacing 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 Echo interecho spacing, example 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 Echo interecho 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 Echo interecho 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 Echo conclusions 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 TR    TI TE inversion recovery conventional SE or FSE sequence

Initial 1800 Flip inversion z y x z y x 0 RF 0 Before ML=M MXY=0 After ML=-M MXY=0 t=t0 t=t0+ 46

T1 Relaxation recovery z y x z y x After ML=-M MXY=0 t=t0+ t=TI 47

900 Flip t=t0 t=t0+ 0 RF 0 Before ML=Msin() After MXY= ML z y x z 48

Second 1800 Flip dephased rephased 900 RF 1800 RF t=0 t=TE/2 t=TE z y x z y x dephased z y x z y x rephased 900 RF 1800 RF t=0 t=TE/2 t=TE

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

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

STIR advantages robust technique high visibility for fluid 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

STIR disadvantages poor S/N incompatible with gadolinium improved with multiple averages FSE improved with shorter TE times incompatible with gadolinium shorter T1 relaxation post-contrast

STIR disadvantages 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

Summary Spin echo Multi-echo spin echo Fast 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