Imaging Sequences part II Gradient Echo Spin Echo

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 RF pulse readout RF pulse signal FID spin echo

Spin Echo pulse timing RF slice phase readout echo signal TE

Spin Echo Contrast T 1 weighted T 2 weighted

Multi Echo Spin Echo rationale • conventional imaging uses a multislice 2 D technique – at a given TR time, number of slices depends on the TE time T 2 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 T 2 weighted imaging • no time penalty for first echo – inserted before second echo • can do multiple echos (usually 4) to calculate T 2 relaxation values

Multi Echo Spin Echo gradient RF pulses RF pulse signal TE 1 TE 2

Multi Echo Spin Echo pulse timing RF slice phase readout signal echo 1 echo 2

Spin Echo Contrast PD weighted T 2 weighted

Multi Echo Spin Echo • Summary – simultaneously generates PD and T 2 weighted images – no time penalty for acquisition of PD weighted image – no mis-registration between echos

Fast Spin Echo • Rationale – importance of T 2 weighted images • most clinically useful • longest to acquire • lowest S/N – need for higher spatial resolution

Fast Spin Echo historical perspective • faster T 2 weighted imaging – gradient echo (T 2*) – 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 …. . echo phase encode n+1 echo n+1 TR …. .

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 only 1 phase encode per TR slice phase readout signal echo 1 echo 2

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

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”

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

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 T 2 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 k. Hz) – 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 T 2 weighted images in a shorter time than conventional SE • great innovation • artifact prone

Inversion Recovery • initially used to generate heavily T 1 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 z 0 Before ML=M MXY=0 0 RF y x t=t 0+ After ML=-M MXY=0

T 1 Relaxation recovery z z After ML=-M MXY=0 y x t=t 0+ y x t=TI

900 Flip z z 0 Before ML=Msin( ) 0 RF y x t=t 0+ After MXY= ML

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

STIR • Short time-to-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 T 1 relaxation time

STIR advantages • robust technique – works better than fat saturation over a large FOV (>30 cms) – better at lower field strengths • high visibility for fluid – long T 1 bright on STIR – long T 2 bright on STIR, given long enough TE

STIR disadvantages • poor S/N – improved with multiple averages • FSE – improved with shorter TE times • incompatible with gadolinium – shorter T 1 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. 5 T – excellent fluid sensitivity in soft tissues

Summary • Spin echo – 90 degree pulse, dephase, 180 degree pulse, rephaseecho • Multi-echo spin echo – 90 degree pulse, dephase, 180 degree pulse, rephase-1 stecho, 180 degree pulse, rephase-2 nd-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