Durgesh Kumar Dwivedi Department of NMR & MRI AIIMS, New Delhi, India FAST SPIN ECHO Durgesh Kumar Dwivedi Department of NMR & MRI AIIMS, New Delhi, India December 2, 2009

Contents Basic terminology of MR pulse sequences RARE/ FSE SE (CSE) vs FSE Contrast in FSE Advantages Disadvantages SSFSE, HASTE

Pulse sequence and timing diagram Four lines are needed radio-frequency (RF) pulse three gradients - slice, phase, frequency/ readout From Abbreviations: One for RF tX and others for Gx Gy Gz Additional lines may be added to indicate other activity such

SPIN ECHO (SE) SEQUENCES Manufacturer Single echo SE Multiple echo SE Echo train SE Siemens Single SE SE Double echo Turbo spin echo (TSE) Half Fourier acquisition turbo SE (HASTE) GE Multiecho multiplanar (MEMP) Variable echo multiplanar (VEMP) Fast SE (FSE) Single shot FSE (SSFSE) Philips SE, Modified SE Multiple SE (MSE) Ultra fast Spin echo (UFSE)

Contrast parameters Two key parameters : repetition time (TR) and echo time (TE) - are key to the creation of image contrast. TR (in milliseconds) is the time between the application of an RF excitation pulse and the start of the next RF pulse TE (in milliseconds) is the time between the application of the RF pulse and the peak of the echo detected From MR pulse sequence page 4 Both parameters affect contrast on MR images because they provide varying levels of sensitivity to differences in relaxation time between various tissues. At short TRs, the difference in relaxation time between fat and water can be detected (longitudinal magnetization recovers more quickly in fat than in water); at long TRs, it cannot be detected. Therefore, TR relates to T1 (Fig 4b) and affects contrast on T1-weighted images. At short TEs, differences in the T2 signal decay in fat and water cannot be detected; at long TEs, they can be detected. Therefore, TE relates to T2 (Fig 4b) and affects contrast on T2-weighted images. When the TR is long and the TE is short, the differences in magnetization recovery and in signal decay between fat and water are not distinguishable (Fig 4b); therefore, the contrast observed on the resultant MR images is predominantly due to the difference in proton density between the two tissue types.

Effect of TR and TE on MR image contrast Imaging technique TR TE T1 weighting Short T2 weighting Long PD weighting * Short TR & long TE produces very low SNR and should be avoided

TR (in ms) TE (in ms) Sequence Short Long SE 250-700 >2000 10-25 >60 GRE <50 >100 1-5 >10

Spin echo 90° pulse flips the net magnetization vector into the transverse plane A 180° pulse is applied at a time equal to one-half of TE to rephase the spinning nuclei When the nuclei are again spinning in phase (at total TE), an echo is produced and read

FSE RARE (Hennig et al 1986) FSE (Fast spin echo) (Mulkern et al 1990) (Rapid Acquisition with Relaxation Enhancement) FSE (Fast spin echo) (Mulkern et al 1990) TSE (Turbo spin echo)

FSE Fast scan (Based on principle of echo imaging) Long TR (Multiple RF pulse) - T2W강조 Conventional SE Fast SE 1 NEX 4 NEX 9 min 28 sec 2 min 25 sec

Frequency encoding axis Spin Echo, k-space RF signal (Echo) (Phase) 부호화 경사 ( Gy ) (Frequency) 부호화 (Gx) RF Pulse 90° 180° Frequency encoding axis Fourier Transform Phase encoding axis ( Ky) + 127 - 127 k-space If 256개의 phase encoding을 채울 경우 TR 256번을 반복 If you fill the 256 phase encoding TR 256 iterations

Fast Spin echo … Echo train length (ETL) Echo train spacing (ETS) 180° 90° Echo train length (ETL) : Number of 180° RF pulse : Scan time ∝ (1/ETL) Echo train spacing (ETS) : Space between 180° RF pulse : Dwell time (in phase encoding direction) Effective echo time (TEeff)          : TE of k-space mid-line    ETS + 127 Overall contrast 0 (Ky) Detailed Description - 127 K-space

Fast Spin Echo ETL=8 TE5 ETS TE eff Centre of k-space RF Pulse 90° 180° 180° 180° 180° 180° 180° ETL=8 RF Pulse TE eff Phase-encoding gradient Echo TE1 TE2 TE3 TE4 TE6 TE7 TE8 TE5 + 127 0 (Ky) - 127 Ky=0 Centre of k-space

Effective TE TEeff 40.36 msec 80.72 msec 96.86 msec 137.22 msec T2 effect ↑, SNR ↓ TEeff 40 TEeff 100 TE 40 TE 100

Echo Train Length ETL 4 ETL 8 ETL 16 ETL 32 10 min 45 sec 5 min 25 sec TR 5000msec, NEX = 2 ETL governs by: (1)T2 relaxation, (2) ETS ETL 4 ETL 8 ETL 16 ETL 32 10 min 45 sec 5 min 25 sec 2 min 45 sec 1 min 25 sec ETL ↑ : Time ↓ Issues : Slice number ↓ Correction: TR ↑, Slice thickness ↑

Scan Time Scan Time(FSE) = (TR)(Ny)(NEX) /ETL Scan Time(SE) = (TR)(Ny)(NEX) Scan Time(FSE) = (TR)(Ny)(NEX) /ETL Ny NEX ETL : Phase-encoding steps Number of excitation Echo Train Length Example: TR = 3000 msec, NEX=1, Matrix 256 X 256.          ETL of 8. Calculate time for CSE and FSE images? CSE 3000(TR) * 256(Ny) * 1(NEX) = 12.8min FSE 3000(TR) * 256(Ny) * 1(NEX) / 8 = 1.6 min

Contrast in FSE FSE SE T1W T2W (17) T1-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 500-msec TR. The images are very similar, even though the fast spin-echo image was obtained four times faster. The latter image shows a slight reduction in resolution due to echo decay throughout the echo train. (18) T2-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 2,000-msec TR. TE was 68 msec for the conventional image, and the TE encoding the center of k space in the fast spin-echo image was also 68 msec. The images are very similar, even though the fast spin-echo image was obtained four times faster. Blurring seen in the Ti-weighted fast spin-echo image is not apparent in the T2-weighted fast spin-echo image because the earlier echoes are used to sample the higher frequency phase-encoding views. T1-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 500-msec TR. Below image: T2-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 2,000-msec TR. TE was 68 msec for the conventional image, and the TE encoding the center of k space in the fast spin-echo image was also 68 msec. Blurring seen in the T1-weighted fast spin-echo image is not apparent in the T2-weighted fast spin-echo image because the earlier echoes are used to sample the higher frequency phase-encoding views. Images illustrate how the various regions of K space (upper row) can be reconstructed, with the corresponding images (bottom row). Reconstructions are shown for all of the data (left), the center of k space (center), and the outer regions of k space (right).

Phase (Phase) Signed slope FSE vs CSE Multiple 180° pulse FSE CSE TR 90° 180° TE30 TE60 TE80 TE100 90° 180° 90° RF Pulse Phase (Phase) Signed slope TE80 K-space TEeff 80 TE 80 Time saving No time saving 18

Advantage Scan time ↓↓: ∝ETL Image quality ↑ : Scan time saving; trade off – ETL and Slice thickness Based on spin echo and similar contrast Artifact (motion, susceptibility) ↓ : by 180° refocusing pulse

Disadvantage Blurring TEeff 50 ms T2 weighted TEeff 150 ms

Disadvantage Bright fat signal : Remedy- Fat suppression image : J-coupling : Remedy- Fat suppression image Conventional SE Fast SE Fast SE Fat suppression

Disadvantage Specific absorption rate (SAR) Use low flip angles : Total RF energy (E) dissipated in a sample over exposure time (texp) per unit mass(M) (watts per kilogram) SAR= E/ (texp*M) Also, SAR α Bo2 * θ (Theta)2 *Bandwidth Use low flip angles

FSE의 변형 3D FSE (+ 3D) SSFSE, HASTE (+ Single shot FSE) (+ Half fourier acquired sigle shot turbo spin echo)

3D FSE Thinner image Phase encoding : z Direction (Nz), add Multiple slices → Slab More scan time Scan time = (TR*NEX*Ny*Nz)/ETL 2D Image 3D Image

SSFSE , HASTE Single shot FSE (Single Shot) + Half fourier acq. Single shot FSE, Half Fourier acquired sigle shot turbo spin echo Frequency encoding ( Kx) Single shot : A very long one echo train (64-128 locations) Ny = ETL, so, Scan time = TR* NEX FSE (Single Shot) + Half fourier acq. Partial fourier technique : K-space Fill in the date part of the Phase encoding ( Ky) K-space Partial fourier technique 64-128 90° 180°

HASTE, SSFSE Ultra-fast : 1-2초 (single breath hold) Abdomen, chest imaging MRCP Liver MR

Summary Characteristics of spin echo Fast scan ∝ETL Advantage : Image quality ↑, Artifact↓ Disadvantage : Fat signal ↑, Slice number ↓ ETL, ETS, TE eff Advancement due to SSFSE & HASTE- better image quality

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