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FAST SPIN ECHO

FAST SPIN ECHO

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FAST SPIN ECHO

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  1. FAST SPIN ECHO Durgesh Kumar Dwivedi Department of NMR & MRI AIIMS, New Delhi, India

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

  3. Pulse sequence and timing diagram • Four lines are needed • radio-frequency (RF) pulse • three gradients - slice, phase, frequency/ readout

  4. SPIN ECHO (SE) SEQUENCES

  5. 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

  6. Effect of TR and TE on MR image contrast * Short TR & long TE produces very low SNR and should be avoided

  7. 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

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

  9. 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

  10. 90° 180° 90° RF Pulse + 127 (Phase) 부호화 경사 ( Gy ) Phase encoding axis ( Ky) 0 (Frequency) 부호화 (Gx) - 127 RF signal (Echo) Spin Echo, k-space Frequency encoding axis Fourier Transform k-space If 256개의 phase encoding을 채울 경우 TR 256번을 반복 If you fill the 256 phase encoding TR 256 iterations

  11. Fast Spin echo … 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 180° 180° 180° 180° 180° 180° 180° 180° 90° ETL ETS + 127 Overall contrast 0 (Ky) Detailed Description - 127 K-space

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

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

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

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

  16. Contrast in FSE FSE SE T1W T2W 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).

  17. TE30 TE80 TE100 TE60 90° 180° 180° 180° 180° 90° K-space TEeff 80 FSE vs CSE Multiple 180° pulse FSE CSE TR 90° 180° 90° RF Pulse Phase (Phase)Signed slope TE80 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

  19. Disadvantage Blurring TEeff 50 ms T2 weighted TEeff 150 ms

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

  21. Disadvantage Specific absorption rate (SAR) : 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

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

  23. 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

  24. SSFSE , HASTE 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 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° 180° Single shot FSE, Half Fourier acquired sigle shot turbo spin echo Frequency encoding ( Kx) Phase encoding ( Ky) K-space Partial fourier technique 64-128 90°

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

  26. 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

  27. THANK YOU…