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

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Presentation on theme: "December 2, 2009 Durgesh Kumar Dwivedi Department of NMR & MRI AIIMS, New Delhi, India."— Presentation transcript:

1 December 2, 2009 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 ManufacturerSingle echo SEMultiple echo SE Echo train SE SiemensSingle SESE Double echo Turbo spin echo (TSE) Half Fourier acquisition turbo SE (HASTE) GESEMultiecho multiplanar (MEMP) Variable echo multiplanar (VEMP) Fast SE (FSE) Single shot FSE (SSFSE) PhilipsSE, Modified SE Multiple SE (MSE) Turbo spin echo (TSE) Ultra fast Spin echo (UFSE)

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 Imaging technique TRTE T1 weighting Short T2 weighting Long PD weighting LongShort * Short TR & long TE produces very low SNR and should be avoided

7 TR (in ms)TE (in ms) SequenceShortLongShortLong SE > >60 GRE<50>1001-5>10

8 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

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

10 FSE Fast scan (Based on principle of echo imaging) Long TR (Multiple RF pulse) - T2W Conventional SEFast SE 1 NEX4 NEX 9 min 28 sec2 min 25 sec

11 Spin Echo, k-space Fourier Transform k-space Frequency encoding axis If 256 phase encoding TR 256 Phase encoding axis ( Ky) RF signal (Echo) (Phase) ( Gy ) (Frequency) (Gx) RF Pulse 90°180°90° If you fill the 256 phase encoding TR 256 iterations

12 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 (TE eff ) : TE of k-space mid-line 180° 90° ETS ETL K-space Overall contrast Detailed Description (Ky)

13 Fast Spin Echo RF Pulse Phase-encoding gradient Echo TE5 TE1TE2TE3TE4TE6TE7TE8 ETS 90°180° TE eff ET L=8 Ky=0 Centre of k-space (Ky)

14 Effective TE TE eff msec80.72 msec96.86 msec msec T2 effect, SNR TE 40TE 100 TE eff 40 TE eff 100

15 Echo Train Length ETL 4ETL 8ETL 16ETL 32 TR 5000msec, NEX = 2 ETL governs by: (1)T2 relaxation, (2) ETS 10 min 45 sec 5 min 25 sec2 min 45 sec 1 min 25 sec ETL : Time Issues : Slice number Correction: TR, Slice thickness

16 Scan Time(SE) = (TR)(Ny)(NEX) Scan Time 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

17 Contrast in FSE 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). 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. FSESE T1W T2W

18 FSE vs CSE 90°180°90° TE80 90°180° 90° TE30 TE60 TE80TE100 CSEFSE TR Phase (Phase) Signed slope RF Pulse Multiple 180° pulse TE 80 Time saving K-space TE eff 80 No time saving

19 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

20 Disadvantage Blurring TE eff 50 ms T2 weighted TE eff 150 ms

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

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

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

24 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

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

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

27 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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