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Assessment of tumoural ADC’s in rectal Tumours using Burst: New methodological Developments

Assessment of tumoural ADC’s in rectal Tumours using Burst: New methodological Developments. SJ Doran 1 , ASK Dzik-Jurasz 2 , J Wolber 2 , C Domenig 1 , MO Leach 2. 1 University of Surrey Department of Physics Guildford, UK. 2 CRC Clinical Magnetic Resonance Research Group

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Assessment of tumoural ADC’s in rectal Tumours using Burst: New methodological Developments

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  1. Assessment of tumoural ADC’s in rectal Tumours using Burst:New methodological Developments SJ Doran1, ASK Dzik-Jurasz2, J Wolber2, C Domenig1, MO Leach2 1 University of Surrey Department of Physics Guildford, UK 2 CRC Clinical Magnetic Resonance Research Group Institute of Cancer Research Sutton, UK

  2. Purpose of the clinical Study • Assessment of locally advanced rectal tumours and surrounding normal tissue prior to chemo- and radio- therapy and after treatment • Changes in ADC may precede conventional morphologicalmeasures of tumour response

  3. The original Burst Sequence (J. Hennig, M. Hodapp, 1993; L. Zha, I. J. Lowe, 1995)

  4. Main Components of Burst-type Sequences • excitation with a series of N low-flip-angel RF pulses (DANTE pulse train) • consecutive acquisition of the N echoes • constant gradient during excitation/acquisition in the readout direction • Intrinsic characteristics: • decay of successive echoes due to T2 and diffusion

  5. Our first Burst-Diffusion Sequence (C. A. Wheeler-Kingshott et al., 2000)

  6. Main Components of the Burst-Diffusion Sequence • Excitation/readout gradient = diffusion gradient • transverse magnetisation dephases during exitation, rephases during readout • dephasing = rephasing due to diffusion motion • Stepped phase-encoding gradient  diffusion gradient • M repetitions of the sequence

  7. Calculation of ADC and T2 map • Data acquisition • N echoes, M repetitions, R points in readout direction per echo  data matrix [M x N x R] • Post processing • reformation of the data in N separate matrices • FT  set of N images of the same slice but with increased diffusion and T2 weighting • double exponential fit  ADC and T2 map

  8. First clinical Results Burst-diffusion image low b-value T2-weighted SE image • Clinical study: • 1.5T Siemens Magnetom Vision MR Scanner • Scanning parameters:  N = 16 echoes  FOV = 180 x 180 mm2  matrix = 128 x 128 • Burst-diffusion MR images:  TR = 2000 ms  TA = 4 min. 16 sec. • T2-weighted SE images:  TR = 1500 ms  TA = 1 min. 54 sec. ADC map T2 map

  9. Problems • Images suffer from low signal-to-noise ratio • Field-of-view and diffusion sensitisation are coupled • Need of different sequence for correcting echo-decay due to T2-relaxation • Only single-slice data can be obtained • Motion during and between scans leads to ghosting artifacts in the phase-encode direction

  10. Problem Solving • Increasing the SNR by: • reducing the number of pulses: 16  9 • increasing the flip angle: 16  21 • reducing the acquisition bandwidth: 100 kHz 25 kHz (12.5kHz) • Uncoupling of readout- and diffusion gradient • diffusion gradient independent of FOV • Sequence can be used to correct for T2-decay

  11. Phantom Study: Comparison of the old and new Sequence in Terms of SNR • Phantom: • Tubes with Acetonitrile, DMSO, H2O and Glycerol • Old Sequence: • N = 16 • BW = 100 kHz • SNR = 7:1 • New Sequence: • N = 9 • BW = 12.5 kHz • SNR = 30:1 low b-value (2 s/mm2) higher b-value (310 s/mm2)

  12. (bj-b0) vs. ln(Sj/S0) Single pixel fit Signal averaged

  13. Gdiff = Gread Gdiff = 3.67 mT/m b-values: 0.95 - 100 s/mm2 Gdiff Gread Gdiff = 10 mT/m b-values: 0.11 - 1272 s/mm2 Acquisition parameters: FOV = 160 x 160 mm2 matrix = 128 x 128 TR = 2000 ms TA = 4 min. 16 sec. Sequence parameters: N = 9 BW = 25 kHz Coupled and uncoupled FOV and Diffusion-Gradient

  14. Coupled and uncoupled FOV and Diffusion-Gradient

  15. The new single-slice Burst-Diffusion Sequence

  16. First abdominal Images 4 differently diffusion-weighted images Comparison in terms of SNR Left: Image obtained with the old sequence Right: Image obtained with the new sequence

  17. Multi-slice Burst-Diffusion Sequence • Replacing the non-selectice a-pulses by slice-selective ones

  18. Conclusions • Burst can be used for abdominal imaging (regions with short T2) • More b-values over a wide range can be obtained (GDiff = 6 mT/m: b-values from 0.05 to 522 s/mm2) • Although longer than EPI the Burst-diffusion sequences are much shorter than the conventional PGSE Method

  19. Future Work • Further testing and clinical evaluation of the new Burst-diffusion sequences • Introduction of navigator echoes in both, the single- slice and multi-slice Burst-diffusion sequence

  20. Acknowledgements • Simon J Doran • Andrzej SK Dzik-Jurasz • Jan Wolber • Martin O Leach

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