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Propeller MRI

Propeller MRI. In Chan Song, Ph.D. Seoul National University Hospital. Contents: Propeller sequence (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) Motion artifact Theoretical basis Applications. Motion

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Propeller MRI

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  1. Propeller MRI In Chan Song, Ph.D. Seoul National University Hospital

  2. Contents: Propeller sequence (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) Motion artifact Theoretical basis Applications

  3. Motion • Periodic: cardiac motion, respiration, blood flow • Sporadic: irritable patients’ motion • Translation, rotation, through-plane  • Artifact in MRI • blurring and ghosting • Cause • Longer encoding step

  4. Scan time= TR x matrix x Average  Long scan time

  5. MR image reconstruction under the assumption of object’s motion-free condition during whole k space coverage

  6. Motion artifacts -Most ubiquitous and noticeable artifacts in MRI due to voluntary and involuntary movement, and flow (blood, CSF) -Mostly occur along the phase encode direction, since adjacent lines of phase-encoded protons are separated by a TR interval that can last 3,000 msec or longer -Slight motion can cause a change in the recorded phase variation across the FOV throughout the MR acquisition sequence

  7. Motion artifact: ghost and blurring

  8. Solution for motion compensation • -Navigator echo usage to estimate the motion or motion related • phase from extra collected data • -Cardiac and respiratory gating • -Respiratory ordering of the phase encoding projections based on • location in respiratory cycle • -Signal averaging to reduce artifacts of random motion • -Short TE spin echo sequences (limited to spin density, • T1-weighted scans). Long TE scans (T2 weighting) are • more susceptible to motion

  9. Motion (abrupt)  phase error  position error Solution Phase information Navigation  Motion correction by phase information

  10. Key ideas in propeller sequence • K space: partial covering for whole image • Motion detection: blade usage • Correction: FFT properties’ usage

  11. Diagram of the PROPELLER collection reconstruction process for motion corrected MRI.

  12. Rectangular filling Propeller filling ky kx Data acquisition

  13. Phase Correct Redundant data must agree, remove phase from each blade image Imperfect gradient balancing, Eddy current effect:  echo center shift

  14. James G. Pipe

  15. Windowing Before After Phase correction

  16. Bulk Transformation Correction • Fourier transform correspondence • Image space  k space • Translation  Phase roll • Rotation  Rotation • Separate estimation of rotation and translation

  17. Fourier Transform Properties rotate imagerotate data

  18. Rotation correction (magnitude image) Reference (only inner circle) Magnitude of the average of strips Rotation (only inner circle) Correlation

  19. Blade by blade operation Rotation at maximum correlation  Correction

  20. Fourier Transform Properties shift image  phase roll across data b is blade image, r is reference image

  21. max at Dx

  22. Translation Complex average k-space data Reference (only inner circle) Complex of the average of strips Multiplication Inverse FT (maximum)

  23. Blade by blade operation Translation at maximum correlation  Correction

  24. Blade Correlation throw out bad – or difficult to interpolate - data

  25. Through-plane motion :low weighting coeff.

  26. Ky Kx Reconstruction (FFT) non-Cartesian sampling requires gridding  convolution

  27. w/motion correction

  28. correlation correction only motion correction only full corrections no correction

  29. Artifact reduction due to head motion T2-FSE T2-Propeller T2-Propeller(corrected)

  30. DWI-EPI B=1000s/mm2 DWI-Propeller (FSE) James G Pipe, 2002

  31. DWI (b=700s/mm2) a. EPI (TR/TE/avg=2700/113/15) b. Propeller EPI (TR/TE/blade=1600/70/26) Wang FN, 2005

  32. Useful application in propeller sequence • Motion- or Bo-inhomogeneities – insensitive • Irritable patient • Diffusion weighted image • Limitations in propeller sequence • Redundant acquisition  • Long scan time: • High SAR: problem in higher field MR system • Solutions  • Undersampling (Konstantinos Arfanakis, 2005) • Parallel imaging • Turbopropeller (James G Pipe, 2006) • Propeller EPI

  33. Propeller sequence • Low sensitivity to image artifacts, • Bo inhomogeneity and motion • T2-, Diffusion-weighted images • (High SNR, low geometric distortion, low SAR)

  34. References 1. Pipe J, MRM 42(5): 963-62,1999. 2. Pipe J, et al., MRM 47(1): 42-53,2002 3. Wu Y, Field AS, Alexander AL. ISMRM, Toronto, Canada, 2003. 2125. 4. Roberts TP, Haider M. ISMRM, Kyoto, Japan, 2004. 946. 5. Sussman MS, White LM, Roberts TP. ISMRM, Kyoto, Japan, 2004. 211. 6. Pipe J and Zwart N. Magn Reson Med 55:380–385, 2006. 7. Cheryaukaa AB, et al. Magnetic Resonance Imaging 22:139-148, 2004

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