In Chan Song, Ph.D. Seoul National University Hospital 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 Cause  Periodic: cardiac motion, respiration, blood flow Sporadic: irritable patients’ motion Translation, rotation, through-plane  Artifact in MRI blurring and ghosting Cause Longer encoding step

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

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

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

Motion artifact: ghost and blurring

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

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

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

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

Data acquisition Propeller filling Rectangular filling kx ky

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

James G. Pipe

Windowing Before After Phase correction

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

rotate imagerotate data Fourier Transform Properties rotate imagerotate data

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

Blade by blade operation Rotation at maximum correlation  Correction

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

max at Dx

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

Blade by blade operation Translation at maximum correlation  Correction

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

Through-plane motion :low weighting coeff.

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

w/motion correction

no correction correlation correction only motion correction only full corrections

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

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

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

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

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

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