Propeller MRI
In Chan Song, Ph.D.
Seoul National University Hospital
Contents: Propeller sequence(Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction)
Motion artifactTheoretical basisApplications
• 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
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 MRIdue 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
SolutionPhase informationNavigation
Motion correction by phase information
Key ideas in propeller sequence
K space: partial covering for whole imageMotion detection: blade usageCorrection: FFT properties’ usage
Diagram of the PROPELLER collection reconstruction process for motion corrected MRI.
Rectangular filling
kx
ky
Data acquisition
Propeller filling
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 rollRotation Rotation
Separate estimation of rotation and translation
rotate imagerotate data
kkx RfRf F F
Fourier Transform Properties
Reference(only inner circle)
Magnitude of the average of strips
Rotation(only inner circle)
Correlation
Rotation correction (magnitude image)
Blade by blade operation
Rotation at maximum correlation Correction
Fourier Transform Properties
shift image phase roll across data
xkkkxx iebb 2 F F
xrbrb * FFF -1 x
b is blade image, r is reference image
xrbrb * FFF -1 x
max at x
TranslationComplex average k-space data
Reference(only inner circle)
Complex of the average of strips
Inverse FT (maximum)
Multiplication
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
T2-FSE T2-Propeller T2-Propeller(corrected)
Artifact reduction due to head motion
DWI-EPIB=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 sequenceMotion- or Bo-inhomogeneities – insensitiveIrritable patientDiffusion weighted image
Limitations in propeller sequenceRedundant acquisition
Long scan time:High SAR: problem in higher field MR system
Solutions Undersampling (Konstantinos Arfanakis, 2005)
Parallel imagingTurbopropeller (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): 42-53,20023. 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