Download - Propeller MRI
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Propeller MRI
In Chan Song, Ph.D.
Seoul National University Hospital
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Contents: Propeller sequence(Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction)
Motion artifactTheoretical basisApplications
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• 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
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Scan time=TR x matrix x Average
Long scan time
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MR image reconstruction under the assumption of object’s motion-free condition during whole k space coverage
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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
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Motion artifact: ghost and blurring
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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
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Motion (abrupt) phase error position error
SolutionPhase informationNavigation
Motion correction by phase information
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Key ideas in propeller sequence
K space: partial covering for whole imageMotion detection: blade usageCorrection: FFT properties’ usage
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Diagram of the PROPELLER collection reconstruction process for motion corrected MRI.
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Rectangular filling
kx
ky
Data acquisition
Propeller filling
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Phase Correct
Redundant data must agree, remove phase from each blade image
Imperfect gradient balancing,
Eddy current effect:
echo center shift
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James G. Pipe
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Windowing
Before After
Phase correction
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Bulk Transformation Correction
Fourier transform correspondence Image space k space
Translation Phase rollRotation Rotation
Separate estimation of rotation and translation
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rotate imagerotate data
kkx RfRf F F
Fourier Transform Properties
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Reference(only inner circle)
Magnitude of the average of strips
Rotation(only inner circle)
Correlation
Rotation correction (magnitude image)
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Blade by blade operation
Rotation at maximum correlation Correction
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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
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xrbrb * FFF -1 x
max at x
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TranslationComplex average k-space data
Reference(only inner circle)
Complex of the average of strips
Inverse FT (maximum)
Multiplication
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Blade by blade operation
Translation at maximum correlation Correction
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Blade Correlation
throw out bad – or difficult to interpolate - data
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Through-plane motion:low weighting coeff.
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Reconstruction (FFT)non-Cartesian sampling
requires gridding convolution
Kx
Ky
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w/motion
correction
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no correction
correlation correction only
motion correction only
full corrections
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T2-FSE T2-Propeller T2-Propeller(corrected)
Artifact reduction due to head motion
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DWI-EPIB=1000s/mm2
DWI-Propeller (FSE)
James G Pipe, 2002
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DWI (b=700s/mm2)
a. EPI (TR/TE/avg=2700/113/15)
b. Propeller EPI(TR/TE/blade=1600/70/26)
Wang FN, 2005
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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
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Propeller sequence
Low sensitivity to image artifacts, Bo inhomogeneity and motion
T2-, Diffusion-weighted images (High SNR, low geometric distortion, low SAR)
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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