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