epi – echo planar imaging
DESCRIPTION
EPI – Echo Planar Imaging. Joakim Rydell 2004-03-26. Echo Planar Imaging. Like ordinary MR, but faster Ordinary MR: approximately one minute EPI: less than one second The Nobel Prize in Medicine 2003 Lauterbur (MR) Mansfield (EPI). Repetition, k-space. Fourier transform of image - PowerPoint PPT PresentationTRANSCRIPT
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EPI – Echo Planar Imaging
Joakim Rydell
2004-03-26
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Echo Planar Imaging
Like ordinary MR, but fasterOrdinary MR: approximately one minuteEPI: less than one second
The Nobel Prize in Medicine 2003Lauterbur (MR)Mansfield (EPI)
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Repetition, k-space
Fourier transform of image
Traversed using magnetic field gradients
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Repetition, pulse sequence
Slice selection Phase encoding Frequency encoding
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Introducing EPI
Acquire all rows in k-space with just one excitation
EPI
Ordinary MR
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EPI – The Movie
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Going even faster – Partial Fourier
Real image hermitian symmetric k-space Acquire half of k-space, compute the rest Not always as good as it seems...
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Partial Fourier, drawbacks
Phase errorsSlowly varyingApproximately 65 % of k-space required
SNR decreaseFull k-space acquisition gives sqrt(2) SNR
improvement
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Why use ordinary MR?
Signal decay Low SNR N/2 ghosting Geometric distortion dB/dt
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Signal decay
Long readout period Weak signal near end
of acquisition Blurring in phase-
encoding direction
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Low SNR
Fast acquisition Less averaging Noisy images
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N/2 ghosting
Alternating acqusition directions
Shifted sampling positions
N/2 ghosting Reference scans
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Geometric distortion
Low bandwidth/pixel Pixels shifted in
phase encoding direction
Field maps
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dB/dt
Fast gradient switching induces eddy currents, causing field inhomogenity
Rapid changes in the magnetic field can cause peripheral nerve stimulation
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k-space trajectories
Cartesian Blurring N/2 ghosting High dB/dt
Spiral Non-uniform sampling
Cartesian spiral
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Ramp sampling
Sampling on gradient ramps
Non-uniform sampling In time In k-space
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Multishot
Both EPI and ordinary imaging Collect several rows per excitation Alignment difficulties
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Applications
Cardiac imaging fMRI ...
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EVI – Echo Volumar Imaging
3D EPI Acquire a full volume
with just one excitation
Not quite possible yet