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