EPI

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EPI Echo Planar Imaging

• Joakim Rydell
• 2004-03-26

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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)

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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
Ordinary MR

• Acquire all rows in k-space with just one
  excitation

EPI
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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 errors
• Slowly varying
• Approximately 65 of k-space required
• SNR decrease
• Full 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