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