EPI – Echo Planar Imaging Joakim Rydell

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 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 errors  Slowly varying  Approximately 65 % of k-space required SNR decrease  Full 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