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Echo Planar ImagingInfoSheet: - Sequences - 
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Echo Planar Imaging Timing Diagram

(EPI) Echo planar imaging is one of the early magnetic resonance imaging sequences (also known as Intascan), used in applications like diffusion, perfusion, and functional magnetic resonance imaging. Other sequences acquire one k-space line at each phase encoding step. When the echo planar imaging acquisition strategy is used, the complete image is formed from a single data sample (all k-space lines are measured in one repetition time) of a gradient echo or spin echo sequence (see single shot technique) with an acquisition time of about 20 to 100 ms. The pulse sequence timing diagram illustrates an echo planar imaging sequence from spin echo type with eight echo train pulses. (See also Pulse Sequence Timing Diagram, for a description of the components.)
In case of a gradient echo based EPI sequence the initial part is very similar to a standard gradient echo sequence. By periodically fast reversing the readout or frequency encoding gradient, a train of echoes is generated.
EPI requires higher performance from the MRI scanner like much larger gradient amplitudes. The scan time is dependent on the spatial resolution required, the strength of the applied gradient fields and the time the machine needs to ramp the gradients.
In EPI, there is water fat shift in the phase encoding direction due to phase accumulations. To minimize water fat shift (WFS) in the phase direction fat suppression and a wide bandwidth (BW) are selected. On a typical EPI sequence, there is virtually no time at all for the flat top of the gradient waveform. The problem is solved by "ramp sampling" through most of the rise and fall time to improve image resolution.
The benefits of the fast imaging time are not without cost. EPI is relatively demanding on the scanner hardware, in particular on gradient strengths, gradient switching times, and receiver bandwidth. In addition, EPI is extremely sensitive to image artifacts and distortions.

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    • Blipped Phase Encoding
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    • Gradient Echo
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Further Reading:
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New Imaging Method Makes Brain Scans 7 Times Faster
Sunday, 9 January 2011   by www.dailytech.com    
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Echo Planar Imaging Factor
 
(EPI Factor) The imaging speed in Echo Planar Imaging (EPI) depends on many factors. Single shot EPI should provide images within 100 ms or less. Because of this limitations, a multi shot EPI approach is in most cases preferred. The parameter 'EPI Factor' is used to specify the number of k-space profiles collected per excitation.
The EPI factor 64 means a measurement time 64 times faster than a normal gradient echo sequence. See also Echo Planar Imaging.

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Further Reading:
  Basics:
Echo Planar Imaging at 4 Tesla With Minimum Acoustic Noise(.pdf)
   by www.bnl.gov    
Echo-planar imaging (EPI) and functional MRI(.pdf)
1998   by www.uib.no    
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Blipped Echo Planar Imaging
 
See Echo Planar Imaging and Blips.
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New Imaging Method Makes Brain Scans 7 Times Faster
Sunday, 9 January 2011   by www.dailytech.com    
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Inversion Recovery Echo Planar ImagingInfoSheet: - Sequences - 
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(IR-EPI) Inversion recovery sequence with echo planar imaging.
See Inversion Recovery and Echo Planar Imaging.
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Blipped Phase Encoding
 
A strategy for incrementing the position of the k-space trajectory of an echo planar imaging (EPI) pulse sequence.
echo planar imaging (EPI) uses a constant gradient amplitude in one direction. This, combined with an oscillating gradient system in the frequency encoding direction, produces a zigzag trajectory in k-space. In the blipped phase encoding variant of EPI, the k-space position in the phase encoded direction is incremented by gradient 'blips' of the appropriate area. These, when timed to occur during the reversals of the read-out gradient, produce a rectilinear path in k-space.
The artifacts in an EPI image can arise from both hardware and sample imperfections. These are most easily understandable from examination of the k-space trajectory involved, which is either a zigzag form (when using a constant phase encoding gradient) or a rastered zigzag (when the phase encoding is performed with small gradients at the end of each scan line, so-called 'blipped' EPI).

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Further Reading:
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Chapter 2 - Principles of Magnetic Resonance Imaging
   by www.fmrib.ox.ac.uk    
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