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Oscillating Gradient System
 
A gradient system, which changes the readout gradient sinusoidally by connecting a capacitor to the self inductance generated by the gradient coil. Oscillating gradient systems were initially used in the development of EPI.
This electrical oscillating circuit can be driven with minimal power to generate the gradient amplitudes and switching frequencies required for echo planar imaging (EPI).
Disadvantages are that it is not possible to use any arbitrary trapezoidal gradient wave form as can be used in standard MRI. Also, the gradients are inflexible and cannot be used to create other ultrafast sequences and beside, nonlinear sampling of the MR signal is required.
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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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