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Asymmetric Sampling
The collection of more data points on one side of the k-space origin than on the other. With fewer k-space data points prior to the center (echo) a shorter echo time can be attained. Asymmetric acquisition in any phase encoding direction followed by partial-Fourier reconstruction leads to a reduction in imaging time.

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Back Projection Imaging
This imaging technique is probably the earliest, but rarely used today. Most of today's imaging techniques are based on the Fourier transform, and fill the Cartesian grid of points in k-space line by line by a sequence of applied gradients. Back projection imaging performs a radial filling of the k-space by a one dimensional field gradient, applied at different angles. Back projection imaging is still in use in laser polarized noble gas imaging (see ventilation agents and lung imaging).


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Further Reading:
Primer on Magentic Resonance Imaging Tomography
The Basics of MRI
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Cartesian Sampling
Cartesian sampling is used to refer to data collection with a fixed value of the phase encoding gradient. In 2D Fourier imaging with common Cartesian sampling of k-space sensitivity encoding by means of a receiver array enables to reduce the number of Fourier encoding steps. This is achieved by increasing the distance of sampling positions in k-space while maintaining the maximum k-values.
The Cartesian coordinates are obtained from the polar coordinates by the operations
x = r sin f
y = r cos f
using the trigonometric functions sine and cosine.

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Echo Planar ImagingInfoSheet: - Sequences - 
Types of, 
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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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Further Reading:
New Imaging Method Makes Brain Scans 7 Times Faster
Sunday, 9 January 2011   by    
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Fractional Nex Imaging
Fractional Nex imaging (GE Healthcare term for imaging with a Nex value less than 1) benefits from the conjugate symmetry of the k-space to reduce the number of phase encoding acquisitions. With fractional Nex imaging (similar to partial Fourier or Half Scan), just over half of the data are acquired and the data from the lower part of k-space are used to fill the upper part, without sampling the upper part. Fractional Nex imaging sequences use a number of excitations values between 0.5 and 1. These values are a bit misleading, because the number of phase encoding steps is reduced, and not the NEX.
Fractional Nex imaging reduces the scan time considerable, by preserving the same contrast between the tissues. The effect by acquiring fewer data points is that the signal to noise ratio decreases.
See also acronyms for 'partial averaging//fractional Nex imaging' from different manufacturers.
Further Reading:
Method and apparatus for subterranean formation flow imaging
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