(PE) The partial echo technique (also called fractional echo) is used to shorten the minimum echo time. By the acquisition of only a part of k-space data this technique benefits (like all partial Fourier techniques) from the complex conjugate symmetry between the k-space halves (this is called Hermitian symmetry).
The dephasing gradient in the frequency direction is reduced, and the duration of the readout gradient and the data acquisition window are shortened. Partial echo gives a better SNR at a given TE when a smaller FOV or thinner slices are selected, allows a longer sampling time, and a larger water fat shift (WFS, see also bandwidth) due to a lower gradient amplitude. The resolution is not affected. This is often used in gradient echo sequences (e.g. FLASH, Contrast Enhanced Magnetic Resonance Angiography) to reduce the echo time and yields a lower gradient moment. The disadvantage of using a partial echo can be a lower SNR, although this may be partly offset by the reduced echo time.
Also called Fractional Echo, Read Conjugate Symmetry, Single Side View.

See also Partial Fourier Technique and acronyms for 'partial echo' from different manufacturers.

Fractional echo (also called asymmetric or partial echo) is used to shorten the echo time in a sequence, by acquiring partial echoes in the frequency direction. The reduction of echo time is possible because if the first part of the echo is not received, the dephasing lobe of the frequency encoding gradient is not to be on for quite as long, and this saves time.

See also Partial Fourier Technique, Read Conjugate Symmetry, Single Side View and acronyms for 'fractional echo' from different manufacturers.

(GE) An echo signal generated from a free induction decay by means of a bipolar switched magnetic gradient. The echo is produced by reversing the direction of a magnetic field gradient or by applying balanced pulses of magnetic field gradient before and after a refocusing RF pulse so as to cancel out the position dependent phase shifts that have accumulated due to the gradient.
In the latter case, the gradient echo is generally adjusted to be coincident with the RF spin echo. When the RF and gradient echoes are not coincident, the time of the gradient echo is denoted echo time (TE) and the difference in time between the echoes is denoted time difference (TD).
Gradient echo does not refocus the effects of main field inhomogeneity and therefore is generally used with a short echo time. Disadvantages of gradient echo imaging are compromised anatomic details and artifacts in regions with varying susceptibility e.g. between the air-containing sinuses and brain and especially between haemorrhages and normal tissue.

See also Susceptibility Artifact.

The partial Fourier technique is a modification of the Fourier transformation imaging method used in MRI in which the symmetry of the raw data in k-space is used to reduce the data acquisition time by acquiring only a part of k-space data.
The symmetry in k-space is a basic property of Fourier transformation and is called Hermitian symmetry. Thus, for the case of a real valued function g, the data on one half of k-space can be used to generate the data on the other half.
Utilization of this symmetry to reduce the acquisition time depends on whether the MRI problem obeys the assumption made above, i.e. that the function being characterized is real.
The function imaged in MRI is the distribution of transverse magnetization Mxy, which is a vector quantity having a magnitude, and a direction in the transverse plane. A convenient mathematical notation is to use a complex number to denote a vector quantity such as the transverse magnetization, by assigning the x'-component of the magnetization to the real part of the number and the y'-component to the imaginary part. (Sometimes, this mathematical convenience is stretched somewhat, and the magnetization is described as having a real component and an imaginary component. Physically, the x' and y' components of Mxy are equally 'real' in the tangible sense.)
Thus, from the known symmetry properties for the Fourier transformation of a real valued function, if the transverse magnetization is entirely in the x'-component (i.e. the y'-component is zero), then an image can be formed from the data for only half of k-space (ignoring the effects of the imaging gradients, e.g. the readout- and phase encoding gradients).
The conditions under which Hermitian symmetry holds and the corrections that must be applied when the assumption is not strictly obeyed must be considered.
There are a variety of factors that can change the phase of the transverse magnetization:
Off resonance (e.g. chemical shift and magnetic field inhomogeneity cause local phase shifts in gradient echo pulse sequences. This is less of a problem in spin echo pulse sequences.
Flow and motion in the presence of gradients also cause phase shifts.
Effects of the radio frequency RF pulses can also cause phase shifts in the image, especially when different coils are used to transmit and receive.
Only, if one can assume that the phase shifts are slowly varying across the object (i.e. not completely independent in each pixel) significant benefits can still be obtained. To avoid problems due to slowly varying phase shifts in the object, more than one half of k-space must be covered. Thus, both sides of k-space are measured in a low spatial frequency range while at higher frequencies they are measured only on one side. The fully sampled low frequency portion is used to characterize (and correct for) the slowly varying phase shifts.
Several reconstruction algorithms are available to achieve this. The size of the fully sampled region is dependent on the spatial frequency content of the phase shifts. The partial Fourier method can be employed to reduce the number of phase encoding values used and therefore to reduce the scan time. This method is sometimes called half-NEX, 3/4-NEX imaging, etc. (NEX/NSA). The scan time reduction comes at the expense of signal to noise ratio (SNR).
Partial k-space coverage is also useable in the readout direction. To accomplish this, the dephasing gradient in the readout direction is reduced, and the duration of the readout gradient and the data acquisition window are shortened.
This is often used in gradient echo imaging to reduce the echo time (TE). The benefit is at the expense in SNR, although this may be partly offset by the reduced echo time. Partial Fourier imaging should not be used when phase information is eligible, as in phase contrast angiography.

See also acronyms for 'partial Fourier techniques' from different manufacturers.

Quick Overview

Materials with magnetic susceptibility cause this artifact. There are in general three kinds of materials with magnetic susceptibility: ferromagnetic materials (iron, nickel etc.) with a strong influence and paramagnetic/diamagnetic (aluminium, platinum etc./gold, water, most organic compounds etc.) materials with a minimal/non influence on magnetic fields. In MRI, susceptibility artifacts are caused for example by medical devices in or near the magnetic field or by implants of the patient. These materials with magnetic susceptibility distort the linear magnetic field gradients, which results in bright areas (misregistered signals) and dark areas (no signal) nearby the magnetic material.

Use a spin echo or a fast spin echo sequence, because gradient echo sequences are more sensitve to susceptibility artifacts. A high bandwidth (small water fat shift) and a short echo time help also to reduce this artifact.
In some cases it is even beneficial to use a gradient echo sequence, e.g. a cavernom contains some iron-rich haemosiderin, which also causes a signal void on gradient echo sequences and for this purpose increases the diagnostic image quality.