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.

(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.

Inhomogeneity is the degree of lack of homogeneity, for example the fractional deviation of the local magnetic field from the average value of the field. Inhomogeneities of the static magnetic field, produced by the scanner as well as by object susceptibility, is unavoidable in MRI. The large value of gyromagnetic coefficient causes a significant frequency shift even for few parts per million field inhomogeneity, which in turn causes distortions in both geometry and intensity of the MR images.
Manufacturers try to make the magnetic field as homogeneous as possible, especially at the core of the scanner. Even with an ideal magnet, a little inhomogeneity is always left and is caused in addition by the susceptibility of the imaging object. The geometrical distortion (displacement of the pixel locations) are important e.g., for some cases as stereotactic surgery. Displacements up to 3 to 5 mm have been reported. The second problem is the undesired changes in the intensity or brightness of pixels, which may cause problems in determining different tissues and reduce the maximum achievable image resolution.

General strategies for reducing field inhomogeneity induced artifacts:
Increasing the strength of the gradient magnetic field.
Decreasing the echo time.
Improving the image resolution. Phase encoding. Postprocessing.