An image in which the signal from two spectral components (such as fat and water) is 180° out of phase and leads to destructive interference in a voxel.
Since fat precesses slower than water, based on their chemical shift, their signals will decay and precess in the transverse plane at different frequencies. When the phase of the TE becomes opposed (180°), their combined signal intensities subtract with each other in the same voxel, producing a signal void or dark band at the fat/water interface of the tissues being examined.
Opposed phase gradient echo imaging for the abdomen is a lipid-type tissue sensitive sequence particularly for the liver and adrenal glands, which puts a signal intensity around abnormal water-based tissues or lesions that are fatty. Due to the increased sensitivity of opposed phase, the tissue visualization increases the lesion-to-liver contrast and exhibits more signal intensity loss in tissues containing small amounts of lipids compared to a spin echo T1 with fat suppression. Using an opposed phase gradient echo also provides the ability to differentiate various pathologies in the brain, including lipids, methaemoglobin, protein, calcifications and melanin.

See also Out of Phase, and Dixon.

Quick Overview
Please note that there are different common names for this artifact.

Black boundary artifacts are black lines following voxels where both water and fat protons are present in the same voxel. This artifact arise along the boundary of organs or tissues perpendicular to the frequency encoding direction, and occurs preferentially in gradient echo sequences with out of phase echo times.

Fat suppression techniques eliminate this artifact. For artifact reducing helps a smaller water fat shift (high bandwidth), a higher matrix or/and an in phase TE.

See also Chemical Shift Artifact.

Quick Overview
Please note that there are different common names for this artifact.

During frequency encoding, fat protons precess slower than water protons in the same slice because of their magnetic shielding. Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain. This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy.
Protons in fat and water molecules are separated by a chemical shift of about 3.5 ppm. The actual shift in Hertz (Hz) depends on the magnetic field strength of the magnet being used. Higher field strength increases the misregistration, while in contrast a higher gradient strength has a positive effect. For a 0.3 T system operating at 12.8 MHz the shift will be 44.8 Hz compared with a 223.6 Hz shift for a 1.5 T system operating at 63.9 MHz.

For artifact reduction helps a smaller water fat shift (higher bandwidth), a higher matrix, an in phase TE or a spin echo technique. Since the misregistration offset is present in the read out axis the patient may be rescanned with this axis parallel to the fat-water interface. Steeper gradient may be employed to reduce the chemical shift offset in mm. Another strategy is to employ specialized pulse sequences such as fat saturation or inversion recovery imaging. Fat suppression techniques eliminate chemical shift artifacts caused by the lack of fat signal.

See also Black Boundary Artifact and Magnetic Resonance Spectroscopy.

(FEDIF) See Out of Phase and Opposed Phase Image.

Water and fat signals being in or out of phase result from the FFE method and the slight difference in resonance frequencies of the protons. It can cause black "outlining" of tissues and decrease in signal from voxel containing both water and fat. At 1.5 T, the water and fat signal are in phase when TE is an even multiple, and out of phase when TE is an odd multiple of 2.3 ms.
1.5T: OUT of PHASE = 2.3, 6.9, 11.5, 16.1, 20.7 ms
1.0T: OUT of PHASE = 3.5, 10.4, 17.3, 24.2 ms
0.5T: OUT of PHASE = 6.9, 20.7 ms

See also Opposed Phase Image, and Dixon.