In a periodic function (such as rotational or sinusoidal motion), the position relative to a particular part of the cycle. An angular relationship describing the degree of synchronism between two sinusoidal waveforms of the same frequency.

Water and fat signals being in or out of phase result from the gradient echo method and the slight difference in resonance frequencies of the protons. 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. With FFE Imaging, it is often advisable to use a TE value equal or close to an in phase value.
1.5T: IN PHASE = 4.6, 9.2, 13.8, 18.4, 23.0 ms
1.0T: IN PHASE = 6.9, 13.8, 20.7, 27.6 ms
0.5T: IN PHASE = 13.8, 27.6 ms

See also Out of Phase and Dixon.

The phase angle f is in turn affected by resonance offsets due to magnetic field inhomogeneity. If f varies throughout the image, the result will be inhomogeneous signal intensity (shading).

The phase shift is the loss of phase coherence in precessing spins. Vascular spins move at variable velocities; faster flow undergo a stronger phase shift than slower flowing spins.

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.