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 term in phase refers to an image in which the signals from two spectral components (such as fat and water) add constructively in a voxel. T1 weighted in phase images are acquired by a gradient echo-based technique with a short TR, TE and a high flip angle greater than 60 degrees. To some degree, in phase sequences are more sensitive to detection of focal hepatic lesions than out of phase for evaluating reduced lesion-to-liver contrast, but the choice for a T1 gradient echo sequence is still based on field strength, advanced imaging techniques (breath hold imaging), and physician preference.

The spins flow with the blood through a slice and experience a RF pulse. If they flow out of the slice by the time the signal is recorded (because the repetition time (TR) is asynchronous with the pulsatile flow), the flowing blood produces intravascular signal void by 'time of flight' effects, turbulent dephasing and first echo dephasing. The liquid flow occasionally produces an intravascular high signal intensity due to flow related enhancement, even echo rephasing and diastolic pseudogating.

See also Flow Artifact and Flow Effects.

(FESUM) See Gradient Recalled Echo Sequence and In Phase Image.

Quantification relies on inflow effects or on spin phase effects and therefore on quantifying the phase shifts of moving tissues relative to stationary tissues.
With properly designed pulse sequences (see phase contrast sequence) the pixel by pixel phase represents a map of the velocities measured in the imaging plane. Spin phase effect-based flow quantification schemes use pulse sequences specifically designed so that the phase angle in a pixel obtained upon measuring the signal is proportional to the velocity. As the relation of the phase angle to the velocity is defined by the gradient amplitudes and the gradient switch-on times, which are known, velocity can be determined quantitatively on a pixel-by-pixel basis. Once, this velocity is known, the flow in a vessel can be determined by multiplying the pixel area with the pixel velocity. Summing this quantity for all pixels inside a vessel results in a flow volume, which is measured, e.g. in ml/sec.
Flow related enhancement-based flow quantification techniques (entry phenomena) work because spins in a section perpendicular to the vessel of interest are labeled with some radio frequency RF pulse. Positional readout of the tagged spins some time T later will show the distance D they have traveled.
For constant flow, the velocity v is obtained by dividing the distance D by the time T : v = D/T. Variations of this basic principle have been proposed to measure flow, but the standard methods to measure velocity and flow use the spin phase effect.
Cardiac MRI sequences are used to encode images with velocity information. These pulse sequences permit quantification of flow-related physiologic data, such as blood flow in the aorta or pulmonary arteries and the peak velocity across stenotic valves.