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.

(PCA) With this method images of the blood flow-velocity (or any other movement of tissue) are produced. The MRI signal contains both amplitude and phase information. The phase information can be used with subtraction of images with and without a velocity encoding gradient. The signal will be directly proportional to the velocity because of the relation between blood flow-velocity and signal intensity.
This is the strength of PCA, complete suppression of stationary tissue (no velocity - no signal), the direct velocity of flow is being imaged, while in TOF (Inflow) angiography, tissue with short T1 (fat or methaemoglobin) might be visualized.
The strength of the gradient determines the sensitivity to flow. It is set by setting the aliasing or encoding velocity (VENC). Unfortunately, phase sensitization can only be acquired along one axis at a time. Therefore, phase contrast angiographic techniques tend to be 4 times slower than TOF techniques with the same matrix.

See also Phase Contrast Sequence, Magnetic Resonance Angiography, Contrast Enhanced Magnetic Resonance Angiography, Flow Effects and Flow Quantification.

(RODEO) The software package Spiral RODEO™ MRI (including a patented fat and ductal tissue suppression technology), developed from Aurora Imaging Technology Inc, improves the spatial resolution and sensitivity for cancer detection in breast MRI examinations.

See also Aurora® 15T Dedicated Breast MRI System.

Quick Overview

Artifacts either by distorting the k-space trajectory (i.e. due to imperfect shimming) or as a consequence of the reduced bandwidth in the phase encode direction, commonly with EPI sequences.
While a standard spin warp-based sequence has an infinitely large bandwidth in the phase encode direction (about 1 or 2 kH), the bandwidth in EPI is related to the time between the gradient echoes (about a millisecond).
Hence even small frequency offsets can result in significant shifts of the signal in the phase encoding direction. Segmentation can introduce ghosting if there are significant difference in the amplitude and phase of the signal. This can be a particular problem when trying to acquire the segments in rapid succession.

Suitable choices of excitation schemes and/or subsequent correction can help to reduce this artifact. The signal from fat can easily be offset by a large fraction of the FOV, and must be suppressed. The effect of frequency offsets can be reduced by collecting data with more than one excitation, which effectively increases the bandwidth in the phase encoding direction.

Sequence of RF (and gradient) pulses designed to produce saturation, typically in a selected region or set of regions, most often by the use of selective excitation followed by a spoiler pulse. Similar to some spectral suppression techniques. Can be used to reduce signal from flowing blood by saturating regions upstream from region being imaged.