A rephasing or refocusing, which occurs when constant velocity spins return to the same starting phase they had directly after the initial exciting RF pulse, as a result of the application of an even number of gradient pulses. This may also result from the application of multiple gradient echo pulses following the RF pulse. The even echo rephasing phenomenon is one of the flow effects observed in MR imaging.

(IR-FSPGR) This sequence uses an inversion pulse followed by a spoiled low flip angle and short TR gradient echo train to collect all partition data. The 3D-IR-FSPGR uses centric ordering of Gz. The selected TI is also the effective TI. The centric reordering strategy can lead to image blurring if a steady state is not achieved quickly with low flip angles (less than 20°).

Short name: AMI-25, generic name: Ferumoxide (SPIO)
Ferumoxides are superparamagnetic (T2*) MRI contrast agents, so the largest signal change is on T2 and T2* weighted images.
The agent distributes relatively rapidly to organs with reticuloendothelial cells primarily the liver, spleen and bone marrow. The liver shows decreased signal intensity, as does the spleen and marrow. The agent is taken up by the normal liver, resulting in increased CNR between tumor and normal liver. Hepatocellular lesions, such as adenoma or focal nodular hyperplasia, contain reticuloendothelial cells, so they will behave similar to the liver, with decreased signal on T2 weighted images. On T1 images, there is typically some circulating contrast agent, and blood vessels show increased signal intensity.
Current MRI protocols involve T1 weighted breath-hold gradient echo images of the liver, and fast spin echo T2 weighted pictures. This requires about 15 minutes. The patient is then removed from the scanner, and the contrast agent administered. After contrast administration, the same pulse sequences are again repeated.

(FE) Echo produced by reversing the direction of the magnetic field gradient to cancel out the position-dependent phase shifts that have accumulated due to the gradient.

See also Gradient Echo Sequence.

The FEER method was the first clinically useful flow quantification method using phase effects, from which all spin phase related flow quantification techniques currently in use are derived.
In this sequence a gradient echo is measured after a gradient with flow compensation. The measured signal phase should be zero for all pixels. A deviation from gradient symmetry by shifting the gradient ramp slightly away from the symmetry condition will impart a defined phase shift to the magnetization vectors associated with spins from pixels with flow.
Slight stable variations in the magnetic field across the imaging volume will prevent the phase angle from being uniformly zero throughout the volume in the flow-compensated image. The first image (acquired without gradient shift) serves as reference, defining the values of all pixel phase angles in the flow (motion) compensated sequence. Ensuing images with gradient phase shifts imparted in each of the 3 spatial axes will then permit measurement of the 3 components of the velocity vector v = (vx, vy, vz) by calculating the respective phases px, py and pz by simply subtracting the pixel phases measured in the compensated image from the 3 images with a well defined velocity sensitization.
The determination of all 3 components of the velocity vector requires the measurement of 4 images.
The phase quantification requires an imaging time four times longer than the simple measurement of a phase image and associated magnitude image. If only one arbitrary flow direction is of interest, it suffices to acquire the reference image plus one image velocity sensitized in the arbitrary direction of interest.

See also Flow Quantification.