Automatic bolus detection is used to trigger the begin of the MRI scan with the time the contrast agent reaches the region of interest. The acquisition process is started once the signal is increased in this region. There are similar methods used by the scanner manufacturer (see MRI Acronyms for Automatic Bolus Detection). After injection of the contrast agent the region of interest is monitored (e.g. with a spin echo or gradient echo sequence). When the signal increases the scan is automatically triggered or the operator is informed.

See also Abdominal Imaging, Bolus Injection, Fluoroscopic Triggering, Care Bolus, and Bolus Tracking.

This method synchronize the heartbeat with the beginning of the TR, whereat the r wave is used as the trigger. Cardiac gating times the acquisition of MR data to physiological motion in order to minimize motion artifacts. ECG gating techniques are useful whenever data acquisition is too slow to occur during a short fraction of the cardiac cycle.
Image blurring due to cardiac-induced motion occurs for imaging times of above approximately 50 ms in systole, while for imaging during diastole the critical time is of the order of 200-300 ms. The acquisition of an entire image in this time is only possible with using ultrafast MR imaging techniques. If a series of images using cardiac gating or real-time echo planar imaging EPI are acquired over the entire cardiac cycle, pixel-wise velocity and vascular flow can be obtained.
In simple cardiac gating, a single image line is acquired in each cardiac cycle. Lines for multiple images can then be acquired successively in consecutive gate intervals. By using the standard multiple slice imaging and a spin echo pulse sequence, a number of slices at different anatomical levels is obtained. The repetition time (TR) during a ECG-gated acquisition equals the RR interval, and the RR interval defines the minimum possible repetition time (TR). If longer TRs are required, multiple integers of the RR interval can be selected. When using a gradient echo pulse sequence, multiple phases of a single anatomical level or multiple slices at different anatomical levels can be acquired over the cardiac cycle.
Also called cardiac triggering.

(CPMG) This type of spin echo pulse sequence consisting of a 90° radio frequency pulse followed by an echo train induced by successive 180° pulses and is useful for measuring T2 weighted images. It is a modification of the Carr-Purcell RF pulse sequence, with 90° phase shift in the rotating frame of reference between the 90° pulse and the subsequent 180° pulses in order to reduce accumulating effects of imperfections in the 180° pulses. Suppression of effects of pulse error accumulation can alternatively be achieved by switching phases of the 180° pulses by 180°.

(CPS) Sequence of a 90° RF pulse followed by repeated 180° RF pulses to produce a train of spin echoes; is useful for measuring T2.

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.