(MSI) The combination of biomagnetic field detection and MR imaging into a merged data set. Most applications of MSI involve the combined use of MRI and measurement of magnetic fields created by electric currents in the brain, so-called magnetoencephalography MEG.
MEG allows calculation of the source of the measured biomagnetic fields, and thereby localization of many regional brain functions, such as mapping of the sensorimotor, auditory and visual cortex and also localization of epileptogenic foci. The MEG coordinate system is defined by anatomical landmarks, which are easily identified also with MRI, making it possible to align the 3D MEG data with the 3D MR image data. The resulting magnetic source images show the spatial relationships between the functional area provided by MEG and the neighboring anatomy and pathology, both provided by MRI.
Cardiac applications of MSI are also being explored. The electric currents in the myocardium create extrathoracic magnetic fields and the source of these fields may be calculated by the same principles as those used in MEG. Possible cardiac applications include mapping of arrhythmogenic sites prior to ablation therapy.

Quick Overview
Please note that there are different common names for this artifact.

Patient motion is the largest physiological effect that causes artifacts, often resulting from involuntary movements (e.g. respiration, cardiac motion and blood flow, eye movements and swallowing) and minor subject movements.
Movement of the object being imaged during the sequence results in inconsistencies in phase and amplitude, which lead to blurring and ghosting. The nature of the artifact depends on the timing of the motion with respect to the acquisition. Causes of motion artifacts can also be mechanical vibrations, cryogen boiling, large iron objects moving in the fringe field (e.g. an elevator), loose connections anywhere, pulse timing variations, as well as sample motion. These artifacts appear in the phase encoding direction, independent of the direction of the motion.

Motion artifacts can be flipped 90° by swapping the phase//frequency encoding directions.
The artifacts can be reduced by using breath holding, cardiac synchronization or respiratory compensation techniques: triggering, gating, retrospective triggering or phase encoding artifact reduction. Flow effects can be reduced by using gradient moment nulling of the first order of flow, gradient moment rephasing or flow compensation, depending of the MRI system.
Peristaltic motion can be reduced with the intravenous injection of an anti-spasmodic (e.g. Buscopan).
By using multiple averages, respiratory motion can be reduced in the same way that multiple averages increase the signal to noise ratio. Noticeable motion averaging is seen when four averages are obtained, six averages are often as good as respiratory compensation techniques and higher averages will continue to improve image quality.
In some cases will help a presaturation of the anatomy that was generating the motion.

See also Phase Encoded Motion Artifact.

(MSMP) Multi slice multiple phases is a cardiac gated sequence with different heart phases and several slices.
(SSMP) Single slice multiple phase is a cardiac gated sequence with different heart phases in one slice.

(PWI - Perfusion Weighted Imaging) Perfusion MRI techniques (e.g. PRESTO - Principles of Echo Shifting using a Train of Observations) are sensitive to microscopic levels of blood flow. Contrast enhanced relative cerebral blood volume (rCBV) is the most used perfusion imaging. Both, the ready availability and the T2* susceptibility effects of gadolinium, rather than the T1 shortening effects make gadolinium a suitable agent for use in perfusion imaging. Susceptibility here refers to the loss of MR signal, most marked on T2* (gradient echo)-weighted and T2 (spin echo)-weighted sequences, caused by the magnetic field-distorting effects of paramagnetic substances.
T2* perfusion uses dynamic sequences based on multi or single shot techniques. The T2* (T2) MRI signal drop within or across a brain region is caused by spin dephasing during the rapid passage of contrast agent through the capillary bed. The signal decrease is used to compute the relative perfusion to that region. The bolus through the tissue is only a few seconds, high temporal resolution imaging is required to obtain sequential images during the wash in and wash out of the contrast material and therefore, resolve the first pass of the tracer. Due to the high temporal resolution, processing and calculation of hemodynamic maps are available (including mean transit time (MTT), time to peak (TTP), time of arrival (T0), negative integral (N1) and index.
An important neuroradiological indication for MRI is the evaluation of incipient or acute stroke via perfusion and diffusion imaging. Diffusion imaging can demonstrate the central effect of a stroke on the brain, whereas perfusion imaging visualizes the larger 'second ring' delineating blood flow and blood volume. Qualitative and in some instances quantitative (e.g. quantitative imaging of perfusion using a single subtraction) maps of regional organ perfusion can thus be obtained.
Echo planar and potentially echo volume techniques together with appropriate computing power offer real time images of dynamic variations in water characteristics reflecting perfusion, diffusion, oxygenation (see also Oxygen Mapping) and flow.
Another type of perfusion MR imaging allows the evaluation of myocardial ischemia during pharmacologic stress. After e.g., adenosine infusion, multiple short axis views (see cardiac axes) of the heart are obtained during the administration of gadolinium contrast. Ischemic areas show up as areas of delayed and diminished enhancement. The MRI stress perfusion has been shown to be more accurate than nuclear SPECT exams. Myocardial late enhancement and stress perfusion imaging can also be performed during the same cardiac MRI examination.

From GE Healthcare;
GE Healthcare has added the Signa HDe 1.5T™, a compact MRI device at an affordable price to its family of MRI products. It has a single electronic cabinet that can be positioned inside the scanner room rather than in a separate equipment room. The Signa HDe 1.5T can be installed in the same physical location as 0.5T MRI systems with minimal construction costs. According to GE, the installation has been simplified to last only 7 days and has a 30 percent smaller footprint than a typical 1.5T system.
The 1.5T Signa™ HDe MRI system is substantially equivalent to the currently marketed GE 1.5T machines. The data acquisition system supports 1, 4, 8 independent receive channels and multiple independent coil elements per channel during a single acquisition series. The gradient specifications of HDe are lower than other GE Signa 1.5T MRI systems, but it can support clinical applications in cardiac and spectroscopy imaging.