Mathematical technique used in projection-reconstruction imaging to create images from a set of multiple projection profiles. The projection profiles are back projected to produce a two (or three) dimensional image. The projection profiles are processed by convolving them with a suitable mathematical function (filtered) prior to back projecting them, in order to improve the image. Widely used in conventional computed tomography (CT).

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

The Gibbs or ringing artifact appears as a series of lines in the MR image parallel to abrupt and intense changes in the object at this location. This artifact does not occur visibly on smooth objects. This artifact is caused by the Gibbs phenomenon, an overshoot or ringing of Fourier series occurring at discontinuities.
In the spinal cord, a small syrinx can be simulated by the Gibbs phenomenon. Gibbs artifacts are also seen in other regions, for example the brain//skull interface.
Fine lines visible in an image may be due to undersampling of the high spatial frequencies, respectively incomplete digitization of the echo.
With more encoding steps the Gibbs artifacts is less intense and narrower. Therefore, e.g. the artifact is more intense in the 256 point dimension of a 256x512 acquisition matrix.

This problem can only be resolved by smoothing filters (LanczosSigmaFactor, 2-D Exponential Filtering, Gegenbauer Reconstruction etc.) or with a higher acquisition matrix and/or a smaller FOV, to smooth the object.

See also Gibbs Phenomenon and Apodization.

MRI hardware includes the electrical and mechanical components of a scanning device.
The main hardware components for the MRI machine are:
The magnet establishing the B0 field to align the spins.
Within the magnet are the gradient coils for producing variations in B0 in the X, Y, and Z directions to make a localization of the received data possible.
Within the gradient coil or directly on the object being imaged is the radio frequency (RF) coil. This RF coil is used to establish the B1 magnetic field necessary to excite the spinning nuclei. The RF coil also detects the signal emitted from the spins within the object being imaged.
The RF amplifier increases the power of the pulses.
The analog to digital converter converts the received analog raw data into digital values.
Depending on the design of the device and the body part being imaged the patient is positioned inside the magnet (e.g. on a movable table or standing upright).
The MRI scan room is surrounded by a RF shield (Faraday cage).
In addition, a computer console, a display, and a film printer belong to the MRI equipment.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

With an open configuration MRI system neurosurgical procedures can be performed using image guidance. Open MRI can be used to guide interventional treatments or procedures, such as a biopsy.
Intraoperative MRI allows lesions to be precisely localized and targeted. Constantly updated images, correlated with images obtained pre-operatively, help to eliminate errors that can arise during framed and frameless stereotactic surgery when anatomic structures alter their position due to shifting or displacement of, e.g. brain parenchyma.
Intraoperative MRI can help with the identification of normal structures, such as blood vessels and is helpful in optimizing surgical approaches, achieving complete resection of intracerebral lesions, determining tumor margins and monitoring potential intraoperative complications.

(IR) Inversion recovery is an MRI technique, which can be incorporated into MR imaging, wherein the nuclear magnetization is inverted at a time on the order of T1 before the regular imaging pulse-gradient sequences. The resulting partial relaxation of the spins in the different structures being imaged can be used to produce an image that depends strongly on T1. This may bring out differences in the appearance of structures with different T1 relaxation times. Note that this does not directly produce an image of T1. T1 in a given region can be calculated from the change in the MR signal from the region due to the inversion pulse compared to the signal with no inversion pulse or an inversion pulse with a different inversion time. This sequence involves successive 180° and 90° pulses. The inversion recovery sequence is specified in terms of three parameters, inversion time (TI), repetition time (TR) and echo time (TE).

See also Inversion Recovery Sequence and FLAIR.