Shimming by adjusting the position of suitable pieces of ferromagnetic metal within or around the main magnet of a MRI system. Typically, the iron is in the form of small plates which are held in removable trays. The magnetic field is plotted, the position and quality of the iron required is then calculated and the appropriate 'shims' put in place. Several repetitions of this process will normally be required in order to achieve the optimum arrangement.
Passive shimming does not replace adjustable shimming, but is installed during the commissioning of a unit to improve the homogeneity of the bare magnet.

(PROPELLER) The PROPELLER MRI technique reduces the sensitivity to various sources of image artifacts (e.g., motion artifact, field inhomogeneity artifact, eddy current artifact). PROPELLER can be used with gradient echo and turbo spin echo sequences in a wide range of applications to improve the image quality, for example cardiac MRI, brain MRI, and pediatric examinations.

(PACS) A system used to communicate and archive medical imaging data, mostly images and associated textural data generated in a radiology department, and disseminated throughout the hospital. A PACS is usually based on the DICOM (Digital Imaging and Communications in Medicine) standard.
The main components in the PACS are:
Acquisition devices, which acquire their data in direct digital format, like a MRI system, are most easily integrated into a PACS.
Short term archives need to have rapid access, such as provided by a RAID (redundant array of independent disks), whereas long term archives need not have such rapid access and can be consigned, e.g. to optical disks or a magnetic.
High speed networks are necessary for rapid transmission of imaging data from the short term archive to the diagnostic workstations. Optical fiber, ATM (asynchronous transfer mode), fast or switched Ethernet, are examples of high speed transmission networks, whereas demographic textural data may be transmitted along conventional Ethernet.
Sophisticated software is a major element in any hospital-wide PACS. The software concepts include: preloading or prefetching of historical images pertinent to current examinations, worklists and folders to subdivide the vast mass of data acquired in a PACS in a form, which is easy and practical to access, default display protocols whereby images are automatically displayed on workstation monitors in a prearranged clinically logical order and format, and protocols radiologists can rapidly report worklists of undictated examinations, using a minimum of computer manipulation.

Time duration of a pulse. For an RF pulse near the Larmor frequency, the longer the pulse length, the greater the angle of rotation of the macroscopic magnetization vector will be (greater than 180° can bring it back toward its original orientation). For an RF pulse of a given shape as a function of time, the longer the pulse length, the narrower the equivalent range of frequencies in the pulse will be.

The quadrature detector is a part of the receiver that converts the high-frequency MRI signal to a lower frequency. This phase sensitive detector or demodulator detects the components of the signal in phase with a reference signal and 90° out of phase with the reference signal. The modulated signal contains i.e. the frequency range across the field of view encoded by the frequency encoding gradient. This may be performed by either analog or digital means.