Mathematically, the eigenvalue is the factor by which a linear transformation multiplies one of its eigenvectors. In an appropriate spatial reference frame, the diffusion tensor is diagonal (contains only three nonzero elements). These elements in diffusion tensor imaging are called the eigenvalues. The vectors characterizing the reference frame are the eigenvectors.

The element helium (He) was discovered 1868 when P.J.C. Janssen and N. Lockyer detected a new line in the solar spectrum during the solar eclipse. Lockyer and E. Frankland suggested the name helium (Gr. Helios, the sun) for the new element. In 1895, helium was discovered in the uranium mineral cleveite and in 1907 it was found out that alpha particles are helium nuclei.
Properties: Helium belongs to the noble gases, is colorless, odorless, and occurs in two naturally isotopes, helium 3 and helium 4. As an inert gas, helium does not react chemically largely and don't burns. Helium 4 makes up over 99% of naturally occurring helium atoms. Helium is extracted from natural gas e.g. present in various radioactive minerals as a decay product. Deposits and sources are in the USA, Poland, the USSR, and a few in India. The rare deposits and increased consumption lead to a shortage of this gas.
K. Onnes worked for many years to liquefy helium, which persisted as a gas to the lowest temperature. Helium does not freeze at atmospheric pressure. The density of helium vapor at his boiling point of 4.2 Kelvin is very high, with the vapor expanding greatly when heated to room temperature. Nb, Tc, Pb, La, V, and Ta are superconductors at liquid helium temperature. Liquid helium is commonly used as a cryogen for superconducting magnets. A rapid evaporation of the cryogen is named Quench. See also Quenching.

Cryogenic liquids and their associated cold vapors can produce effects on the skin similar to a thermal burn and can cause frostbite. Prolonged breathing of extremely cold gases may damage the lungs and in absence of enough air or oxygen, asphyxiation and death can occur. Unprotected skin can stick to very cold metal (e.g. cooled by liquid helium) and then tear when pulled away.

From Siemens Medical Systems;
Received FDA clearance in 2010.
The MAGNETOM Aera is a patient friendly, comfortable 1.5 Tesla MRI system with advanced radio frequency chain.
The system is equipped with the Tim 4G and Dot system (Total imaging matrix + Day optimizing throughput), to enhance both productivity and image quality.
Tim 4G technology provides improved SNR. The standard system configuration of 48 radio frequency channels and 204 coil elements creates an imaging matrix that allows maximum use of coil elements at full field of view. Dot provides improved image consistency through new features like auto align, auto FoV and automatic bolus detection.

In parallel MR imaging, a reduced data set in the phase encoding direction(s) of k-space is acquired to shorten acquisition time, combining the signal of several coil arrays. The spatial information related to the phased array coil elements is utilized for reducing the amount of conventional Fourier encoding.
First, low-resolution, fully Fourier-encoded reference images are required for sensitivity assessment. Parallel imaging reconstruction in the Cartesian case is efficiently performed by creating one aliased image for each array element using discrete Fourier transformation. The next step then is to create an full FOV image from the set of intermediate images. Parallel reconstruction techniques can be used to improve the image quality with increased signal to noise ratio, spatial resolution, reduced artifacts, and the temporal resolution in dynamic MRI scans.
Parallel imaging algorithms can be divided into 2 main groups:
Image reconstruction produced by each coil (reconstruction in the image domain, after Fourier transform): SENSE (Sensitivity Encoding), PILS (Partially Parallel Imaging with Localized Sensitivity), ASSET.
Reconstruction of the Fourier plane of images from the frequency signals of each coil (reconstruction in the frequency domain, before Fourier transform): GRAPPA.
Additional techniques include SMASH, SPEEDER™, IPAT (Integrated Parallel Acquisition Techniques - derived of GRAPPA a k-space based technique) and mSENSE (an image based enhanced version of SENSE).

Substances exhibiting paramagnetic properties are used as contrast agents in MR imaging. They have a small but positive magnetic susceptibility (magnetizability - tends to align along the magnetic field). Typical paramagnetic substances usually possess an unpaired electron and include atoms or ions of transition elements, rare earth elements, some metals, and some molecules including molecular oxygen and free radicals.

See also Paramagnetism.