A constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field.
Definition: The ratio of the magnetic moment (field strength = T) to the angular momentum (frequency = ν) of a particle.
The gyromagnetic effect happens if a magnetic substance is subjected to a magnetic field. Upon a change in direction of the magnetic field, the magnetization of the substance must change. In order for this to happen, the atoms must change their angular momentum. Since there are no external torques acting on the system, the total angular momentum must remain constant. This mass rotation may be measured. The gyromagnetic ratio is different for each nucleus of different atoms. The value of the gyromagnetic ratio for hydrogen (1H) is 4,258 (Hz/G) (42.58 MHz/T).

(HASTE) A pulse sequence with data acquisition after an initial preparation pulse for contrast enhancement with the use of a very long echo train (Single shot TSE), whereat each echo is individually phase encoded. This technique is a heavily T2 weighted, high speed sequence with partial Fourier technique, a great sensitivity for fluid detection and a fast acquisition time of about 1 sec per slice. This advantage makes it possible for using breath-hold with excellent motionless MRI, e.g. used for liver and lung imaging.

See also Segmented HASTE.

(HS) A method in which approximately one half of the acquisition matrix in the phase encoding direction is acquired. Half scan is possible because of symmetry in acquired data. Since negative values of phase encoded measurements are identical to corresponding positive values, only a little over half (more than 62.5%) of a scan actually needs to be acquired to replicate an entire scan. This results in a reduction in scan time at the expense of signal to noise ratio. The time reduction can be nearly a factor of two, but full resolution is maintained.
Half scan can be used when scan times are long, the signal to noise ratio is not critical and where full spatial resolution is required. Half scan is particularly appropriate for scans with a large field of view and relatively thick slices; and, in 3D scans with many slices. In some fast scanning techniques the use of Half scan enables a shorter TE thus improving contrast. For this reason, the Half scan parameter is located in the contrast menu.

More information about scan time reduction; see also partial fourier technique.

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.

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Hitachi Medical Systems America, Inc. (HMSA), was a major provider of magnetic resonance imaging systems in the United States. Hitachi had more than 2300 installed permanent magnet MR imaging systems worldwide. As a full-line supplier of medical imaging equipment in Japan, Hitachi Medical Corporation (HMC) founded HMSA to provide a direct link to the U.S. marketplace. Altaire™ , the open MR system from Hitachi, extends the family of open MRI products.

In December 2019 Japan's Fujifilm announced the acquisition of Hitachi's diagnostic imaging business for 179 billion yen ($1.63 billion). This includes Hitachi’s CT, MRI, X-ray, and ultrasound imaging operations, also its electronic health record business. Fujifilm expects the deal to close in July 2020 subject to regulatory clearances.

MRI Scanners:

0.3T to 0.7T:
1.5T: