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Helium
 
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.


MRI Safety Guidance
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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• Related Searches:
    • Cryostat
    • Quenching
    • Boil off Rate
    • MRI Safety
    • Cryogen
 
Further Reading:
  Basics:
Cryogenic Liquids and their Hazards
   by www.ccohs.ca    
Liquid Helium
   by hyperphysics.phy-astr.gsu.edu    
  News & More:
MR Solutions new pre clinical MRI scanners overcome helium shortage
Thursday, 20 June 2013   by www.heraldonline.com    
Tech firms, medical research threatened by helium shortage
Thursday, 19 September 2013   by www.livemint.com    
Cooling MRI magnets without a continuous supply of scarce helium
Tuesday, 13 August 2013   by www.wired.co.uk    
Helium shortage has wide impact Effects medical research, MRIs, party supplies
Wednesday, 15 August 2012   by www.niagarathisweek.com    
How 9/11 Made The Global Helium Shortage Worse
Thursday, 3 July 2014   by www.popsci.com    
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Quenching
 
A quench is the rapid helium evaporation and the loss of superconductivity of the current-carrying coil that may occur unexpectedly, or from pressing the emergency button in a superconducting magnet. As the superconductive magnet becomes resistive, heat will be released that can result in boiling of liquid helium in the cryostat. This may present a hazard if not properly planned for.
The evaporated coolant requires emergency venting systems to protect patients and operators. Quenching can cause total magnet failure and cannot be stopped. MRI systems are designed such that all of the escaping cryogenic gas is directed out of the building (quench pipe through the roof or the wall). In the event of a burst of the tank (possible in the case of an accident) or a blockage of the pipes, the helium gas will be forced into the scanner room, giving rise to a large white cloud of chilled gas. Under such circumstances it is essential that the scanner room is evacuated, also caused by the displacement of oxygen, which under extreme conditions could lead to asphyxiation. The force of quenching can be strong enough to destroy the walls of the scanner room or the MRI equipment.
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Superconducting Magnet
 
Superconducting magnets are electromagnets that are partially built from superconducting materials and therefore reach much higher magnetic field intensity.
The coil windings of superconducting magnets are made of wires of a type 2 superconductor (mostly used is niobium-titanium - up to 15 Tesla the critical temperature is less then 10 Kelvin). These coils have no resistance when operated at temperatures near absolute zero (-273.15°C, -459°F, 0 K).
Liquid helium (4.2 K) is commonly used as a coolant (sometimes in addition with a second cryogen liquid nitrogen as an intermediate thermal shield to reduce the boil-off rate of liquid helium), which consequently conclude refilling (intervals: liquid helium ~ 3 month, liquid nitrogen ~ 2 weeks). There are cryogen-free superconducting magnets with a closed-cycle refrigerating system at the horizon. Superconducting magnets typically exhibit field strengths of greater than 0.5 T, operate clinically up to 3 T, and have a horizontal field orientation, which makes them prone to missile effects without significant magnetic shielding.
See also Quenching.
See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'
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Further Reading:
  Basics:
Superconducting Magnets
   by hyperphysics.phy-astr.gsu.edu    
Magnetic Field of the Strongest Magnet
2003   by hypertextbook.com    
  News & More:
Age of Super Conductors has began
Monday, 1 February 2016   by www.albanydailystar.com    
A hot time for cold superconductors
Tuesday, 9 December 2003   by www.brightsurf.com    
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Boil off Rate
 
Rate of cryogen (liquid helium) evaporation in superconducting magnets, usually measured in liters of liquid per hour. The boil off rate of cryogen increases during ramping of the magnet and with eddy currents induced in the cryoshields by pulsed field gradients. In calculating cryogen consumption additional transfer and filling losses have to be considered.
See also Cryoshielding and Helium.

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Further Reading:
  Basics:
Cryogenic Liquids and their Hazards
   by www.ccohs.ca    
Zero Boil Off Cryostats
Friday, 23 September 2011   by www.cryogenicsociety.or    
  News & More:
A hot time for cold superconductors
Tuesday, 9 December 2003   by www.brightsurf.com    
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Cryogen
 
A cooling agent, typically liquid helium or liquid nitrogen used to reduce the temperature of the magnet windings in a superconducting magnet. All cryogenic liquids are gases at normal temperatures and pressures. Different cryogens become liquids under different conditions of temperature and pressure, but all have two properties in common: they are extremely cold, and small amounts of liquid can expand into very large volumes of gas. The boiling points of cryogens are commonly below -150°C(- 238°F).
See also Helium.
Radiology-tip.comCryotherapy
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• View the NEWS results for 'Cryogen' (1).Open this link in a new window.
 
Further Reading:
  Basics:
Cryogenic Liquids and their Hazards
   by www.ccohs.ca    
  News & More:
A hot time for cold superconductors
Tuesday, 9 December 2003   by www.brightsurf.com    
New imaging and spectroscopy magnets eliminate liquid helium
Thursday, 29 August 2013   by www.spectroscopynow.com    
MRI Resources 
Musculoskeletal and Joint MRI - Libraries - MRI Accidents - Calculation - Manufacturers - Cardiovascular Imaging
 
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