Paper 4 Answers
James Day, Amy Thomson, Tamsin McAllister, Nawal Bahal in Get Through, 2014
The magnetic field is generated by a superconducting electromagnet that produces a field more than 10 000 times stronger than the earth’s magnetic field. When a magnetic field power is increased above 0.5 Tesla, power is dissipated in the windings of the magnet, resistance builds in the conducting elements and heat is produced. Superconductivity is the process of cooling metals to reduce electrical resistance. The superconducting magnet is cooled to near absolute zero by immersion in liquid helium (which has a boiling point of below 4.3 kelvin). As liquid helium is expensive, in practice liquid nitrogen is added to reduce the temperature of gaseous helium to below its boiling point. The magnet can be easily ramped down in emergency situations, rather than switched off. In a life-threatening emergency, the magnet can also be ‘quenched’, in which the coolant is vented off and the magnet may be damaged. The vent reaches extremely low temperatures and in the event of vent failure the coolant will expand, causing hypothermia and asphyxiation to anyone in the room. Note that a residual magnetic field may still be present after a quench. The cost of reactivating a magnet after a quench is considerable.
Electron Paramagnetic Resonance of Copper Proteins
René Lontie in Copper Proteins and Copper Enzymes, 1984
Operation with liquid helium is more difficult, the two most common systems involving either the total immersion of the cavity and its connecting waveguide into liquid helium or the use of a continuous flow system. The first method uses a double dewar assembly in which the liquid helium section is shielded by liquid nitrogen and has the advantage of small liquid helium consumption. In a good dewar, with minimum heat losses, the liquid helium consumption may be 250 mℓ/hr or even less. Such a system is advantageous if repeated measurements on a particular sample are to be made, e.g., measurements of the angular dependence of the resonances of a single crystal, or for ENDOR and spin-echo measurements. A further advantage is that temperatures below 4.2 K can be obtained relatively easily by pumping the liquid helium bath. These lower temperatures will give further increases in sensitivity and will increase spin-lattice relaxation times, which may be advantageous for ENDOR experiments. The disadvantages of a double dewar system are that samples are not readily changed once the dewar contains liquid helium, and that considerable ancillary equipment, such as a vacuum system capable of pumping the cryostat down to better than 10−5 Torr, is required. An example of this type of system for single crystal EPR studies is described by Brill and Venable.161
Safety Considerations on Siting and Shielding
Bertil R. R. Persson, Freddy Ståhlberg in Health and Safety of Clinical NMR Examinations, 2019
To maintain the superconducting conditions, the magnet must be filled with liquid cryogens which are usually transferred into the cryostat from transport vessels (dewar) using routine top-pressure methods. During this operation one must put the cryogens in the correct inlets and not blow warm helium gas onto the magnet. These mistakes could result in a magnet quench. Liquid nitrogen consumption will be of the order of 200 to 250 ℓ/week and can be obtained from bulk storage either directly by a fixed, insulated transfer pipe or transport dewar of suitable capacity. The cryostat capacity of liquid helium is 200 to 450ℓ and typical consumption is between 50 to 100 ℓ/week with a monthly refill period. The costs of cryogens vary considerably but are in the order of 6 U.S. dollars/ℓ of liquid helium and 0.7 U.S. dollars/ℓ of liquid nitrogen. In order to reduce the cost it may be effective to install a helium recovery system. Physical data for the cryogenics are given in Table 1.
Anaesthesia in the MRI suite
Published in Southern African Journal of Anaesthesia and Analgesia, 2018
Reinier Swart, William Ian Duncombe Rae
The superconducting magnets are cooled to below 4.2 Kelvin by super-cooled liquids or cryogens, with liquid helium being the most commonly used agent.1 Quenching is the process through which the liquid is rapidly boiled off and large volumes of gas are produced, which are generally vented into the atmosphere outside the building. Subsequently, the magnets lose their superconducting ability and the magnetic field is deactivated. This process may be initiated manually by means of an emergency switch generally located in the chamber, or may occur because of an error of installation or service.3 This process rapidly cools the gases in the MRI chamber if the vent is exposed to the environment and may lead to condensation of liquid oxygen. The descending vapour cloud may be harmful to the patient and staff, as it may create a hypoxic environment in the MRI suite. As a result of this possibility, oxygen sensors should be present in the MRI suite to alert the operators when a hypoxic environment develops.6 The implications of this process should also caution the anaesthetic care provider not to view quenching as a first-line option in the event of an emergency. It is, however, imperative that quenching be reserved for dire emergencies only, for example, in the case of impingement of a person to the magnet by another object. Each unit should have a quenching policy that is readily available.
Related Knowledge Centers
- Cryogenics
- Helium
- Magnetoencephalography
- Mass Spectrometry
- Nuclear Magnetic Resonance
- Superfluidity
- Refrigerant
- Cryocooler
- Magnetic Resonance Imaging
- Absolute Zero