Cryogens Flashcards
Cryogens
• Superconductivity (refer to nites above)
• Cryogens are used to cool the coil and maintain it at low temperatures
• Coil material in all superconductive conductive materials is Naiobium titanium
• The main magnet windings are baited in liquid helium at around 4 degrees calvin in a structure called cryostat
• Cryostat -multi compartmental structure that functions like a thermos bottle
• Contains various insultating layers to thermally shield the coils from the warm external environment
• 7 litres of liquid helium is now being used due to global warming-helium is sealed into the scanner and doesn’t need to be refilled
• The magnet coils sit in a vacuum and not a liquid helium bar as before- reduces maintenance costs
What are cryogens?
• Substances used to produce extremely low temperatures typically below 150 degrees celcius
• Cryogens are gases that have been cooled to a liquid state at very low temperatures
• Superconductive state achieved by using cryogens to cool the MRI coil ensures that the magnetic field remains stable within minimal resistivity in the coil over time without fluctuations i.e. chnages in resistivity that could disrupt the uniformity of the magnetic field and degrade image quality
• The stability of the magnetic field is essential for producing high quality images
• Imp to regularly replenish cryogens to maintain the superconducting state and the magnet’s performance over time
• Cryogogens therefore play a crucial role in cooling superconducting magents and maintaining magnetic performance
Types of cryogens
- Liquid Helium
- Liquid Nitrogen
Liquid helium
• Liquid helium is most commonly used
• Boiling point of -269 deg C - few deg above absolute zero
• Used as it can cool superconducting coils to their critical temperature-allowing them to maintain their superconductive state
• Liquid helium shows Superfluidity – characteristic property of a fluid with 0 viscosity (thickness) and therefore flows without losing kinetic energy
Liquid nitrogen
Used in precooling rather than direct involvement in the MRI magnet
MRI ramping
• The process of gradually increasing the current in the superconducting magnets to generate the required magnetic field strength for imaging
• This process is crucial when an MRI magnet is bought online or after it has quenched
Quenching
When the magnet loses its superconductivity and becomes resistive
Purpose of the ramping process
• When an MR iis first brought in it undergoes an initial cool down
• Where cryogens are used to cool it down to its operating temperature
• Liquid nitrogen may be used to preheat the system before liquid helium is used
• During regular mr operation liquid helium is continuously used to keep the superconducting magnet at a low temperature -often involves periodic refills of liquid helium- advanced mri systems have cryocoolers to recondense the helium gas back into liquid – reducing the need for frequent refills
• During the ramping process the mr magnet will become operational -therefore allstaff need to be screened
During ramping
• The current in the magnet coils is gradually increased to reach the desired magnetic field strength – to prevent any sudden damaging changes and anything that can effect the stability of the magnetic field
• During ramping the magnet is maintained at cryogenic temperatures-typically using liquid helium to ensure that the coils remain superconductive – minimises electricity resistance and power loss – enabling the generation of a strong and stable magnetic field
• The ramping process is carefully monitored to ensure the magnet’s stability and safety
• Sensors and control systems track the temperature,current and field strength to prevent any issues that could lead to a quench
• In summary – ramping in MRI is the essential procedure of gradually increasing the electric current in the superconducting magnet to establish the desired magnetic field strength for imaging whilst ensuring stability and safety throughout the process
Safety precautions
• Personnel safety and training protocols – only authorised professionals should be performing ramping
• Equipment and environment preparation- ensure that liquid helium levels are adequate to maintain superconductive temperatures throughout the ramping process
• Ensure proper ventilation – to prevent a asphyxiation(gas that can cause unconsciousness) hazard – due to the potential release of cryogens – emergency stops systems easily accessible
• Monitoring and control systems- monitoring the temperature of the superconductive coils to orevent heating
• Monitor thr current and voltage levels during ramping to detect any abnormalites early
• Gradually increase the magnetic field strength to avoid sudden changes that could cause system inabilities or quenching
• Area access control – restrict access to personnalss
• Warnign signs
• Emergency procedures – clear communication and clear plans in place in case of quenching
• Magnetic safety – during ramping magnetic field strength is slowly buikding up to maintain the desired magnetic field – all ferromagnetic objects should be removed
• Individuals with implants or magnetic devices are not present during ramping
Quenching in MRI
• Comes about when there is a sudden rise in temperature due to the deliberate or sudden escape of helium =magnet coils heat up and become resistive
• The heat produced rapidly converts liquid helium into it gaseous form
Quenching cases
• Emergency ec fire or leak, critically low helium levels
• Emergency button is only pressed when patient safety is compromised -intentional quenching
• Spontaneous quench – mr system quenches on its own- such as a leak or escape of liquid helium (serious expense and potential loss of magnet strength)
• First option instead of quenching- turn off power supply
Dangers of magnetic quenching
• During a quench the superconductive magnet loses its superconductivity and becomes resistive – causing the liquid helium to rapidly boil off and expand into gas and potentially escape into the mr room – can displace a large volume of exygen in a short time
• Cryogens can cause a significant risk related to oxygen levels in the MRI suite – if there is a leak of liquid nitrogen or helium – the cryogen can rapidly vaporize and expand
• If the quench fails or is overhwhelmed this vapour can displace oxygen in the mri room leading to a reduction in breathable air
• The displacement of o2 creates an asfixation risk for anyone in the room
• Low 02 can lead to hypoxia
• O2 monitors are therefore very imp
• O2 monitors continusly o2 levels – personnels are contacted if below normal levels
Emergency procedures in quenched magnet
• Immediate detection and response; Alarm activation and inform personnel
• Evacuation – stay low – to prevent the inhalation of helium moved to a well ventilated area – difficult due to the fog
• Reporting and documentation
• Oxygen monitoring
• Ventilation management
• Securing the area
• Medical assistance
• Assessment and recovery
