MRI Safety Overview - Quench

Transcription

1 MRI Safety Overview - Quench Zachary W. Friis, Ph.D., DABR Quench What is a Quench? Quench is term we give to the situation that occurs (accidentally or purposefully) in which a super conducting magnet suddenly loses it s superconductivity. Quench During a quench a superconductive magnet suddenly reverts back to its resistive state. This in turn is accompanied by Joule Heating in which all of the magnetic energy is converted to heat, there will also be a loud banging noise. As the temperature suddenly increases all the cryogen is rapidly boiled off. Warning!! just because the magnet is quenched does not guarantee that the static field is completely off. There may be a residual magnetic field so care must still be taken with ferrous materials until it is fully verified that there is no magnetic field. This can be done with a gauss meter or even a simple ferrous dog chain.

2 Superconducting magnets are similar to resistive magnets except they are operated at a superconducting state. This means there is essentially ZERO electrical resistance.

3 This enables them to store very large currents without using a lot of power. Since current is directly proportional to the magnetic field, this is how we are able to get high field magnets. Two conditions need to be meet in order to achieve the superconductive state. First the wire must be made out of a special alloy. This is typically niobium-titanium (Nb-Ti) embedded in copper. Second the temperature must be maintained below the supercritical temperature. There are actually lots of types of super conductors! With lots of different supercritical temperatures.

4 Unfortunately, the only ones available for MRI are the Low Temperature Kind (LTS) So, when we say Low Temperature How low is low??? We get such low temperature with cryogen pumps circulating the liquid helium. If the cryogen system fails, a chain reaction starts. As the temperature increases so does resistance.

5 The resistance increase causes the system to give off even more heat. This becomings a forward feeding reaction that can cause a collapse of the magnetic field. This is why it is imperative to make sure you maintain your cryogen levels and to use proper procedures when filling them. Once the magnet is no longer superconducting, all that store energy has to go somewhere! But, where does it go? Some of the force from the quench is absorbed by the scanner itself. Most of the energy though has to go somewhere else. All the current that was once flowing through a system with zero resistance now has to start flowing in a conductor with a lot of resistance. This creates a very hot wire immersed in a bath of liquid helium, and we all remember the 2nd law of thermo right? All that helium starts heating up and expanding! It NEEDS somewhere to go!

6 Asphyxiation One of the largest dangers of a quench is asphyxiation. This can occur if the vent for the helium is clogged or not working properly. If this happens all of the boiled off helium will be funneled into the room. Helium by itself is nontoxic, however in the event of a quench the massive and instantaneous filling of the scan room with helium can displace oxygen and lead to suffocation. Another consequence of this malfunction is a quick pressure build up inside the scan room. P= F/A This is why if you have a superconducting magnet your door to the scan room should swing outside when opening. If you have a door that opens in to the scan room it maybe impossible to open the door during a vent malfunction depending on how high the pressure buildup. If this occurs the last resort is to break the glass between the control room and the scan room. MRI Scan room door. A second consequence of this heat transfer is the potential liquefaction of air! Helium may begin to boil at around 4.22 degrees kelvin, and air liquifies at around 82 degrees kelvin. So, the helium steam rushing through the quench pipes is cold enough to cause the air to liquify. Why s this important?

7 A second consequence of this heat transfer is the potential liquefaction of air! Helium may begin to boil at around 4.22 degrees kelvin, and air liquifies at around 82 degrees kelvin. So, the helium steam rushing through the quench pipes is cold enough to cause the air to liquify. Why s this important? Can you imagine putting the Quench pipe outlet next to a smoking area?!!!! Quenching Policy and Procedure It is vital that MRI personnel know what to do in the event of a quench. POLICY: It is the policy of this facility to provide guidelines for proper action in the event of a sudden flooding of the unit with helium gas (a quench). PROCEDURE: Respond to visual or audible (each unit is equipped with a sensor and audible alarm system) warning of a quench by recognizing that patient safety is the first concern. Remove the patient from the unit and room quickly and carefully. Remain calm and reassure the patient. Remember: Helium is not toxic; the hazard is that it will displace the oxygen in the area and create a possibility of suffocation. Also, be aware that gaseous helium will be very low temperature. Close all doors when leaving the area. Evacuate patient and personnel to a safe location in the Contracted Site or host facility. Notify the Operations Manager of damage report and anticipated downtime. Notify the MR manufacturer/service organization of the quench. Notify the Contracted Site Safety Officer. Complete and file Occurrence Report to the Director of Risk Management.

8 Quench/Emergency Stop Button Some machines have two buttons. One for a quench and another for shutting down electricity to scan room. Know which is which if your system has two separate ones. Electrical shut down to scan room Some machines have a button that immediately shuts down electricity to the system power distribution unit (PDU). This is useful if you have a fire or sparks contained to the scan room or in the event of the sprinklers going off or a flood. However due to the nature of superconducting magnets this does not impede the static magnetic field. As long as the temperature of the magnet is below supercritical there is minimal electrical resistance and the magnetic is still fully functional. The same current is running in the wires. 5-Year-Old Girl Dies During MRI In Sri Lanka February 13, 2013 Written by: Steve Millburg, Filed in: Diagnostic Imaging, Medical Ethics, Neuroradiology, Pediatric Radiology A mother pleaded Monday with a Sri Lankan magistrate not to let the money power obstruct an investigation into the death of her 5-year-old daughter during an MRI procedure. The woman, Indumathi Ekanayake of Heiyantuduwa, Sri Lanka, said she took her daughter on January 31 to the Nawaloka Hospital in Colombo, Sri Lanka, because the girl, Buddhini Ratnayake, was suffering from a possible epileptic fit. Ekanayake said a doctor recommended three blood tests and an MRI scan. She said her daughter was anesthetized and put into the scanner. After 30 minutes, she said, she heard a sound. The Daily Mirror, an English-language newspaper in Colombo, quoted Ekanayake as testifying: I heard the doctor shouting that the balloon had exploded and nobody was there to assist him. I pushed off a ward attendant towards the room because there was nobody inside the room to help the doctor. When my daughter was pulled out from the machine, I saw she had turned blue and the belly had become swollen. The Daily Mirror quotes Ekanayake as saying that the girl was admitted to the intensive care unit and put on a vacuum machine apparently a reference to a ventilator. Ekanayake said the child was kept on the ventilator for four days, even though she had already died. On the fourth day, Ekanayake said, I saw ants entering her nose. The newspaper said Ekanayake testified that a doctor told her the death was not the hospital s fault and that hospital management declared that the girl had died four days after the MRI scan. The hospital is part of a large business group in Sri Lanka, which may be the source of Ekanayake s worry about the money power. DOTmed News cites a different report saying the mother heard the doctor screaming, The oxygen pipes have burst. DOTmed says postmortem reports found that the child died of oxygen deprivation. -

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