What is liquid helium and how is it used in MRI? More importantly, how does the current liquid helium market affect MRI, along with the healthcare sector? This article provides expert insights into the integral relationship between MRI and liquid helium. We address the most commonly asked questions about helium in MRI and detail the critical role of ultra-pure liquid helium in the functionality of MRI scan machines.
What is Liquid Helium?
Liquid helium is a colorless, odorless, inert, and nonflammable cryogenic liquid comprised of helium particles in a liquefied state. At standard atmospheric pressure, liquid helium boils off at 4.2 Kelvin (-269.96 Celsius, -452.07 Fahrenheit). When it transitions from its liquid to gaseous state, helium expands roughly 757 times in volume. Additionally, the density of liquid helium is approximately 125 grams per liter, making it one of the lightest liquids on earth. (.125g/cc LHe)
Interesting Liquid Helium Facts
Liquid helium is not naturally occurring; it is produced by liquefying extracted helium gas through a process called cryogenic distillation.
When cooled below 2.17K, liquid helium demonstrates superfluidity, flowing with zero viscosity and capable of climbing out of containers.
This cryogen finds applications across various industries including aerospace, scientific research, and particle accelerators, but its most prominent use is in the healthcare sector. As a matter of fact, the healthcare sector consumes over 34% of the global liquid helium supply. Now, let’s explore its specific role in MRI. But first, let’s address associated risks.
Liquid Helium Safety Risks
It’s important to note that liquid helium poses several safety risks that require careful handling and precautions.
Firstly, due to its extremely low temperatures, it can cause severe frostbite upon contact with skin or other materials. Direct exposure to liquid helium can result in tissue damage cold burns.
Additionally, liquid helium can displace oxygen in poorly ventilated areas, leading to asphyxiation if inhaled in high concentrations. When it transitions from its liquid to gaseous state, it expands roughly 757 times in volume. Proper ventilation and monitoring of oxygen levels are crucial when working with liquid helium to prevent such risks.
Furthermore, liquid helium containers must be securely sealed and handled with caution to avoid leaks. This prevents the rapid release of helium gas and potential hazards such as pressure build-up or oxygen displacement in the surrounding area.
Only trained personnel should handle liquid helium. Adhere to safety protocols, including wearing appropriate PPE, ensuring proper ventilation, and following careful handling procedures, to minimize associated risks.
Why Is Liquid Helium Used in MRI?
Liquid helium is the ideal cryogen to keep the superconductive Niobium-Titanium core of the MRI magnet below it’s critical temperature (9.2 K). As an inert and non flammable cryogen with a boiling point of 4.2 K, it is uniquely suited for the strict requirements of MRI. Sustaining MRI magnet core temperatures below this critical point enables superconductive properties that allow the magnet to generate a strong and stable magnetic field without any resistance.
Liquid helium fill performed on a GE S Series Magnet
Why not use liquid nitrogen or other cryogenics in MRI?
Other cryogens, such as liquid nitrogen, lack the requisite low boil-off temperature to sustain the Niobium-Titanium MRI magnet core below its critical temperature. While liquid nitrogen is more readily available and less expensive, its higher boil-off temperature renders it inadequate for achieving the necessary superconductive state in the magnet core. Additionally, other cryogens may pose safety risks or compatibility issues within the MRI environment. Thus, while liquid helium may entail higher initial costs, its unique properties and safety make it the clear choice for MRI applications.
Liquid helium stands out as the optimal choice for MRI applications due to its unmatched low boil-off temperature, inertness, and non-flammability. Unlike other cryogenics, its boil-off temperature is the lowest, ensuring efficient cooling of the niobium titanium MRI magnet core below its critical temperature of 9.2 Kelvin. This characteristic is essential for sustaining the superconductive properties necessary for generating a powerful magnetic field without resistance. Furthermore, the unique properties of liquid helium make it the safest and most effective option for maintaining the stringent operational demands of MRI systems. See the chart below for a comparison of cryogenic boil-off temperatures and elemental properties.
| Cryogen | (K) | (°C) | (°F) | Properties |
|---|---|---|---|---|
| Helium | 4.2 | -269.1 | -452.1 | Inert, low-boiling point, non-flammable. |
| Hydrogen | 20.3 | -252.9 | -423.2 | Not inert, boiling point >9.2K, flammable. |
| Nitrogen | 77.4 | -195.8 | -320.4 | Inert, boiling point >9.2K, non-flammable |
| Oxygen | 90.2 | -182.9 | -297.4 | Not inert, boiling point >9.2K, supports combustion. |
| Methane | 111.7 | -161.5 | 258.6 | Not inert, boiling point >9.2K, flammable. |
Most importantly, helium is inert and non-flammable, even in a condensed liquid state. It meets the golden triangle criteria for safer superconductive environments.
How is Liquid Helium Kept Cold in MRI?
In MRI machines, liquid helium is kept cold through the use of a coldhead and a compressor assembly. The coldhead is an essential MRI scanner component responsible for supercooling the liquid helium to maintain the required low temperatures for the superconducting magnet. It provides the rhythmic chirping MRI sounds that can be heard in the background of every MRI.
Coldhead
The coldhead operates based on the principle of thermodynamics, specifically the Joule-Thomson effect. It consists of a series of stages or chambers through which helium gas is cycled.
The coldhead initially compresses the helium gas, raising its pressure and temperature. Then, it circulates this cooled gas around the superconducting magnet, absorbing heat and maintaining the desired low temperatures.
The compressed gas rapidly expands through a small orifice, causing a drop in temperature due to the Joule-Thomson effect. It then circulates this cooled gas around the superconducting magnet, absorbing heat and maintaining the desired low temperatures.
Helium Compressor
To maintain the continuous cooling process, a compressor is employed to pump helium gas through the coldhead. The compressor increases the pressure of the helium gas, initiating the cooling cycle in the coldhead. It plays a vital role in ensuring a constant flow of cooled helium gas to sustain the supercooling process.
How Much Liquid Helium Does MRI use?
An average MRI scanner consumes helium at a rate of 0.1% to 0.14% per day, with an expected monthly loss of 3% to 4%. The average MRI scanner holds 1500 Liters of liquid helium in the magnet vessel. On the higher end, systems like the Toshiba Vantage with an OR76 magnet holds 1800 Liters LHe, where as a GE Signa HDx with an LCC magnet holds a total of 1200 liters.
It is important to note that liquid helium capacity is determined by magnet type and not necessarily by MRI scanner model. Additionally, MRI magnet vessel capacity determines the percentages reflecting the total volume of MRI liquid helium consumption.
The advent of low cryogen consumption (LCC) MRI magnets dramatically reduces the average helium boil-off.
Under ideal conditions, LCC MRI magnets effectively eliminate liquid helium loss, potentially saving $30,000 in cryogen costs per year. Additionally, helium-free MRI systems, such as the Philips Ingenia Ambition 1.5T MRI, utilize a closed loop cryogenic system that requires only 8 liters of liquid helium.
When estimating how much liquid helium does MRI use, it’s important to note that the number reported above are estimates based on ideal clinical conditions. Component failure or maintenance issues can impact efficiency results.
How often are MRI liquid helium fills required?
MRI scanner magnets with lower capacity or lower efficiency require helium fills on average every 14 months. Meanwhile, MRI magnets with higher capacity or higher efficiency systems require helium fills on average every 30 months. MRI helium fill schedules are determined by minimum levels set forth by equipment manufacturers. Consult the operator manual for your specific MRI magnet or contact the OEM for specific recommendations.
MRI Liquid Helium Cost
The current average cost for liquid helium is $49 per liter when purchased in smaller quantities. However, bulk orders offer a discount, with an average liquid helium price of $31 per liter. Due to high demand and global market shortages, the price of liquid helium is at all-time highs.
Why is liquid helium in MRI so expensive?
MRI requires the use of ultra-high purity helium rated at 99.999% pure. Helium from conventional sources, like those used for balloons, may contain impurities that could solidify under the ultra-low temperatures within an MRI. This can pose a risk of permanent damage to the magnet.
As a result, there are a limited number of helium suppliers that can meet the stringent requirements within magnetic resonance imaging environments. The limited supply of liquid helium coupled with the increasing demand of the healthcare sector, among others, leads to elevated pricing.
With new liquid helium refinement plants expected to reach full capacity by 2025 and the integration of new helium recovery plants, the global helium market is expected to see a relief in the cost of liquid helium for MRI applications.
Where Can I Buy Liquid Helium?
Major distributors like Praxair, Airgas, or Linde offer bulk quantities of liquid helium for MRI purchases. Alternatively, reclamation centers like the University of Illinois Liquid Helium Facility provide smaller quantities.
Because of its rapid boiloff rate in atmospheric conditions, liquid helium requires transportation in a sealed, vacuum-insulated vessel called a Dewar. The vacuum-jacketed design ensures minimal heat transfer and maintains the extremely low temperatures require to keep helium in its liquid state during transportation.
Key Takeaways
Liquid helium in MRI enables the superconductive properties that make MRI scan machines a powerful diagnostic tool. The remarkable properties of liquid helium make it uniquely suited for applications in the strong superconductive magnet of an MRI scan machine. With MRI accounting for over 34% of the global helium supply, the sustainability of the healthcare sector hinges on the prosperity of both industries. If you would like to learn more, see our related resources link below the MRI liquid helium FAQ section.
How is Liquid Helium Made?
Liquid helium is produced through a refinement process that involves liquefying helium gas via cryogenic distillation. However, due to the complexity and energy-intensive nature of this process, coupled with the limited natural reservoirs of helium, liquid helium is produced by a limited number of refinement plants and recovery centers across the globe.
Why is liquid helium used in MR?
Liquid helium is used in MRI machines because it cools the superconducting magnets to ultra-low temperatures around 4.2 Kelvin. This enables MRI magnets to generate the strong magnetic fields necessary for imaging the body’s tissues.
How much helium does an MRI use?
The amount of helium used in an MRI machine can vary, but on average, an MRI scanner consumes approximately 0.1% to 0.14% of its helium supply per day, with an expected monthly loss of 3% to 4%. This can equate to approximately 600L to 1000L per year depending on MRI magnet type.
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