An MRI coldhead is one of the MRI scanner components that you hear in the background of almost every scan but somehow never really notice. It’s also referred to as a cryocooler, cryogenic refrigerant, or a helium condenser, and It creates the signature rhythmic sound associated with MRI and at its core, it really only serves one purpose: To keep the liquid helium inside of the MRI scanner for as long as possible.
What is A MRI Coldhead?
A coldhead is the cryogenic refrigeration unit used in an MRI scanner to maintain the superconducting magnet coils at -269 degrees celsius. Modern MRI scan machines rely on superconducting coils made from niobium-based alloys that must be kept below 9 kelvin, that’s -264.15 degrees Celsius and about -443.47 degrees Fahrenheit. The MRI coldhead is the component that removes heat from the magnet vessel and recondenses helium, which allows the MRI machine to operate continuously, often for multiple years, before needing another liquid helium fill.
Despite the obvious limitations posed by the powerful MRI magnetic field, the coldhead is one of the few MRI scanner components that requires a large motor and internal components which contain ferromagnetic material. While the casing is often made of non-ferrous metals such as aluminum and stainless steel alloys, each coldhead contains up to 25 lbs of ferrous metal components.
How MRI Coldhead works
An MRI coldhead works by using thermodynamic staging, regenerative heat exchange, oscillatory helium flow, and strict entropy management to force heat to move from a super cold MRI magnet vessel to room temperature.
In an MRI system, the coldhead is mounted externally in a vacuum-sealed jacket and thermally coupled to the MRI cryostat with a 99.99% pure indium gasket. The coldhead assembly is also connected to an external helium compressor, that sits inside the MRI equipment room, via high-pressure helium lines.
Inside the coldhead, high-pressure helium gas is compressed and expanded at about 60 cycles per second. During expansion, the helium cools and absorbs heat from the cold surfaces. This heat is transported back to the equipment room through the low pressure helium line. The helium compressor filters any contaminants via the absorber, and then removes excess heat before recirculating the gas through the high pressure helium line and feeding it back out to the coldhead, where the process starts all over again.
The coldest point of the coldhead is thermally coupled to the helium vessel, or recondenser assembly to be more specific, where it recondenses boiled-off helium gas back into liquid, which helps to maintain the liquid helium bath inside the MRI scanner.
Single Stage MRI Coldhead
Single stage coldheads provide cooling at a single temperature level, typically in the 40 kelvin to 80 kelvin range. These systems are used to primarily cool the radiation shield and reduce the overall heat load on the cryostat.
Single stage coldheads are more commonly integrated in older, or lower-field MRI designs. They were typically mounted on the bottom half of the magnet at about a 45 degree angle into the cryostat, as opposed to the more commonly seen top placement of 2 stage coldheads in modern MRI scanners.
Advantages of single-stage MRI coldhead
On the positive side, single stage coldhead are much simpler from a design and mechanical standpoint, they have shorter dimensions which require less volume in the MRI vessel, and are generally quieter with less vibration.
On the negative side, single stage coldheads are not as efficient in maintaining the liquid helium vessel at ultra-low temperatures which leads to significantly higher liquid helium boil-off. But the biggest drawback of a single stage coldhead is that they can’t reach temperatures as low as 4 Kelvin, which is absolutely critical for a low cryogenic consumption MRI scanner. At least not without a second stage.
Two Stage MRI Coldhead
Two stage MRI coldheads are the standard in modern MRI scanners. The first stage operates at around 40 to 50 kelvin and is used to cool the thermal radiation shields and intercept heat conducted through supports and wiring, The second stage operates at temperatures near 4 kelvin and is connected to the helium condenser via an indium gasket.
By using two separate cooling stages, the two stage design significantly improves cooling efficiency at low temperatures and enables nearly zero helium boil-off. The 2 stage coldhead design improves uptime, and allows MRI systems to function up to 2 years without needing a refill or top-off.
Key Takeaways
The coldhead produces the characteristic chirping, or train-like MRI sound, due to the mechanical cycling of helium gas in the MRI vessel. Inside the coldhead, rotary and reciprocating valves create pressure pulses that transmit vibration through the coldhead structure. These pressure oscillations occur at low frequencies, typically in the 1 to 2 hertz range, and create a unique signature sound that can be heard in the background of almost every MRI scanner. Additional noise can be generated from resonance within the cryostat, mechanical wear over time, contaminants in the liquid helium lines, or even partial loss of vacuum inside the coldhead jacket.
The MRI coldhead is one of the few MRI scanner components that contains ferrous metals in the motor assembly, internal components and casing. As a result, handling of an MRI coldhead in an MRI scan room when it is not mounted on the MRI scanner creates a safety hazard.
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