Helium Reclaim in Magnetic Resonance Imagers

Dan Hazen, MKS Instruments, Inc.

INTRODUCTION Magnetic Resonance Imagers (MRIs, Figure 1), first reported by Paul Lauterbur in 1973 using small test tube samples [1], can be found in most major hospitals today. They have become indispensable for accurate and effective medical diagnoses. Worldwide, there are about 25,000 MRI units and around 80 million MRIs are performed annually around the world [2].

MRIs are critically dependent on superconducting magnets to achieve their scanning function [3] and these magnets must be maintained at cryogenic temperatures near absolute zero (-273.15°C, -459.67°F) to remain superconducting. The only cryogenic fluid suitable to cool the magnets in MRIs is liquid helium, owing to its very low boiling point (4.22°K, -268.93°C) and safe usage characteristics. Figure 2 shows a cross-section of a modern MRI, showing the bore in which the patient lies and the arrangement of the surrounding superconducting magnet and liquid helium coolant channels. A typical MRI requires up to 1500 to 2000 liters of liquid helium for operation. MRI’s share of the world helium market is thought to be around 20-30% [4].

Helium Resources, Usage and CostHelium is a trace component in natural gas (up to ~3%); it has a global annual demand of about 170 million cubic meters [5]. Applications for helium reside primarily in cryogenics (including MRI) and high tech process industries such as the space program and semiconductor and fiber optics manufacturing [Figure 3, Ref. 6, 7]. Roughly one third of this demand is supplied by the world’s largest helium resource, the US Government’s Federal Helium Reserve, located near Amarillo, Texas. The Reserve was established in 1925 as a strategic supply for military use. US lawmakers decided to get out of the helium business in 1996; since then the Federal Helium Reserve has been selling its stockpile to cover off the site’s $1.3 billion debt which was fully repaid by October 2013. However, by 2013 the helium market had changed dramatically from that of 1996, and helium was once again considered a critical resource due to its use in the strategic industries denoted in Figure 3. The continuing high demand for helium by these industries, when coupled with the depletion of the US reserve and the limited number of new helium resources that have come on-line between ’96 and ’13, has produced a tight demand/supply imbalance and this has increased helium prices by a factor of 10 in the past few years.

Helium Conservation and Losses in MRIsLooming helium shortages and high costs have prompted helium users and equipment suppliers in the NMR/MRI, semiconductor, and fiber optics manufacturing sectors, to develop system designs that limit operational helium loss. MRI manufacturers have developed very effective cryogenic containment systems that employ vacuum Dewars cooled by cryocoolers that eliminate the need for a liquid nitrogen thermal buffer (at -196°C); which, in turn, reduces helium losses. Liquid helium hold times in these systems are now reported to be 3 to 4 years [3].

Figure 1 - A medical magnetic resonance imager (image fromWikipedia)

Figure 2 - Cross section of an MRI showing superconducting magnet and liquid helium cooling (from Ref. 3)

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While initial cool-down and operational losses of helium from MRIs has been drastically reduced in modern system designs, there remains one mechanism that frequently leads to unacceptable helium losses. The superconducting magnet in an MRI can unintentionally enter a state known as ''quench'' in which it reverts to normal conductivity, causing a rapid rise in the temperature of the magnet. The electrical current circulating in the superconducting wire winding of a MRI magnet holds a great deal of potential energy (according to Siemens [8], a 4.0T MRI magnet has a stored energy of 26 MJ). If for any reason, some section of the wire reverts to normal resistivity, the resistive heating that occurs is conducted to neighboring sections of the wire, raising its temperature to the point where it loses its superconducting property producing even greater resistive heating in the magnet. It is easy to see how this effect can rapidly cascade through the

entire superconducting winding, dissipating all of the stored energy in the MRI magnet as Joule heating. When this happens, the magnet temperature can rise from 4°C above absolute zero to several hundred degrees in a matter of seconds or minutes. The temperature rise can boil off the entire volume of cryogenic fluid in the magnet and can result in damage to the magnet. Even if there is no damage, bringing the magnet back down to 4°C produces significant boil-off losses of liquid helium that represent an unacceptable operational cost. Additionally, if cold cryogenic gas escapes from the venting system during a rapid quench, it represents a serious safety risk to those in the nearly hermetically sealed MRI room. Thus, safety issues, the limitations of supply and the high cost of helium, all dictate that the recovery of cryogenic helium lost from MRI systems is a critical design feature.

Figure 3 - US helium usage (source: US Geological Survey)

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HELIUM RECLAIM INTERFACERecently, one of the largest manufacturers of MRI systems has made strong efforts to reduce the number of quenching events experienced in their manufacturing process. The manufacturer had a number of production bays producing MRI systems and was averaging two to three quenches per day across the entire assembly line, with occasional rises in the frequency to up to ten quench occurrences per day. This quenching frequency resulted in significant losses of helium with obvious consequences for manufacturing costs. As well, when a quench occurred, the helium was being vented into the manufacturing bay and this represented a significant safety risk on the manufacturing floor. Furthermore, in addition to the increased helium costs, the manufacturer was also subject to a helium quota and so their ability to replace the cryogen when these accidental losses occurred was severely restricted. This limitation in supply effectively reduced the number of MRI systems that could be produced within a given period of time.

To address their helium loss issue, the MRI manufacturer contracted a specialty gas supplier and requested the installation of an on-site Helium Recovery System that could capture and reclaim the helium that would otherwise be vented during a quench. The Recovery System chosen was a generalized unit, designed for helium recovery in a variety of

manufacturing settings. MKS Instruments was contracted to produce the custom and complex interface that would mate the Helium Recovery System with the MRI manufacturing environment. MKS Instruments was chosen to provide the interface due to the company’s tremendous breadth of products and its demonstrated ability to quickly and effectively provide single, integrated solutions in similar situations.

A typical configuration developed for integrated helium recovery is shown in Figure 4. It employs three MKS components from three separate product lines: an RMU2 controller [9], 740C Baratron® manometer(s) [10] and a customized high flow valve [11] specifically modified to meet the needs of this application. The Baratron manometer monitors the pressure within the helium cooling vessel, detecting a quench event by the pressure rise that occurs as the helium boils off. The RMU2 controller, which uses a simple web-based interface for set-up and control of the interface, takes the pressure signal from the Baratron manometer and activates the custom valve when a pressure rise signaling a quench event occurs. Activation of the custom valve directs the escaping helium from the MRI system into the Helium Recovery System. Each RMU2 controller monitors eight Baratron manometers and controls eight valves.

Figure 4 - Final configuration for an MRI Helium Recovery System (a) schematic; (b) components and (b) connectivity

(a) (b)

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Configuring the helium recovery procedure is simple using the input interface of the RMU2 unit’s SenseLink™ software. Figures 5(a) through (c) show screen captures of a typical configuration procedure. Figure 5(a) is a screen capture of the Analog Input screen of the Senselink interface with the 740C Baratron manometer set up as an Analog Input. It can be seen that different pressure modes can be configured for up to 8 analog inputs per board in the RMU2. Figure 5(b) shows the configuration profile for the 2 digital outputs used as valve open and close signals for the customized high flow valve. Figure 5(c) shows the configurable Events/Triggers profile in which two triggers are set up to turn the Valve Open and

Valve Close digital outputs on. The triggers are set to remain on for 1 second (configurable). Finally, Figure 5(d) shows how 2 events are set up under the Events/Events profile to set when the Triggers (digital outputs for valve open and close) will turn ON. Valve Open will occur when AI1_1 reaches 8 psig and Valve Close will occur when AI1_1 reaches 1 psig.

We estimate that the implementation of a Helium Recovery System using the MKS Instruments’ interface can produce cost savings of up to $1M/month from reduced helium costs in a large MRI production facility.

Figure 5 - Helium recover method configuration: (a) Analog input profile; (b) Digital output profile; (c) Events/Trigger profile; and (d) Events/Events profile.

(b)(a)

(d)(c)

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CONCLUSIONThe success of the interface to the Helium Recovery System in this application demonstrates the strategic advantage that arises from the unmatched range of products and technologies that MKS can draw on to develop its integrated solutions. Furthermore, its ability to integrate tools from diverse product lines and to quickly customize products such as the high flow valve to meet novel application is a unique capability that facilitates rapid and effective solutions to MKS customers’ manufacturing problems.

REFERENCES[1] Lauterbur P.C., "Image Formation by Induced Local

Interactions: Examples of Employing Nuclear Magnetic Resonance", Nature, 242 (1973), 190–1.

[2] "Magnetic Resonance – A Peer-Reviewed, Critical Introduction", Rinck, P.A., Magnetic Resonance in Medicine. The Basic Textbook of the European Magnetic Resonance Forum. 8th edition; 2014, Chapter 21, http://www.magnetic-resonance.org/ch/21-01.html

[3] Hornack, J. P., "The Basics of MRI", available at https://www.cis.rit.edu/htbooks/mri/inside.htm

[4] "Helium – Softening Demand Ahead?", Gasworld, pp. 42-43, January, 2014.

[5] Peplow, M., "Helium Reserves Under Pressure", The Royal Society of Chemistry chemistryworld, 30 May, 2013, published online at http://www.rsc.org/chemistryworld/2013/05/helium-reserve-supply-shortage-price-rise

[6] Garvey, M.D., "The 2014 Worldwide Helium Market", Cryogas International, June, 2014, pp. 32 – 36.

[7] Bowe, D.J., "Helium Recovery and Recycling Makes Good Business Sense", IndustrialHeating.com, Sept. 2004, pp. 79-81.

[8] "Siemens Magnet Technology Ltd., Company Profile", November, 2007, published online at ” http://metrolab.com/newsletters/2008-2/articles/Siemens_Magnet_Technology_Company_Profile.pdf

[9] The RMU Controller, http://www.mksinst.com/product/product.aspx?ProductID=199

[10] The 740C High-Pressure Baratron® Gauge Capacitance Manometer (20 – 3000 psig), http://www.mksinst.com/product/product.aspx?ProductID=666

[11] http://www.mksinst.com/docs/ur/VALVE-HFV.aspx

For a general catalog of MKS products, follow this link http://www.mksinst.com

Some Baratron® capacitance manometer products may not be exported to many end user countries without both US and local government export licenses under ECCN 2B230. mksinst™ and SenseLink™ are trademarks and Baratron® is a registered trademark of MKS Instruments, Inc.