Friday, March 8, 2013

Helium Shortage!

A recent article in the New York Times discusses the looming shortage of helium.
“A global helium shortage has turned the second-most abundant element in the universe (after hydrogen) into a sought-after scarcity, disrupting its use in everything from party balloons and holiday parade floats to M.R.I. machines and scientific research….

Experts say the shortage has many causes. Because helium is a byproduct of natural gas extraction, a drop in natural gas prices has reduced the financial incentives for many overseas companies to produce helium. In addition, suppliers’ ability to meet the growing demand for helium has been strained by production problems around the world. Helium plants that are being built or are already operational in Qatar, Algeria, Wyoming and elsewhere have experienced a series of construction delays or maintenance troubles.”
One medical use of helium is discussed in the 4th edition of Intermediate Physics for Medicine and Biology. In Chapter 8, Russ Hobbie and I write about the role of helium in magnetoencephalography--the biomagnetic measurement of electrical activity in the brain--using Superconducting Quantum Interference Device (SQUID) magnetometers.
“The SQUID must be operated at temperatures where it is superconducting. It used to be necessary to keep a SQUID in a liquid-helium bath, which is expensive to operate because of the high evaporation rate of liquid helium. With the advent of high-temperature superconductors, SQUIDS have the potential to operate at liquid-nitrogen temperatures, where the cooling problems are much less severe [for additional information, see here].”
A more wide-spread use of helium in medicine is during magnetic resonance imaging. Chapter 18 of our book discusses MRI, but it does not describe how the strong, static magnetic field required by MRI is created. In a clinical MRI system, a magnetic field (typically 2 to 4 T) must exist over a large volume. Producing such a magnetic field using permanent magnets would, if possible, require giant, massive, expensive structures. A more reasonable method to create this field is using coils carrying a large current. One way to minimize the resulting Joule heating losses in the coils is to make them out of superconducting wire, which must be cooled cryogenically. An article on the Time Magazine online newsfeed states
Liquid helium has an extremely low boiling point — minus 452.1 degrees Fahrenheit, close to absolute zero — which makes it a perfect substance for cooling the superconducting magnets found in MRI machines. Hospitals are generally the first in line for helium, so the shortage isn’t affecting them yet. But prices for hospital-grade helium may continue to go up, leading to higher health-care costs or, in the worst-case scenario, the need for a backup plan for cooling MRI machines.”
More detail about the use of helium during MRI can be found in an online book titled The Basics of MRI by Joseph Hornak. Below I quote some of the text, but you will need to go the book website to see the pictures and animations.
“The imaging magnet is the most expensive component of the magnetic resonance imaging system. Most magnets are of the superconducting type. This is a picture of a first generation 1.5 Tesla superconducting magnet from a magnetic resonance imager. A superconducting magnet is an electromagnet made of superconducting wire. Superconducting wire has a resistance approximately equal to zero when it is cooled to a temperature close to absolute zero (-273.15° C or 0 K) by immersing it in liquid helium. Once current is caused to flow in the coil it will continue to flow as long as the coil is kept at liquid helium temperatures. (Some losses do occur over time due to infinitely small resistance of the coil. These losses can be on the order of a ppm of the main magnetic field per year.)

The length of superconducting wire in the magnet is typically several miles. The coil of wire is kept at a temperature of 4.2 K by immersing it in liquid helium. The coil and liquid helium is kept in a large dewar. The typical volume of liquid Helium in an MRI magnet is 1700 liters. In early magnet designs, this dewar was typically surrounded by a liquid nitrogen (77.4 K) dewar which acts as a thermal buffer between the room temperature (293 K) and the liquid helium. See the animation window for a cross sectional view of a first generation superconducting imaging magnet.

In later magnet designs, the liquid nitrogen region was replaced by a dewar cooled by a cryocooler or refrigerator. There is a refrigerator outside the magnet with cooling lines going to a coldhead in the liquid helium. This design eliminates the need to add liquid nitrogen to the magnet, and increases the liquid helium hold time to 3 to 4 years. The animation window contains a cross sectional view of this type of magnet. Researchers are working on a magnet that requires no liquid helium.”
With the discovery of high temperature superconductivity (HTS), MRI magnets cooled at higher temperatures, avoiding the need for liquid helium, are possible. The ideal solution to the helium shortage would be superconducting coils cooled with liquid nitrogen. Nitrogen makes up 80% of our atmosphere, so it is free and virtually limitless. However, a 2010 article by scientists at the MIT Francis Bitter Magnet Laboratory (FBML) suggests that a more practical solution might be the use of solid nitrogen to reach temperatures of 20 K, for which superconducting materials such as magnesium diboride (MgB2) exist that have the properties required for magnet coils.
“A tremendous progress achieved in the past decade and is continuing today has transformed selected HTS materials into 'magnet-grade' conductors, i.e., meet rigorous magnet specifications and are readily available from commercial wire manufacturers [1]. We are now at the threshold of a new era in which HTS will play a key role in a number of applications— here MgB2 (Tc=39 K) is classified as an HTS. The HTS offers opportunities and challenges to a number of applications for superconductivity. In this paper we briefly describe three NMR/MRI magnets programs currently being developed at FBML that would be impossible without HTS: 1) a 1.3 GHz NMR magnet; 2) a compact NMR magnet assembled from YBCO [yttrium barium copper oxide] annuli; and 3) a persistent-mode, fully-protected MgB2 0.5-T/800-mm whole-body MRI magnet.”
Even if new MRI magnets using solid nitrogen or some other abundant substance as the coolant were developed, there are thousands of existing MRI devices that still would require liquid helium and would be very expensive to replace. Congress is currently considering legislation to address the helium shortage (see article here). We urgently need to preserve our helium supply to ensure its availability for important medical devices.

P.S. I saw this article just a few days ago. High temperature superconductors for MRI may be just around the corner!

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