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NCF > News > Tiny Tech News

Star Trek-like Medical Scanner Coming from NIST

By Dan Ray
Published 4/10/2009

In the 2150s, aboard the USS Voyager spacecraft, if a patient happens to be suffering from recurring headaches, memory-loss or seizures, the Chief Medical Officer will simply wave a hand-held scanner in front of the unfortunate individual’s head and immediately pinpoint any irregularities – such as brain tumors or hemorrhages.

Today, on Earth, medical practitioners can detect these same problems using non-invasive “scanning” techniques as well. However, many existing devices for monitoring brain activity or other electrical processes of the human body are big, bulky, incredibly expensive or, sometimes, imprecise.

This is a schematic of the most common form of magnetometer, a fluxgate magnetometer. This type of device was invented during WWII and was used aboard Allied bombers to detect the presence of German submarines below. It is also used in modern metal detectors. Essentially, two magnetized bars are placed close to one another, but their polarities are opposite. So, once these two bars are placed in an external magnetic field, one of the bar's orientations will be parallel to the field and one will be opposite. This change in the bars' magnetic fields can be detected by the change in the external coil's current.

But probably not for long.

Researchers at the Institute of Standards and Technology (NIST) in Boulder, Colo. have developed a tiny medical sensor, capable of detecting the impossibly small electrical activity of a fetal heart or an adult’s thoughts – and this microchip-sized invention may yet gain even greater sensitivity.

“We’ve developed a chip-scale magnetometer with sensitivity that makes it competitive with large commercial systems,” said John Kitching, NIST physicist and the project’s leader. “We could make a portable MRI for the battlefield or detect bombs with very weak magnetic fields.”

The NIST magnetometer has already demonstrated the ability to detect the electric activity of a mouse’s beating heart, and is sensitive to magnetic fluxuations as small as 70 femtoteslas – this about ten billion times weaker than the magnetic field of the Earth and ten trillion times weaker than the strength of a refrigerator magnet. Refining the fabrication and measurement techniques of the magnetometer will also likely lead to further improvements in sensitivity, Kitching said.

“A commercial magnetometer is of the order of $10,000 and it’s the size of a coke can,” he said. “Ours only requires 10 watts of power and is 300 cubic millimeters.

“Our cause is to reduce the size, power and cost,” Kitching said.

Above is a schematic of NIST's chip-scale magnetometer and its components (a), along with a picture of the same device (b). The components are: 1–the VCSEL, 2–the optics package, 3–the Rb-87 vapor cell with transparent heaters above and below it and 4–the photodiode assembly. This assembly works on an entirely different principle than the fluxgate device. Section 3 holds the Rumidium vapor, which is sensitive to tiny external magnetic fields. The atoms orient themselves in the direction of the field and specialized lasers can detect this change to determine the presence of a magnetic field, its strength, its precise location and if it is changing. (NIST).

It is unclear yet exactly what the commercial price of the chip-scale magnetometer will be, but the fact that it is constructed via processes that are already well-established in microchip production suggests that mass producing them is possible – which would almost certainly drive costs quite low. Furthermore, the performance and potential applications of NIST’s invention may significantly outpace both types of magnetometers that are widely used today – fluxgate magnetometers and SQUIDs.

Most commercial magnetometers are based on fluxgate technology, a relatively simple, yet reliable method for detecting and measuring the presence of magnetic fields or magnetic materials – most metal detectors use some form of fluxgate magnetometer to function.

Basically, fluxgate technology involves the arrangement of two magnetic materials – similar to bar magnets – placed close to each other. Both of these bars have a current-carrying coil of wire placed around them. Depending on which way the current in these wires is flowing, the magnetic bars have different < a href="http://encyclopedia.farlex.com/Magnetic+polarity" target="_blank">magnetic polarities. Placing this system in an external magnetic field – for example, the magnetic field created by the electronics of a bomb – causes a change in the strength of the magnetic field created by the two magnetic bars. This change can be detected with the aid of a third current-carrying coil, which is wrapped around the entire system, and the source of the external magnetic field can be located.

This type of magnetometer has the advantage of being quite cheap, easy-to make and reliable. However, it is not very sensitive or accurate, and it cannot be used to do the sort of high-tech imaging required for MRIs. Furthermore, some objects that people might want to detect – such as bombs – may create magnetic fields so weak that fluxgate magnetometers simply wouldn’t be able to find them.

For decades the cutting-edge in magnetometers has been the Superconducting Quantum Interference Devices, knows as SQUIDs. These small gadgets work on an entirely different principle than the relatively unsophisticated fluxgate magnetometers. Small loops of superconducting material are placed very close to each other, but these loops are not directly connected – a thin insulating material separates them. Due to what is known as the Josephson effect, when two such superconducting loops are placed in a magnetic field, a certain amount of electricity manages to jump the insulating barrier between them. The amount of electricity that makes this escape from one loop to the other can indicate the strength of the external magnetic field and its location.

This process is sensitive to very small magnetic fields and, thus, has made it ideal for modern Magnetic Resonance Imaging devices. These MRIs have virtually revolutionized the medical imaging industry and allowed for non-invasive diagnoses of numerous health disorders – such as tumors, hemorrhages, broken bones, torn ligaments and many more. In MRI machines arrays of dozens of SQUIDs are used – usually arranged all around the patient’s body in a tube – to detect the minute magnetic fields generated by different bodily structures.

Although SQUIDs are very small and sensitive, they have one glaring drawback: cost. Despite the apparent simplicity of the Josephson effect, it only occurs in superconducting materials; and superconducting materials only function at extraordinarily cold temperatures. Many SQUIDs only work when the metal superconducting alloys comprising their loops are at about 4 degrees Kelvin. This is a mere 4 degrees above the lowest temperature possible in the universe, known as absolute zero. To cool these metals at roughly the same temperature as outer space, SQUIDs must use liquid helium or nitrogen to virtually freeze every single superconducting loop. And liquid helium isn’t cheap.

But driving the cost down is possible by employing yet another, and entirely different type of technology to the problem of sensing magnetic fields: atomic clocks.

In 2004 NIST perfected the process for making atomic clocks based on Micro-Mechanical Machines – better known as MEMs. Essentially, atomic clocks keep extremely precise time, allowing for far more accurate and reliable communications networks or GPS systems through synchronization. The NIST clock relied on simple cesium atoms to keep some of the most precise time yet achieved.

The most advanced form of magnetometers used commonly today are SQUIDs, which use a superconducting lead alloy to make very precise measurements of magnetic field strength, flux and location. Arrays of SQUIDs are used in MRI imaging machines, shown above. However, for the metal alloys used in these to function as superconductors, they need to be extraordinarily cold -- about 4 degrees above the coldest temperature physically possible, known as absolute zero. This can only by done by cooling the metal with liquid helium or, in some cases, nitrogen. This process has the drawback of making SQUIDs extremely expensive and not nearly as portable as SERFs (NIST's invention) may be. (National Cancer Institute).

Although many of us may not be aware of it, all atoms spin, similar to how the Earth spins on a daily basis. And just as we keep time by the rate at which the Earth spins, in terms of days, we can also keep time by the rate at which certain atoms spin, in terms of billionths of seconds. In fact, we can keep time far more accurately with atoms, since they are generally insensitive to differences in gravitational pull, magnetic fluxuations or other sources of error.

“The reason atoms are used is because they’re largely insensitive to these sort of things,” Kitching said. “In fact, if you had an atom that was just suspended in space it would be a great clock because the frequency is so regular.”

Suspending an atom is space is virtually what Kitching’s team did in 2004 when it created its chip-scale atomic clock. A very small hole was etched, with a laser, into a silicon wafer – using a process very much like what is used to create microchips. Cesium was put in this hole and trapped inside. Next, the chamber was heated so that the cesium would vaporize. Using a series of lasers to monitor and keep the cesium vapor in place, Kitching’s team was able to accurately keep track of the number of rotations the atoms made through time. Thus, creating a miniature atomic clock with amazing accuracy.

Since this process involved preexisting fabrication techniques – many of which are used in making microchips – this made NIST’s atomic clock the smallest and cheapest.

The next step was to refine this invention so that it could measure things besides just time, Kitching said.

“Because the frequency is so well defined, we could use it to measure all sorts of different things,” he said. “We could measure temperature, electric fields, rotation or, of course, magnetic fields.”

The chip-scale magnetometer was the next logical step. The atomic clock was refined to be sensitive to magnetic fields by replacing the cesium vapor with a rubidium vapor. These atoms are more sensitive to magnetic fields; placing them near a magnetic source disrupts their rotation patterns.

“If you put it in a magnetic field, you get it to do what a top does, which is precess,” Kitching said.

Precession in a top is simply the wobbling that occurs when it begins to lose speed. With a top spinning on the ground, this precession is caused by friction. Cesium atoms in a vapor don’t experience this. Instead, they can be made to be most sensitive to magnetic disruptions, and the way they precess can be used to determine the strength and location of various magnetic sources.

“It’s like pulling apart a complicated pendulum and seeing how it wobbles around and seeing how (the atoms) interact,” he said.

NIST’s chip-scale magnetometer already rivals the accuracy of the vastly more expensive SQUIDs, and could even surpass it, according to Kitching. There are still techniques available to reduce the amount of power it consumes, and to make it more effective in strong external magnetic fields – right now it works best when only small magnetic fields are present.

If these next advancements can be perfected, we may one day have cheap, easy, hand-held medical imaging systems saving lives in real life, not just fictional lives on Star Trek.


Related Sites


Further Information for Students or Professionals

  • NIST: Institute's homepage
  • John Kitching : optical frequency measurements group homepage

Selected Publications

  1. Kitching's NIST publications are available at http://tf.nist.gov/cgi-bin/showpubs.pl

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John Kitching

NIST physicist and project leader for the Optical Frequency Measurements Group.

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