jump to navigation



Images on monitors are jittering… excessive hum in the audio system… strange problems with network data systems… perhaps caused by magnetic fields?
When reports are received of wavy or jittery images on monitors, or audible “hum” in an audio system, maintenance engineers often scramble for an explanation and a solution. Grounding systems are meticulously checked and inspected, yet the monitor distortion remains or excessive audio hum persists. Monitors are swapped, isolation amplifiers substituted, surge protectors installed, all to no avail. Everything is within specification. The mysterious interference problem persists. Eventually, everyone involved concludes that the source of interference isn’t the affected equipment, the system it’s connected to or the power source. What’s left? What could it possibly be? It just seems to be in the air. Perhaps the interference problem stems from low frequency magnetic fields emanating from some near-by power source.

Broadcast engineers and technicians are generally knowledgeable about equipment interference problems often associated with electronic grounding issues or external environmental sources (RF, microwave, radar systems, etc). However, few engineers and technicians are aware of interference that may be caused from elevated low frequency magnetic field conditions.

AC and DC Magnetic Fields

Equipment interference in a technical facility may be caused by elevated levels of either alternating current (AC) magnetic fields or by direct current (DC) magnetic fields in a building’s interior. A wide variety of equipment interference problems can occur when levels of either type of magnetic field are excessive.Magnetic fields occur throughout nature and are among the basic forces of nature. Any object with an electric charge on it has a potential or “voltage” at its surface and can create an electric field. A change in voltage over distance is known as the electric field. When electrical charges move together (defined as “current”), additional forces are generated. These additional forces are represented by magnetic fields; hence all electric currents create magnetic fields. Depending on the source of the field, a magnetic field’s strength and direction can be static (direct current, DC) or alternating (alternating current, AC). AC magnetic fields are a natural consequence of distribution and use of electricity. At 60 Hz or extremely low frequency (ELF), the electric and magnetic fields generated by an AC circuit operate independently. In other words, it is possible to reduce or eliminate one without materially affecting the other. However, at radio frequency (RF) and higher, a fixed relationship exists between the electric field and the magnetic field. A reduction in the magnetic field also results in a reduction in the electric field.

AC electric fields are generated by voltage and are measured in volts/meter while magnetic fields are caused by current flow and are measured in milliGauss (mG). At ELF or power frequency, electric fields emanating from an AC circuit are quite easy to shield, as nearly all standard building materials will substantially reduce the electric field strength. Conversely, ELF magnetic fields are extremely difficult to reduce. At 60 Hz power frequency, AC magnetic fields pass relatively undiminished through nearly all common building materials, including lead.

Elevated ELF magnetic field conditions are normally present in areas adjacent to high current-carrying conductors. Typical high current sources in commercial buildings include electrical equipment rooms and closets, utility substation and transformer vaults, distribution bus-ducts, wire ducts and feed conducts. Electrical transmission and distribution lines passing near the exterior of a building may also create elevated magnetic field conditions in a building’s interior. In some instances, the source of elevated ELF magnetic fields in a building may not be readily apparent. Wiring errors in a building’s AC power system, even in low current distribution circuits, can cause substantial ELF magnetic field conditions to exist in large areas of a building. Such wiring errors including improper ground-to-neutral connections and crossed neutral conductors, can create “net-current” conditions wherein all of the current in a circuit is not returning via the same path. Since the strength of an ELF magnetic field is directly proportional to the amount of current flowing in the source circuit, fluctuations in the use of power during the day or seasonally, can cause changes in elevated magnetic field conditions. Because of this temporal variation, it is not uncommon for an equipment magnetic field interference problem to appear as intermittent.

Static or DC magnetic fields commonly occur in nature. The earth has a natural static magnetic field which, depending on location, will range from 400 to 500 mG. DC magnetic fields may also be generated by DC current flow in subway or train rails, elevators, and battery based power systems. Powerful magnets associated with MRI and NMR medical magnetic imaging systems typically produce elevated levels of DC magnetic fields. As a consequence, structural and reinforcing steel members in commercial building structures can become magnetized as a result of containing such equipment over a length of time. Structural steel can be magnetized by placing it in a strong external dc magnetic field that will essentially capture and align the magnetic domains in the material. Steel can also become magnetized by putting strong DC currents through the material such as grounding welding equipment to structural steel during construction.

Common Magnetic Field Interference Problems

Interference with computer or Cathode Ray Tube (CRT) video display monitors is the dominant problem associated with elevated magnetic field environments. Screen interference caused by magnetic fields is of two categories: AC magnetic fields can cause the image to “jitter” on a display, while DC magnetic field monitor interference problems are manifested as stationary image tilt or color purity problems (changes or blotches of color in various areas of the screen). Thresholds for computer monitor interference will vary by different magnetic field intensities, depending upon the type, size, make and model of the monitor. In general, CRT’s are much more sensitive to AC magnetic fields than to DC magnetic fields. Many CRT’s will exhibit signs of image jitter interference when placed in external AC magnetic field conditions of 10 mG and most will be unstable in fields of 30 mG. Some high-end large screen graphics monitors tend to be much more sensitive and interference will often be noticed at thresholds as low as 3 to 5 mG. As ambient AC magnetic field conditions in most commercial buildings range from 1 to 4 mG, the chances for interference with such monitors is high.

Typical DC magnetic field interference on CRT’s can be observed in magnetic fields as low as 1,000 mG, (500-600 mG above background Earth DC field) with increasing interference as DC magnetic field levels increase. Although relatively infrequent, residual, elevated DC fields in buildings can be in the range of 2 to 5,000 mG.

A wide variety of audio equipment may experience interference problems when located in an elevated AC magnetic field environment. Most notably, sensitive preamplifier sections of professional and broadcast audio mixing consoles may experience increases of audible 60 Hz “hum” or increased levels of signal-to-noise when located in areas with elevated AC magnetic field conditions. Such hum and increased signal-to-noise conditions are created by the induction of an interference voltage at 60 Hz in sensitive components of analog audio amplifiers. Similarly, sensitive or poorly shielded microphones and musical instrument audio pick-up transducers can experience undesirable levels of 60 Hz hum when used in environments with elevated levels of AC magnetic fields. Professional musicians have long been aware of this phenomena in performance venues and have learned to shift or orient amplifiers and sensitive musical instruments to locate areas with lower AC magnetic field levels or “null” points.

Interference problems may be present in audio/video/data cabling when placed in close adjacency to conduits, bus-ducts or other electrical distribution equipment containing high AC current conductors. ELF magnetic fields naturally emitted from such conduits or bus-ducts may be sufficient in magnitude to induce troublesome levels of interference AC voltage on adjacent signal cabling. The potential for such interference is markedly greater when signal cable runs closely parallel AC power conduits for extended distances. Although not well documented or understood, there have been numerous anecdotal reports of a wide variety of possible interference manifestations in CPU and digital equipment when placed in elevated AC magnetic field environments. Such interference problems have been known to affect the operation of high-speed CPU and certain computer disk drives, loss of data resulting in increased error rates and slower transmission speed of LAN digital signal networks. Most equipment manufactures including companies making CRT monitors, unfortunately do not publish AC or DC sensitivity levels for equipment.

Lastly, elevated levels of magnetic fields present in archive areas may affect the long-term storage of magnetic media including magnetic tape, floppy-discs, etc. Typical specifications for magnetic storage media including floppy-disks and hard-disk drives, range from about 6,000 to 10,000 mG for magnetic field levels in the frequency range of 0 Hz (dc) to 700 kHz (which includes the power-frequency of 60 Hz). Below 6,000 mG data corruption on storage media is typically not observed as reported by hard-disk drive manufacturers.

Dealing with Magnetic Field Interference Problems

Resolving equipment interference problems due to the presence of an external magnetic field source is often difficult and expensive. In some instances a choice may have to be made between eliminating the source of a magnetic field or minimizing the interference effects of the source magnetic field. The first step in identifying and resolving a suspected magnetic field interference problem, is to locate possible magnetic field sources and to measure magnetic field intensities in the area of concern. Magnetic fields are measured by a gauss meter which range in cost from several hundred to several thousand dollars. Different instruments are required to measure AC and DC magnetic field values and directions. Prior to purchasing such measurement equipment, it may be preferable to contact the local utility company and request magnetic field measurement assistance. Many utilities have an “EMF specialist” who will provide at no-cost or for a minimal fee, initial magnetic field measurement services. If a sizable problem is identified, such as an audio/video equipment room located above or immediately adjacent to a major electrical service room, it may be prudent to engage the services of a professional engineering firm. A specialized and experienced engineering firm can conduct a detailed assessment of the problem and evaluate possible solutions. If the source of interference is an AC magnetic field, three general strategies may be considered to mitigate the problem: (1) increase distance, (2) decrease the magnetic field strength, or (3) shielding.

Magnetic fields decrease in strength at increased distances from the source. It may be possible therefore, to simply move or relocate affected equipment away from a magnetic field source until interference problems are minimized or eliminated. This solution may be effective, for example, in instances where a monitor is near a transformer or electrical panel, but this effort may prove ineffective if the source is a transmission line passing outside the building. In certain instances, it may be possible to decrease the magnetic field strength from a source by implementing electrical modifications to increase natural cancellation of opposing conductors. In the instance of magnetic fields caused by net-current electrical circuit conditions, dramatic reductions in magnetic field levels typically result as a consequence of correcting wiring errors that create net current conditions. As a third possible solution, consideration may be given to shielding the affected equipment, shielding the source of magnetic fields or shielding the area in which the affected equipment is located. In the case of monitors, special external shields made of permeable materials that attract magnetic fields and provide an alternate path around the monitor are available from a number of manufacturers. Shielding large areas such as an electrical room or an entire space containing sensitive equipment is generally difficult to implement and should be designed and installed by an experienced and qualified magnetic field shielding engineering company.

Monitor interference from external AC magnetic field sources may be minimized by two additional possibilities. In some computer systems, it is possible to set the vertical refresh rate of the monitor to 60 Hz power frequency without serious compromises to the image quality. However, resolution of the monitor may be reduced and in almost all cases, cure of the jitter problem will be at the expense of increased “flicker” from area lighting. Further, if the external AC magnetic fields are strong, setting the refresh rate to 60 Hz will not remove all jitter interference. As a second possibility, it may be acceptable to replace CRT monitors with LCD monitors. LCD technology monitors are generally not affected by an external magnetic field. However, quality and system compatibility issues should be considered prior to purchase of a replacement LCD monitor.

In instances where monitor interference is from a DC magnetic field, it may be possible to degauss the affected monitor to temporarily restore color purity or install a shield around the monitor. If the source of DC magnetic fields is from a structural steel building member that has become magnetized, it may be necessary to consider degaussing the magnetized steel to permanently to eliminate the interference DC magnetic field. This degaussing option removes residual magnetism from steel objects.

In new facility construction projects, consideration should be given to careful design of electrical facilities such that high-current carrying equipment is not located adjacent to areas which may contain sensitive equipment. Documenting AC magnetic field conditions at a proposed project site prior to design and construction may insure that passing transmission lines or near by utility electrical facilities won’t present a problem. It may also be a good idea to measure DC magnetic field levels near all structural steel members during building construction. If excessively high values of DC magnetic field levels are present due to magnetized steel members, it is much more feasible and cost effective to remove the magnetism by degaussing while such steel members are exposed and accessible.

A. Problem/ Issue

Interference with computer of Cathode Ray Tube (CRT) video displays monitors is the dominant problem associated with the elevated magnetic field environments.  Screen Interference caused by magnetic fields is of two categories: AC magnetic fields can cause the image to “jitter” on a display, while the DC magnetic field monitor interference problems are manifested as stationary image tilt or color purity problems.

B. How big is the problem? How does it affect you? Why is it important to study the problem?
A wide variety of equipment interference problems can occur when levels of either type of magnetic field are excessive. It affects us in our work when we encounter this kind of problem. It is important to study this problem because it might lead to bigger problems if not given proper attention and so we can minimize the problem.
C. Causes of the Problem

Equipment interference in a technical facility may be caused by elevated level of either alternating current (AC) magnetic fields or by direct current (DC) magnetic fields in a building’s interior.

D. Solution

In new facility construction projects, consideration should be given to careful design on electrical facilities such that high-current carrying equipment is not located adjacent t areas which may contain sensitive equipment. Documenting AC Magnetic Field conditions at a proposed project site prior to design and construction may insure that passing transmission lines or near by utility electrical facilities won’t present a problem. It may also be a good idea to measure DC magnetic field levels near all structural steel members during building construction. If excessively high values of DC Magnetic Field levels are present due to magnetized steel members, it is much more feasible and cost effective to remove the magnetism by degaussing while such steel members are exposed and accessible. 


Magnetic Contamination: The Ghost of MRI Past

By Matthew Robb
Radiology Today
Vol. 5 No. 21 Page 22

After delivering a two-hour seminar on the mysterious topic of retired MR equipment and magnetic contamination, medical architect Toby Gilk saw one attendee make the connection.

“We were all standing around,” Gilk recalls, “when suddenly this administrator comes up to us and says, ‘You guys hit the nail right on the head. You solved the problem that we’ve been dealing with for more than a year now.’”Her new PET/CT suite was haunted by the ghost of its previous tenant. As it turns out, the imaging center had started having trouble just after mothballing its old MRI magnet.

“The original equipment manufacturer who decommissioned the MRI decided to site a new PET/CT scanner in the same suite,” says Gilk, an associate architect with Kansas City-based Junk Architects. “Over the coming months, they had all kinds of problems with image quality. The CT resolution was so bad, in fact, that the vendor’s technicians were pulling their hair out, asking, ‘What’s going on here? Why aren’t we having these problems at any of our other installations?’”

Gilk’s presentation at May’s meeting of the Magnetic Resonance Managers Association opened at least one administrator’s eyes to a curious scientific footnote that few people in the industry had ever heard of, much less discussed over coffee and Danish. But now, everything clicked. The administrator’s old MRI suite, she told the group, was lined with thick steel plating. Over the years, the MR unit must have transformed this room from a benign diagnostic chamber into a hostile environment for CT and PET technology, office computers, and possibly more.

“This administrator said she didn’t think the vendor had ever checked for passive magnetization or magnetic contamination,” Gilk observes. “The entire topic just slipped by, completely unnoticed.”

The technical implications are evident. But as word of magnetic contamination slowly spreads, landlords, business insurance firms, and maybe even a few trial attorneys may reformulate their strategies. In short, the imaging community has been served notice about the possibilities of magnetic contamination.

While best estimates suggest that a statistically small but numerically significant number of MRI facilities today have some issue with “residual magnetization,” Gilk notes that lack of general awareness is nearly universal and perhaps equally understandable.

“This issue has been known for decades, but that knowledge has resided in tiny, isolated pockets,” he says. “There was a scattering of people who had these bizarre war stories, but even today most MR administrators and technologists will say, ‘Magnetic pollution? I’ve never heard of that happening.’” Gilk says this information disconnect can be found even among “experts” in the technical design community. In some cases, Junk Architects encounters professionals playing ostrich. Gilk cited an incident earlier this year in which Junk Architects ran across a government bid solicitation for a new MRI suite. Scanning the attached blueprints, the staff became concerned about passive magnetization. They alerted the contracting officer and warned, “These issues might come back to bite you five to 10 years down the road.” The response? A collective yawn, recalls Gilk, “Word got back to us from their architect to the effect that, ‘We’ve done MRI suites before. Go away,’” Gilk said.

He concludes, “The people involved in this project ran the risk of permanently magnetizing that suite.” The last Gilk had heard, the ill-conceived renovation was moving full steam ahead, as originally planned.

Nuts and Bolts of Magnetism
Explaining the phenomenon of MRI-induced passive magnetization, Gilk provides a common reference point: the Earth’s magnetic field. Averaging approximately 0.5 gauss in the central United States, this weak background force has only a minor (and largely inconsequential) effect on the steel skeleton of commercial buildings.

But MRI units generate magnetic fields exponentially stronger than those experienced naturally on Earth. And while newer magnets generally contain magnetic spillage better than earlier models, even the best are leaky. Exhaustive research establishes the 5-gauss threshold—corresponding with the FDA’s “exclusion zone”—as the watershed in MRI design and construction. “You want to keep any significant quantity of steel outside that 5-gauss bubble,” Gilk notes. “A 5-G [gauss] magnetic exposure seems relatively low, but remember that if you run an MRI for 10 years in the same space, that gentle, constant magnetization is going to build and build the passive magnetic field in any nearby ferrous material.” A suitable analogy might conjure the drip, drip, dripping of mineral-rich water from a cave ceiling, which eventually gives birth to imposing stalactites.

A contemporary 1.5-Tesla magnet will generate a 5-gauss magnetic “bubble” that reaches out in a circle roughly 8 to 12 feet from the center of the magnet bore. “But if you go to a powerful research MRI or an installation older than, say, 10 years old,” he notes, “this unshielded magnetic bubble might extend 30 or more feet.” Debunking cheesy 1950s science fiction images of an entire building turned into a powerful magnet, Gilk notes that even among older, unshielded magnets, “we’re probably talking about the magnet room itself and the immediately adjacent spaces becoming magnetized—no more. And you’re not going to get stuck to the wall if you’re wearing a big ol’ rodeo belt buckle.”

Passive magnification becomes a potential problem only when people fail to detect it and take appropriate corrective measures. “This is an operational and financial issue,” Gilk says. “There are no direct physical safety concerns arising from magnetic contamination. The only potential health consequences are indirect and could come from a diminished clinical value from CT/PET scans, for example, where magnetic ‘noise’ degrades image quality. Obviously we all believe that we should use our diagnostic tools to the very best of their abilities to maximize the benefits for patient care.”

When hospitals or group practices introduce computers, shift modalities, or start talking about upgrades, problems can arise. Notably, administrators are reluctant to move MRI units because of their tremendous heft, tipping the scales at 6 to 8 tons. Swapping old technology for new becomes an attractive alternative.

One client of Junk Architects received its schooling in the wily ways of passive magnetism when administrators decided to convert an abandoned MRI suite into an office. Only later did they learn that the room’s magnetic field registered above 5 gauss. “The entire room had become a huge refrigerator magnet,” Gilk wryly notes. Connecting the dots, he adds, “I’ve got refrigerator magnets at home that are 30 years old and still stick to the fridge.” For the rest of the building’s useful life, he says, “this room is only good for paper storage or as a janitor’s closet.”

Look Out Below
As director of new product development at ETS-Lindgren, Inc., Joe Weibler has helped companies identify, resolve, and avert issues of residual magnetism. Located just outside of Chicago, ETS-Lindgren conducts magnetic field surveys and provides primary radiofrequency shielding in medical, industrial, and government applications.

According to Weibler, “The biggest potential problem with residual magnetism” is found with new equipment—CT and PET scanners, for instance—slated to go into old MR spaces. Part of the problem is that today’s ultra-precise technology is also ultra-sensitive. Problems arise, he says, when MRI units are retired, but their magnetic signatures keep working overtime. These confounding magnetic fields can be traced to steel beams in walls, steel reinforcement bars in floors, or steel structural supports in ceilings.

“We’ve seen instances,” he says, “of residual magnetic fields really impacting the performance of 3-D rotational angiography systems.” Variations in passive magnetic fields interfere with the rotational arc as these systems create their fantastically detailed 3-D images. “We only saw the effect of residual magnetism when [our clients] actually did rotational 3-D scans,” he notes. “When used as a straight fluoroscope or x-ray machine, there were no issues.” In short, all it takes to garble one of the world’s most sophisticated devices is a magnetized chunk of steel rebar hidden under the floor.

“The crucial thing to understand,” Gilk says, “is that today’s movement toward active shielding is not a panacea. Instead of eliminating passive magnetism, it just concentrates it into a smaller footprint. And with the rise of 3-Tesla magnets, it becomes especially important that these issues are addressed in advance or you’re going to magnetize a larger area with a much greater field strength much more quickly.”

Assessing the Risk
While acknowledging the issue of passive or residual magnetization, Weibler says he hasn’t heard much public concern—not at this point anyway. Similarly, he questions the impact of passive magnetization in most day-to-day operations. “From everything I’ve experienced,” he says, “it would take a pretty phenomenal event to screw up a laptop computer going by. I’ve had credit cards in my pockets when I’ve walked into an MRI suite. We’re talking magnetic fields easily 10 to 50 times stronger than any residual magnetism you would find, yet my credit cards worked fine after that.” Left unsaid is the issue of long-term exposure.

Emanuel Kanal, MD, FACR, agrees with Weibler’s conservative appraisal. Says the chairman of the American College of Radiology MRI Safety Committee and director of magnetic resonance services at the University of Pittsburgh Medical Center department of radiology, “It might be a real problem, but only in limited cases. The number of times where a site finds itself affected by induced magnetization of the environment that previously housed an MR imaging unit is likely quite small. Although for affected sites it might not be a trivial matter, I don’t believe this issue has caused overwhelming difficulties.”

Kanal argues that administrators can usually accommodate for any passive magnetization issues, but adds, “No question, with color CRT [cathode ray tube] monitors, it would not have been a trivial issue. They are sensitive to roughly 1-gauss magnetic fields. But today everyone seems to be using LCD [liquid crystal display] monitors and optical storage media. The things that once were so sensitive have significantly less of a presence today.”

An Ounce of Prevention
While risk assessments of magnetic contamination vary, Michael L. Hiles says the issue generates much concern—and business—for his Los Angeles-based firm, Field Management Services.

“It’s clear and obvious that magnetism inside a building can be a problem,” he says. “Depending on the kind of equipment you’re putting in a particular suite, it can be a serious problem. It’s easy for scientists to scoff, to stand back and say, ‘I wouldn’t worry about that.’ But their heads aren’t on the chopping block when something goes awry. It’s not their ‘home’ that they are betting. It’s someone else’s home.” Gilk agrees and says awareness of passive magnetism just isn’t where it needs to be, hence Junk Architects’ ramped-up industry awareness initiative.

The bottom line, Hiles cautions, is the need for administrators to “think about the history of the building.” Gilk echoes Hiles’ “pay us now or pay us later” refrain. “With extra time, forethought, and money put into the initial construction, we’re talking a marginal difference in extra materials cost of a few thousand dollars. Compared with the MRI cost of $1.5 to $2 million, my question is: ‘Is that amount really going to break the project?’ With a little bit of effort, you’ve preserved the value of that property indefinitely.” Without it, he says, an MRI room “may become an MRI room for life.” Savvy designers, both Hiles and Gilk say, can design a building so that once a retired MRI is moved out, the magnetized structural steel elements embedded in a floor or wall can be easily removed.

Shielding and Degaussing
Once passive magnetism is identified as an issue, administrators have two basic options: dedicate the room to nonimaging (and possibly, noncomputer use) or remediate the problem.

Junk Architects, ETS-Lindgren, and FMS—all specialists in identifying and resolving issues of magnetic contamination—say the traditional solution is to cover magnetized construction elements with steel plating. Options include low-carbon steel, silicon steel, and fiberglass and polyester-reinforced concrete that substitute for steel rebar.

“The amount of steel used is a function of how much you want to reduce the magnetic field,” Gilk says. “If all you’re concerned about is a magnet that generates 7 gauss on the outside of the magnetic room and want to get that down to 5 gauss to make sure you don’t have the FDA exclusion zone, you can probably do that with a couple of 1/16 sheets of silicon steel. If, however, you have 20, 30, 40, 50 gauss outside the magnet room, now you’re talking anywhere up to 3 to 4 inches of steel plate, which means you also have to beef up the support structure in the building. This can get very expensive.”

Hiles notes that in some cases, merely gaining access to magnetized floor beams or rebar just isn’t feasible. At this point, the three firms might pull out the big guns and attempt to manually “degauss” any suspect building elements. All firms acknowledge that this process—which essentially dampens the passive magnetic field by way of high-amperage currents—is as much art as it is science. Hiles ballparks the cost of degaussing an MRI suite in the $100,000 range. “If the problem is localized, you can think about degaussing,” he says. “But if the magnetism is located throughout the entire structure, you might [have to] redirect the field with steel plate or ferromagnetic shielding.”

Taking the long view, Gilk advocates prevention. Administrators with a 20- to 30-year perspective can avoid costly bloopers and blunders. He adds that when facilities do plan on adding or decommissioning an MR unit, it’s important to consult with qualified firms.

Hinting at what might become an issue in years ahead, he adds, “I’d be very curious to find out about medical office buildings where imaging centers once leased space—and where, today, the building has been magnetized to the point that it interferes with the next tenant’s operations.”





No comments yet — be the first.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: