Gremlins in Your Medical Equipment: Technical Note 123

by Dr. Margot Stock

RF radiation is everywhere. We can't smell it, see it, hear it or touch it, yet we know it is out there. Medical diagnostic equipment provides unreliable results. Weird audio can be heard through testing equipment. Rock and roll music accompanies a medical diagnostic test. These gremlins are caused by high-power RF installations several miles away, such as AM, FM and shortwave radio, TV, microwave links and other such sources. Even local sources such as: EMT trucks, WLAN's, MRI, and linear accelerators can have an impact.

Case Study: A neurologist was testing his new Neuro Conduction/EMG machine. When applying the concentric needle to his wrist, he noted that his arm was dancing to clearly heard rock music. As an amateur radio operator, he was quite conversant with radiofrequency and understood that this behavior by the machine was connected in some way. This particular radio frequency interference was traced by LBA engineers to an AM radio station a mile away. A telephone call to the builder leading to the installation of custom radio frequency shielding, by LBA, of the exam room solved the problem. On a further, interesting note, the engineer detected a signal of approximately 1 V/m emanating from the radio station. Federal (FCC) guidelines suggest that RF interference problems with electrical medical equipment can be elicited within the 1 V/m field of a radio station. This case study was an obvious instance of RF interference, but not all interference is so easily detected.

Diagnostic tests are an invaluable tool in identifying patient problems. The results must be accurate if the diagnosis is to be correct. The tests are too expensive to be repeated, particularly if they are due to outside RF interference which can be prevented.

Case Study: Several years ago, an incident occurred in a local hospital when an Ultrasound machine used for tracking fetal growth developed noise in the display that obscured the picture. This problem was traced to a nearby radio station. As it happened, the station went out of business before architectural shielding was installed. The old adage that "an ounce of prevention is worth a pound of cure" is definitely true in the case of RF emissions and medical technology. It is never too early to plan for effective RF interference abatement.

Medical offices are, by virtue of their focus, usually clustered in specific areas in any urban setting. Frequently, this cluster of medical facilities includes a hospital with its abundance of medical technology. Urban areas are a hotbed of constantly evolving RF sources both local and ephemeral. This provides a distinct possibility of higher RF interference which will have an increasingly greater impact on the medical equipment within the area. In addition to the cases outlined above, RF interference might manifest itself as false positive or negative readings, transient, undesirable audio-on signals, aberrant readings on ECG from unshielded leads to name but a few.

Conclusion

Obviously, the answer to RF interference is appropriate shielding of equipment pieces or exam rooms containing diagnostic and recording equipment. The trick is to ensure that the passage of unwanted RF signals are small enough to cause no harm to medical equipment. New innovations in shielding make it possible to lower unwanted RF interference to manageable levels at a reasonable cost. Using architectural shielding such as fiber attenuation composites, and conduction coatings can achieve attenuations of 40dB or more in existing facilities. While not to CIA specifications, such installations are often a cost effective way to prevent RF ingress into your medical equipment. For small pieces of equipment, stand alone Faraday cages such as LBA Technology's EMFaracage^TM can be employed to block virtually all RF signals from entering. In addition to protecting your investment in expensive medical equipment, reducing maintenance costs and increasing reliability and patient satisfaction.

If possible, shielding for specific spaces should be part of the original architectural design. If a potential source of interference is introduced into the area then a competent consulting engineer with experience in RF compatibility should be retained immediately. His role will be to study the electromagnetic environment and provide expert guidance in achieving compatibility between the new potential source of interference and the medical office with its' diagnostic equipment. Prompt attention to this will prevent surprises in medical equipment behavior and reassure the owners that RF interference is not an angry gremlin sent to make life untenable. 

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Dr. Margot Stock is Director of Radio Frequency Environmental Support at Lawrence Behr Associates, Inc.

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