Interference of Another Type
But not all ghosts are due to radiated or conducted electromagnetic fields. For example, a report was received of intermittent interference to an EMG device in the physical therapy department of the hospital. The department felt that the culprit was the MRI system located directly above the area containing the victim device. During footprinting, measurements entailing more than the normal broad spectrum procedures were indicated. In this application, the fingerprinting equipment was set up to measuring any radiated electromagnetic fields at the resonant frequency of the MRI that might leak from the shielded room environment that contains the MRI system. During a period of several hours, no leakage was detected. A broader spectrum scan showed that this department was in a remarkably quiet location with respect to radio frequency energy. Further interviews with the doctors and staff led the clinical engineer to perform a somewhat unorthodox series of tests. The filtration on the EMG was broadened and the leads laid out, unterminated, on the couch on which patients were placed. During this procedure the clinical engineer observed that, when pressure was applied to the couch cushion, the baseline of the EMG machine would vary synchronously. Several additional adjustments to the sensitivity (gain) of the device and the time base were made. It then appeared that any motion in the immediate vicinity of the device would cause this baseline shift. Based on the results of these tests and of an investigation that determined the material composition of the environment (vinyl cushions on the couch and highly waxed vinyl floor), the engineer concluded that the cause of the interference with the device was electrostatic, not electromagnetic. Furthermore, the intermittent appearance of the problem was attributed to the fact that hydrotherapeutic baths were located two doors away from the EMG room. Their intermittent operation would raise the humidity sufficiently to reduce the potential for the generation of intense electrostatic charges. This was a decidedly different ghost hunt.
Interference Not Caused by EMI
One of the recent trends related to electromagnetic interference is that of a manufacturer’s technical problems being attributed to EMI. Although EMI may play a significant role in the performance degradation in certain clinical devices, it is not always the cause. One example involved radiological equipment. X-ray films from one of the radiology labs displayed an artifact described by the radiologists as “chicken scratches.” This artifact was present primarily during a Temporo-Mandibular Joint (TMJ) procedure. This artifact was initially attributed to either conducted or radiated electromagnetic interference, specifically conducted EMI from the power lines. An outside consultant measured