Biomedical Engineering Reference
In-Depth Information
5.6
Chapter 5.6
Electromagnetic interference
in the hospital
W. David Paperman, Yadin David, and James Hibbetts
The density of occupancy of the electromagnetic spec-
trum is increasing because of the expansion of wireless
services (the culprits). Despite the efforts of manufac-
turers to harden clinical devices to the effects of elec-
tromagnetic interference (EMI), reports of new incidents
of interference to previously unaffected medical devices
(the victims) appear in medical literature and anec-
dotally (CBS, 1994; Paperman et al, 1994). The role
and the knowledge base of the clinical engineer must
expand to understand and manage these ever-increasing
challenges.
The periodicity or frequency of alternation is ex-
pressed in Hertz per second, which allows the calculation
of the wavelength of the radiated electromagnetic field.
Knowing the wavelength versus the physical length of
both the radiator and the secondary conductor into
which the radiated energy is induced provides an esti-
mate of the potential amplitude developed by or within
a device at risk. Waveforms other than linear (sinusoidal)
can produce multiple and variable frequencies. The
foremost example of a nonlinear waveform is the square
wave. The square wave produces many multiples, or
harmonics, which have a range many times that of the
fundamental frequency. The majority of digital devices
generate square waves.
Depending on the amplitude at the point of genera-
tion, the efficiency of the auxiliary radiator (antenna),
the generated frequency, and the waveform, the poten-
tial for interference between digital devices and clinical
devices can exist for many kilometers from the source of
electromagnetic radiation.
EMI threats come from a multitude of sources. These
are broadly divided into two categories: Devices that
emit intentional electromagnetic radiation for commu-
nications and control, and devices that, as a byproduct of
their operation, emit unintentional (incidental) radiation
(
Figure 5.6-1
). Some of the major and more common
sources of EMI encountered in the clinical environment
follow.
Electromagnetic radiation
Electromagnetic radiation occurs when an alternating
current is generated. An electromagnetic field is created
in the vicinity of the source. The range, or distance of the
radiated electromagnetic field can be increased when
coupled to a conductor. The magnetic field is further ra-
diated by the flow of the current along this conductor.
Even at a reduced amplitude, a corresponding current will
be generated when another conductor is subjected to the
field of the radiating conductor. The radiated field will
have many characteristics. The most important of those
characteristics are amplitude, periodicity, and waveform.
The amplitude of the impressed alternating current
defines the energy of a radiated field. This field is
generally expressed in volts per meter (V/M). Amplitude
at the source point (transmitting conductor) will define,
in conjunction with frequency and distance, the ampli-
tude of the electromagnetic field induced in the sec-
ondary (receiving) conductor subject to the Inverse
Square Law.
Intentional radiators:
Television broadcast stations: Analog and digital
Commercial radio stations: Analog and digital
