Biological Effects - RF/Microwave Interaction with Biological Tissues - page 127

suites. These devices often produce broadband energy at frequencies below
10 MHz but may use higher frequency narrow-band energy as part of the oper-
ation. For example, an MRI system uses a 64-MHz source, and lasers often use
13- or 27-MHz sources. If well shielded, these systems do not usually pose a
threat to nearby electronic equipment; if unshielded, however, devices such as
electrosurgical units can cause serious interference.
As expected, the principal sources of radiated EMI are intentional radia-
tors , usually situated within the hospital, such as telemetry and paging trans-
mitters, hand-held radios, and cellular phones. Electrosurgical units are also a
major source of radiated energy. In all cases, the proximity to the source is the
key factor affecting the radiated EMI levels. Such sources can cause monitor-
ing, therapeutic, and diagnostic medical devices to malfunction. Rather sur-
prising, however, are the very low levels of radiated EMI from equipment such
as lasers, X-ray and MRI equipment, and computed tomography scanners.
While these systems use high levels of energy to perform their functions,
the resulting radiated EMI levels are well below the levels from nearby
transmitters.
There is a concern about EMI induced by cellular phones, due in part to
the rapid expansion in use and proliferation of these devices. There is also
another reason. A hand-held two-way radio emits RF energy only when the
user intentionally transmits; hence, the user can use discretion about operat-
ing the radio in a clinical area. However, the cellular telephone can transmit
without the user's knowledge when placed in the standby mode because the
phone periodically transmits to the system receiver to indicate that it is in the
cell and ready to receive calls. Equipment responses have resulted when a
cellular phone was operated within a distance of 1 m [106]. It should be noted
that a directive from the European Union about EMI imposes electric
and electronic equipment to be exposed to an electric field of 3 V m -1 up to
1 GHz without dysfunction to obtain a quality label. This value is obtained at
a distance of about 2.5 m from a transmitting cellular phone of the GSM
type.
GSM cell phones can also interfere with ionizing radiation dose-
monitoring equipment. A total of 13 personal electronic dosimeters, portable
dose monitors, and contamination monitors were assessed. Six of the personal
dosimeters showed abnormal responses when exposed to mobile phone
transmission. This should be taken into account when distributing these
devices and when assessing results generated by them. Electromagnetic com-
patibility testing should form part of the commissioning and specification pro-
tocol for new dose-monitoring equipment [107].
Electromagnetic interference of pacemakers by mobile phones has also
been investigated, in particular for European GSM mobile cellular phones.
Two studies investigated about 250 different models of pacemakers from more
than 20 manufacturers [108, 109]. The result was that about 30% of patients
might have problems with a cellular phone at a distance of 10-15 cm from
the pacemaker. A distance of 20 cm is sufficient to guarantee integrity of the