359
Medical Devices and Systems Exposure and Dosimetry
• Maintaining a safe distance from RF EMF emitting devices. In the frequency
range 800 MHz–2.5 GHz, depending on the RF EMF output power P and the
immunity level E i , the medical devices should maintain a safe distance d:
k × P
d =
(6.7)
E i
with k = 23 for life-supporting and k = 7 for all other medical devices. As an
example, this results in a safe distance of medical devices to GSM handsets of
1.2 m for 900 MHz (output power, 0.25 W) or 0.8 m for 1.8 GHz. Walkie talkies
used by rescue services should even be kept at a distance of 5.1 m (IEC 60601-1-2)
• Immunity of a device cannot be recognized from its appearance. Therefore, maintaining a safe distance is an important preventive measure. Because walls are no
major shield, this needs to be observed also in rooms and aisles adjacent to critical
locations (e.g., intensive care units)
• Reducing the ambient field level in locations with critical assessment of biosignals.
Standards limit rms power frequency magnetic levels for biosignal recording to
141 nT for electrocardiograms (ECG), 71 nT for electroencephalograms (EEG),
and 35 nT for electromyograms (EMG) (ÖVE E 8007). This requires already
timely measures such as appropriate site selection, design, construction, and layout of health care facilities (Leitgeb 2010; Cesar, Ramos, and Tahan 2009).
6.5 Summary
Medical applications make use of EMFs over a wide range of frequencies and amplitudes
(Figure 6.9). Medical device development is characterized by the fact that exposure of
the patient may not only be caused by external devices operated at some distance or
in contact to the body such as transmitting coils for delivering energy and commands
to implanted stimulators. Medical devices may also be incorporated, and they become
EMF sources in close contact to tissue and organs such as transmitters of electronic
implants for data exchange, permanent magnets for navigation, or magnetic nanoparticles for tracking, targeting, heating, and MRI contrast enhancement.
Magnetic therapy devices apply static and alternating magnetic fields in the frequency
range of a few Hz until several kHz. With regard to amplitudes, two ranges can be differentiated: Weak-field applications below existing limits with lacking evidence of clinical efficiency, and emissions above limits usually for wound healing and bone fracture
treatment. Diathermy and hyperthermia applications are restricted to ISM frequencies
and generate intended exposures of the patient well above limits but may also cause relevant exposures of medical staff and unintended exposure of sensitive regions within the
patient. The use of ferromagnetic parts and superparamagnetic (nano)particles offered
new possibilities of targeted EMF heating for cancer treatment or drug delivery and
moved tumor tracking. Required RF EMF amplitudes increase with decreasing size of
particles. A major EMF source is MRI imaging with high static magnetic fields and
intense RF EMF as well as high magnetic gradients with development toward higher
static fields as well as toward low-cost low-field devices.
Medical Devices and Systems Exposure and Dosimetry
• Maintaining a safe distance from RF EMF emitting devices. In the frequency
range 800 MHz–2.5 GHz, depending on the RF EMF output power P and the
immunity level E i , the medical devices should maintain a safe distance d:
k × P
d =
(6.7)
E i
with k = 23 for life-supporting and k = 7 for all other medical devices. As an
example, this results in a safe distance of medical devices to GSM handsets of
1.2 m for 900 MHz (output power, 0.25 W) or 0.8 m for 1.8 GHz. Walkie talkies
used by rescue services should even be kept at a distance of 5.1 m (IEC 60601-1-2)
• Immunity of a device cannot be recognized from its appearance. Therefore, maintaining a safe distance is an important preventive measure. Because walls are no
major shield, this needs to be observed also in rooms and aisles adjacent to critical
locations (e.g., intensive care units)
• Reducing the ambient field level in locations with critical assessment of biosignals.
Standards limit rms power frequency magnetic levels for biosignal recording to
141 nT for electrocardiograms (ECG), 71 nT for electroencephalograms (EEG),
and 35 nT for electromyograms (EMG) (ÖVE E 8007). This requires already
timely measures such as appropriate site selection, design, construction, and layout of health care facilities (Leitgeb 2010; Cesar, Ramos, and Tahan 2009).
6.5 Summary
Medical applications make use of EMFs over a wide range of frequencies and amplitudes
(Figure 6.9). Medical device development is characterized by the fact that exposure of
the patient may not only be caused by external devices operated at some distance or
in contact to the body such as transmitting coils for delivering energy and commands
to implanted stimulators. Medical devices may also be incorporated, and they become
EMF sources in close contact to tissue and organs such as transmitters of electronic
implants for data exchange, permanent magnets for navigation, or magnetic nanoparticles for tracking, targeting, heating, and MRI contrast enhancement.
Magnetic therapy devices apply static and alternating magnetic fields in the frequency
range of a few Hz until several kHz. With regard to amplitudes, two ranges can be differentiated: Weak-field applications below existing limits with lacking evidence of clinical efficiency, and emissions above limits usually for wound healing and bone fracture
treatment. Diathermy and hyperthermia applications are restricted to ISM frequencies
and generate intended exposures of the patient well above limits but may also cause relevant exposures of medical staff and unintended exposure of sensitive regions within the
patient. The use of ferromagnetic parts and superparamagnetic (nano)particles offered
new possibilities of targeted EMF heating for cancer treatment or drug delivery and
moved tumor tracking. Required RF EMF amplitudes increase with decreasing size of
particles. A major EMF source is MRI imaging with high static magnetic fields and
intense RF EMF as well as high magnetic gradients with development toward higher
static fields as well as toward low-cost low-field devices.