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Medical Devices and Systems Exposure and Dosimetry
with consequential risks. In addition, inside the patient, ferromagnetic implants such
as aneurysm clips could be twisted, rip the blood vessel, and cause even fatal internal
bleeding (Sommer et al. 2004). Such risks can occur already in static fields of several millitesla. Therefore, gates are installed preventing metallic objects from approaching the
magnet. However, experience shows that a residual risk might still remain, for example,
if in an emergency case a ferromagnetic fire extinguisher is brought into the vicinity of
the magnet (ICNIRP 2009a, IEC 2010).
6.3.1.3.2 Switched Magnetic Gradient Fields
Time-varying magnetic fields induce electric currents in conducting objects. Induced
intracorporal current densities may become large enough to stimulate nerve and
muscle cells and hence cause involuntary movements and/or painful sensations. This
limits enhancing switched magnetic gradients. In addition, they contribute to potential excess heating. Their relative contribution to heating increases with decreasing
static field strength (and decreasing RF frequency); however, their contribution can
be neglected in comparison to RF EMF heating. Therefore, the safety standard of MRI
devices limits gradient fields, in particular, the time derivative dB/dt, which depends
on the duration and mode of device operation (IEC 60601-2-33) because of their stimulatory effects.
6.3.1.3.3 Pulsed Radio-Frequency Electromagnetic Fields
In MRI, the frequency of RF EMFs extends from 42.6 kHz until 426 MHz. Their energy
is absorbed by tissues and converted to heat. The exposure is usually characterized by
the average power deposited per unit mass, which is called specific absorption rate and is
given in watt per kilogram. Depending on the kind of exposure, the reference mass for
SAR assessment may be the whole body (SAR WB ), partial body such as head, trunk, and
extremities, or local such as any 1 g (SAR 1g ) or 10 g (SAR 10g ) tissue mass. In pregnant
women, the fetus merits specific attention. Therefore, prevention from excess heating
as described by SAR values becomes one of the limiting factors in MRI development
(Pediaditis, Leitgeb, and Cech 2008; Hand et al. 2006).
Numerical investigations into an anatomical model of a pregnant woman (SILVY)
showed that for the same magnetic induction in the center of the birdcage coils, the SARs
exhibit a quadratic increase with frequency (Figure 6.5). The largest local SAR 10g values
were found in the extremities. They exceeded the whole-body SAR by about 24-fold.
The whole-body SAR WB of the mother and the fetus were almost similar; however, high
partial-body SAR values were identified in the trunk of the mother. Local SAR 10g values
of the fetus were about fourfold smaller (Pediaditis, Leitgeb, and Cech 2008).
6.3.1.3.4 Contrast Agents
To improve contrast in magnetic resonance images, the most commonly used contrast
agents are based on chelates of gadolinium (Gd), which are delivered intravenously.
However, due to their unique magnetic properties and the ability to function at the cellular and molecular level of biological interactions, magnetic nanoparticles have become
an attractive alternative both as contrast agents for MRI and as carriers for drug delivery.
Medical Devices and Systems Exposure and Dosimetry
with consequential risks. In addition, inside the patient, ferromagnetic implants such
as aneurysm clips could be twisted, rip the blood vessel, and cause even fatal internal
bleeding (Sommer et al. 2004). Such risks can occur already in static fields of several millitesla. Therefore, gates are installed preventing metallic objects from approaching the
magnet. However, experience shows that a residual risk might still remain, for example,
if in an emergency case a ferromagnetic fire extinguisher is brought into the vicinity of
the magnet (ICNIRP 2009a, IEC 2010).
6.3.1.3.2 Switched Magnetic Gradient Fields
Time-varying magnetic fields induce electric currents in conducting objects. Induced
intracorporal current densities may become large enough to stimulate nerve and
muscle cells and hence cause involuntary movements and/or painful sensations. This
limits enhancing switched magnetic gradients. In addition, they contribute to potential excess heating. Their relative contribution to heating increases with decreasing
static field strength (and decreasing RF frequency); however, their contribution can
be neglected in comparison to RF EMF heating. Therefore, the safety standard of MRI
devices limits gradient fields, in particular, the time derivative dB/dt, which depends
on the duration and mode of device operation (IEC 60601-2-33) because of their stimulatory effects.
6.3.1.3.3 Pulsed Radio-Frequency Electromagnetic Fields
In MRI, the frequency of RF EMFs extends from 42.6 kHz until 426 MHz. Their energy
is absorbed by tissues and converted to heat. The exposure is usually characterized by
the average power deposited per unit mass, which is called specific absorption rate and is
given in watt per kilogram. Depending on the kind of exposure, the reference mass for
SAR assessment may be the whole body (SAR WB ), partial body such as head, trunk, and
extremities, or local such as any 1 g (SAR 1g ) or 10 g (SAR 10g ) tissue mass. In pregnant
women, the fetus merits specific attention. Therefore, prevention from excess heating
as described by SAR values becomes one of the limiting factors in MRI development
(Pediaditis, Leitgeb, and Cech 2008; Hand et al. 2006).
Numerical investigations into an anatomical model of a pregnant woman (SILVY)
showed that for the same magnetic induction in the center of the birdcage coils, the SARs
exhibit a quadratic increase with frequency (Figure 6.5). The largest local SAR 10g values
were found in the extremities. They exceeded the whole-body SAR by about 24-fold.
The whole-body SAR WB of the mother and the fetus were almost similar; however, high
partial-body SAR values were identified in the trunk of the mother. Local SAR 10g values
of the fetus were about fourfold smaller (Pediaditis, Leitgeb, and Cech 2008).
6.3.1.3.4 Contrast Agents
To improve contrast in magnetic resonance images, the most commonly used contrast
agents are based on chelates of gadolinium (Gd), which are delivered intravenously.
However, due to their unique magnetic properties and the ability to function at the cellular and molecular level of biological interactions, magnetic nanoparticles have become
an attractive alternative both as contrast agents for MRI and as carriers for drug delivery.