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Medical Devices and Systems Exposure and Dosimetry
6.3.1.2 Exposure
The functional principle of MRI involves different kinds of magnetic fields and EMFs:
1. Static magnetic field (exhibiting a longitudinal gradient). The amplitude of this
field must be increased to get higher signal amplitudes, improved signal-to-noise
ratio, or a resolvable chemical frequency shift. Consequently, MRI development
goes toward higher static magnetic fields. The conventional devices today apply
1.5 T or 3 T, whereas experimental devices with fields up to 11 T are already available. Because of excessively increasing resistive heating loss, such high fields must
be generated by superconducting magnetic coils cooled down to temperatures of
liquid Helium (–269°C).
On the other hand, costs and weight of devices increase with increasing magnetic induction. Therefore, for commercial reasons, attempts are made to get sufficient image quality with lower induction. Although currently low-field MRI
already use only some 100 mT, future ultralow-field low-cost devices could operate with static magnetic fields well below 100 mT down to 1 mT, for example,
by overcoming the requirement for a constant static magnetic field (Trahms and
Burghoff 2010; Matlashov et al. 2009; Bindonisti et al. 2004; Durand et al. 2002).
2. Switched gradient fields are applied in the three spatial directions (longitudinal, transverse lateral, and transverse anterior–posterior). The shape of these
g radients varies with the applied RF pulse sequence. The gradient rise time is
typically 0.2–1 ms with rates of change between 20 and 150 Tm –1 s −1 (HPA 2008;
ICNIRP 2004).
3. RF EMFs depending on the assessed tissue parameter exhibit a complex time signature. Due to the proportionality of the magnetic resonance frequency and the static
magnetic field, the frequency of the applied RF EMF increases with the amplitude of
the static magnetic field. Conventional devices apply 63.9 MHz (1.5 T) and 127.8 MHz
(3 T). However, frequencies reached 462 MHz (11 T) in already available experimental devices. On the other hand, there has been development of low-cost MRI with low
fields that need only 12.8 MHz (300 mT); in the future with further field reductions,
frequencies may be even much lower, such as 42.58 kHz for 1 mT MRI.
RF fields are usually generated by birdcage coils driven in quadrature, which
produce circular polarized fields. Apart from its linear dependence on field amplitude, inductive heating increases with the square of frequency. Therefore, prevention from excess heating SARs is limited for different reference masses: whole
body, partial body, or local. In fact, heating is one of the limiting factors in MRI
development (Pediaditis, Leitgeb, and Cech 2008; Hand et al. 2006).
6.3.1.3 Unintended Side Effects
Because the intended purpose of MRI does not include therapeutic effects, any associated adverse health-relevant effect needs to be considered an unintended side effect.
Such effects can arise from fields used for imaging such as static magnetic fields,
switched gradients, and RF EMF pulses but include also acoustic noise, electromagnetic
interference (EMI) with other devices, and attractive forces on ferromagnetic objects.
Medical Devices and Systems Exposure and Dosimetry
6.3.1.2 Exposure
The functional principle of MRI involves different kinds of magnetic fields and EMFs:
1. Static magnetic field (exhibiting a longitudinal gradient). The amplitude of this
field must be increased to get higher signal amplitudes, improved signal-to-noise
ratio, or a resolvable chemical frequency shift. Consequently, MRI development
goes toward higher static magnetic fields. The conventional devices today apply
1.5 T or 3 T, whereas experimental devices with fields up to 11 T are already available. Because of excessively increasing resistive heating loss, such high fields must
be generated by superconducting magnetic coils cooled down to temperatures of
liquid Helium (–269°C).
On the other hand, costs and weight of devices increase with increasing magnetic induction. Therefore, for commercial reasons, attempts are made to get sufficient image quality with lower induction. Although currently low-field MRI
already use only some 100 mT, future ultralow-field low-cost devices could operate with static magnetic fields well below 100 mT down to 1 mT, for example,
by overcoming the requirement for a constant static magnetic field (Trahms and
Burghoff 2010; Matlashov et al. 2009; Bindonisti et al. 2004; Durand et al. 2002).
2. Switched gradient fields are applied in the three spatial directions (longitudinal, transverse lateral, and transverse anterior–posterior). The shape of these
g radients varies with the applied RF pulse sequence. The gradient rise time is
typically 0.2–1 ms with rates of change between 20 and 150 Tm –1 s −1 (HPA 2008;
ICNIRP 2004).
3. RF EMFs depending on the assessed tissue parameter exhibit a complex time signature. Due to the proportionality of the magnetic resonance frequency and the static
magnetic field, the frequency of the applied RF EMF increases with the amplitude of
the static magnetic field. Conventional devices apply 63.9 MHz (1.5 T) and 127.8 MHz
(3 T). However, frequencies reached 462 MHz (11 T) in already available experimental devices. On the other hand, there has been development of low-cost MRI with low
fields that need only 12.8 MHz (300 mT); in the future with further field reductions,
frequencies may be even much lower, such as 42.58 kHz for 1 mT MRI.
RF fields are usually generated by birdcage coils driven in quadrature, which
produce circular polarized fields. Apart from its linear dependence on field amplitude, inductive heating increases with the square of frequency. Therefore, prevention from excess heating SARs is limited for different reference masses: whole
body, partial body, or local. In fact, heating is one of the limiting factors in MRI
development (Pediaditis, Leitgeb, and Cech 2008; Hand et al. 2006).
6.3.1.3 Unintended Side Effects
Because the intended purpose of MRI does not include therapeutic effects, any associated adverse health-relevant effect needs to be considered an unintended side effect.
Such effects can arise from fields used for imaging such as static magnetic fields,
switched gradients, and RF EMF pulses but include also acoustic noise, electromagnetic
interference (EMI) with other devices, and attractive forces on ferromagnetic objects.