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Medical Devices and Systems Exposure and Dosimetry
In addition, inside high-field MRI, RF pulses may induce thermoelastic pressure
waves inside the head, which also may contribute to auditory sensations. For a given
SAR, the peak acoustic pressures generated in the anatomic head model were essentially
the same at 64, 300, and 400 MHz (1.5, 7.0, and 9.4 T). Exposing the cochlea to 4 W/kg
generates pressures in the anatomic head that are comparable to the auditory threshold
pressure of 20 mPa of human sound perception. Moreover, results indicate that the peak
acoustic pressure in the brain is two to three times the auditory threshold of the FDA
guideline (8 W/kg). However, even at a high SAR of 20 W/kg, causing an acoustic pressure in the brain of more than seven times the auditory threshold, at the cochlea, the
sound pressure levels would not be more than 17 dB above the perception threshold (Lin
and Wang 2010).
6.3.1.3.7 Contraindications
In general, patients with metallic implants are contraindicated for MRI due to the risk
of excess heating by induced electric currents and the consequential tissue damage. In
the case of ferromagnetic implants, risks due to magneto-mechanical effects are added.
Electromagnetic compatibility problems may arise with electronic implants due to
induction of interference voltages and consequently cause malfunction or damage of
electronic components. In the case of cardiac pacemakers, the risk associated with static
magnetic field induced switching to another operating mode and/or alterations of programmed parameters is added. For functional testing, switching from the programmed
synchronous mode (triggered or demand) to the fixed-frequency asynchronous mode is
foreseen by placing a permanent magnet onto the chest wall. In static magnetic fields,
above 1 mT internal reed relays can be activated unintentionally. Therefore, pacemaker
patients should keep a safe distance large enough to assure that the static magnetic fringe
field has decreased below 0.5 mT (IEC 60601-2-33). The first MRI compatible pacemakers are becoming available.
It could be shown that meeting exposure limits of the mother does not protect the
fetus from overexposure (Figure 6.6). Exposure of the mother is limited by the partialbody local SAR 10g value. If the mother is exposed up to the limit for normal operation
conditions, the fetal SAR exceeds limits such as 1.8-fold the local SAR 10g and 7.4-fold
the fetal whole-body SAR WB . Therefore, the fetal exposure turns out to be the limiting
parameter for MRI investigations of the mother (Pediaditis, Leitgeb, and Cech 2008).
6.3.1.4 Limiting Exposure
For the general population and workers, there are already guidelines, recommendations,
standards, or legal regulations limiting exposures to nonionizing EMFs in the entire
range from static fields until the border to optical infrared radiation (ICNIRP 1998; IEEE
2002a,b, 2005; EC 1999, 2004). However, patients are exempted from such regulations.
Medical devices may expose patients even to much higher levels provided their benefit
outweighs potential risks (IEC 60601-1, IEC 60601-2-33). Manufacturers have to systematically analyze and assess risks according to existing rules (Leitgeb 2010). In medical
practice, it is the physician who is responsible to balance risks and benefits to an individual patient. With regard to MRI examinations, a three-tier approach is defined to