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Medical Devices and Systems Exposure and Dosimetry
• Exposure to pulsed fields from a body-worn device to operate the medical implant
(e.g., to trigger pulse delivery of implanted functional stimulators and stimulate
nerves (e.g., for breathing) or control skeletal muscles)
• Continuous wireless transmission of signals to achieve the intended use (e.g.,
acoustic signals received by the belt-worn master device to the in-the-ear
hearing aid)
• Remote wireless data transmission to read out stored data or to monitor devices’
operation, for which active implants are equipped with functions, for example, for
saving events, parameter sequences, and electrocardiograms
• Occasional EMF transmission to (re)program implanted devices (e.g., cardiac
pacemakers or cardioverter defibrillators)
6.3 Medical Diagnosis
6.3.1 Magnetic Resonance Imaging
6.3.1.1 Principle
MRI has become a major medical imaging modality, which allows noninvasive anatomic, functional, and molecular imaging with still growing areas of application.
The operation principle of MRI makes use of intrinsic nuclear magnetic moments, in
particular of hydrogen. These are usually in random positions with magnetic fields compensating each other. If a strong external static magnetic field is applied (e.g., along the
body axis), these magnetic moments align either parallel or antiparallel to it. Because
there is a surplus of parallel alignment, the atomic contributions add to a macroscopic
magnetization, which can be measured. The efficiency of alignment (and magnetization
amplitude) increases with increasing external static magnetic induction.
If an adequate (resonant) external RF EMF is acting on the aligned nuclei, they are
able to absorb energy and start spinning around the field lines of the external static field.
This leads to precession of the macroscopic magnetization. However, due to basic atomic
laws, nuclear absorption of RF EMF quantum energy is only possible in case of (magnetic)
resonance. It occurs if the frequency of the RF EMF equals the nucleus-specific Larmor
frequency ω L . For hydrogen, ω L amounts to 42.58 MHz/T. This resonance frequency
strictly depends on the atomic gyromagnetic ratio γ and the magnetic field B at the site
of the nucleus according to the following:
ω L = γ ⋅ B
(6.6)
The spinning nuclei generate a macroscopic magnetization, which is inclined relative
to the direction of the external field and hence exhibits a transverse magnetic component. The angle of inclination (flip angle) increases proportional to the duration of the
RF pulse. In a receiving coil, the transversal RF EMF component induces a damped
sinusoidal signal, which is used as primary information for MRI.
One of the advantages of MRI is that more than just one tissue-characterizing parameter can be derived from the amplitude and time course of the received signal.
Medical Devices and Systems Exposure and Dosimetry
• Exposure to pulsed fields from a body-worn device to operate the medical implant
(e.g., to trigger pulse delivery of implanted functional stimulators and stimulate
nerves (e.g., for breathing) or control skeletal muscles)
• Continuous wireless transmission of signals to achieve the intended use (e.g.,
acoustic signals received by the belt-worn master device to the in-the-ear
hearing aid)
• Remote wireless data transmission to read out stored data or to monitor devices’
operation, for which active implants are equipped with functions, for example, for
saving events, parameter sequences, and electrocardiograms
• Occasional EMF transmission to (re)program implanted devices (e.g., cardiac
pacemakers or cardioverter defibrillators)
6.3 Medical Diagnosis
6.3.1 Magnetic Resonance Imaging
6.3.1.1 Principle
MRI has become a major medical imaging modality, which allows noninvasive anatomic, functional, and molecular imaging with still growing areas of application.
The operation principle of MRI makes use of intrinsic nuclear magnetic moments, in
particular of hydrogen. These are usually in random positions with magnetic fields compensating each other. If a strong external static magnetic field is applied (e.g., along the
body axis), these magnetic moments align either parallel or antiparallel to it. Because
there is a surplus of parallel alignment, the atomic contributions add to a macroscopic
magnetization, which can be measured. The efficiency of alignment (and magnetization
amplitude) increases with increasing external static magnetic induction.
If an adequate (resonant) external RF EMF is acting on the aligned nuclei, they are
able to absorb energy and start spinning around the field lines of the external static field.
This leads to precession of the macroscopic magnetization. However, due to basic atomic
laws, nuclear absorption of RF EMF quantum energy is only possible in case of (magnetic)
resonance. It occurs if the frequency of the RF EMF equals the nucleus-specific Larmor
frequency ω L . For hydrogen, ω L amounts to 42.58 MHz/T. This resonance frequency
strictly depends on the atomic gyromagnetic ratio γ and the magnetic field B at the site
of the nucleus according to the following:
ω L = γ ⋅ B
(6.6)
The spinning nuclei generate a macroscopic magnetization, which is inclined relative
to the direction of the external field and hence exhibits a transverse magnetic component. The angle of inclination (flip angle) increases proportional to the duration of the
RF pulse. In a receiving coil, the transversal RF EMF component induces a damped
sinusoidal signal, which is used as primary information for MRI.
One of the advantages of MRI is that more than just one tissue-characterizing parameter can be derived from the amplitude and time course of the received signal.