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Electromagnetic Fields in Biological Systems
6.3.2 Transcranial Magnetic Stimulation
In medical diagnosis, TMS is a noninvasive method to study brain functions and functional connectivity, for example, to investigate motor pathways or to evaluate diagnostic
parameters such as central motor conduction time (CMCT), motor threshold (MT), motor
evoked potential (MEP) amplitude, central silent period threshold (cSPT), and duration.
The CMCT is a sensitive parameter to detect myelopathic abnormalities even in the
case of negative radiological findings. It may also be useful to detect upper motor neuron
involvement in amyotrophic lateral sclerosis. The diagnostic sensitivity may be increased
by using the triple stimulation technique (TST), by combining several parameters such
as CMCT, MT, and silent period, or by studying multiple muscles. In peripheral facial
nerve palsies, TMS may be used to localize the site of nerve dysfunction and clarify
the etiology. TMS parameters may also help in detecting lesions in multiple sclerosis.
Abnormalities in CMCT or TST may correlate with motor impairment and disability.
Cerebellar stimulation may detect lesions in the cerebellum or the cerebellar output
pathway (Chen et al. 2008). TMS may also be useful in diagnosis of neurophysiological
disorders such as Parkinson and Alzheimer diseases. The ipsilateral silent period that
measures transcallosal inhibition is a potential method to distinguish between different
Parkinson syndromes. Prolonged CMCT might help identifying the genetic Parkinson
type (Perretti et al. 2010). Short-latency afferent inhibition (SAI), which is related to
central cholinergic transmission, is reduced in Alzheimer’s disease.
TMS may also be used to create a map of the somatotopic organization of the motor
cortex. Advantageous over other noninvasive brain imaging techniques is the functional
response of the stimulated cortical area. TMS has been shown to be effective in mapping
cortical motor areas and in the functional assessment of motor tracts, for example, in
the follow-up of recovery from stroke (Ruohonen and Karhu 2010).
The amplitudes of magnetic fields required for diagnostic TMS are lower than for
TMS therapy. The amplitude of the magnetic gradient depends on pulse time and depth
of the stimulation target zone but needs to exceed 20 T/s.
6.3.3 Magnetic Tracking
Magnetic tracking systems determine the location of objects (e.g., a catheter) that contain a
magnetic sensor or marker. This requires a magnetic field of known geometry generated by
orthogonal field coils. In spite of the use of magnetic fields, due to widespread unfortunate
habit, this technique is frequently called electromagnetic tracking (EMT). In contrast to an
optical tracking system, this approach does not need line of sight and hence allows tracking of intracorporal objects. When an object is placed inside the magnetic field, voltages
are induced in the sensor, which are indicating the position and orientation of the object.
Sensors might be coils in which signals are induced either by 8–12 kHz AC fields or by
switched DC fields. Another solution is tracking the position of a permanent magnet or transponder incorporated in the medical device. Inaccuracies might occur if the tracking field is
disturbed by the presence of metallic objects or EMI (Zhang et al. 2006; Wagner et al. 2002).
Magnetic tracking is applied for a variety of applications such as minimally invasive
surgery, catheterization, and assisting radiation therapy by tracking movable tumors
(Keall et al. 2010).
Electromagnetic Fields in Biological Systems
6.3.2 Transcranial Magnetic Stimulation
In medical diagnosis, TMS is a noninvasive method to study brain functions and functional connectivity, for example, to investigate motor pathways or to evaluate diagnostic
parameters such as central motor conduction time (CMCT), motor threshold (MT), motor
evoked potential (MEP) amplitude, central silent period threshold (cSPT), and duration.
The CMCT is a sensitive parameter to detect myelopathic abnormalities even in the
case of negative radiological findings. It may also be useful to detect upper motor neuron
involvement in amyotrophic lateral sclerosis. The diagnostic sensitivity may be increased
by using the triple stimulation technique (TST), by combining several parameters such
as CMCT, MT, and silent period, or by studying multiple muscles. In peripheral facial
nerve palsies, TMS may be used to localize the site of nerve dysfunction and clarify
the etiology. TMS parameters may also help in detecting lesions in multiple sclerosis.
Abnormalities in CMCT or TST may correlate with motor impairment and disability.
Cerebellar stimulation may detect lesions in the cerebellum or the cerebellar output
pathway (Chen et al. 2008). TMS may also be useful in diagnosis of neurophysiological
disorders such as Parkinson and Alzheimer diseases. The ipsilateral silent period that
measures transcallosal inhibition is a potential method to distinguish between different
Parkinson syndromes. Prolonged CMCT might help identifying the genetic Parkinson
type (Perretti et al. 2010). Short-latency afferent inhibition (SAI), which is related to
central cholinergic transmission, is reduced in Alzheimer’s disease.
TMS may also be used to create a map of the somatotopic organization of the motor
cortex. Advantageous over other noninvasive brain imaging techniques is the functional
response of the stimulated cortical area. TMS has been shown to be effective in mapping
cortical motor areas and in the functional assessment of motor tracts, for example, in
the follow-up of recovery from stroke (Ruohonen and Karhu 2010).
The amplitudes of magnetic fields required for diagnostic TMS are lower than for
TMS therapy. The amplitude of the magnetic gradient depends on pulse time and depth
of the stimulation target zone but needs to exceed 20 T/s.
6.3.3 Magnetic Tracking
Magnetic tracking systems determine the location of objects (e.g., a catheter) that contain a
magnetic sensor or marker. This requires a magnetic field of known geometry generated by
orthogonal field coils. In spite of the use of magnetic fields, due to widespread unfortunate
habit, this technique is frequently called electromagnetic tracking (EMT). In contrast to an
optical tracking system, this approach does not need line of sight and hence allows tracking of intracorporal objects. When an object is placed inside the magnetic field, voltages
are induced in the sensor, which are indicating the position and orientation of the object.
Sensors might be coils in which signals are induced either by 8–12 kHz AC fields or by
switched DC fields. Another solution is tracking the position of a permanent magnet or transponder incorporated in the medical device. Inaccuracies might occur if the tracking field is
disturbed by the presence of metallic objects or EMI (Zhang et al. 2006; Wagner et al. 2002).
Magnetic tracking is applied for a variety of applications such as minimally invasive
surgery, catheterization, and assisting radiation therapy by tracking movable tumors
(Keall et al. 2010).