Siemens MAGNETOM Skyra fit Operator's Manual page 111

Peripheral nerve stimulation
Monitoring
Operating modes
MAGNETOM Skyra fit | syngo MR E11
Print No. M6-04401.621.02.02.02
MR system operation
Stimulation threshold: The electrical field affects the patient. If the
strength of the electrical field exceeds a certain threshold
(stimulation threshold), the patient experiences peripheral nerve
stimulation. Nerve stimulation manifests itself as e.g. tingling
sensations or slight muscle spasms in the ribs, side, abdomen, hip,
buttock, or thoracic regions, along the upper arms or the back
muscles in the shoulder region. Depending on physiological
conditions, the stimulation threshold may vary greatly from patient to
patient.
Stimulation limits: So-called stimulation limits were determined by
averaging the individual stimulation thresholds of test subjects during
an extensive clinical trial. Based on the statistical distribution, it can
be expected that up to 50 % of all patients will experience at least
mild stimulations after reaching this stimulation limit.
The MR system software includes a monitoring feature (stimulation
monitor) which monitors how close patients are to the stimulation
limit.
Look Ahead monitoring: Prior to starting an MR measurement
protocol, the stimulation monitor checks whether the stimulation
limits may be exceeded. If so, the measurement cannot be started. To
perform the examination, the parameters of the measurement
sequence must be adjusted accordingly.
Online monitoring: If the stimulation limit is exceeded while a
measurement is in progress, the active measurement is aborted.
The MR system can be operated in two operating modes which differ
with respect to different levels of stimulation.
The power limits are based on stimulation models derived from the
statistically determined stimulation limits. The higher the gradient
output is, the higher the probability and the intensity of the effects
(for example, peripheral nerve stimulation). To minimize the
occurrence of peripheral nerve stimulations, it is recommended to
operate the system in the Normal Operating Mode. However, heart
stimulations can be excluded.
Normal Operating Mode: In the Normal Operating Mode, the limit is
set to 80 % of the stimulation limit according to IEC 60601-2-33. At
the maximum performance allowed in this operating mode, the ratio
of patients affected by peripheral nerve stimulation is rather low.
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