First Level Controlled Operating Mode: In the First Level Controlled
Operating Mode, patients may experience noticeable stress levels
depending on the measurement programs selected. This usually
includes perspiration accompanied by an increase in pulse rate.
Patients with reduced thermoregulatory capability and higher
sensitivity toward increases in body temperature (e.g. patients with
fevers or cardiac decompression, patients with perspiratory
impairments, or pregnant women) would experience additional
effects. Therefore, these patients must not be examined in First Level
Controlled Operating Mode.
Stimu: Exposure to low frequency electromagnetic
fields
During the measurement, patients are exposed to an electrical field
created by the time-varying magnetic fields of the gradient coils.
Assuming all other conditions remain constant, the strength of the
electrical field is directly proportional to the change of the magnetic
flux (dB/dt).
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.
5.5.3
Peripheral nerve stimulation
Monitoring
MR system operation 5
MAGNETOM Avanto, MAGNETOM Verio | syngo MR E11
121
Print No. MR-01501G.621.02.02.02
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