First Level Controlled Operating Mode
: In the First Level Controlled
Operating Mode, the performance limits are determined directly from
the statistically determined stimulation limits. Accordingly, at the
maximum performance allowed in this operating mode, up to 50 % of
all patients may experience stimulations.
Exposure to RF electromagnetic fields
During the course of an MR measurement, the patient’s body absorbs
energy from the RF field of the transmitter coil. Depending on the
type of transmitter coil used, the absorption is either concentrated
locally (when using so-called “Local RF Transmit Coils”) or relatively
uniform across the part of the body examined (when using volume
coils, for example, head, extremity, or body coil).
The Specific Absorption Rate (SAR), expressed as W/kg, serves as a
stress indicator.
Unacceptably high local SAR values may lead to RF burns. High global
SAR values (head, exposed part of the body, whole body) may lead to
overstress the patient’s thermoregulation and the cardiovascular
system.
The B1+ rms value (root mean square value of the MR-relevant
component of B1, which is indicated by the “+”) is displayed at the
syngo Acquisition Workplace for each sequence and may serve as an
indication of the RF magnetic field intensity. This can be used to
assess the risk of scanning a patient with an active or passive implant.
The energy absorbed in the course of the MR measurement warms
the tissue. The heat generated is dissipated by the thermoregulation
mechanisms of the patient, e.g. through increased perspiration and
blood flow.
The body temperature increases if the patient absorbs more energy
per unit of time than can be dissipated through thermoregulation.
The longer this condition lasts, the greater the increase in
temperature.
The increase in core body temperature is usually well below 1°C
during the course of the MR examination (if the SAR limits described
below are maintained). Nevertheless, patients with a core
temperature higher than 39.5°C must not be scanned and patients
with a core temperature higher than 39.0°C must only be scanned in
the Normal Operating Mode.
5.5.3
Warming of body tissue
5
MR system operation
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MR System | Operator Manual
Print No. M6-04401.621.02.02.02