ViewRay Inc. MRIdian® system L-0009 Release 4.5 45
Chapter 2: Safety Information and
2.10.1 Warming of Body Tissue
During the radio frequency pulses that are part of the MR scan, the patient’s body is exposed
to RF output in the form of electromagnetic radiation. The output absorbed by the patient
depends on body weight and is indicated as the
specific absorption rate (SAR), expressed as
W/kg.
The specific absorption rate obtainable with the MRIdian® system is set to
the acceptable limits for normal operating mode, according to currently
publi
shed literature including IEC guidelines.
The body temperature increases when the SAR r
ate exceeds the limit set by the body’s
cooling capacity. The patient’s ability to dissipate surplus heat is made more difficult as the
room temperature and relative humidity increase. Ensure the room temperature does not
exceed 24C (75F) and the relative humidity does not exceed 60%.
The use of an air conditioning system and/or appropriate temperature control devices may be
necessar
y to ensure the appropriate temperature and humidity.
If the room temperature and/or relative humidity is higher than the levels
specified above, compliance with the SAR limits according to FDA (Food and
Drug Administration) or IEC (International Electrotechnical Commission)
regulations may no longer be assured.
The Operator is responsible for monitoring the functionality of the air conditioning system as
w
ell as the temperature and relative humidity inside the treatment room for the comfort and
safety of patients.
The energy absorbed in the course of MR imaging warms the tissue. The heat generated is
dissip
ated by the thermoregulation mechanisms of the patient, such as, increased
perspiration and blood flow.
Unacceptably high local SAR values may lead to RF burns. At high values, the SAR that is
e
xposed evenly across the body adversely affects the patient's thermoregulation and the
cardiovascular system.
The body temperature increases if the patient absorbs more energy per unit of time than can
be dissip
ated through thermoregulation. The longer this condition lasts, the greater the
increase in temperature.
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