Esaote O-scan eXP, User Manual

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28 / 48 Chapter 3
It is generally accepted that no published evidence supporting the
occurrence of cumulative and/or long-term effects after exposure to EMF
emitted by the MR systems exists.
MR personnel and patient exposure to contact currents
Contact current is the current produced when a person comes into contact
with conductor with a different electrical potential within a
electro-magnetic field.
Conformance to the directive 2013/35/EC requires not to exceed a
threshold, defined as the action level, determined by the frequency.
Taking into consideration the set of frequencies at which the O-scan
system operates, the action level is 1.0 mA.
O-scan is however designed in conformance with the standard
IEC 60601-1, which establishes an upper limit for contact currents of
100 µA (0.1 mA) in normal operation, and of 500 µA (0.5 mA) in single
fault conditions.
This means that the action levels prescribed by the directive are never
exceeded.
Precautions against localized cutaneous heating
In general, when a patient subject to an MR examination creates closed
conductive loops (and thereby of electrical circuits) via the parts of the
body, localized cutaneous heating may occur in the vicinity of contact
points.
Closed conductive loops may also be created through contact between the
patient and the receiving coil cable and/or ECG terminals.
Currents caused by radio frequency circulate in these loops passing via
the contact surfaces: the smaller the contact surfaces, the greater the RF
heating on the patient's skin.
In fact possible burns/irritations are caused by the density of current
which increases in proportion to the decrease in size of the contact surface
passed through by the current.
The O-scan system may be critical in this case when, during an
examination of the knee or other area of the lower limb, the proximal part
of the thigh and/or calf (or other area of the lower limb) may touch the
front and/or rear frame of the gantry.
The creation of a contact surface would in fact complete a closed electric
circuit between the gantry frame (connected to the equipment mass) and
the patient, causing localized heating which increases in proportion to the
decrease in dimensions of the contact area.
Other factors may increase the likelihood of excessive local RF heating: