Esaote O-scan eXP, Руководство пользователя

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24 / 48 Chapter 3
Patient
The field intensity increases on approach to the side walls of the gantry,
where the gradient coils are located, and therefore - during examinations
of larger size patients - it is necessary to minimize the dB/dt influence on
parts of the body not being examined, but that are close to the gantry
walls during the examination.
In particular, during examinations of the knee, the following precautions
must be observed:
MR Personnel
In the worst conditions, MR worker are exposed to an electric field E value
equal to the gradient output limit, i.e. 4.20 V/m.
The user must ensure observation of all state, federal and/or local
regulations governing exposure of personnel to time varying magnetic fields.
The user must also bear in mind that the limits of MR worker exposure to
electromagnetic fields may not apply to pregnant women.
In some nations, there may be a requirement that the fetus is not exposed
to electromagnetic fields exceeding the limit specified for the public.
In any event the user is recommended to envisage special precautions for
pregnant employees, even though there is currently no epidemiological
proof of negative health effects.
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.
Conformance to directive 2013/35/EC requires that fields induced by the
gradient do not exceed a threshold dependent on the frequency, defined
as the action level.
This action level is a decreasing function of the frequency up to 10 Mhz.
However, for the O-scan system, the most rapid variation possible, which
corresponds to the current rising edge, has a estimated maximum
frequency of 5 kHz, which determines action values of the fields induced
by the gradient equal to 0.1 mT for the whole body and head and 0.3 mT
for the limbs.
The following graphs show the spatial distribution of the field gradient
along the Z axis of the magnet (front/rear axis relative to the front view of
the magnet).
W a r n i n g
To avoid excessive dB/dt exposure of the thigh during knee
examinations, especially for larger patients, place the insulating strap
around the thigh. Otherwise place standard cushions between the
gantry frame and thigh.