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frequency range. Due to the large variety of environmental RF signals in this frequency range
that may interfere with the desired signal, the RF shield is an integral part of any MR system.
As RF interference is a very common artifact in MR images, the RF shielding should be thor-
oughly tested.
The RF shield, in essence, is a Faraday cage completely surrounding the MR scanner.
Most commonly this shield is in the form of copper sheeting in the walls, ceiling, and floor;
copper mesh in the window(s); and specially designed RF doors which, when closed, maintain
the integrity of the shield. Most vendors specify the performance of the shield in terms of
attenuation (in decibels [dB]) at a particular frequency. For example, for
≤1.5 tesla (T) scan-
ners, common acceptance criteria are 100 dB attenuation at 100 MHz plane-wave. (The test
frequency typically increases to 150–170 MHz for 3.0T systems.) In addition, the shield is
typically required to be electrically isolated from building ground (before scanner installation)
at DC frequency. (A typical requirement is 1 kΩ isolation at DC.) Note that both of these cri-
teria are specified for tests performed before the MR scanner is actually installed. Therefore,
the testing must be performed after the completion of the RF shield (most commonly by a
shielding vendor, not the MR system vendor), but before the MR scanner is installed and the
MR scan room construction completed.
The actual test of RF shield performance is accomplished by placing an antenna on one
side of the shield and broadcasting RF test signals (at the agreed-upon test frequency) through
the shield. A second RF antenna on the other side of the shield is used as the receiver and the
attenuation of the signal is determined by comparing these signals to unattenuated reference
signals obtained, for example, through the open RF door. Therefore, the test requires a fre-
quency generator, RF amplifier, two tuned antennas, and a spectrum analyzer. Such equipment
is not commonly available to the practicing medical physicist. Hence, this test is typically per-
formed by the shield vendor or an independent contractor. However, medical physicist
involvement is quite important in the thorough testing of the shielded room. The medical
physicist should ensure that the weak points of the room, not just the strongly attenuating
regions, are tested and the results documented on a drawing of the room under test. Weak
points are most commonly the RF door, the window(s), the area near the cryogen vent pene-
tration through the shield, the shield penetrations for the room ventilation ducts, and the pen-
etration panel where the MR scanner connections to the rest of the system are ultimately
made. If a dry pipe fire sprinkler system is not used, the RF shield test should be performed
with water in the sprinkler system. The physicist should insist on a copy of the test results, not
just the certificate of compliance, and such results should be made part of the acceptance test
and maintained in the facility.
Recall, however, that this test is performed after the RF shield is in place but before the
room is completed and the scanner installed. During scanner installation, a section of the
shield must be removed to install the MR system. The shield is then closed again and the con-
tractors complete the finish-out of the room. In finishing out the room, it is not uncommon for
construction workers to accidentally short the RF shield to ground by, for example, driving a
nail or screw through a grounded structure and into the RF shield. The shield integrity will be
violated by this action and, if undetected until the first MR images demonstrate excessive RF
noise, such a mistake can be difficult to find and repair. Therefore, to assist in preventing such
problems, a common alarm circuit can be installed after the shield tests. Essentially, the alarm
AAPM REPORT NO. 100