capabilities.
The 38 cm SSD also applies to lateral
plane x-ray tubes found in biplane fluoroscopic systems.
Since automatic collimators
required on
certified
stationary fluoroscopic units
are usually large in size,
the minimum SSD in these
cases
usually
cannot be
violated.
In fact, many current under table fluoroscopic
units are designed with an SSD of 46 cm (18").
The Federal Standard makes one exception to the
mobile image-intensified fluoroscopic
SSD when used for
specific surgical application. The
minimum SSD may be
reduced to as little as
20 cm (8") (Reference #l).
Please note that this
standard applies only to specific
surgical situations.
Therefore,
the reduced SSD should
be allowed only if the surgeon can demonstrate that a 30
cm (12”) SSD
would
render
the
specific procedure
impossible.
Be aware that the surgeon is concerned with
surgical procedures and may be unaware
of the high
patient skin exposure at short SSDs.
II-5-A. Measurement of SSD
On a mobile C-arm type fluoroscopic systems, the SSD
can be directly measured if the location of the focal
spot is known.
If this location
is not marked on the x-
ray tube housing,
triangulation (Section I-5-A) can be
used to locate the
position
of
the
focal spot.
Triangulation may be
used to measure the SSD on an
undertable X-ray tube fluoroscope.
One radiopaque ruler
is placed on the table top and another is taped on the
underside of the spot film device
(Reference #5). Their
relative magnification can be varied by
raising or
lowering the image
intensifier
tower.
When
the
fluoroscopic image of the markings on the tabletop ruler
are twice those on the ruler taped to the spotfilm
device, the vertical distance between the rulers is equal
to the SSD.
This method can be used if the SSD exceeds
the maximum vertical distance between the tabletop and
spotfilm device by using rulers with appropriately spaced
markings and an appropriate multiplier.
If this test is
completed when the
equipment is
new,
it should be
repeated only if the type of x-ray
tube or its mounting
is changed.
CAUTION:
The image intensifier must be protected from
excessive radiation levels during the test to protect the
TV camera.
This can be achieved by using the automatic
brightness control mode and by placing l-2 mm copper or
l-2 in Aluminum sheets of appropriate cross section on
the table top.
24
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