lock out exposures
until
the operator has manually
adjusted the x-ray field to be smaller or equal to the
size of the image receptor.
The function and sizing accuracy of the PBL should
be checked at least annually or after any repairs to the
PBL sensors in the cassette holder, drive motors in the
collimator, or electronics of the PBL.
I-7. Primary Radiation Beam Characteristics
The penetrating quality of an x-ray beam is characterized
by its half value layer (HVL). Undesirable low energy x-
rays are edliminated from
the beam by adding a filter
which increases the HVL.
specified in Reference #1.
Minimum half-value layers are
These specifications are
based on recommendations for single phase
found in Reference #2.
generators
The minimum required half value
layers for diagnostic x-ray equipment are listed in Table
I below (Reference #l).
The minimum HVL requirements can be met if the
equipment is installed
with
the proper
amount of
filtration in the x-ray tube housing and collimator. The
total equivalent Aluminum filtrations that will meet the
minimum HVL requirements of Table I are listed in Table
II (Reference #2).
No maximum
total Aluminum filtration values are
established.
However,
excessive
filtration needlessly
reduces the x-ray intensity with little gain in the
effective beam energy and possible shortened x-ray
tube
life.
The HVL is a function of x-ray tube age (use), the
tube voltage waveform, and the kVp.
As the x-ray tube
ages,
the target's surface pits and becomes rough which
increases the inherent
filtration of the tube.
Therefore, if the kV calibration of the generator and the
added filtration remain constant
during the lifetime of
the tube, the HVL will increase and the radiation output
will decrease.
Thus,
a modest change in HVL from one
survey to the next does not necessarily mean that the kV
calibration has changed.
The values for minimum HVL in Table I under the
"Other X-ray systems"
heading for the
kVp
apply to dedicated mammographic units
range of 30-50
(Reference #6).
The radiologic physicist
added filtration is
should verify that Molybdenum
used
with
anode
mammographic x-ray tubes if
the
Molybdenum
machine
allows the
operator to choose,
Materials other
than Aluminum (Copper,
and rare earth elements),
Molybdenum,
or specialty filters (trough
filters, and wedge filters) are occasionally used. If
such a filter is not permanent, it must be removed during
11
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Kommentare zu diesen Handbüchern