Caution:
Some PBL collimators do not have a key designed
to-disable their automatic sizing as required in step 3
of Method II.
If this key is not provided and the
automatic collimation feature cannot be readily disabled,
method I or III must be used. Method I has the same
precision,
but less accuracy than Method II. Method III
is both less precise and less accurate than Method II.
Regardless which method is used, the PBL sizing
evaluation should be completed by placing the appropriate
size cassettes in each cassette holder (eg.
table, wall
unit, other, etc.)
which
is interfaced to the PBL
collimator.
If any of these cassette holders are used at
more than one SID, the PBL sizing for each commonly used
SID should be checked.
The above methods cannot evaluate the PBL sizing for
a 35 cm x 43 cm cassette without additional effort.
Special PBL testing
devices have been fabricated to
address this and the numerous films which must be made.
Lin, Kriz and Storzum (Reference #5) have described one
device. Hendee and Rossi (Reference #3) and Gray et. al.
(Reference #4) describe test methods similar to Method I.
The measured length or width of the x-ray field in
the plane of the image receptor compared to the length or
width respectively of the film in the cassette tray must
agree to within 3% of the SID used.
In addition, the
absolute value of
the sum of the length and width
differences must not exceed 4% of the SID (Reference #l).
If the collimator is designed to allow the operator
to override the PBL function, a key must be
ided for
this function.
prov
When the PBL function is overridden, the
collimator must not allow removal of the key (Reference
#1).
PBL collimators
allow the operator
reduce the x-ray field
size with
to manually
receptor in the
cassette holder without
respect to the image
override key,
using the
When this is done, the collimator should
revert to the normal PBL mode when the cassette or SID is
changed (Reference #l).
Both
functions
should be checked.
Most units are designed to bypass the PBL function
when the central x-ray is not perpendicular to the image
receptor.
As above,
the collimator should revert to the
normal PBL mode when the
central
perpendicular to the
image receptor.
ray is realigned
This function
should also be checked.
Some units
are equipped with semi-automatic PBL
collimators which do not automatically adjust the x-ray
field size to the image receptor size.
Instead, they
10
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