intersecting at the center of the field of view shall not
exceed 4 percent of the SID (Reference #1).
II-6-C. Evaluation of Spotfilm Alignment
Steps 13-19 of Section II-6-A allow evaluation of
the alignment of the central
ray of the x-ray field and
the center of each selected portion of the image receptor
within the spotfilm device as a function of the SID. On
each image, the diagonals drawn from the corner of each
darkened area mark the center of the x-ray field. The
center of the
selected portion
of the
film can be
determined from diagonals
and measurements on the film.
The alignment of the x-ray field center and center of the
selected portion of the image
receptor in the spotfilm
device should agree to within 2% of the SID (Reference
#1).
II-6-D. Evaluation of
Spotfilm
Field Sizing
Steps 20-27 of Section II-6-A allow evaluation of
the size of the x-ray field with respect to the size of
each selected portion
of the image receptor within the
spotfilm
device as a function of the SID.
On the images
one can measure the x-ray field size at the location of
the test tool. The
magnification
factor
can be
calculated since the following distances are known:
(a) From focal spot to tabletop (Section II-5),
(b) From tabletop to test tool top, and
(c) From test tool top to image receptor.
The magnification factor is applied to the x-ray field at
the test tool
to calculate the field size at the image
receptor level in the spotfilmer.
Either the length or
the width of the x-ray field shall not differ by more
than 3% of the SID with respect to the length or the
width of the selected portion of the image receptor. The
sum of the absolute values
of these length and width
differences shall not exceed 4% (Reference #1).
CAUTION: This parameter cannot be measured by evaluating
the degree of overlap of individual exposure fields made
on a loaded cassette placed in the spot film device.
Many units contain a
second square or
rectangular
diaphragm between the patient and image receptor in the
spotfilm
device.
If present,
this diaphragm reduces the
x-ray field to the appropriate size at the image receptor
in the
spotfilm
device if the x-ray field size is too
large at the patient position.
28
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Kommentare zu diesen Handbüchern