Fluke Biomedical 07-611 - Manual (Page 17)

Operation
Procedure-Star MeasurementTechnique
2
2-11
[ ] ]
For the transverse dimension:
D
2
= 34 mm
2 34
F
2
=
57.3
x
1.18
=
1.01
mm
The focal spot is 0.8 mm x 1.0 mm at the kVp and mA setting used.
10. Since these measurements yield the equivalent homogenous focal spot size they cannot be
compared directly to specified tolerances in Table 2-2, or Table 2-3. However, any changes noted
in the focal spot size with this technique should be indicative of changes in the actual size. In this
case, the size should be verified using the slit measurement technique.
Figure 2-11. Star Pattern Image Showing the Measurement of
the Diameter of the Image of the Pattern and the Diameter at Zero Contrast.
1. Misalignment of the test stand with respect to
the central beam of the x-ray tube can alter the
measurements.
2. The focal spot size may change with mA and
kVp. Refer to methods for evaluating x-ray tube
blooming characteristics
1 .
3. The manufacturer may require the use of direct
x-ray exposed film instead of an extremity or
mammography cassette system for acceptance
testing.
4. With the Star Test Pattern, several regions of
zero contrast may be found on a single film. It is
extremely important that the largest one be
used. If there is any doubt (i.e., the focal spot
calculation yields low values), then a second
exposure with a lower magnification should be
made and evaluated.
[
NOTE