Fluke Biomedical 07-611 - Manual (Page 14)

Nuclear Associates Radiographic and Mammographic Focal Spot Measurement Products
Operators Manual
2-8
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15. Compare the measured dimensions to the Focal Spot Size Tolerance Limits in Table 2-2. The
manufacturer may not accept the screen-film slit image measurements for acceptance testing
purposes. In this case it may be necessary to repeat the procedure using direct exposure x-ray film.
2.1.8 Procedure - Pinhole Measurement Technique
1. Replace the test stand alignment device with the appropriate pinhole assembly. (The size of the
pinhole diameter is engraved on the pinhole assembly.)
The following pinhole assemblies should be used:
Nominal Focal Spot Size (F, in
mm)
Model Pinhole Diameter
0.3 < F < 1.2 07-613 0.030 mm
1.2 < F < 2.5 07-617 0.075 mm
F > 2.5 07-611 0.100 mm
2. Cover the localization holes only, not the pinhole insert, with 0.25 mm lead-equivalent rubber. (This
reduces the exposure and film density of the localization holes so that it will be easy to measure the
distance between them.)
3. Place a cassette under the base for over-table x-ray tubes or on top of the stand for under- table
tubes.
4. Expose the cassette. A suggested x-ray setting for the 0.075 mm pinhole assembly (for use with a
focal spot size between 1.0 mm and 2.5 mm) is 75 kVp and 100 mAs using Kodak Lanex Fine
screens and Kodak TML film.
5. Develop and examine the resulting image of the focal spot (Figure 2-10). If the density of the
densest portion is between 0.8 and 1.2, the exposure is satisfactory. If the density range is
unsatisfactory, adjust the exposure conditions and repeat.
6. Measure the center-to-center distance between localization holes on the radiograph using the ruler
and determine the magnification using the formula:
image hole separation
Sin (T + A)
- 1 = Magnification
where 13 mm is the separation of the localization holes in the pinhole assembly.
7. Place the focal spot image on an illuminated surface and measure with a 6X magnifier with graticle
scale. Most focal spots consist of two fairly dark lines having a relatively lighter area between them.
The focal spot dimensions are determined by measuring all perceptible portions of the image width
and length.
8. Follow step 14 of the Slit Measurement Procedure in Section 2.1.7.
9. For line-focus tubes, the measured focal spot length should be multiplied by a correction factor of
0.7.
10. Suggested focal spot size tolerances are given in Table 2-3. However, these measurements may
not be accepted by the manufacturer for acceptance test purposes. In this case, the size should be
verified using the slit measurement technique.