Fluke Biomedical 07-611 - Manual (Page 16)

Nuclear Associates Radiographic and Mammographic Focal Spot Measurement Products
Operators Manual
2-10
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The minimum focal spot size measurable with the
2° star is 0.5 mm. For smaller focal spots, a 1.5°
star (07-543), a 1° star (07-542) or a 0.5° star (07-
503) should be used.
2.3 Procedure - Star Measurement Technique
1. Replace the test stand alignment device with the appropriate star test pattern.
2. Place a direct exposure x-ray film under the base for over-table x-ray tubes or on top of the stand
for under-table tubes.
3. Select a technique of 75 kVp, one-half of the maximum mA and about 40 mAs.
4. Expose the x-ray film at the selected technique factors.
5. Process the film and examine the image (See Figure 2-10).
6. Determine the magnification (M) by dividing the diameter of the radiographic image of the star
pattern by the true diameter (45 mm) of the spokes only, i.e., do not include the outer edge of the
pattern (See Figure 2-10).
7. Scan image of the star pattern inward from the periphery to find the outer-most region at which the
image of the sectors bends or disappears. This is the region of zero contrast. Measure the diameter
of this region across its greatest extent. Repeat the measurement in the perpendicular direction. Let
these diameters be called D
1
and D
2
.
8. The focal spot size corresponding to the individual diameter dimensions can be determined by
formula:
N D
F =
57.3
x
(M-1)
Where: F is the focal spot size in mm.
N is the angle of the star pattern line (which is marked on the test pattern: 0.5°, 1°, 1.5°, or
2°).
D is the diameter of the zero contrast region in mm.
M is the magnification.
9. For the radiograph shown:
N = 2
98 mm
Magnification M =
45 mm
= 2.18
(M-1) = 1.18
For the anode-cathode dimension:
D
1
= 28 mm
2 28
mm F
1
=
57.3
x
1.18
F
1
= 0.83 mm
NOTE
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